Protozoan infections (Toxoplasma gondii, Neospora caninum and Sarcocystis spp.) in sheep and goats: recent advances

Abstract

The protozoan parasite Toxoplasma gondii is a serious cause of fetal mortality in sheep and goats. Oocysts, the parasite stage responsible for initiating infection, are produced following a primary infection in cats. A primary infection in pregnant sheep and goats can establish a placental and fetal infection which may result in fetal death and resorption, abortion or stillbirth. Diagnosis is aided by the clinical picture, the presence of characteristic small white necrotic foci in placental cotyledons, the possible presence of a mummified fetus and on fetal serology and histopathology. Development of the polymerase chain reaction (PCR) specific for T. gondii may also provide a valuable diagnostic tool. Measures to control abortion include improved management of farm cats, fodder and water. Vaccination of sheep with the live vaccine is an effective preventive measure and the use of decoquinate in feed may be useful in some situations. Neospora caninum is related to T. gondii and while its asexual life cycle is similar to that of the latter it is currently not known whether it has a similar sexual life cycle in a definitive host. Neospora is an important cause of fetal loss in cattle and parallels that of T. gondii infection in sheep and goats. While it does not appear to cause frequent losses in these latter animals, experimental infection is readily induced in them and if initiated during pregnancy provides a very good model of the bovine infection. Furthermore clinical signs and pathological lesions in sheep and goats are similar to those induced in them by T. gondii, although there are subtle histopathological differences. These changes will aid possible diagnosis as will specific serological tests such as the indirect immunofluorescent antibody test and the enzyme linked immunosorbent assay and the PCR. Sarcocystis, which exists as numerous species, undergoes a coccidian-like life cycle with each having a distinctive definitive (usually carnivore) host which excretes sporocysts into the environment. Clinical sarcocystiosis is much less commonly diagnosed than toxoplasmosis and neither is it normally associated with fetal infection or abortion in either sheep or goats. However, infection is extremely common throughout the world and follows ingestion of food or water contaminated with sporocysts. Clinical signs, when seen, include fever, anaemia, inappetance and weight loss or reduced weight gain. Central nervous signs (hind limb weakness, ataxia, paresis), acute myopathy and death may occur. Diagnosis is difficult as infection is so common and clinical signs absent, mild or non-specific. Serology may be useful in some situations and histopathology/immunohistochemistry is valuable for confirming the cause of death. Control relies on preventing contamination of pasture and water with faeces of dogs, foxes and cats or by controlling access of young susceptible stock to contaminated land. Relatively little is known of the immunity induced by infection with Sarcocystis spp. but research indicates that protective immunity does develop and that cell-mediated mechanisms are probably important. It is likely that sarcocystiosis is underdiagnosed as a problem and that better diagnostic methods are needed to show the true extent of the losses caused. Neosporosis on the other hand would appear not to be so common in sheep and goats. The value of experimental infections in these animals may be to provide a comparative model of the infection in cattle in the same way that our understanding of toxoplasmosis in sheep provides a superior model of human toxoplasmosis.