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Neospora caninum is a microscopic protozoan parasite with worldwide distribution. Many domestic (dogs, cattle, sheep, goats, horses, chickens) and wild animals (deer, rodents, rabbits, coyotes, wolves, foxes) can be infected. Neosporosis is one of the most common causes of bovine abortion, especially in intensively farmed cows. Neosporosis abortion also occurs in sheep, goats, water buffalo and South American camelids, although they may be less susceptible than cattle.

Epidemiology: Neosporosis in cattle herds manifests in both endemic and epidemic abortion patterns, but it is also possible for a herd to have a high infection prevalence without a noticeable abortion problem. Both endemic and epidemic transmission patterns in cattle are positively associated with the presence and number of dogs in and around farms. Endemic abortion is mainly associated with endogenous transplacental transmission, although occasional transmission from dogs or other canids may compound the problem. Epidemic abortion is a possible consequence of sudden large-scale transmission to pregnant cattle, presumably by ingestion of a mixed ration or water that has been contaminated with infected canine feces. The use of mixed rations in dairy herds probably accounts for the greater prevalence of Neosporosis in dairy cattle than in extensively grazed beef cattle.

Transmission: Dogs are definitive hosts of N. caninum and are capable of shedding oocysts in feces after eating tissues of infected animals. Wild canids also are suspected to be, and coyotes have been confirmed as, definitive hosts. Neospora oocysts have an impervious shell that enables survival in soil and water for prolonged periods after canine feces have decomposed. Intermediate hosts such as cattle become infected by ingesting oocysts. Cattle do not produce oocysts and thus do not transmit infections horizontally to other cattle, but latent infection endures permanently in their tissues and is transmitted to canids by carnivorism.

In cattle, N. caninum can be transmitted transplacentally from an infected cow to the developing fetus, an event that may occur in multiple pregnancies of the same cow. Because the majority of congenital infections are subclinical, congenitally infected heifer calves may remain in the breeding herd and in turn may pass infections transplacentally to their own offspring. This endogenous transplacental transmission enables transgenerational maintenance of the parasite even if the herd does not have frequent transmission from dogs. Exogenous transplacental transmission may occur when a previously uninfected cow ingests Neospora oocysts during pregnancy and the fetus becomes infected.