TOXOPLASMA GONDII

TOXOPLASMOSIS

Toxoplasma gondii-- Obligate intracellular parasite produces a wide range of clinical syndromes in humans, land and sea mammals, and various bird species.

Nicolle and Manceaux first described the organism in 1908 after they observed the parasites in the blood, spleen, and liver of a North African rodent, Ctenodactylus gondii.

The parasite was named Toxoplasma (arc like form) gondii (after the rodent) in 1909.

Toxoplasma Gondii

Scientific classification

Kingdom: Protista

Phylum: Apicomplexa

Class: Conoidasida

Subclass: Coccidiasina

Order: Eucoccidiorida

Family: Sarcocystidae

Genus: Toxoplasma

Species: T. gondii

T. gondii tachyzoites

Toxoplasma gondii 

                                                                                                 

farm animals

Toxoplasma gondii is a protozoa. The definitive host, in which the sexual stages takes place is the domestic cat.

There are many intermediate hosts, including man rodent and farm animals.

Man Rat

A cat becomes infected with toxoplasmosis by eating the raw flesh of infected animalsCat transmits the infection to her unborn kittens through the placenta or to her new litter through her milk.An infected kitten may die, An infected otherwise healthy cat may show no signs of disease.

Inside the cat Toxoplasma infects the intestinal epithelial cells and divides. In the cat the sexual phase of the life cycle occurs resulting in the production of oocysts (eggs).  The eggs are excreted in fecal matter. The intermediate host becomes infected by eating the oocysts.

Oocyst contains two sporocysts with 4 sporozoites

Definitive host

Intermediate host

Tissue cysts containing BradyzoitesEaten by cat

1) Tachyzoite: the intracellular parasitic stage which rapidly proliferates by binary fission, usually seen in the early acute stage of infection - size 4-8 µm.

2) Bradyzoite: quiescent forms within a cyst which is formed during chronic latent (asymptomatic) infection; persists for years in human tissues (brain, retina, muscle etc.)

3) Oocysts: shed in the stool of cats, They  sporulate in 3 days after passage in stool and then can be infective in the environment for a year. Man swallows oocysts to become infected.

 

                                            

Three forms of parasite involve man:

MORPHOLOGYTachyzoite- 3-7micrometre in lengthOval to crescent shaped with a pointed anterior end and a rounded posterior end..Ovoid nucleus at the posterior endIt is the actively multiplying form (tachy = fast, zoite = organism) and can invade any cell in the host

Media file 1:  Toxoplasmosis. Toxoplasma gondii tachyzoites (Giemsa stain).

Tissue cyst After repeated divisions, tachyzoites become encysted in the body, mostly in muscle and neural tissues.

The encysted stage (tissue cyst) contains relatively slowly multiplying organisms called bradyzoites,( brady = slow).

Tissue cysts persist in the body as long as the host lives. The tissue cyst is a dead end phase of the parasite in the intermediate host waiting to be eaten by the definitive host, cat

In man found in brain, skeletal muscle and heart muscle.

PSEUDOCYST AND TISSUE CYST

In pseudocyst there are actively multiplying tachyzoites within a cytoplasmic vacuole of the cell they infect. Here they may appear rounded or enclosed in a host cell. In tissue cyst it is the bradyzoites which are present and encysted within the tissue especially in the brain and muscles.Large in size:-12-100 micrometre in size with several 1000 bradyzoites.

Pseudocyst in human heart

pseudocyst

Pseudocyst in human heart -magnified

Pseudocyst of toxoplasma

OocystsIn the environment, cysts take 48-72 hours to sporulate and become infective.  Mature oocysts measure 10-12 µm in diameter and contain two sporocysts. . Cats may excrete millions of oocysts after ingesting as few as 1 bradyzoite or 1 tissue cyst, and many tissue cysts may be present in one infected mouse. Oocysts are shed for only a short period (1-2 weeks) in the life of the cat, however, the enormous numbers shed assure widespread contamination of the environment. 

A:  Toxoplasma gondii sporulated oocyst in an unstained wet mount B:  Toxoplasma gondii sporulated oocyst  

Epidemiology: Very common throughout the world; up to 50+% in other developed or developing countries.This is a zoonosis affecting almost all mammalian carnivores and mammals in contact with cats. Toxoplasma gondii infection occurs commonly in many animals used for food (for example, sheep, goats, pigs, and rabbits). The cat is the definitive host. Many mammals and birds serve as intermediate hosts

Mode of infection

Ingestion of oocysts through contamination of food, hands, etc. with cat feces.

Ingestion of bradyzoites in uncooked meat, e.g. lamb, pork, beef, caribou

Transplacental when mother develops acute infection during pregnancy

Blood transfusion, Organ transplant.

Life Cycle The sexual cycle takes place exclusively in the intestinal enterocytes of many members of the cat family 1) Ingestion of tissue cysts,

2) The parasites invade the enterocytes and divide repeatedly

3) Differentiate into microgametocytes and macrogametocytes. 4 )The gametocytes fuse to form a zygote or ‘oocyst’ that is shed into the environment with the cat’s faeces.

5) The oocyst undergoes meiosis, producing eight infectious ‘sporozoites’ that are resistant to environmental damage and may persist for years in a moist environment. 6)Ingestion (by a secondary host such as a mouse),

7) Sporozoites differentiate into the rapidly dividing ‘tachyzoite’ form, which establishes and sustains the acute infection. 8) During the acute infection, congenital transmission to the developing fetus can occur

9) In many hosts, a chronic phase of the disease ensues, as the tachyzoite changes into a slowly dividing form known as the ‘bradyzoite’. Latent bradyzoite tissue cysts persist for the life of the host.

10) Ingestion of tissue cysts can lead to the infection of a new host, allowing for an indefinite nonsexual propagation of T. gondii.

11) In the cat, this will initiate the sexual cycle.

12) In man it is a dead end infection unless its in a pregnant female.

Toxoplasma gondii in infected monolayers of HeLa cells (Giemsa stain).

B. Tissue cysts in section of muscle. The tissue cyst wall is very thin (arrow) and encloses many tiny bradyzoites (arrowheads).

C. Tissue cyst separated from host tissue by homogenization of infected brain. Note tissue cyst wall (arrow) and hundreds of bradyzoites (arrowheads

D. Schizont (arrow) with several merozoites (arrowheads) separating from the main mass.

E. A male gamete with two flagella (arrows). Impression smear of infected cat intestine

F. Unsporulated oocyst in fecal float of cat feces.

G. Sporulated oocyst with a thin oocyst wall (large arrow), 2 sporocysts (arrowheads). Each sporocyst has 4 sporozoites

A. Tachyzoites Note crescent-shaped individual tachyzoites (arrows), dividing tachyzoites (arrowheads) compared with size of host red blood cells and leukocytes.

PathophysiologyOocysts are ingested in material contaminated by feces from acutely infected cats. Sporozoites are released from oocysts and enter gastrointestinal cells. They multiply, rupture cells, and infect contiguous cells. They are transported via the lymphatics and disseminated hematogenously throughout the tissues.

Tachyzoites proliferate, producing necrotic foci surrounded by a cellular reaction. With the development of a normal immune response, tachyzoites disappear from tissues. In immunodeficient individuals and in some apparently immunologically healthy patients, the acute infection progresses and may cause potentially lethal consequences such as pneumonitis, myocarditis, or necrotizing encephalitisWhen a mother acquires the infection during gestation, the organism may be disseminated hematogenously to the placenta. When this occurs, infection may be transmitted to the fetus transplacentally or during vaginal delivery

Clinical 1) Majority are asymptomatic In humans, severe disease is usually observed only in congenitally infected children and in immunosuppressed individuals, including patients with acquired immune deficiency syndrome (AIDS).

2. Acute Toxoplasmosis: Fever, Lymphadenopathy Any node can be infected, but the deep

cervical nodes are the most commonly involved. Infected nodes are tender and discrete but not painful; the infection resolves spontaneously in weeks or months

Fatigue, Muscle pains, Sore throat, and Headache. can Rarely cause specific organ inflammation, e.g. encephalitis,

myocarditis.

3. Reactivation toxoplasmosis: occurs in immunosuppressed such as AIDS, transplant and cancer patients:

CNS disease occurs in 50% of patients— encephalitis, meningoencephalitis, or mass lesions.Hemiparesis, seizures, and mental status changes are reported.Patients report visual changes.Myocarditis and pneumonitis are reported.

4. Choreoretinitis: Occurs later in life in individuals who acquired toxoplasmosis congenitally;

Focal lesion  in retina presenting as Decreased visual acuity; Rarely occurs during acute toxoplasmosis.

5. Congenital toxoplasmosis: Transmission from mother to fetus when mother has developed acute toxoplasmosis during pregnancy --- increased transmission rate in third trimester, but increased severity of fetal disease in first trimester.

Infection in the first trimester - approximately 17% of fetuses are infected, and disease in the infant is usually severe.Infection in the third trimester --65% of fetuses are infected, and involvement is mild or inapparent at birth.

These different rates of transmission are most likely related to placental blood flow, the virulence and amount of T gondii acquired, and the immunologic ability of the mother to restrict parasitemiaPresents as

Hydrocephalus, Hepatomegaly, Cerebral calcifications, Mental retardation Choreoretinitis

LAB DIAGNOSIS

• Demonstration of the T. gondii organism in blood, body fluids, or tissue.

• Isolation of T. gondii from amniotic fluid is diagnostic of congenital infection by mouse inoculation.

• Lymphocyte transformation to T. gondii antigens is an indicator of previous toxoplasmosis in adults

•Detection of T. gondii antigen in blood or body fluids by enzyme-linked immunosorbent assay (ELISA) technique indicates acute infection.

•The Sabin-Feldman dye test – Sensitive and specific neutralization test. It measures IgG antibody and is the standard reference test for toxoplasmosis. Live virulent tachyzoites of T gondii are used as antigen and are exposed to dilutions of the test serum and to a complement accessory factor resembling complement that is obtained from Toxoplasma-antibody free-human serum. Its main disadvantages are its high cost and the human hazard of using live organisms.

•The indirect fluorescent antibody test measures the same antibodies as the dye test. Titers parallel dye test titers.

•The IgM fluorescent antibody test detects IgM antibodies within the first week of infection, but titers fall within a few months.

•The indirect hemagglutination test measures a different antibody than the dye test. Titers tend to be higher and remain elevated longer.

•The results from a double-sandwich IgM ELISA are more sensitive and specific than the results from other IgM tests.

Diagnosis in pregnancyDiagnosis is primarily made by serologic investigations.

Antibody DetectionThe detection of Toxoplasma-specific antibodies is the primary diagnostic method to determine infection with Toxoplasma. Antibodies are detected by numerous serologic tests and most of the test kits are commercially available to detect T. gondii specific IgG, IgM , IgA or IgE antibodies. All suspected patients should be initially tested for the presence of Toxoplasma-specific IgG antibodies to determine their immune status. The presence of IgG antibodies only means exposure because asymptomatic humans can develop very high (>100,000) and titres may remain elevated for several years or even whole life if repeated exposures are encountered.

A 8-fold rise in antibody titre, taken two weeks apart, is indicative of a recent infection, Compared to IgG antibodies, IgM antibodies are short-lived, and they appear before IgG antibodies

A negative IgM test essentially excludes recent infection, but a positive IgM test is difficult to interpret because Toxoplasma-specific IgM antibodies may be detected by EIA for 6-12 months but rarely as long as 18 months after acute acquired infection. Two situations occur frequently: i) positive IgM but negative IgG, and ii) positive IgG and IgM.

In the first situation, the patient's blood should be redrawn two weeks after the first and tested together with the first specimen. If the first specimen was drawn in early infection, the patient should have highly positive IgG and IgM antibodies in the second sample.

If the IgG is negative and the IgM is positive in both specimens, the IgM result should be considered to be a false positive and the patient should be considered to be not infected.

In a pregnant women whose sample is taken in second or third trimester is found IgG positive but IgM negative, it is more advisable to perform IgG avidity test. High avidity IgG tests indicate that she acquired the infection more than 4 months ago. But the low avidity is not a confirmatory test for recent infection.

New born infants suspected of congenital toxoplasmosis should be tested by both an IgM- and an IgA-capture EIA. Detection of Toxoplasma-specific IgA antibodies is more sensitive than IgM detection in congenitally infected babies.

Polymerase chain reaction on body fluids, including CSF, amniotic fluid, BAL fluid, and blood, may be useful in the diagnosis.

Control 1)Sulfonamides and 2)pyrimethamine (Daraprim) are two drugs widely used to treat toxoplasmosis in humans.

Although these drugs are helpful when given in the acute stage of the disease, usually they will not eradicate infection when active multiplication of the parasite occurs.

Because sulfa compounds are excreted within a few hours of administration, they must be administered in daily divided doses.

No killed vaccine is currently available to reduce or prevent congenital infections in humans and animals, but research to develop such an agent is under way. A live vaccine using a nonpersistent T gondii strain is available in Europe and New Zealand to reduce abortion in sheep.

Precautions 1)Meat should be cooked to 66°C throughout before eating.

2)Hands should be washed with soap and water after handling meat.

3)Raw meat should never be fed to cats; only dry or canned food or cooked meat should be fed. 4)Cats should be kept indoors and litter boxes changed daily.

5)Cat feces should be flushed down the toilet or burned.

6)Litter pans should be cleaned by immersing them in boiling water.

7)Gloves should be used while working in the garden.