Toxoplasmosis is a zoonotic, parasitic infection caused by the intracellular, apicomplexan parasite Toxoplasma gondii, which infects all homeothermic animals including humans. The parasite has a major economic impact on the livestock industry. This is especially true for small ruminants (sheep, goats) as it is one of the most likely reasons for reproductive disorders in these animals. Primary infe
ction in sheep and goats can result in a fetus that is mummified or macerated, fetal embryonic death, abortion, stillbirth, or the postnatal death of neonates, all of which threaten sheep and goat rearing globally. Humans can also become infected by ingesting bradyzoite-containing chevon or mutton, or the contaminated milk of sheep or goats, highlighting the zoonotic significance of this parasite. This article reviews the advances in vaccine development over recent decades and our current understanding of the immune response to toxoplasmosis in small ruminants (sheep, and goats).In a small ruminant, acute toxoplasmosis is manifested by a brief episode of fever, apathy, anorexia, diarrhea, and coughing (1, 9). In pregnant sheep species, the tachyzoites invade and proliferate within the tissues of the feto-maternal junction , resulting in fetal abortion, mummification, maceration, stillbirth, premature birth, or the delivery of a weak lamb that fails to survive long after birth . gondii typically causes early abortions in sheep that usually occur 1 month after infection but recent studies have also described a clinical presentation of early abortions only 14 days after experimental infection while late, or classical abortion occurs 19–26 days post-infection . In their study, sheep were inoculated with 50 oocysts of M4 strain in early (40 days of gestation), mid (90 days of gestation), and late gestation (120 days of gestation), and abortion was observed in all groups. However, the distribution of parasite was controlled in a better way in early and mid-gestation with lesser lesions than in late gestation. On the other hand, the dissemination of parasites was faster in late gestation to the placenta and fetus causing abortion and the lesions found in the fetuses were more visible in late gestation than those caused by early gestation. The mechanisms that trigger these variations are still unknown, however, it has been hypothesized that modulation of the immune response that occurs during pregnancy might be responsible . The histopathological findings involve leukoencephalomalacia in the brain of lambs infarcts and thrombosis in the caruncular villi of the placentomes, and ischemic lesions of ewes during the acute phase of abortion as a consequence of hypoxic damage to the fetus resulting abortion . It has also been described that, in the ovine, early abortions (40 days of gestation) cause increased infiltration of macrophages in caruncular septa, whereas in late (120 days of gestation) abortions the placentas containing the parasite had an increase of T lymphocytes and macrophages primarily in the fetal cotyledons It has also been reported that the differences in peripheral and placental immune responses following T. gondii infection at different gestational ages in sheep may play role in pathogenesis The immune response is mediated by the interaction of Th1 (IFN-γ and TNF-α), Th2 (IL4), and Treg cytokine. The severity of infection is determined by the stage of pregnancy . The fetus is immunologically weak during early pregnancy; hence, the consequences are most severe if infection occurs early. However, as the pregnancy progresses, the fetal immune system grows stronger, as a result of which the clinical effects are less severe
In 2005, a Toxoplasma-induced abortion storm was suspected among sheep in the United States . In the case of goats, a severe outbreak of clinical toxoplasmosis was documented in Brazil and Parana The farming industry suffers financial losses as a result of morbidity and mortality among lambs and kids Practically, it is difficult to assess the real losses caused by Toxoplasma in the ovine industry as the disease appears to be unpredictable, few aborted fetal samples are available for diagnosis, the samples that are submitted are often not adequately examined, and some submitted samples are not appropriate for diagnostic evaluation, and non-specific serodiagnosis occurs (10). Thus, this parasite is not only a public health burden but also an economic burden on the ovine industry
To alleviate this economic burden, it is necessary to control the acute and congenital transmission of T. Vaccination might be one of the effective strategies to prevent abortion, vertical transmission, as well as acute and chronic infection, and tissue cyst formation. Effective vaccination could thereby help to protect sheep and goats and to prevent the risk of zoonosis. gondii vaccines currently available for sheep and goats and explores our understanding of the immune responses triggered by these vaccines.
