Arthropod-Borne Viruses

Part 1 lectureDr.T.V.Rao MD

What Is An Arbovirus?• Arbovirus = arthropod-borne viruses • Arboviruses are maintained in nature

through biological transmission between susceptible vertebrate hosts by blood-feeding arthropods

• Vertebrate infection occurs when the infected arthropod takes a blood meal

Arboviral Infections.

• 100s of Arbovirus,• Around 100 are Human pathogens,• Prevalent in Temperate and Tropical

areas.• Most common in tropics,• Out of Many 10 are very important.

Arthropod-borne VirusesArthropod-borne viruses (arbovirus) are viruses that can be transmitted to man by arthropod vectors. “

The WHO definition is as follows

• Viruses maintained in nature principally, or to an important extent, through biological transmission

between susceptible vertebrate hosts by haematophagus arthropods or through trans ovarian and possibly venereal transmission in arthropods.”

Arbovirus belong to • Arbovirus belong to three families• 1. Toga viruses e.g. EEE, WEE, and VEE• 2. Bunya viruses e.g. Sandfly Fever, Rift

Valley Fever, Crimean-Congo Hemorrhagic Fever

• 3. Flavivirus e.g. Yellow Fever, dengue, Japanese Encephalitis

Disease Mechanisms of Toga viruses and Flavivirus

Viruses are cytolytic, except for rubella.

Viruses establish systemic infection and viremia.

Viruses are good inducers of interferon, which can account for the flulike symptoms of infection.

Viruses, except rubella and hepatitis C, are arboviruses.

Flaviviruses can infect cells of the monocyte-macrophage lineage. Non-neutralizing antibody can enhance flavivirus infection via Fc receptors on the macrophage.

Flulike Syndrome Encephalitis Hepatitis Hemorrhage Shock

Dengue +   + + +

Yellow fever +   + + +

St. Louis encephalitis + +      

West Nile encephalitis + +      

Venezuelan encephalitis + +      

Western equine encephalitis + +      

Eastern equine encephalitis + +      

Japanese encephalitis + +  

Man-Arthropod-Man Cycle

Animal-Arthropod-Man Cycle

Arthropod VectorsMosquitoesJapanese encephalitis, dengue, yellow fever, St. Louis encephalitis, EEE, WEE, VEE etc.

TicksCrimean-Congo hemorrhagic fever, various tick-borne encephalitis's etc.

SandfliesSicilian sandfly fever, Rift valley fever.

Examples of Arthropod Vectors

Aedes AegytiAssorted Ticks

Phlebotmine SandflyCulex Mosquito

Animal ReservoirsIn many cases, the actual reservoir is not known. The following animals are implicated as reservoirs

Birds Japanese encephalitis, St Louis encephalitis,

EEE, WEE

Pigs Japanese encephalitis

Monkeys Yellow Fever

Rodents VEE, Russian Spring-Summer encephalitis

Major Arboviral Diseases1.Yellow fever

2.Dengue,3.Japanese B Encephalitis,4.St Louis Encephalitis,5.Russian spring summer encephalitis.6.Eastren Equine Encephalitis,7.West Nile Fever,8.Sand fly Fever

Major Arboviruses That Cause Encephalitis

• Flaviviridae– Japanese encephalitis– St. Louis encephalitis– West Nile

• Togaviridae– Eastern equine encephalitis– Western equine encephalitis

• Bunyaviridae– La Crosse encephalitis

http://www.cdc.gov/ncidod/dvbid/arbor/schemat.pdf

http://www.cdc.gov/ncidod/dvbid/arbor/worldist.pdf

St. Louis Encephalitis

St. Louis Encephalitis• Flavivirus• Most common

mosquito-transmitted human pathogen in the US

• Leading cause of epidemic flaviviral encephalitis

Eastern Equine Encephalitis• Toga virus• Caused by a virus transmitted to

humans and horses by the bite of an infected mosquito.

• 200 confirmed cases in the US 1964-present

• Average of 4 cases per year• States with largest number of

cases – Florida, Georgia, Massachusetts, and New Jersey.

• Human cases occur relatively infrequently, largely because the primary transmission cycle takes place in swamp areas where populations tend to be limited.

Western Equine Encephalitis• Toga virus• Mosquito-borne• 639 confirmed cases in

the US since 1964 • Important cause of

encephalitis in horses and humans in North America, mainly in the Western parts of the US and Canada

West Nile virus (WNV)

• West Nile virus (WNV) is a mosquito-borne zoonotic arbovirus belonging to the genus Flavivirus in the family Flaviviridae. This flavivirus is found in temperate and tropical regions of the world. It was first identified in the West Nile sub-region in the East African nation of Uganda in 1937.

How the disease manifests• Approximately 80 percent of West Nile virus infections

in humans are subclinical, which cause no symptoms. In the cases where symptoms do occur – termed West Nile Fever in cases without neurological disease – the time from infection to the appearance of symptoms (incubation period) is typically between 2–15 days. Symptoms may include fever, headaches, fatigue, muscle pain of aches, malaise, nausea, anorexia vomiting myalgias and rash. Less than 1% of the cases are severe and result in neurological disease when the central nervous system is affected.

La Crosse Encephalitis• Bunya virus• On average 75 cases per year

reported to the CDC• Most cases occur in children under 16

years old• Zoonotic pathogen that cycles

between the daytime biting tree hole mosquito, and vertebrate amplifier hosts (chipmunk, tree squirrel) in deciduous forest habitats

• Most cases occur in the upper Midwestern state, but recently cases have been reported in the Mid-Atlantic region and the Southeast

• 1963 – isolated in La Crosse, WI from the brain of a child who died from encephalitis

Togaviridae

Togaviridae

• The name Togaviridae derived from Toga meaning roman mantle or claok refers to the viral surface

TOGAVIRIDAEChikungunya virus Infection

• 1952 Epidemic in Tanzania.• Manifest as Bend Up with Severe

Joint pains.• Spread from wild primates –

Mosquito-Man• Appears , reappears,

Indian outbreaks• The virus first appeared in India in

1958 the virus caused large epidemics in Thailand

• In 1963 India Chikungunya outbreaks occurred at irregular intervals along the east coast of India and in Maharashtra

Viral Morphology

• Spherical 50 - 70 nm

• Bears the Nucleocapsid, 42 capsomeres

• Positive sense ss stranded RNA

Prevalence of Chikungunya

Transmission of Infection

Man to Man infection with Mosquito bites

• Chikungunya virus requires an agent for transmission and hence direct human to human transmission is not possible. Usually transmission occurs when a mosquito bites an infected person and then later bites a non infected person.

Clinical Manifestations

•Crippling Joint pains •Conjunctivitis • Lymphadenopathy

•Hemorrhagic tendencies.

Diagnosis

• Isolation of viruses,

• Serology Ig M • Nt and HI

tests,

Control and Prevention.• Mosquito control • No vaccines,• Other diseases like Chikungunya 1 Onyong Nyong Viruses 2 Simliki Forest Viruses

Japanese B Encephalitis

FlaviviridaeGenus – Flavivirus,

• Important Diseases,

1. St Louis encephalitis, 2.Ilheus virus 3.West Nile Virus, 4.Murray valley encephalitis, 5.Japanese B encephalitis,

• Flaviviridae– Flavivirus

• The name is derived from the Latin ‘flavus’ – Flavus means “yellow”

• Refers to yellow fever virus

• Enveloped• Single stranded RNA virus• Morphology not well

defined

Center for Food Security and Public Health Iowa State University - 2007

Japanese Encephalitis belongs to Genus Flavivirus

• First discovered and originally restricted to Japan. Now large scale epidemics occur in China, India and other parts of Asia.

• Flavivirus, transmitted by culex mosquitoes.

• The virus is maintained in nature in a transmission cycle involving mosquitoes, birds and pigs.

• Most human infections are subclinical: the in apparent to clinical cases is  300:1

• In clinical cases, a life-threatening encephalitis occurs.

• The disease is usually diagnosed by serology. No specific therapy is available.

• .

Japanese Encephalitis

• 1870s: Japan– “Summer encephalitis” epidemics

• 1924: Great epidemic in Japan– 6,125 human cases; 3,797 deaths

• 1935: First isolated– From a fatal human encephalitis case

• 1938: Isolated from Culex tritaeniorhynchus

Center for Food Security and Public Health Iowa State University - 2007

History

Dr.T.V.Rao MD 40

Genus - Flavivirus• Japanese B encephalitis

virus is Spherical, 40 – 60 nm in

diameter Contain a positive sense

Single stranded RNA, 11 kb in size

RNA genome is infectious Several viruses in this group

are related.

Dr.T.V.Rao MD 41

Infection is caused by a flavivirus, a single stranded RNA virus. It is transmitted by the bite of the Culex tritaeniorhynchus mosquito. The virus multiplies at the site of the bite and in regional lymph nodes before viraemia develops. Viraemia can lead to inflammatory changes in the heart, lungs, liver, and reticuloendothelial system.

Japanese B virus Infection

Dr.T.V.Rao MD 42

• The outer envelope is formed by envelope (E) protein and is the protective antigen. It aids in entry of the virus to the inside of the cell. The genome also encodes several non-structural proteins also (NS1,NS2a,NS2b,NS3,N4a,NS4b,NS5). NS1 is produced as secretary form also. NS3 is a putative helicase, and NS5 is the viral polymerase.

Structure of Virus

Dr.T.V.Rao MD 43

• Japanese encephalitis ( previously known as Japanese B encephalitis is a disease caused by the mosquito-borne Japanese encephalitis virus. The Japanese encephalitis virus is a virus from the family Flaviviridae. Domestic pigs and wild birds are reservoirs of the virus; transmission to humans may occur

A Flavivirus

INDIAN SCENARIO

• Japanese encephalitis ( previously known as Japanese B encephalitis is a disease caused by the mosquito-borne Japanese encephalitis virus. The Japanese encephalitis virus is a virus from the family Flaviviridae. Domestic pigs and wild birds are reservoirs of the virus; transmission to humans may occur

D 44

• 1940-1978– Disease spread with epidemics in China,

Korea, and India• 1983: Immunization in South Korea

– Started as early as age 3– Endemic areas started earlier

• 1983-1987: Vaccine available in U.S. on investigational basis

Center for Food Security and Public Health Iowa State University - 2007

History

Dr.T.V.Rao MD 46

• Japanese encephalitis is the leading cause of viral encephalitis in Asia, with 30,000–50,000 cases reported annually. Case-fatality rates range from 0.3% to 60% and depends on the population and on age.

A leading cause of viral Encephalitis

Animal-Arthropod-Man Cycle

Dr.T.V.Rao MD 48

Cycle of Infection in Japanese B Viral Infection

• Vector-borne disease• Enzootic cycle

– Mosquitoes: Culex species• Culex tritaeniorhynchus

– Reservoir/Amplifying hosts• Pigs, bats• Ardeid (wading) birds• Possibly reptiles and amphibians

– Incidental hosts• Horses, humans, others

Center for Food Security and Public Health Iowa State University - 2007

Transmission

Dr.T.V.Rao MD 50

A Vector born- Arbovirus Infection

• Culex tritaeniorhynchus a rural Mosquito that breeds in rice fields, is the principle vector.

In India in 1955 the virus were isolated from Culex vishnui mosquitoes in Vellore region in Tamil Nadu

• Most important global cause of arboviral encephalitis with > 50,000 cases and 15,000 deaths reported each year.

• Only about 1 in 250 JE infections result in symptomatic illness.

• Primarily affects children 1 to 15 years of age.

• Incubation period is 5 to 14 days.

Japanese Encephalitis (JE)

• If unrecognized, mortality is up to 30% with half of survivors sustain severe neurological sequelae.

Dr.T.V.Rao MD 52

• Leading cause of viral encephalitis in Asia with 30-50,000 cases reported annually

• Fewer than 1 case/year in U.S. civilians and military personnel travelling to and living in Asia

• Rare outbreaks in U.S. territories in Western Pacific

INCIDENCE

Dr.T.V.Rao MD 53

Dr.T.V.Rao MD 54

Cycle of Events in Japanese B Encephalitis

Dr.T.V.Rao MD 55

Pass through two prominent Hosts

• Herons act as reservoir hosts and pigs as amplifier hosts.

• Human infection is a tangential ‘dead end’ and infections are spread when the infected mosquitoes reach high density.

Dr.T.V.Rao MD 56

• The incubation period is 6 to 16 days. • There is a prodrome of fever, headache, nausea,

diarrhoea, vomiting, and myalgia, which may last for several days.

• This may be followed by a spectrum of neurological disease ranging from mild confusion, to agitation, to overt coma.

• Two thirds of patients have seizures. It is more common in children, while headache and meningism are more common in adults.

Clinical Manifestations

LethargySudden fever

Vomiting and diarrhea

Tremors or convulsions

Headache Change in consciousness

Irritability or restlessness

Common symptoms of encephalitis

Dr.T.V.Rao MD 58

• Tremor or other involuntary movements are common.

• Mutism has been described as a presenting symptom. So has a syndrome of acute flaccid paralysis.

• Fever resolves by the second week, and choreoathetosis or extra pyramidal symptoms develop as the other neurological symptoms disappear.

Can lead to Neurological damage

Dr.T.V.Rao MD 59

• The isolation of virus from Blood, CSF, or tissues.

• Detection of Arbovirus specific RNA in blood,CSF, or Tissue

• However very few reference laboratories can perform the isolation in view of the biosafety considerations

Diagnosis of Japanese B Encephalitis

Dr.T.V.Rao MD 60

• IgM capture enzyme-linked immunoassay (ELISA) of serum or CSF is the standard diagnostic test. Sensitivity is nearly 100% when both serum and CSF are tested. False-negatives may result if the samples are tested too early, as in the first week of illness.

• New IgM dot enzyme immunoassays for CSF and serum are portable and simple tests that can be used in the field. Compared with ELISA as the gold standard, the sensitivity and specificity are around 98 and 99% respectively.

Serology by ELISA

Dr.T.V.Rao MD 61

• Viral RNA is extracted from serum or from suspected tissues of the patients or mosquito homogenates.

• The product is amplified by RTPCR and the products analyzed by restriction digestion and determined by nucleotide sequence of PCR product.

• The identified sequence is compared with nucleotide sequence found in Gene bank or other data bases

Arbovirus Specific RNA detection

Dr.T.V.Rao MD 62

• Japanese Encephalitis B Vaccine has been produced since 1992. The vaccine is effective but not without risks and the substantial risks of the disease and the risks of the vaccine have to be balanced, especially for stays of brief duration. These are discussed more fully in the article on that subject.As with malaria, prophylaxis must be supplemented by techniques to avoid being bitten by mosquitoes.

Japanese Encephalitis B Vaccine

Dr.T.V.Rao MD 63

• Preventive measures include mosquito control and locating piggeries away from human dwellings

• A formalin inactivated mouse brain vaccine using the Nakayama strain has been employed in human immunization in Japan – Two doses at two week’s interval followed by a booster 6 – 12 months later constitute a full course.

• However the immunity was short lived

Preventive measures

• Two vaccines are manufactured and distributed exclusively in People’s Republic of China– Inactivated vaccine grown in primary hamster kidney cells– Live attenuated vaccine (SA14-14-2) grown in hamster

kidney cells• The third is manufactured in Japan and distributed

abroad by arrangement with Sanofi-Pasteur– Licensed as JE-VAXR and is the only FDA approved vaccine

for use in the U.S.– Has been in wide use worldwide since the 1960’s– Three subcutaneous injections over a month with a

booster at 3 years– 91% efficacy in a large field trial in Thailand

Emerging Vaccines for JE virus

• Live attenuated vaccine– Used in equine and swine– Successful for reducing incidence

• Inactivated vaccine (JE-VAX)• Used for humans• Japan, Korea, Taiwan, India, Thailand• Used for endemic or epidemic areas

– Recommended for travelers• Visiting endemic areas for > 30 days

Center for Food Security and Public Health Iowa State University - 2007

Vaccination

Dr.T.V.Rao MD 66

• A live attenuated vaccine has been developed in China from JE strain SA 14-14-2, passed through weanling mice

• The vaccine is produced in primary bay hamster kidney cells.

• Administered in two doses, one year apart, the vaccine has been reportedly effective in preventing clinical disease

Later vaccines

• Vector control– Eliminate mosquito breeding areas– Adult and larvae control

• Vaccination– Equine and swine– Humans

• Personal protective measures– Avoid prime mosquito hours– Use of repellants containing DEET

Center for Food Security and Public Health Iowa State University - 2007

Prevention

Yellow Fever

Yellow Fever,Flaviviridae - Family

• Mosquito Borne disease • Present in Africa, Central

and South America.• Absent in India.

Flavivirus• Spherical 40-60 nm in diameter

glycosylated. Diameter,• Ss-RNA positive sense • Three or Four structural polypeptides,

Two are glycosylated.• Replicates in Cytoplasm.• Produces Councilman bodies

Pathogenesis and Pathology

• Mosquito ( Ades aegypti )Through skin- Lymphatic's, Lymph nodes, circulation liver,

Spleen, Kidney, Bone marrow, Lymph glands. Necrotic lesions in liver , kidney,Mid zone – liverFatty degeneration – kidney,Hemorrhages/Circulatory collapse.Injury to Myocardium

Clinical Features• Incubation period 3-6 days,• Fever, chills,• Intoxication, Fever, Jaundice• Clotting disorders,• Mortality > 20%• May recover totally

Laboratory Diagnosis

• Intracerebral inoculation,• Mosquito cell lines,• P C R• Serology – ELISA Ig M Raise of

titers,

Immunity and Epidemiology

• Nt Antibodies protects,• Epidemiology Urban yellow fever, Jungle yellow fever. Monkey, Not Invaded Asia Not present in India.

Immunity and Epidemiology

• Nt Antibodies protects,• Epidemiology Urban yellow fever, Jungle yellow fever. Monkey, Not Invaded Asia Not present in India.

Treatment and Prevention.

• No Antiviral drugs,• Mosquito control• Vaccine 17 D strain of yellow fever

vaccine.• A single dose protect 95% of vaccinated.• Not to be given in infants < 9 months

age.

• Programme Created By Dr.T.V.Rao MD for Undergraduate Teaching in

Medical Sciences • Email

• doctortvrao@gmail.com