Study of oral infection with Toxoplasma gondii in sheep: evaluation of congenital transmission in experimental infections by different strains in Brazil

Abstract

Toxoplasma gondii is considered as the primary cause of congenital disease, miscarriage and stillbirth in humans and farm animals. In congenital diseases neurological manifestations such as eye disease and hydrocephalus are common. Toxoplasma gondii infections do not always result in clinical manifestations. Rather, it is quite frequent occurrence of asymptomatic infections in both animals and humans. Studies of seroprevalence of T. gondii infection in sheep in Brazil show that it varies on average between 20 and 40%, but is widespread in more than half of the herds analyzed. Toxoplasmosis in sheep usually manifests itself clinically after primary infection of a pregnant female. The sheep usually become infected after birth with oocysts available in the environment. In a pregnant female, the parasite infects the placenta, the maternal placentomes and reaches the fetal trophoblast cells (transplacental transmission). After multiplying in these tissues, the parasite infects other fetal tissues, producing symptoms such as miscarriage, stillbirth and birth of weak lambs. Chronically infected sheep may suffer a recrudescence of the infection and transmit the parasite to the pregnant reproductive system. This condition, transplacental transmission caused by recrudescence of a persistent infection in sheep, is the subject of intense debate in the scientific community today. In this field, two opposing point of views are recognized. Some research groups claim that sheep suffer reproductive problems only after primary infection, because once infected the sheep mount efficient and durable immune responses that prevents the occurrence of congenital transmission, or make this event being rare. Another current suggests that a primary infection does not confer protection to subsequent progeny, with the risk of vertical transmission in animals chronically infected. In this case, the primary infection produce an immune response that would not prevent the reactivation of latent infection that would lead to a congenital transmission, so that ewes infected before pregnancy were at risk of abortion with greater frequency. Other form of transplacental transmission can occur as a result of a new infection of chronically infected animals by oocysts. The consequences of this situation are barely known by the scientific community. This project aims the experimental reproduction of infection by T. gondii in sheep, in order to study aspects related to the occurrence of congenital transmission in this species. In our proposal we intend to evaluate infections caused by strains of T. gondii isolated from sheep in Brazil, using oocysts as inoculum. Seronegative ewes will be prime-infected with oocysts and, after the chronicity of infection, the animals will be mated. A second challenge, also with oocysts will be held at the final third of pregnancy. The effects of reinfection by the same strain as well as different strains of T. gondii will be observed and evaluated. The work proposed here should contribute to the knowledge useful in formulating programs for the control of toxoplasmosis in sheep breeding. Once the role of infected females in the transmission of this disease is better known, more efficient decision-making may be taken with regard to the sanitary management of properties at risk. (AU)