Veterinary Epidemiology and Zoonosis

TOXOPLASMOSIS

Type of zoonosis

• Direct anthropozoonosis, Saprozoonosis, Facultative cyclozoonosis, Food-borne zoonosis

Definition

• Toxoplasmosis is a congenitally and postnatally acquired zoonosis, caused by Toxoplasma gondii. It is characterized by retinochoroiditis and encephalitis, and abortion in pregnant women (especially at first trimester). In animals, it causes abortion (storms of abortion), specially in sheep.

Etiology

• Toxoplasma gondii (Subphylum: Apicomplexa, Family: Eimeriidae).
• It is an obligate intracellular parasite.
• Only one species of Toxoplasma present worldwide.

Reservoir and incidence

• Toxoplasmosis in animals and man is distributed worldwide. It has wide host range.
• Cat and wild felines act as definitive hosts (final hosts), and reservoirs for Toxoplasma gondii.
• Domestic cat predominates as reservoir for zoonotic transmission.
• All mammals, man, sheep, dogs, pigs, including cats and birds act as intermediate hosts.
• Prevalence is more common during warm and humid climates.
• Infection is common in older persons, immunodeficient people and HIV positive people.
• Seropositivity increases with age.

Transmission and life cycle

• Faecal-oral route: Ingestion of meat containing cysts or tachyzoites or ingestion of oocysts.
• Ingestion of food materials and water contaminated with faeces of animals.
• Eating of under cooked meat or raw meat containing cysts.
• Ingestion of raw milk.
• Ingestion of sporulated oocysts in the soil contaminated with faeces of animals.
• Blood transfusion.
• Transplacental (in-utero) transmission.
• Spread by organ transplantation.
• Life cycle consists of two phases of infection, intestinal and extra-intestinal phase
  • Intestinal infection in the definitive host (felids) results in shedding of unsporulated oocysts, and then sporulated (oocysts become infective after sporulation, in 2 to 3 days) in the environment (soil). Sporulated oocysts contain two sporocysts, each containing four sprozoites, which are infective to the intermediate hosts. Sexual development of Toxoplasma gondii occurs only in the definitive hosts.
  • Extra-intestinal infection in the intermediate hosts (man, mice, rats, hamsters, guinea pigs, rodents, rabbits, dogs, sheep, cattle, birds and cats) results in replication of sporozoites asexually in the striated muscles and various organs, and develops "pseudocyst" with tachyzoites (banana-shaped). Continuous replication of tachyzoites (called, "proliferative form" or active phase of infection) results in development of immunity, and tachyzoites are replaced in to bradyzoites (called, "cystic form" or chronic phase of infection or latent infection). Immunosuppression leads to active proliferation of bradyzoites in to tachyzoites.

Disease in animals

• Prepatent period is variable.
  • Tissue cysts with bradyzoites: 3 to 5 days.
  • Tachyzoites: 5 to 10 days.
  • Sporocysts: 20 to 24 days.
• Cats are asymptomatic carriers for toxoplasmosis.
• Shedding occurs for 1 to 2 weeks following ingestion of sporulated oocysts.
• In the intermediate hosts, infection is clinically inapparent or sporadic clinical infection.
• In sheep and goats, it causes congenital infection, which leads to abortion and perinatal infection, which leads to infection of offspring.

Disease in man

• Usually it is inapparent or asymptomatic infection.
• Sporadic clinical incidence in humans, however, the infection is very common.
• Congenital infection may lead to systemic disease, or often lead to encephalitis.
• Ocular or systemic toxoplasmosis can be acquired by older people or immunocompromised people.
• Clinical signs include fever, skin eruption, malaise, myalgia, arthralgia, cervical lymphadenopathy, pneumonia, myocarditis, meningoencephalitis and chorioretinitis.

Diagnosis

• Serology with paired sera.
  • Indirect fluorescent antibody test
  • Sabin- Feldman dye test (most sensitive test, but rarely used)
  • ELISA
  • Compliment fixation test
  • Latex agglutination test
• Toxoplasmosis skin test for epidemiological studies.
• Faeces floatation technique to identify oocysts. The demonstration of cysts does not establish a causal relationship to clinical illness, since cysts may be found in both acute and chronic infections. However, only finding tachyzoites in blood or body fluids confirms active infection.
• TORCH test for the diagnosis of toxoplasma, rubella, cytomegalovirus and herpesvirus.

Treatment in man

• The drugs of choice are pyrimethamine and sulfadiazine. Combination of these drugs is more potent than either alone, and acting on dividing Toxoplasma gondii tachyzoites.
• In those patients who do not tolerate sulfadiazine, a combination of pyrimethamine and clindamycin is advised.
• Pyrimethamine is generally not given to pregnant women.
• Trimethoprim-sulfamethaxozole combination is effective for cerebral toxoplasmosis, led to complete regression of clinical signs.
• Folinic acid is given to avoid the haematologic effects of pyrimethamine-induced folate deficiency.

Prevention and control

• Avoid eating raw or under cooked meat.
• Freezing of meat to -20°C for 2 days or -15°C for 3 days kills cysts.
• Cooking of meat to 60°C kills cysts.
• Oocysts voided in cat faeces can remain infective for a year or more under appropriate environmental conditions.
• Children's play areas should be protected from cat and dog faeces.
• Clean the litter pans daily.
• Wear gloves while cleaning and removing cat faeces.
• Hands should be washed after handling raw meat and before eating or touching the face.
• Pregnant women should be checked for both IgM and IgG antibodies against toxoplasmosis. If the IgM antibody is negative but the IgG titre is present less than 1:1000, no further evaluation is necessary.
• Pregnant women with negative titres for both IgM and IgG antibodies, should take all precautionary measures to prevent infection (by avoiding contact with cat faeces and avoid working in soil or gardens that could be contaminated by cats, etc. and by thoroughly cooking meat).