Virus linked to Thoroughbred abortion epidemic

June 22, 2006

CORVALLIS, Ore. - 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.

In a recent publication in the Journal of Medical Virology, Smith and other researchers outlined how vesivirus or the antibodies to it were found in human blood samples, most often in those individuals with liver damage or hepatitis of unknown cause. In persons who previously had transfusions or dialysis, and then developed hepatitis of unknown cause, 47 percent had antibodies to vesivirus. In the newest study with Thoroughbred horses, when a different criterion for a "positive" exposure to vesivirus was used, allowing samples with a slightly lower level of antibody to be included as positive, it showed that 81 percent of abortion-associated mares in the study group were seropositive for vesivirus exposure, the scientists said.

In a second study involving experimental exposure of pregnant mares to Eastern tent caterpillars where 17 of 29 aborted, the association of vesivirus antibodies with abortion problems appeared to be stronger than the association with Eastern tent caterpillars, the scientists said.

The most common point source of infection with vesivirus, the study said, would be animals with acute or chronic infections with persistent shedding of virus. Transmission via direct contact has been often reported in other animal species, such as swine, cattle and reptiles.

There is no vaccine or medical treatment for vesivirus control in horses, the researchers said, so vesiviral problems that are identified through accurate diagnosis would probably be managed by those methods used for any infectious disease, such as animal quarantines and other herd management techniques.

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Oregon State University

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