Neospora Abortion in Dairy Cattle

Steven L. Berry, DVM, MPVM;
John H. Kirk, DVM, MPVM;
Mark C. Thurmond, DVM, PhD

Department of Animal Science and
School of Veterinary Medicine, UC Davis

Neospora caninum is a coccidian protozoa that causes abortion in dairy cattle. Neospora has been found worldwide and is the most common cause of abortions on dairies in many areas including California. The organism was first identified i n 1988 as a cause of abortion in dogs and, shortly after; a Neospora–like organism was described as causing abortions in dairy cows. The organism causing abortion in cattle is now known to be the same species but a different strain as that ca using abortion in dogs.

Neospora caninum is a parasite that very closely resembles Toxoplasma gondii which is known to cause abortion in sheep and goats but has not been diagnosed as a cause of abortion in cattle. The definitive host for a parasite is on e which the parasite requires to complete its life cycle. No definitive host has not been found for Neospora. The cat is the definitive host for Toxoplasma gondii but Neospora caninum has never been found to occur naturally in cats. Cats are an important part of rodent control on many dairies and if the cats are removed from the dairy, rodents will be more of a problem. It is also possible that the rodents could be the definitive host for the parasite. Neospora caninum can in fect a wide variety of animals (dogs, cats, cattle, sheep, goats, horses, mice) and is found world wide. We might, therefore, expect that the definitive host would be a species or group of related species that have a worldwide distribution. We would also expect the definitive host to be one in which the parasite would have a low pathogenicity.

The economic loss to the California dairy industry from Neospora caninum has been estimated to be more than $35,000,000 per year due to failure to start lactation due to early abortion. This estimate does not consider the loss of calves, lower milk production, and increased culling not related to abortion. One California study found that cows that had antibodies against (seropositive) Neospora caninum produced at least 2.5 lb/day less milk that those that were seronegative. Another stud y found that seropositive cows were culled 6.3 months earlier and had a 1.6 times greater risk of being culled than seronegative cows in the same herd.

It appears that vertical transmission (from cow to fetus through the placenta) is the main means of disease transmission. Infection can thus be maintained on dairies even without the definitive host since the infection can pass from cow to calf throug h many generations. How cows acquire the initial infection is unknown at this time. There has only been one report of limited postnatal transmission in some herds.

Neosporosis is the most commonly diagnosed cause of abortions in California dairy herds. The presence of antibodies to Neospora caninum in cows does not ensure protection against the disease. It is possible that protection might depend on anoth er form of immunity, cell-mediated immunity, although the mechanism has not been shown. There are some herds which have endemic infection (many cows have antibody titers to Neospora caninum) which have a fairly steady abortion rate due to Neosp ora caninum and other causes. There are also herds in which the disease will cause epidemic abortion storms. Seropositive cows are 2-3 times more likely to abort than seronegative cows in a herd with endemic infection. The odds of a seropositive cow aborting are much higher in herds that have epidemic abortions. Herds that experience abortion storms are probably the result of recent exposure of naïve cows or some sort of immunosuppression such as concurrent infection with BVD.

There are several tests used to detect the disease in dairy herds. Immunofluorescent antibody tests (IFAT) and enzyme linked immunosorbent assay (ELISA) will detect antibodies against Neospora caninum in cows blood serum. Positive IFAT or ELISA results mean that the cows were exposed to the disease but do not necessarily indicate infection status at the time of the test. Abortion diagnosis should be made with a combination of the serological tests and immunohistochemistry and histopathology of the aborting fetuses.

Many producers live with the disease in endemically infected herds and consider Neospora to be a cost of production. Abortions tend to occur during mid-lactation and it appears that fetuses exposed in early gestation are more likely to abort an d that fetuses exposed later are more likely to survive. It is uncertain whether fetal exposure is related to ingestion of Neospora by the cow or if it is due to recrudescence of a latent infection. Although most calves born to infected cows will appear to be clinically normal, there may be some neurological disease that is not readily apparent. Normal calves with congenital infections usually have high antibody titers which indicate exposure in the uterus. Thus far, there is no evidence of disea se progression or risk of other diseases in clinically normal, infected calves. In one California study, seropositive calves grew up to give birth to seropositive calves and seronegative calves grew up to give birth to seronegative calves. None of the se ronegative calves became seropositive during the study.

Prevention recommendations are difficult to make without knowing the definitive host and also vertical transmission of the disease can maintain the disease on a dairy even in the absence of the definitive host. The most common recommendation is to try to prevent fecal contamination of feed on the dairy. There is no evidence that culling seropositive cows or heifers will help control the disease and in all likelihood would be economically disastrous. It is possible that seropositive cows act as a natu ral form of vaccination and may help prevent abortion storms, although Neospora caninum can still cause sporadic abortions on the dairy.

There is some evidence that Neospora caninum may be overdiagnosed in some abortion storms. California researchers investigated 20 herds with epidemic abortion storms characteristic of Neospora and found that 6/20 had no relationship betw een the abortion epidemic and antibody titers in the herd. BVD was involved in three of the herds and leptospirosis was involved in one herd. Another study found that 80% of seropositive cows did not abort. It is important when taking blood samples from aborting cows that samples also be taken from non-aborting cows so that antibody titers can be compared between the groups to assess the probability of Neospora caninum being the cause of the abortions. The take home message is that vaccination pr ograms on dairies should be in place to control other abortion causing organisms and that investigation into the cause of epidemic or endemic abortions needs to include organisms other than Neospora caninum.