Toxoplasma gondii
The final host for T.Gondii is the cat. Sexual reproduction occurs in the epithelial cell sof the small intestine. Oocysts are produces that are passed in the faeces. The cat usually shows no sign of infection and normally, after excreting oocysts for about 10days, becomes immune and stops production. Asexual reproduction occurs in the extra-intestinal (outside of the intestine) tissue of almost any animal. Following ingestion of oocysts or asexual stages the sporozoites leave the intestine and travel to tissue, particularly muscle or brain. Here they divide to form tachyzoites. Once an immune response is started by the host these undergo slower division; they are then known as bradyzoites. these remain in the tissue in hope that they will one day be eaten by a cat.
These cysts in tissue are minute and cause little problem except in certain circumstances:
- A ewe is infected for the first time during pregnancy. Some cysts may occur in the placenta and may cause abortion.
- A woman is infected for the first time during pregnancy, e.g. by eating meat containing bradyzoites or accidentally swallowing sporulated oocysts. Infection of the foetus may result and, depending on the stage of pregnancy, this may result in abortion, severe abnormalities or no clinical signs at all. Fortunately, infections during human pregnancy are not common. Further information and information leaflets can be obtained from the Toxoplasmosis Trust.
- Infection in humans may be associated with malaise and flu-like symptoms that vary in severity from individual to individual.
- Cysts in immunosuppresed individuals may once again begin to undergo rapid division and cause severe tissue lesions.
In order to try and prevent these infections occuring:
- Farmers are advised to prevent cats, particularly young cats, from getting into food stores intended for sheep. There is now a vaccine against toxoplasma for sheep.
- Pregnant woman are advised to take precautionary measures. For example, they should not clean out cat litter trays, they should wear gloves when gardening and they should ensure all meat is thoroughly cooked before eating it.
There is no effective treatment to prevent oocysts shedding in the cat. Children that have been infected prenatally are treated with antibacterials to prevent any long-term efects.
It is now recognised that there is a seperate parasite, Neospora Caninum, that may cause incoordination in young dogs and abortion in cattle. In the past, infection was normally ascribed to Toxoplasmia gondii. It is believed that the dog may be the final host for this parasite.
