Oregon State Study Links Virus to 2001 Kentucky Abortions

Veterinary researchers at Oregon State University (OSU) announced today that they have linked a major abortion epidemic in Central Kentucky mares in 2001 to vesivirus infection. This marks 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, in spring 2001, 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, dubbed mare reproductive loss syndrome (MRLS) eventually dissipated, and researchers have been working to identify the cause ever since.

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% of the mares with no reported abortion problems tested positive for vesivirus antibodies, but 64% 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 vesiviruses must now be considered pathogenic viruses associated with abortion in mares. It indicates that broodmares are commonly exposed to vesiviruses 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," 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--in swine, marine mammals, cats, and potentially other species, including cattle and humans. Abortion is one of the leading concerns in these species. The host range for vesivirus is broad--including, but not limited to, fish, seals, whales, reptiles, birds, primates, swine, cattle, and humans. According to the researchers, severe disease conditions 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 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% 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, the researchers showed that 81% of abortion-associated mares in the study group were seropositive for vesivirus exposure.

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.

(edited press release)

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