Study Links Virus to 2001 Thoroughbred Abortion Epidemic

A new study by veterinary researchers at Oregon State University has linked a major epidemic of abortion a few years ago in Kentucky Thoroughbred mares to infection with vesivirus, the first time the virus has been suggested to cause this type of problem in horses.

The findings, which were just published in the American Journal of Veterinary Research, add another concern to the health issues associated with this virus, which is a part of the Caliciviridae viral family that can infect and cause health problems in many animal species, including humans.

About five years ago, mares in Kentucky's billion-dollar Thoroughbred industry were losing foals at an alarming rate, with abortions of unknown cause happening up to 10 times more than usual on some farms. The problem eventually dissipated, and researchers since then have worked to identify the cause.

Many agents known to cause abortion in horses were considered at the time and ultimately dismissed, but some studies suggested at least part of the problem might be exposure to toxins in Eastern tent caterpillars found on some farms.

One new study examined 112 horses, both normal and those that had suffered abortions. It found that 40 percent of the mares with no reported abortion problems tested positive for vesivirus antibodies, but 64 percent of those from areas with high rates of abortion, or that had aborted their foals, had vesivirus exposure. All male horses and younger females not of breeding age tested negative for vesivirus antibodies.

According to researchers, these data suggest that vesivirus must now be considered a pathogenic virus associated with abortion in mares. It indicates that broodmares are being commonly exposed to vesivirus from unknown sources, they said, and that vesiviruses should be added to the panel of diagnostic tests for horses that abort.

It is not known whether exposure to horses, especially broodmares that abort, is an increased public health risk, the scientists said. However, vesiviruses have been shown to infect more than one animal species and there is evidence of their association with abortions in humans. Because of that, exposure to such horses might be a concern for pregnant women, they said in their report.

"Reproduction loss in valuable Thoroughbred horses is taken very seriously, as yearling Thoroughbreds can be worth thousands of dollars," said Alvin Smith, a professor of veterinary medicine at OSU and co-author of the paper with Andreas Kurth, a former doctoral student at OSU.

"Prior to this it has never been demonstrated that caliciviral infection in horses can occur in a natural setting," Smith said. "These data clearly suggest that pregnant mares are susceptible to infection, that they can replicate a high viral load, and that elevated antibody levels and increased incidences of abortion in mares are associated with this."

The genus vesivirus has been shown to cause a wide range of health problems in multiple animal species, and abortion is one of the leading concerns - in swine, marine mammals, cats, and potentially other species, including cattle and humans. The host range for vesivirus is broad - including but not limited to fish, seals, whales, reptiles, birds, primates, swine, cattle and humans. And severe disease conditions, the study said, have been positively correlated with the virus, including hepatitis, pneumonia, diarrhea, myocarditis and encephalitis.