A112 - Toxoplasma gondii

Toxoplasma gondii is an intracellular coccidian parasite and is one of the most common parasitic diseases of animals and man. The definitive hosts for the parasite (the only animals in which the organism reproduces sexually) are members of the Felidae family (mainly domestic cats). T. gondii is of primary importance since the range of intermediate hosts which can become infected encompasses virtually all warm-blooded animals. Cats usually become infected with T. gondii by ingestion of encysted organisms present in the tissues of a chronically infected intermediate host (eg, rodents, birds, sheep, pigs, cattle). Surveys generally show a 20-60% prevalence of infection with T. gondii in cats. The prevalence will depend on many factors but is primarily related to the availability and ingestion of infected intermediate hosts (rodents, birds etc). Infection is therefore more common in stray and feral cats than in pet cats, and the prevalence also increases with age. Although there is generally a high prevalence of infection in cats, most surveys show a less than 1% incidence of oocyst shedding. This is to be expected as infected cats generally do not re-shed oocysts following their first exposure to T. gondii.
Despite the high prevalence of Toxoplasma gondii infection, significant clinical disease in cats (and other species) appears to be very rare. When disease does occur, it may develop either following primary infection (where an inadequate immune response fails to arrest the invasive tachyzoites), or as a result of reactivated infection (where compromised immunity allows the reactivation of infection from encysted bradyzoites with the formation of invading, multiplying tachyzoites). Clinical disease appears to be most common in young cats (less than 2 years of age), and this may be due, in part, to a poorly developed immune response in these cats. Reactivation of infection in older cats may be linked to co-infection with feline leukaemia virus or feline immunodeficiency virus in some cats. The most commonly reported clinical signs associated with feline toxoplasmosis are anorexia, weight loss, lethargy, dyspnoea (due to pneumonia), ocular signs (iritis, chorioretinitis) and pyrexia. Other less common features include gastrointestinal signs (vomiting and diarrhea), neurological signs, lymphadenopathy, jaundice, myositis and abortion.
Toxoplasma gondii is also reported from most bird species but is more common in birds that eat meat or have access to the ground or insects that can mechanically transfer the oocysts. Infected birds may display no clinical signs, but toxoplasmosis should be considered in acute onset of "sick bird syndrome" with possible neurological signs or acute death. Specific signs are variable because of the wide variety of body systems that can be affected. Clinical signs of toxoplasmosis in birds have included anorexia, prostration, weight loss, diarrhea, dyspnea, pneumonia, blindness, and conjunctivitis, and toxoplasmosis should be considered in birds that present with these clinical signs.

The diagnosis of toxoplasmosis is problematic and a definitive diagnosis rests on demonstration of the active form of the organism in tissues taken at post mortem examination or in biopsy samples. Laboratory tests may also be used as diagnostic aids. Diagnosis is usually done by ELISA test. Serological diagnosis is difficult in animals with severe immune dysfunction. Recently, the PCR conformation test was developed and it is based on the DNA amplification of the Toxoplasma gondii B1 gene. A single organism can be detected directly from a crude cell lysate or as few as 10 parasites in the presence of 100,000 leukocytes.