The Dog and Bovine Neospora Abortion

Dr. Clell V. Bagley, D.V.M.
USU Extension Veterinarian

    A recently published article (Intnl J Parasitology 28:1473-1478, 1998) identified the dog as the definitive host of Neospora caninum. Neospora was first identified in 1988, but has become recognized as a major cause of bovine abortion in several areas. It occurs across the United States and in many other countries. One set of 50 samples from a Utah dairy showed 50% of them to be seropositive. Neospora is a protozoan parasite that closely resembles toxoplasmosis. Dogs have previously been suspected of being involved in the pathogenesis, but their role had not been proven until this recent research. It now becomes even more important to prevent contamination of cattle feed with dog feces.

     Abortions induced by Neospora have been observed in cattle, horses and goats. Some cases of neurologic and neuromuscular disease have been observed in neonatal calves, dogs, sheep, foals, and deer, but this is not a usual clinical sign in adult animals. The fetal infections may result in fetal death, mid-gestation abortion, or occasionally in the birth of calves with congenital brain disease.

     The majority of fetal infections result in the birth of healthy calves with latent infections that are maintained and eventually passed on, when these become dams, to their fetus, which continues the cycle. This vertical transmission (transplacental) can continue for several generations without the parasite passing through the definitive host (dog). Apparently, almost every infected dam will pass Neospora to its calf. Stress and lowered resistance seem to be the factors which determine whether abortion will occur or a carrier fetuses will be born. The incidence of BVD infection in the herd seems to be especially correlated with initiation of abortion problems, but its exact role has not been determined. Probably anything that weakens the cow�s resistance might predispose to an abortion. There is no evidence of horizontal spread of Neospora directly from one cow to another in the herd.

     In the dog, ingestion of Neospora infected tissue (probably placenta or aborted fetus) results in an intestinal infection (mild or subclinical) with subsequent shedding of oocysts in the feces. The contamination of this onto feed can initiate the infective cycle in cattle. It may also be possible for the oocytes to be passively spread by birds over long distances. In the intermediate host the parasite would become systemic for a time (and could cause abortion during that time) but then would localize and become dormant. When these latently infected cows become pregnant, their fetuses become infected virtually every time. The feeding of oocyst infected feed to a group of pregnant cattle could result in an explosive abortion storm, much like lepto, BVD, or IBR.

     Abortion occurs typically in mid-gestation, but may occur anytime after the second month. It may take 2-4 weeks for the fetus to die after infection and another 2-4 weeks before it is aborted from the dam. This makes positive diagnosis more difficult. Some cows abort repeatedly in subsequent pregnancies; others do not.

     Serological tests for Neospora determine only whether the animal has antibodies. But these antibodies develop rapidly and to a high level, so if an aborting dam is negative it is very doubtful that Neospora was the cause. A fetus with typical lesions, positive immunohistochemical tests and no evidence of BVD, IBR, or lepto could be considered a Neospora abortion. Fetal brain is the most important tissue for diagnosis.

     There is still much to learn about prevention but the following are essential:

     * Protect feed and water sources from contamination with dog feces.
     * Prevent dogs from ingesting aborted fetuses or placentas.
         (Probably applies to other canids as well.)
     * Promptly remove and dispose of aborted fetuses and placentas.
     * Control rodents.

     It is also good to try to prevent birds from defecating in the feed. Serological testing and culling of positive animals may be feasible in some herds as a means for eradication of Neospora. ©