Toxoplasmosis is a common parasitic disease affecting man and animals in all parts of the world. It is an infection caused by the single-celled parasite Toxoplasma gondii. In humans it causes a usually mild fever that can result in swollen glands. After the first infection, the person is "exposed"; the disease can recur not through reinfection but reactivation if the immune system becomes suppressed. The disease can produce lymphadenopathy and in some parts of the world is reported to be responsible for up to 15% of otherwise unexplained cases of this condition. It can also cause more severe symptoms - fever, rash, malaise, muscular pains, pneumonia, myocarditis or meningoencephalitis.
By far the most serious form of the infection, however, is congenital toxoplasmosis, which is acquired by the foetus during a mild infection of the mother. The baby may be stillborn, or it may suffer from hepatosplenomegaly, purpura, jaundice, lesions of the central nervous system, or destroyed areas of the retina. These conditions may be present at birth or may appear weeks or months afterwards. Once tissue has been destroyed - for example, in the brain or the eye - the effects do not regress.
No region is exempt from the toxoplasma parasite except possibly the Antarctic. It has been estimated that between 15 and 50% of the adult human population in most places in the world have been infected, as evidenced by serological studies. Most at risk are pregnant women. Infection can lead to brain damage and death of the foetus. Toxoplasmosis in people with AIDS is usually a reactivation. Several epidemiologic and serologic studies of the disease in animals and man, and studies identifying environmental reservoirs of the disease, have suggested that agricultural workers are at increased risk of acquiring this disease as compared to the general population.
Many species of wildlife and domestic cat and certain other members of the family Felidae are the only hosts in which the parasite will complete the sexual stage of the life cycle. The cat is an important source for the widespread dissemination of the organism in the environment. Infection in cats and other animals is transmitted either by ingestion of oocysts in faeces, in soil, or by consumption of infected animals which have toxoplasma cysts in their tissues. Infection is quite common in cattle, sheep, goats, swine and chickens. Serological surveys in several countries have shown that the average positive reaction rate recorded for the food animal species ranged from 22 to 39%, with sheep being the highest.