151Fenner’s Veterinary Virology. DOI:© Elsevier Inc. All rights reserved.2011

10.1016/B978-0-12-375158-4.00007-9

Chapter ContentsProperties of Poxviruses 152

Classification 152Virion Properties 152Virus Replication 152

Members of the Genus Orthopoxvirus 155Vaccinia Virus and Buffalopox Virus 155Cowpox Virus 156Camelpox Virus 156Ectromelia Virus (Mousepox Virus) 156Monkeypox Virus 157Members of the Genus Capripoxvirus 157Sheeppox Virus, Goatpox Virus, and Lumpy Skin

Disease (of Cattle) Virus 157Clinical Features and Epidemiology 158Pathogenesis and Pathology 159Diagnosis 160Immunity, Prevention, and Control 160

Members of the Genus Suipoxvirus 160

Swinepox Virus 160Members of the Genus Leporipoxvirus 160Myxoma Virus, Rabbit Fibroma Virus, and Squirrel

Fibroma Virus 160Members of the Genus Molluscipoxvirus 161Molluscum Contagiosum Virus 161Members of the Genus Yatapoxvirus 161Yabapox and Tanapox Viruses 161Members of the Genus Avipoxvirus 162Fowlpox and Other Avian Poxviruses 162Members of the Genus Parapoxvirus 163Orf Virus (Contagious Ecthyema/Contagious Pustular

Dermatitis Virus) 163Pseudocowpox Virus 164Bovine Papular Stomatitis Virus 164Poxviruses of Fish 164Other Poxviruses 165Squirrel Poxvirus 165

Poxviridae

The family Poxviridae includes numerous viruses of veteri-nary and/or medical importance. Poxviruses are large DNA viruses that are capable of infecting both invertebrates and vertebrates. Poxvirus diseases occur in most animal spe-cies, and are of considerable economic importance in some regions of the world. Sheeppox, for example, has been erad-icated in many countries, whereas it remains enzootic in Africa, the Middle East, and Asia. A characteristic of many of these viruses is their common ability to induce charac-teristic “pox” (pockmark) lesions in the skin of affected animals.

The history of poxviruses has been dominated by small-pox. This disease, once a worldwide and greatly feared disease of humans, has now been eradicated by use of the vaccine that traces its ancestry to Edward Jenner and the cowsheds of Gloucestershire in England. Prior to Jenner’s innovations, immunization of humans required the danger-ous practice of “variolation”—specifically, the deliberate exposure to infectious smallpox virus. Although Jenner’s first vaccines probably came from cattle, the origins of mod-ern vaccinia virus, the smallpox vaccine virus, are unknown. In his Inquiry published in 1798, Jenner described the clini-cal signs of cowpox in cattle and humans and how human

infection provided protection against smallpox. Jenner’s discovery soon led to the establishment of vaccination pro-grams around the world. However, it was not until Pasteur’s work nearly 100 years later that the principle was used again—in fact it was Pasteur who suggested the general terms vaccine and vaccination (from vacca, Latin for cow) in honor of Jenner. Other important discoveries came from early research on myxoma virus, an important cause of dis-ease and high mortality in domestic rabbits, described first by Sanarelli in 1896. Myxoma virus is the cause of myxo-matosis in European rabbits (Oryctolagus cuniculus) and was the first viral pathogen of a laboratory animal to be described. Rabbit fibroma virus was first described in 1932 by Shope, as the cause of large wart-like tumors of the face, feet, and legs of affected North American Sylvilagus spp. rabbits, the first virus shown to cause tissue hyperplasia.

With the eradication of smallpox in the second half of the 20th century, use of the smallpox vaccine was discon-tinued throughout the world. However, vaccinia and other poxviruses are now used as vectors for delivering a wide range of microbial antigens in recombinant DNA vaccines. For example, a vaccinia virus vectored rabies vaccine has been widely used in some enzootic areas, to control rabies in

Chapter 7

PART | II Veterinary and Zoonotic Viruses152

wildlife. Similarly, canarypox virus vectored vaccines have been developed for canine distemper, West Nile, and equine influenza viruses, amongst others, as have recombinant raccoonpox virus vectored vaccines for rabies and feline panleukopenia. Additional potential future uses for pox-viruses include gene therapy and tissue-targeted oncolytic viral therapies for cancer treatment.

PROPeRTIes OF POxvIRuses

Classification

The family Poxviridae is subdivided into two sub-families: Chordopoxvirinae (poxviruses of vertebrates) and Entomopoxvirinae (poxviruses of insects). The subfamily Chordopoxvirinae is subdivided into eight genera (Table 7.1). Each of the genera, includes species that cause diseases in domestic or laboratory animals. Because of the large size and distinctive structure of poxvirus virions, negative-stain electron microscopic examination of lesion material is used in many veterinary and zoonotic virology laboratories for diagnosis—this method allows rapid visualization of pox-viruses in various specimens, but it does not allow specific verification of virus species or variants. Hence, diagnostic specimens are frequently left with a diagnosis of “poxvirus,” “orthopoxvirus,” or “parapoxvirus,” with further identifica-tion only pertaining to the species of origin. Characterization of these viruses with molecular methods will identify addi-tional pathogenic poxvirus species; for example, a poxvi-rus was recently associated with proliferative gill disease in farmed salmon, and preliminary evaluation is suggestive it is a member of Entomopoxvirinae.

virion Properties

Most poxvirus virions are pleomorphic, typically brick-shaped (220–450 nm 140–260 nm) wide with an irregular surface of projecting tubular or globular structures, whereas those of the genus Parapoxvirus are ovoid (250–300 nm long and 160–190 nm in diameter) with a regular surface (Figure 7.1; Table 7.2). Virions of the members of the genus Parapoxvirus are covered with long thread-like surface tubules, which appear to be arranged in crisscross fash-ion, resembling a ball of yarn. Virions of some ungrouped viruses from reptiles are brick shaped but have a surface structure similar to that of parapoxviruses (Figure 7.1). The virion outer layer encloses a dumbbell-shaped core and two lateral bodies. The core contains the viral DNA, together with several proteins. There is no isometric nucleocapsid conforming to either icosahedral or helical symmetry that is found in most other viruses; hence poxviruses are said to have a “complex” structure. Virions that are released from cells by budding, rather than by cellular disruption, have

an extra envelope that contains cellular lipids and several virus-encoded proteins.

The genome of poxviruses consists of a single mol-ecule of linear double-stranded DNA varying in size from 130 kbp (parapoxviruses), to 280 kbp (fowlpox virus), up to 375 kbp (entomopoxviruses). Poxvirus genomes have cross-links that join the two DNA strands at both ends; the ends of each DNA strand have long inverted tandemly repeated nucleotide sequences that form single-stranded loops. Poxviruses have more than 200 genes in their genomes, and as many as 100 of these encode proteins that are contained in virions. Many of the viral proteins with known functions are enzymes involved in nucleic acid syn-thesis and virion structural components. Examples of the former are DNA polymerase, DNA ligase, RNA polymer-ase, enzymes involved in capping and polyadenylation of messenger RNAs, and thymidine kinase. The genomes of members of the Poxviridae also include a remark-able number of genes encoding proteins that specifically counteract host adaptive and innate immune responses. The activities of these immunomodulating proteins are diverse, and include complement and serine protease inhibitors, proteins that modulate chemokine and cytokine activity, and those that specifically target innate immune pathways such as toll-like receptor complex signaling and interferon-induced antiviral resistance. As an exam-ple, poxviruses encode a variety of proteins that modu-late chemokine activity by functioning as: (1) chemokine receptor homologs that bind chemokines, (2) biologically inactive chemokine homologs that block authentic cellular receptors, or (3) chemokine binding proteins that neutralize chemokine activity. By interfering with normal chemok-ine responses and activity, these poxvirus-encoded immu-nomodulatory proteins inhibit the migration of leukocytes into areas of infection or injury. These proteins have no known mammalian homologs.

Poxviruses are transmitted between animals by sev-eral routes: by introduction of virus into skin abrasions, or directly or indirectly from a contaminated environment. Several poxviruses, including sheeppox, swinepox, fowlpox, and myxoma virus, are transmitted mechanically by biting arthropods. Poxviruses are generally indigenous to specific host niches, but, in many cases, they are not species-specific. Poxviruses are resistant in the environment under ambient temperatures, but can survive for many years in dried scabs or other virus-laden material.

virus Replication

Replication of poxviruses occurs predominantly, if not exclusively, in the cytoplasm. To achieve this independ-ence from the cell nucleus, poxviruses, unlike other DNA viruses, have evolved to encode the enzymes required for

Chapter | 7 Poxviridae 153

transcription and replication of the viral genome, several of which must be carried in the virion itself. Virus replica-tion begins after fusion of the extracellular enveloped vir-ion with the plasma membrane, or after endocytosis, and the virus core is then released into the cytoplasm where it “uncoats” (Figure 7.2).

Transcription is characterized by a cascade in which the transcription of each temporal class of gene (“early,” “intermediate,” and “late” genes) requires the presence of specific transcription factors that are transcribed from the preceding temporal class of genes. Intermediate gene tran-scription factors are encoded by early genes, whereas late

Table 7.1 Poxviruses: Host Range and Geographic Distribution

Genus Virus Major Hosts Host Range

Geographic Distribution

Orthopoxvirus Variola (smallpox) virus Humans Narrow Eradicated globallyVaccinia virus Numerous: humans, cattle,a buffalo,a

swine,a rabbitsaBroad Worldwide

Cowpox virus Numerous: rodents, domestic cats and large felids, cattle, humans, elephants, rhinoceros, okapi, mongoose

Broad Europe, Asia

Camelpox virus Camels Narrow Asia, AfricaEctromelia virus Mice, voles Narrow EuropeMonkeypox virus Numerous: squirrels, monkeys,

anteaters, great apes, humansBroad Western and

central AfricaUasin Gishu disease virus Horses Broad Eastern AfricaTatera poxvirus Gerbils (Tatera kempi) ? Western AfricaRaccoon poxvirus Raccoons Broad North AmericaVolepox virus Voles (Microtus californicus) ? CaliforniaSkunkpox virus Skunks (Mephitis mephitis) ? North America

Capripoxvirus Sheeppox virus Sheep, goats Narrow Africa, AsiaGoatpox virus Goats, sheep Narrow Africa, AsiaLumpy skin disease virus Cattle, Cape buffalo Narrow Africa

Suipoxvirus Swinepox virus Swine Narrow Worldwide

Leporipoxvirus Myxoma virus, rabbit fibroma virus Rabbits (Oryctolagus and Sylvilagus spp.) Narrow Americas, Europe, Australia

Hare fibroma virus European hare (Lepus europaeus) Narrow EuropeSquirrel fibroma virus Eastern and western gray (Sciurus

carolinensis), red (Tamaiasicuris hudsonicus), and fox (S. niger) squirrels

Narrow North America

Molluscipoxvirus Molluscum contagiosum virus Humans, non-human primates, birds, kangaroos, dogs and equids

Broad Worldwide

Yatapoxvirus Yabapox virus and tanapox virus Monkeys, humans Narrow West Africa

Avipoxvirus Fowlpox virus, canarypox, crowpox, juncopox, mynahpox, pigeonpox, psittacinepox, quailpox, sparrowpox, starlingpox, turkeypox (etc.) viruses

Chickens, turkeys, many other bird species

Narrow Worldwide

Parapoxvirus Orf virus Sheep, goats, humans (related viruses of camels and chamois)

Narrow Worldwide

Pseudocowpox virus Cattle, humans Narrow WorldwideBovine papular stomatitis virus Cattle, humans Narrow WorldwideAusdyk virus Camels Narrow Africa, AsiaSealpox virus Seals, humans Narrow WorldwideParapoxvirus of red deer Red deer Narrow New Zealand

Currently unclassified

Poxviruses of fish—carp edema and proliferative gill disease viruses

Koi (Cyprinus carpio), Atlantic salmon (Salmo salar)

Narrow Narrow

Japan, Norway

Squirrel Poxvirus Red and gray squirrels Narrow Europe and North America

aInfected from humans; now that smallpox vaccination has been discontinued for the civilian populations of all countries, such infections are unlikely to be seen.

PART | II Veterinary and Zoonotic Viruses154

transcription factors are encoded by intermediate genes. Transcription is initiated by the viral transcriptase and other factors carried in the core of the virion that mediate the pro-duction of messenger RNAs within minutes after infection. These early transcripts are synthesized from both DNA

strands, and extruded from the virus core particle before translation by host-cell ribosomes. Proteins produced by translation of these messenger RNAs complete the uncoat-ing of the core and transcription of about 100 early genes; all this occurs before viral DNA synthesis begins. Early

Surfacetubules

Lateralbody

EnvelopeSurface membrane

Nucleoprotein 100 nm Coremembrane

Nucleoprotein

Surfacemembrane

Envelope

Lateralbody

Surfacefilament

genus: Parapoxvirusgenus: Orthopoxvirus

Core membrane

(A)

(C)

(B)

FIGuRe 7.1 Poxviridae (bar 100 nm). (A) Negatively stained vaccinia virus virions showing surface tubules characteristic of member viruses of all genera except the genus Parapoxvirus. (B) Negatively stained orf virus showing characteristic surface tubules of the member viruses of the genus Parapoxvirus. (C, left) Schematic diagram, genus Orthopoxvirus (and all other vertebrate poxvirus genera except the genus Parapoxvirus). (C, right) Schematic diagram, genus Parapoxvirus. Part of the two diagrams shows the surface structure of an unenveloped virion, whereas the other part shows a cross-section through the center of an enveloped virion.

Table 7.2 Properties of Poxviruses

Virions in most genera are brick-shaped (220–450 140–260 nm), with an irregular arrangement of surface tubules. Virions of members of the genus Parapoxvirus are ovoid (250–300 160–190 nm), with regular spiral arrangement of surface tubules.

Virions have a complex structure with a core, lateral bodies, outer membrane, and sometimes an envelope.

Gernome is composed of a single molecule of linear double-stranded DNA, 170–250 kbp (genus Orthopoxvirus), 300 kbp (genus Avipoxvirus), or 130–150 kbp (genus Parapoxvirus) in size.

Genomes have the capacity to encode about 200 proteins, as many as 100 of which are contained in virions. Unlike other DNA viruses, poxviruses encode all the enzymes required for transcription and replication, many of which are carried in the virion.

Cytoplasmic replication, enveloped virions released by exocytosis; non-enveloped virions released by cell lysis.

Chapter | 7 Poxviridae 155

proteins include DNA polymerase, thymidine kinase, and several other enzymes required for replication of the genome. Some viral proteins require post-translational modification by proteolytic cleavage, phosphorylation, gly-cosylation, etc. Host macromolecular synthesis is inhibited with the production of viral proteins.

Poxvirus DNA replication involves the synthesis of long concatameric intermediates, which are subsequently cut into unit-length genomes that are ultimately covalently linked. With the onset of DNA replication there is a dra-matic shift in gene expression. Transcription of “interme-diate” and “late” genes is controlled by binding of specific viral proteins to promoter sequences in the viral genome. Some early gene transcription factors are made late in infection, packaged in virions, and used in the subsequent round of infection.

Because poxvirus virions are composed of a very large number of proteins, it is not surprising that virus assem-bly is a complex process that requires several hours to be completed. Virion formation involves coalescence of DNA within crescent-shaped immature core structures, which then mature by the addition of outer coat layers. Replication and assembly occur in discrete sites within the cytoplasm (called viroplasm or virus factories), and virions are released by budding (enveloped virions), by exocytosis, or by cell lysis (non-enveloped virions). Most virions are not enveloped and

are released by cell lysis. Both enveloped and non-enveloped virions are infectious, but they apparently infect cells by dif-ferent pathways; enveloped virions are taken up by cells more readily and appear to be more important in the spread of virions through the body of the animal.

MeMbers of The Genus OrthOpOxvirus

vACCInIA vIRus And BuFFAlOPOx vIRus

Because of its widespread use and its wide host range, vac-cinia virus sometimes has caused naturally spreading dis-eases in domestic animals (e.g., teat infections of cattle) and also in laboratory rabbits (“rabbitpox”). Outbreaks of dis-ease associated with “vaccinia-like” viruses (Aracatuba and Cantagalo viruses) have been reported among dairy cattle and humans in Brazil, and genetic analyses of selected viral genes showed these viruses to be related most closely to vaccinia virus. Before human vaccination against smallpox had been discontinued, putative instances of cowpox were frequently caused by vaccinia virus infection.

Outbreaks of buffalopox affecting buffalos, cows, and humans have been recorded regularly in the Indian subcon-tinent and Egypt. The causative agent is an orthopoxvirus

Uncoating I

Uncoating IIDNA

replication

Recombination

Virion proteinsCore

Membranes

Growth factorsand hostresponsemodifiers

Membranes

Early enzymes+ proteins

Nuclear factors?

Morphogenesis

Lipids A-typeinclusion

body

Late enzymesearly

transcriptionsystem,

latevirion protein

LatemRNA

IntermediatemRNA

Latetranscription

factors

Actintail

Golgi

Golgi-wrapped

or

IMV

EEV

CEV IEV

IEV

IMV

Host cellmodification

Early mRNA

Early RNAtranscriptionmachinery

FIGuRe 7.2 The infectious cycle of vaccinia virus. IEV, intracellular enveloped virus; EEV, extracellular enveloped virus; CEV, cell-associated envel-oped virions; IMV, intracellular mature virus. [From Virus Taxonomy: Eighth Report of the International Committee on Taxonomy of Viruses (C. M. Fauquet, M. A. Mayo, J. Maniloff, U. Desselberger, L. A. Ball, eds.), p. 120. Copyright © Elsevier (2005), with permission.]

PART | II Veterinary and Zoonotic Viruses156

that is related so closely to the vaccinia virus that it is con-sidered a clade. The disease is characterized by pustular lesions on the teats and udders of milking buffalo. Lesions also can occur at the base of the ear and in the inguinal region. Rarely, especially in calves, a generalized disease occurs. Outbreaks still occur in India (even though vaccinia virus is not used for any type of vaccination in the coun-try), sometimes producing lesions on the hands and face of milkers who are no longer protected by vaccination against smallpox.

COwPOx vIRus

Inappropriately named, cowpox virus has as its reservoir hosts rodents, from which the virus occasionally spreads to domestic cats, cows, humans, and zoo animals, includ-ing large felids (especially cheetahs, ocelots, panthers, lynx, lions, pumas, and jaguars), anteaters, mongooses, rhinoceroses, okapis, and elephants. Cowpox virus infec-tion is enzootic in Europe and adjacent regions of Russia. During an outbreak at the Moscow zoo, the virus was also isolated from laboratory rats used to feed the big cats, and a subsequent survey demonstrated infection in wild sus-liks (Spermophilus citellus and S. suslicus) and gerbils (Rhombomys opimus) in Russia. In Germany, transmission of cowpox virus occurred from rat to elephant to human. The elephant exhibited disseminated ulcerative lesions of the skin and mucosal membranes. In the United Kingdom, the reservoir species are bank voles (Clethrionomys glareolus), field voles (Microtus agrestis), and wood mice (Apodemus sylvaticus). Zoonotic transmission of cowpox virus from pet rats has been reported with increasing frequency from sev-eral countries in Europe. Lesions in humans usually appear as single maculopapular eruptions on the hands or the face with minimal systemic reaction, except in immunosup-pressed patients.

Clinical cowpox disease in cattle is extremely rare, but occurs sporadically in enzootic areas. Cowpox virus pro-duces lesions on the teats and the contiguous parts of the udder of cows, and is spread through herds by the process of milking. Cowpox virus infection in domestic cats is often a more severe disease than in cattle or humans. There is typically a history of a single primary lesion manifest as necrotizing dermatitis, generally on the head or a forelimb, but by the time the cat is presented for veterinary attention, widespread skin lesions have usually developed. Pulmonary infection and even disseminated systemic infection some-times occur in cats, typically with fatal consequences.

Cowpox virus, like smallpox, monkeypox and other pathogenic orthopoxviruses, encodes a unique family of ankyrin repeat-containing proteins that inhibit the nuclear factor B (NF-B) signaling pathway and so inhibit inflammation at the sites of viral infection.

CAMelPOx vIRus

Camelpox virus infection causes a severe generalized disease in camels and dromedaries that is characterized by extensive skin lesions. It is an important disease, especially in countries of Africa, the Middle East, and southwestern Asia, where the camel is used as a beast of burden and for milk. The more severe cases usually occur in young animals, and in epizoot-ics the case-fatality rate may be as high as 25%. The causative virus is a distinctive orthopoxvirus species, and comparative genome analysis shows camelpox virus to be closely related to other orthopoxviruses, including variola virus (smallpox virus). Genomic differences that distinguish camelpox from other orthopoxviruses occur in genes that probably determine either host range or virulence. Camelpox virus has a narrow host range, and despite the frequent exposure of unvaccinated humans to florid cases of camelpox, human infection has not been described. A parapoxvirus (Ausdyk virus) also infects camels, producing a disease that can be confused with camel-pox (Table 7.1).

eCTROMelIA vIRus (MOusePOx vIRus)

Ectromelia virus, the cause of mousepox, has been spread around the world inadvertently in shipments of laboratory mice and mouse products, and has been repeatedly reported from laboratories in the United States, Europe, and Asia. Outbreaks in mouse colonies in the United States have resulted from importation of infected mice or products from other countries—for example, via mouse tumor material and commercial sources of mouse serum from China. The origin of ectromelia virus remains a mystery. It first appeared in a laboratory mouse colony in England, involving mice with amputation of limbs and tails. The name is derived from the Greek designations ectro, which means abortion, and melia, which means limb (Figure 7.3). The disease has since spread throughout the world, but its occurrence is sporadic and rare.

There are several named strains of ectromelia virus that vary in virulence, including NIH-79, Wash-U, Moscow,

FIGuRe 7.3 Ectromelia: healed amputating lesions of the distal extremities of a mouse that survived natural ectromelia virus infection. [From Pathology of Laboratory Rodents and Rabbits, D. H. Percy, S. W. Barthold, 3rd ed., p. 127. Copyright © Wiley-Blackwell (2007), with permission.]

Chapter | 7 Poxviridae 157

Hampstead, St. Louis-69, Bejing-70, and Ishibashi I–III. Disease severity is determined by virus strain, but mouse geno-type and age are also important determinants. Susceptible mouse strains include C3H, A, DBA, SWR, CBA, and BALB/c. Resistant mouse strains include AKR and C57BL/6. Infection is acquired primarily through skin abrasions and direct contact. Virus may be shed from skin, respiratory secre-tions, feces, and urine. Highly susceptible genotypes of mice develop disseminated infection, but die rapidly within hours and shed little virus. Highly resistant genotypes of mice develop more limited infections and recover before shedding virus. Mice with intermediate susceptibility are therefore critical for outbreaks of disease, in that they develop dissemi-nated infections and survive long enough to spread virus to other animals. Under these circumstances, such mice develop multifocal necrotizing lesions in all organs, particularly liver, lymphoid tissues, and spleen, as well as disseminated rash and gangrene of limbs. Necrosis of Peyer’s patches in the intestine may result in intestinal hemorrhage. Colonies that contain mice of various genotypes and immune perturbations are most at risk for high mortality, in that they may contain semi-susceptible mice that sustain infection, and highly sus-ceptible mice that contribute to high mortality. Under these circumstances, the typical clinical picture within the popula-tion is a spectrum of clinical disease, ranging from subclinical infections to high mortality.

The consequences of the introduction of ectrome-lia virus into a mouse colony are sufficiently serious that rapid and definitive diagnosis is required. Mousepox can be diagnosed by the histopathologic examination of tissues of suspected cases, its diagnostic features being the typi-cal clinical signs and gross lesions and, histologically, the presence of multifocal necrosis of many tissues, with dis-tinctive eosinophilic cytoplasmic inclusion bodies in epithe-lial cells at the edges of skin lesions and mucosa. Electron microscopy is also a valuable diagnostic adjunct: distinc-tive virions may be seen in any infected tissue. Virus may be isolated in mouse embryo cell cultures and identified by immunological means.

Because mice are infected readily by inoculation, virus-contaminated mouse serum, hybridoma lines, trans-plantable tumors, or tissues constitute a risk to laboratory colonies previously free of infection. Prevention and control of mousepox are based on quarantine and regulation of the importation and distribution of ectromelia virus, mice, and materials that may be carrying the virus. However, because such precautions offer no protection against unsuspected sources of infection, regular serologic testing (enzyme-linked immunosorbent assay) is performed in many colonies housing valuable animals. In immunocompetent strains of mice, infection is acute and animals recover with no car-rier status. Thus seropositive animals can be quarantined, held without breeding for a few weeks, and then used to

re-establish breeding colonies. Vaccination with vaccinia virus (IHD-T strain) has been used to protect valuable col-onies against severe clinical disease, but vaccination will not prevent ectromelia virus infection or transmission. Vaccination, however, will also obscure serosurveillance, because vaccinia virus is transmissible among mice and may remain enzootic within the population.

MOnkeyPOx vIRus

Monkeypox virus is a zoonotic agent with a broad host range that includes humans. Outbreaks of human disease occur in villages in the tropical rain forests of west and central Africa, especially in the Democratic Republic of Congo. The virus was discovered in 1958, when it was iso-lated from pox lesions of cynomolgus macaques imported into Denmark. The first human cases were recognized in the 1970s. The signs and symptoms are very like those of smallpox, with a generalized pustular rash, fever, and lym-phadenopathy. Monkeypox virus is acquired by humans by direct contact with wild animals killed for food, especially squirrels and monkeys. The virus is maintained in rodents and non-human primate species.

The human disease is relatively uncommon, although more than 500 human cases were reported in the Congo in 1996–1997, the largest reported outbreak of the disease. In 2003, a widely publicized outbreak of monkeypox virus infection occurred in the United States. In this outbreak, monkeypox virus was transmitted from imported African rodents [Funisciurus spp. (rope squirrel), Cricetomys spp. (giant pouched rat), and Graphiurus spp. (African dormouse)] to co-housed prairie dogs (Cynomys spp.). Infected prairie dogs then transmitted the virus to humans. A total of 82 infections in children and adults occurred dur-ing the outbreak, which subsequently resulted in a ban on the importation of African rodents into the United States.

MeMbers of The Genus CapripOxvirus

sheePPOx vIRus, GOATPOx vIRus, And luMPy skIn dIseAse (OF CATTle) vIRus

Although the geographic distribution of sheeppox, goatpox, and lumpy skin disease is very different, suggesting that they are caused by distinct viruses, the causative viruses are indistinguishable by conventional serological assays and are genetically very similar. The African strains of sheeppox and lumpy skin disease viruses are related more closely to each other than sheeppox virus is to goatpox virus. Although sheeppox and goatpox are considered to be host specific, in parts of Africa where sheep and goats are herded together, both animal species may show clinical signs during an

PART | II Veterinary and Zoonotic Viruses158

outbreak, indicating that some virus strains may infect both sheep and goats.

Sheeppox, goatpox and lumpy skin diseases are consid-ered to be the most important of all pox diseases of domestic animals, because they cause significant economic loss and high mortality in young and/or immunologically naïve ani-mals. Furthermore, these viruses are currently expanding their distribution, with recent outbreaks of sheeppox or goatpox in Vietnam, Mongolia, and Greece, and outbreaks of lumpy skin disease in Ethiopia, Egypt, and Israel (Figure 7.4).

Lumpy skin disease affects cattle breeds derived from both Bos taurus and Bos indicus, and was first recognized in an extensive epizootic in Zambia in 1929. An epizootic in 1943–1944 that involved other countries, including South Africa, emphasized the importance of this disease, which remained restricted to southern Africa until 1956, when it spread to central and eastern Africa. Since the 1950s, the virus has continued to spread progressively throughout

Africa, first north to the Sudan and subsequently westward, to appear by the mid-1970s in most countries of western Africa. In 1988, the disease was confirmed in Egypt, and in 1989 a single outbreak occurred in Israel, the first report outside the African continent.

Clinical Features and epidemiology

In common with most poxviruses, environmental contami-nation can lead to the introduction of sheep or goat pox-virus into small skin wounds. Scabs that have been shed by infected sheep remain infective for several months. The common practice of herding sheep and goats into enclosures at night in countries where the disease occurs provides suf-ficient exposure to maintain enzootic infection. During an outbreak, the virus is probably transmitted between sheep by respiratory droplets; there is also evidence that mechanical transmission by biting arthropods, such as stable flies, may

(A)

(B)

FIGuRe 7.4 Map showing likely global distribution of (A) sheeppox and goatpox, and (B) lumpy skin disease (LSD) viruses. Recent outbreaks are marked with arrows. [From S. L. Babiuk, T. R. Bowden, S. B. Boyle, D. B. Wallace, R. P. Kitchen. Capripoxviruses: an emerging worldwide threat to sheep, goats and cattle. Transbound. Emerg Dis. 55, 263–272 (2008), with permission.]

Chapter | 7 Poxviridae 159

be important. Lumpy skin disease has shown the potential to spread outside continental Africa. It is likely that the virus is transmitted mechanically between cattle by biting insects, with the virus being perpetuated in a wildlife reservoir host, possibly the African Cape buffalo. Because it is transmitted principally by insect vectors, the importation of wild rumi-nants to zoos could establish new foci of infection, if suit-able vectors were available.

Clinical signs vary in different hosts and in different geographical areas, but the signs of sheeppox, goatpox, and lumpy skin disease of cattle are similar. Sheep and goats of all ages may be affected, but the disease is generally more severe in young and/or immunologically naïve animals. An epizootic in a susceptible flock of sheep can affect over 75% of the animals, with mortality as high as 50%; case-fatality rates in young and/or naïve sheep may approach 100%. After an incubation period of 4–8 days, there is an increase in temperature, an increase in respiratory rate, edema of the eyelids, and a mucous discharge from the nose. Affected

sheep may lose their appetite and stand with an arched back. One to 2 days later, cutaneous nodules about 1 cm in diam-eter develop; these may be distributed widely over the body (Figure 7.5). These lesions are most obvious in the areas of skin where the wool hair is shortest, such as the head, neck, ears, axillae, and under the tail. These lesions usually scab and persist for 3–4 weeks, healing to leave a permanent depressed scar. Lesions within the mouth affect the tongue and gums, and ulcerate. Such lesions constitute an impor-tant source of virus for infection of other animals. In some sheep, lesions that develop in the lungs progress to multicen-tric areas of pulmonary fibrosis and consolidation. Goatpox is similar clinically to sheeppox.

Lumpy skin disease of cattle is characterized by fever, followed shortly by the development of nodular lesions in the skin that subsequently undergo necrosis (Figure 7.6). Generalized lymphadenitis and edema of the limbs are com-mon. During the early stages of the disease, affected cat-tle show lacrimation, nasal discharge, and loss of appetite. The skin nodules involve the dermis and epidermis; they are raised and later ulcerate, and may become infected secondar-ily. Ulcerated lesions may be present in the mouth and nares. Healing is slow and affected cattle often remain debilitated for several months (Figure 7.6). Morbidity in susceptible herds can be as high as 100%, but mortality is rarely more than 1–2%. The economic importance of the disease relates to the prolonged convalescence and, in this respect, lumpy skin disease is similar to foot-and-mouth disease.

Pathogenesis and Pathology

Sheeppox, goatpox, and lumpy skin disease viruses all have tropism for epithelial cells. Sheeppox and goatpox virus infection in immunologically naïve animals leads to concurrent fever and skin papules, followed by rhinitis,

FIGuRe 7.5 Sheeppox, with characteristic raised skin lesions. (Courtesy of D. Rock, University of Illinois.)

(A) (B)

FIGuRe 7.6 (A) Acute lumpy skin disease in cattle. (B) Animal approximately 2 months after infection with lumpy skin disease virus. (Courtesy of M. Scacchia, Namibia.)

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conjunctivitis, and hypersalivation. Although pox lesions can be widespread, the more common presentation is a few nodules beneath the tail. Pox lesions also develop in the lungs and gastrointestinal tract. High viral loads occur in the skin and viremia is probably cell associated. Lesions seen at necropsy include tracheal congestion and patchy discoloration of the lungs. The spleen and lymph nodes are enlarged, with multifocal to coalescing areas of necrosis. The essential histological lesion is necrosis, with depletion of lymphocytes in the paracortical regions and absence of germinal centers in the spleen and lymph nodes.

Lumpy skin disease is most commonly recognized by widespread skin lesions. Disease is characterized by fever, lymphadenopathy, and skin nodules that persist for many months. Certain breeds of cattle such as Jersey and Guernsey have enhanced susceptibility.

diagnosis

Apart from occasional outbreaks in partly immune flocks—in which the disease may be mild—or when the presence of orf (contagious ecythema) complicates the diagnosis, sheeppox and goatpox present little difficulty in clinical diagnosis. For presumptive laboratory diagnosis, negative-contrast electron microscopy can be used to demonstrate virions in clinical material, as the virions are indistinguish-able from those of vaccinia virus. The viruses can be iso-lated in various cell cultures derived from sheep, cattle, or goats; the presence of virus is indicated by cytopathology and cytoplasmic inclusion bodies. The clinical diagnosis of lumpy skin disease also presents few problems to clinicians familiar with it, although the early skin lesions can be con-fused with generalized skin infections of pseudo lumpy skin disease, caused by bovine herpesvirus 2.

Immunity, Prevention, and Control

Control of sheeppox, goatpox, and lumpy skin disease in free countries is by exclusion; these are notifiable diseases in most countries of the world, with any suspicion of disease requir-ing disclosure to appropriate authorities. Control in countries where the diseases are enzootic is by vaccination; attenuated virus and inactivated virus vaccines are used. Two vaccines are currently available: in South Africa an attenuated virus vaccine (Neethling) is used, and in Kenya a strain of sheep/goatpox virus propagated in tissue culture has been used.

MeMbers of The Genus suipOxvirus

swInePOx vIRus

Swinepox virus is the sole member of the Suipoxvirus genus within the subfamily Chordopoxvirinae. Swinepox-virus-induced disease occurs worldwide and is associated

with poor sanitation. Comparative genetic analyses indi-cate that swinepox virus is most closely related to lumpy skin disease virus, followed by yatapoxvirus and leporipox- viruses. Many outbreaks of poxvirus disease in swine have been caused by vaccinia virus, but swinepox virus is now the primary cause of the disease. Swinepox is most severe in pigs up to 4 months of age, in which morbidity may approach 100%, whereas adults usually experience a mild disease with lesions restricted to the skin. The typical “pox” lesions may occur anywhere, but are most obvious on the skin of the abdomen. A transient low-grade fever may pre-cede the development of papules which, within 1–2 days, become vesicles and then umbilicated pustules, 1–2 cm in diameter. The pocks crust over and scab by 7 days; healing is usually complete by 3 weeks. The clinical picture is char-acteristic, so laboratory confirmation is seldom required.

Swinepox virus is transmitted most commonly between swine by the bite of the pig louse, Hematopinus suis, which is common in many herds; the virus does not replicate in the louse, but sporadic vertical transmission has been reported. No vaccines are available for swinepox, which is controlled most easily by elimination of the louse from the affected herd and by improved hygiene. As with other poxviruses of livestock, swinepox virus is being developed as a recom-binant vaccine vector for expression of heterologous genes.

MeMbers of The Genus LepOripOxvirus

MyxOMA vIRus, RABBIT FIBROMA vIRus, And squIRRel FIBROMA vIRus

Myxoma virus causes localized benign fibromas in its natu-ral hosts, wild rabbits in the Americas (Sylvilagus spp.); in contrast, it causes a severe generalized disease in European rabbits (Oryctolagus cuniculus), with a very high mortal-ity rate. Myxoma virus originated in the Americas, but is now enzootic on four continents: North and South America, Europe, and Australia. The characteristic early signs of myxomatosis in the European rabbit are blepharoconjunc-tivitis and swelling of the muzzle and anogenital region, giving animals a leonine appearance (Figure 7.7). Infected rabbits become febrile and listless, and often die within 48 hours of onset of clinical signs. This rapid progression and fatal outcome are seen especially with the California strain of myxoma virus. The myxoma virus genome encodes a number of immunomodulatory proteins that target host cytokines, host-cell signaling cascades, and apoptosis, and these probably contribute to the virulence of individual virus strains. In rabbits that survive longer, subcutaneous gelati-nous swellings (hence the name myxomatosis) appear all over the body within 2–3 days. The vast majority of rabbits (over 99%) infected from a wild (Sylvilagus spp.) source of myxoma virus die within 12 days of infection. Transmission

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can occur via respiratory droplets, but more often via mechanical transmission by arthropods (mosquitoes, fleas, black flies, ticks, lice, mites).

Diagnosis of myxomatosis in European rabbits can be made by the clinical appearance or virus isolation in rab-bits, on the chorioallantoic membrane of embryonated hens’ eggs, or in cultured rabbit or chicken cells. Electron microscopy of exudates or smear preparations from lesions reveals virions morphologically indistinguishable from those of vaccinia virus.

Laboratory or hutch rabbits can be protected against myxomatosis by inoculation with the related rabbit fibroma virus or with attenuated myxoma virus vaccines developed in California and France.

Myxoma virus was the first virus ever introduced into the wild with the purpose of eradicating a vertebrate pest, namely the feral European rabbit in Australia in 1950, and in Europe 2 years later. History confirms the long-term fail-ure of this strategy.

Although myxoma virus receives the most attention, there are many antigenically distinct but related poxviruses of wild Oryctolagus and Sylvilagus rabbits and Lepus spp. hares in Europe and the Americas, including rabbit fibroma virus (or Shope fibroma virus), hare fibroma virus, and myxoma virus. Myxoma virus is considered a variant of rabbit fibroma virus; indeed, California myxoma virus is also termed “California rabbit fibroma virus.” Myxoma and rabbit fibroma viruses originated in the Americas, whereas hare fibroma virus was originally indigenous to Europe. All leporid species are susceptible to infection with these various leporipoxviruses. Less virulent viruses and those that infect their natural hosts tend to produce localized fibromatous lesions, whereas virulent isolates tend to pro-duce myxomatous lesions in aberrant Oryctolagus hosts.

American gray squirrels (Sciurus spp.) and red squir-rels (Tamiasciurus spp.) develop natural outbreaks of squir-rel fibromatosis as a result of a virus that is closely related to

myxoma and rabbit fibroma leporipoxviruses. The animals develop multifocal to coalescing, nodular, tan cutaneous lesions, often involving the head, and disseminated lesions in internal organs, characterized by focal proliferation of mesen-chymal cells with cytoplasmic inclusions. Natural outbreaks of squirrel fibromatosis occur periodically in some regions of the United States, resulting in declines in squirrel populations.

MeMbers of The Genus MOLLusCipOxvirus

MOllusCuM COnTAGIOsuM vIRus

Molluscum contagiosum virus is a human pathogen, but it has been documented as naturally producing similar lesions in birds (chickens, sparrows, and pigeons), chimpanzees, kan-garoos, dogs, and horses, among other species. Infection is characterized by multiple discrete nodules 2–5 mm in diam-eter, limited to the epidermis, and occurring anywhere on the body except on the soles and palms. The nodules are pearly white or pink in color and painless. The disease may last for several months before recovery occurs. Cells in the nodule are hypertrophied greatly and contain pathognomonic large hya-line acidophilic cytoplasmic masses called molluscum bodies. These consist of a spongy matrix divided into cavities, in each of which are clustered masses of virus particles that have the same general structure as those of vaccinia virus. The disease is seen most commonly in children and occurs worldwide, but is much more common in some localities—for example, parts of the Democratic Republic of Congo and Papua New Guinea. The virus is transmitted by direct contact, perhaps through minor abrasions and sexually in adults. In developed countries, communal swimming pools and gymnasiums have been sources of contagion. Infection in animals is rare, and is typically associated with human contact.

MeMbers of The Genus YatapOxvirus

yABAPOx And TAnAPOx vIRuses

Yabapox and tanapox occur naturally only in tropical Africa. The yabapox virus was discovered because it pro-duced large benign tumors on the hairless areas of the face, on the palms and interdigital areas, and on the mucosal surfaces of the nostrils, sinuses, lips, and palate of Asian monkeys (Cercopithecus aethiops) kept in a laboratory in Nigeria. Subsequent cases occurred in primate colonies in California, Oregon, and Texas. Yabapox is believed to cause epizootic infection in African and Asian monkeys. The virus is zoonotic, spreading to humans in contact with diseased monkeys and causing similar lesions as in affected monkeys.

Tanapox is a relatively common skin infection of humans in parts of Africa, extending from eastern Kenya to the Democratic Republic of Congo. It appears to be spread

FIGuRe 7.7 Myxomatosis in a laboratory rabbit (Oryctolagus cunicu-lus), showing generalized facial lesions. (Courtesy of S. Barthold and D. Brooks, University of California.)

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mechanically by insect bites from an unknown wild animal reservoir, probably a species of monkey. In humans, skin lesions start as papules that progress to vesicles. There is usually a febrile illness lasting 3–4 days, sometimes with severe headache, backache, and prostration.

MeMbers of The Genus avipOxvirus

FOwlPOx And OTheR AvIAn POxvIRuses

Serologically related poxviruses that specifically infect birds have been recovered from lesions in all species of poultry and many species of wild birds, with natural pox virus infections having been described in 232 species in 23 orders of birds. Viruses recovered from various species of birds are given names pertaining to their respective hosts, such as fowlpox (chickens), canarypox, turkeypox, pigeon-pox, magpiepox, etc. Differences in the genome sequences and biological properties of individual viruses confirm that there are several different species of avian poxviruses. Mechanical transmission by arthropods, especially mos-quitoes, provides a mechanism for transfer of the viruses between different species of birds.

Fowlpox is a serious disease of poultry that has occurred worldwide for centuries. Fowlpox virus is highly infec-tious for chickens and turkeys, rarely so for pigeons, and not at all for ducks and canaries. In contrast, turkeypox virus is virulent for ducks. There are two forms of fowlpox, probably associated with different routes of infection. The most com-mon form, the cutaneous form—which probably results from infection by biting arthropods or mechanical transmission to injured or lacerated skin—is characterized by small papules on the comb, wattles, and around the beak; lesions occa-sionally develop on the legs and feet and around the cloaca. The nodules become yellowish and progress to a thick dark scab. Multiple lesions often coalesce. Involvement of the skin around the nares may cause nasal discharge, and lesions on the eyelids can cause excessive lacrimation and predispose poultry to secondary bacterial infections. In uncomplicated cases, healing occurs within 3 weeks. The second form of fowlpox is probably caused by droplet infection and involves infection of the mucous membranes of the mouth, pharynx, larynx, and sometimes the trachea (Figure 7.8A). This is often referred to as the diphtheritic or wet form of fowlpox because the lesions, as they coalesce, result in a necrotic pseudomem-brane, which can cause death by asphyxiation. The prognosis for this form of fowlpox is poor. Extensive infection in a flock may cause a slow decline in egg production. Cutaneous infec-tion causes little mortality, and these flocks return to normal production on recovery. Recovered birds are immune.

Under natural conditions there may be breed differences in susceptibility; chickens with large combs appear to be

more affected than those with small combs. The mortality rate is low in healthy flocks, but in laying flocks and in chickens in poor condition or under stress the disease may assume serious proportions with mortality rates of 50% or even higher, although such mortality is rare.

(A)

(B)

FIGuRe 7.8 Avian poxvirus disease. (A) Avian pox affecting the oral cavity and stomach. (B) Histological appearance of avian pox disease; epidermal hyperplasia with characteristic eosinophilic (red) intracytoplas-mic inclusion bodies. (A: Courtesy of L. Woods, University of California.)

Chapter | 7 Poxviridae 163

The cutaneous form of fowlpox seldom presents a diag-nostic problem. The diphtheritic form is more difficult to diagnose, because it can occur in the absence of skin lesions and may be confused with vitamin A, pantothenic acid, or biotin deficiency, T-2 mycotoxicosis-induced con-tact necrosis, and several other respiratory diseases caused by viruses such as infectious laryngotracheitis herpesvirus. Histopathology and electron microscopy are used to con-firm the clinical diagnosis. Typical lesions include extensive, local hyperplasia of the epidermis and underlying feather follicle epithelium, with accompanying ulceration and scab-bing. Histologically, the hyperplastic epithelium contains cells with characteristic large, intracytoplasmic eosinophilic inclusion bodies (Figure 7.8B). The virus can be isolated by the inoculation of avian cell cultures or the chorioallantoic membrane of embryonated eggs.

Fowlpox virus is extremely resistant to desiccation: it can survive for long periods under the most adverse envi-ronmental conditions in exfoliated scabs. The virus is transmitted within a flock through minor wounds and abra-sions, by fighting and pecking, mechanically by mosqui-toes, lice, and ticks, and possibly by aerosols.

Several types of vaccine are available. Non-attenuated fowlpox virus and pigeonpox virus vaccines prepared in embryonated hens’ eggs and attenuated virus vaccines pre-pared in avian cell cultures are widely used for vaccination. Vaccines are applied by scarification of the skin of the thigh. One vaccine can be administered in drinking water. In flocks with enzootic infection, birds are vaccinated during the first few weeks of life and again 8–12 weeks later. Recombinant vaccines for poultry have been developed using either fowl-pox or canarypox viruses as vectors. In poultry, fowlpox-vectored vaccines have been licensed with gene inserts for Newcastle disease virus (paramyxovirus), H5 and H7 avian influenza virus (orthomyxoviruses), infectious laryngotra-cheitis (herpesvirus), infectious bursal disease virus (birna-virus), and Mycoplasma spp. These viruses have also been utilized as vaccine vectors in mammals.

Other than fowlpox, the most economically significant reports of pox have been canarypox, turkeypox, quail-pox, and psittacinepox in Amazon parrots. These poxvirus infections are typically the cutaneous form, but in canaries the cutaneous form is rare and the systemic form is com-mon and may produce 80–90% mortality. In canaries, the systemic disease presents with hepatic necrosis and pulmo-nary nodules. Vaccination is practiced in canary aviaries.

MeMbers of The Genus parapOxvirusParapoxviruses infect a wide range of species, generally causing only localized cutaneous lesions. Disease in sheep, cattle, goats, and camels can be of economic significance. Parapoxviruses also infect several species of terrestrial and

marine wildlife (e.g., chamois, red and black-tailed deer, seals, and reindeer), but their clinical importance in these species is more conjectural. These viruses are zoonotic; farmers, sheep shearers, veterinarians, butchers, and others who handle infected livestock or their products are espe-cially at risk and can develop localized lesions, usually on the hand. The lesions, which are identical irrespective of the source of the virus and resemble those in the animal host, begin as an inflammatory papule, and then enlarge before regressing. They may persist for several weeks. If the infec-tion is acquired from milking cows, the lesion is known as “milker’s nodule;” if from sheep, it is known as “orf.”

ORF vIRus (COnTAGIOus eCThyeMA/COnTAGIOus PusTulAR deRMATITIs vIRus)

Orf (syn. contagious pustular dermatitis, contagious ecythema, scabby mouth) is an important disease in sheep and goats, and is common throughout the world wherever sheep and goats are raised. Orf, which is Old English for “rough,” commonly involves only the muzzle and lips, although lesions within the mouth affecting the gums and tongue can occur, especially in young lambs and kids. The lesions can also affect the eyelids, feet, and teats. Human infection can occur among persons exposed occupationally.

Lesions of orf progress from papules to pustules and then to thick crusts (Figure 7.9). The scabs are often friable and mild trauma causes the lesions to bleed. Orf may pre-vent lambs from suckling. Severely affected animals may lose weight and be predisposed to secondary infections. Morbidity is high in young sheep, but mortality is usually low. Clinical differentiation of orf from other diseases sel-dom presents a problem, but electron microscopy can be used, if necessary, to confirm the diagnosis.

FIGuRe 7.9 Orf lesion on the lip of a lamb. (Courtesy K. Thompson, Massey University.)

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Sheep are susceptible to reinfection and chronic infec-tions can occur. These features, and the resistance of the virus to desiccation, explain how the virus, once introduced to a flock, can be difficult to eradicate. Spread of infection can be by direct contact or through exposure to contami-nated feeding troughs and similar fomites, including wheat stubble and thorny plants.

Ewes can be vaccinated several weeks before lambing, using commercial non-attenuated virus vaccines derived from infected scabs collected from sheep or from virus grown in cell culture—in a manner analogous to pre-Jennerian vacci-nation for smallpox. Vaccines are applied to scarified skin, preferably in the axilla, where a localized lesion develops. A short-lived immunity is generated; ewes are thus less likely to develop orf at lambing time, thereby minimizing the risk of an epizootic in the lambs.

Orf virus is zoonotic; however, infections are frequent in humans, especially when they are in contact with sheep (e.g., during shearing, docking, drenching, slaughtering) or wildlife. In humans, after an incubation period of 2–4 days, the follow-ing stages may be observed: (1) macular lesions; (2) papular lesions; (3) rather large nodules, becoming papillomatous in some cases. Lesions are, as a rule, solitary, although multiple lesions have been described. The duration of lesions ranges from 4 to 9 weeks. Healing takes place without scarring, but secondary infections may retard healing. Severe complica-tions, such as fever, regional adenitis, lymphangitis, or blind-ness when the eye is affected, are seen only rarely.

PseudOCOwPOx vIRus

Pseudocowpox occurs as a common enzootic infection in cattle in most countries of the world. It is a chronic infec-tion in many milking herds and occasionally occurs in beef herds. The lesions of pseudocowpox are characterized by “ring” or “horseshoe” scabs, the latter being pathognomonic for the disease. Similar lesions can occur on the muzzles and within the mouths of nursing calves. Infection is transmitted

by cross-suckling of calves, improperly disinfected teat clus-ters of milking machines, and probably by the mechanical transfer of virus by flies. Attention to hygiene in the milking shed and the use of teat dips reduce the risk of transmission.

BOvIne PAPulAR sTOMATITIs vIRus

Bovine papular stomatitis is usually of little clinical importance, but occurs worldwide, affecting cattle of all ages, although the incidence is higher in animals less than 2 years of age. The development of lesions on the muzzle, margins of the lips, and the buccal mucosa is similar to that of pseudocowpox (Figure 7.10). Immunity is of short duration, and cattle can become reinfected. Demonstration by electron microscopy of the characteristic parapoxvirus virions in lesion scrapings is used for diagnosis.

Poxviruses of fishTwo poxviruses of significance to the culture of fish have been reported: the first associated with disease in koi (Cyprinus carpio) that is characterized by edema, and the second with a proliferative gill disease in Atlantic salmon (Salmo salar). Although the viruses involved in both dis-ease syndromes are only partially characterized, they share similarities in their virion morphogenesis and genetic makeup that are more similar to those of viruses in the subfamily Entomopoxvirinae than those of viruses in the subfamily Chordopoxvirinae. This association of the fish poxviruses with entomopoxviruses may reflect the long evolutionary co-existence of fish with aquatic insects.

The disease syndrome associated with the carp edema virus has been designated as “sleepy disease” as, before death, affected fish lie on their sides on the pond bottom. The disease was first recognized in 1974 among cultured koi pop-ulations in Japan. Affected fish developed swollen bodies and proliferation of the gill epithelium, the latter beginning from the most distal tips and preceding to the base of the lamella.

(A) (B) (C)

FIGuRe 7.10 Bovine papular stomatitis. (A) Gross appearance of hard palate. (B) Histologic appearance of normal buccal epithelium. (C) Histologic appearance of affected buccal mucosal epithelium. (A: Courtesy of M. Anderson, University of California.)

Chapter | 7 Poxviridae 165

Electron microscopy of the affected gill epithelium revealed pleomorphic mulberry-like virions of 335 265 nm, with an envelope and surface membrane surrounding the core. In severe outbreaks, mortality ranged from 80 to 100% among juvenile koi at water temperatures in the range 15–25°C. The virus has not been isolated in cell culture, but the disease can be transmitted to naïve koi by injections with filtrates from the gills of infected koi. Current diagnostic methods include the characteristic clinical signs in juvenile koi, and elec-tron microscopic examination of tissues from affected fish. A polymerase chain reaction (PCR) assay has been devel-oped to detect viral DNA in fish. Control is currently reliant upon extended treatment of the water of affected ponds with the addition of 0.5% NaCl, a process that prevents virus-induced mortality but probably does not affect infection of carrier fish.

An emerging proliferative gill disease first recognized in 1998 has continued to increase in prevalence such that 35% of Atlantic salmon farms in Norway reported the con-dition in 2003. The disease is most frequent shortly after juvenile fish are transferred to sea water, and it occurs at water temperatures from 8.5 to 16°C, with mortality ranging from 10 to 50%. Protozoa (amoeba) and bacteria (chlamydia) may contribute to disease expression, but a recently described poxvirus is likely to be the true causa-tive agent. The hyperplasia and hypertrophy of the gill epi-thelium in Atlantic salmon are similar to those described in koi with carp edema virus infection. Virions with a similar morphology but smaller in size than those from koi have been identified in the gill epithelium of affected Atlantic salmon. Although a PCR has been developed to detect this virus, it has not been widely used, and control measures for the disease have not been described to date.

oTher PoxvirusesPoxvirus infections also have been described in raccoons, skunks, voles, and various species of deer, seals, horses, donkeys, and other animal species. The number of spe-cies of poxviruses will unquestionably grow as additional viruses are characterized and new viruses are isolated.

squIRRel POxvIRus

Squirrelpox is a fatal disease of red squirrels in the United Kingdom. It is a highly significant wildlife disease, in that the mortality rate is nearly 100%, and is responsible for local extinctions of red squirrel populations. The virus is carried by an introduced non-native species, the gray squir-rel from North America. Gray squirrels develop only mild disease when infected with the virus. The historical origins of the virus have not been determined. Although the virus is considered to have been introduced by gray squirrels, serological evidence of infection of gray squirrels in North America has been only recently identified, and the virus has not been identified among gray squirrels introduced to other parts of Europe. Although squirrel poxvirus initially was classified as a member of the genus Parapoxvirus, sub-sequent genetic studies have shown it to be distinct from other poxviruses and that it belongs in its own clade. The virus is notable in that it encodes homologs of both protein kinase (PKR) and 2–5 oligoadenylate synthetase, which are host-cell enzymes that mediate interferon-induced anti-viral resistance. These viral homologs disrupt host innate antiviral immunity (see Chapter 4); for example, the three enzymatically active sites of authentic oligoadenylate syn-thetase enzyme are all inactivated in the viral homolog.