Update on Neosporosis
By Steve Wikse, DVM, DACVP, beef cattle clinician and Extension veterinarian,
College of Veterinary Medicine, Texas A&M University



 

Neospora caninum may not sound familiar, but this organism causes significant economic losses in the Texas beef industry.

In 1989, it was the reported cause of abortions on a New Mexico dairy. A flurry of research followed, especially on California dairies, that declared neosporosis to be the most common cause of dairy cattle abortions.

This new disease was heralded as a major breakthrough in our understanding of the most difficult group of cattle diseases to control: Abortions.

Transmission
Neospora caninum is a coccidian parasite that causes intestinal infection in canids, members of the dog family (dogs, foxes, coyotes, etc.). Following initial infection, the canid sheds infective oocysts, or eggs, in its feces.

Research implicates coyotes and gray foxes as the probable sources of infection of cattle in Texas. In Australia, ranchers that noticed abortions after dingoes were seen in their cattle herds referred to neospora abortions as “wild dog disease.”

In early research outside Texas, ranches with dogs were more likely to be infected. Texas studies, however, found herds with dogs to be less likely to have neosporosis.

Cattle become infected with N. caninum two ways: Postnatal and congenital. Ingestion of oocysts in fecal-contaminated feed leads to “postnatal infections.” A cow that ingests neospora for the first time may later abort. It,s suspected that a herd mate that licks the aborted fetus or placenta may also become infected.

Infected cows become life-long carriers of neospora. A carrier cow passes infection to her fetus 85 to 95 percent of the time. A very small percentage of these “congenital infections” lead to abortion, but usually they result in healthy calves that are also life-long carriers. A heifer calf, however, who is a healthy carrier of neospora will be many times more likely to abort her first or second pregnancy than a herd mate who is not a carrier.

Studies in the Texas Ranch to Rail Program revealed 59 percent of Texas beef ranches were infected with neospora. Other work showed 10 to 20 percent of cows are chronic carriers in beef herds infected with N. caninum.

Losses
Abortions, the main problem caused by neosporosis, occur anytime between three months of gestation and term, but are primarily concentrated at five to six months.

Fetuses may die in the uterus and be reabsorbed or mummified with no external sign of fetal loss. Stillbirths can also occur. Rarely, calves with congenital infection are born with incoordination.

Texas A&M University researchers estimate reproductive effects of neosporosis result in losses to the Texas beef industry of $13.75 to $23.29 per cow annually.

Studies in Texas have associated neospora in feedlot steers with reduced growth rates that could result in annual losses of $13 million.

With increasing awareness of biosecurity, purebred producers may face future losses from reduced value of infected replacement heifers.

How do I know if my herd has neosporosis?
Action is recommended when abortion rates in beef herds exceed three percent.

A very accurate test used to measure antibodies against N. caninum in the blood can detect chronic carriers. Finding antibodies to neospora in a cow that aborted, however, doesn't prove neospora caused the abortion.

Accurate diagnosis of neospora as the cause of abortions in a herd can be made by comparing the percentage of cows with antibodies to N. caninum in the group that aborted to the group that didn't abort. If neospora is the culprit, a much higher percentage of cows that aborted will have antibodies than cows that didn't abort.

Control of neosporosis
We do not have a complete understanding of the biology of this new disease. Various combinations of biosecurity, testing and culling carrier cows and vaccination are being tested.

There have been no published independent studies on efficacy of neospora vaccines. Use of a vaccine in a herd could interfere with diagnosis of neospora as a cause of abortions and make a test and cull program impossible because vaccinated cattle may have antibodies for long periods of time.

Consider these suggested steps to control neospora, minimizing congenital and postnatal transmission of the disease:

1) Biosecurity: Purchase only antibody-negative females.
2) Reduce the number of carrier cows in the herd.

• Implement one of these two methods:
  Test breeding females and do not keep offspring of infected animals as replacement heifers. It,s highly likely they are infected! This is the most economical control program for a commercial cow-calf herd.

• Or test breeding females, cull all infected animals and replace them with animals that are antibody-negative. This option is best for a purebred herd using embryo transfer into recipient cows because the value of the fetus is too high to risk losing. There is no need, however, to cull infected donor cows. Their embryos are safe to use.

3) Protect feed and water sources from fecal contamination from canids.
4) Reduce the number of wild canids.
5) Promptly and properly dispose of aborted fetuses and placentas.

I have found testing and culling infected cows plus reducing the wild canid population is effective in controlling abortions due to N. caninum in a purebred beef herd. However, we have created a herd with no immunity against neospora, vulnerable to abortions from ingestion of oocysts.

It is important to accompany a test and cull program with an effort to reduce the number of potential wildlife carriers on the ranch. These considerations indicate that when an effective vaccine becomes available, vaccination will become an important part of a neospora control program.

A neosporosis control program for a beef herd should only be implemented under the direction of the herd,s veterinarian.