Neosporosis

Etiology

• The cow is an intermediate host for N. caninum Neospora caninum: the parasite.
• In the cow, Neospora caninum appears to localize to neuromuscular tissue in its slow growing stage (bradyzoite), but in its initial fast growing stage (tachyzoite) it can pass the placenta and infect the fetus.
• N. caninum does not always cross the placenta; uninfected calves can therefore be born, but the majority of fetuses in seropositive dams will be infected via the placenta.
  • The majority of these infected fetuses will develop without any clinical signs but are persistently infected.
• Other intermediate and definitive hosts can become infected by ingesting infected tissue (such as placenta, aborted fetus, and neuromuscular tissue).
• Clinical signs in these intermediate hosts are uncommon; however, they do provide a source for definitive hosts to become infected.
• Data from a recent GB study showed that almost all the wild carnivorous mammal species tested were intermediate hosts for N. caninum and are therefore capable of acting as reservoirs of infection for other species. Some of the intermediate hosts include: ferrets, red foxes, polecats, mink, badgers, wild rabbits, house mice, wood mice, water buffalo, sparrows, horses, goats, deer, cats, sheep magpies, common buzzard, crows, domestic chicken, shrews, harvest mice, voles, and pigeons.
• Horizontal cow to cow transmission has not been described. However, it is speculated cattle can infect themselves in a similar way to other intermediate hosts; by, for example, ingesting infected placenta or fetal fluids.
  • This has not yet been confirmed, and therefore currently the most likely route a cow is infected is via vertical transmission in utero. This means the calf is already infected at birth and therefore preventive measures, such as vaccination to prevent infection, are of limited use.

Predisposing factors

General

• The reported chances of a fetus becoming infected and the occurrence of fetal death are variable and appear to be depending on a variety of factors; for example, beef cattle are less likely to abort than dairy cattle and concurrent infection with BVDv Bovine viral diarrhea virus may aggravate disease; these associated risk factors appear to be related to immune system health.

Timecourse

• Calves will be born healthy or will be weak from birth, having been infected in utero.
• Cows can abort within a week after being infected, but once they are infected, the time to abortion can be anytime as the animal remains infected for life.
• Survival times of oocysts on pasture and in silage are unknown but are suggested to be long, as they are expected to be similar to other apicomplexan parasites such as Toxoplasma gondii. Toxoplasma gondii.

Epidemiology

• Cattle get infected by ingesting sporulated oocysts or by vertical transmission via the placenta.
• Although conflicting results are presented in studies, semen and milk appear to be unlikely sources of infection.
• Oocysts are only found in fecal material from definitive hosts. Thus far only the dingo, coyote and domestic dog have been confirmed to be definitive hosts.
• Cows that have aborted once due to N. caninum Neospora caninum are more likely to have either subsequent abortions or infected fetuses in future pregnancies.