Study: Antibiotics in Extenders can Prevent CEM Transmission

In the study, none of the mares inseminated with extended or frozen semen from the CEM-positive stallion or mares in the control group inseminated with raw CEM-negative semen developed clinical signs of vaginitis

Photo: Anne M. Eberhardt/The Horse

Researchers from the University of Kentucky Gluck Equine Research Center recently investigated whether antibiotics in a semen extender could inhibit the growth of the bacteria Tayorella equigenitalis, which causes contagious equine metritis (CEM), and escape detection of the bacteria in horses bred by artificial insemination.

What is CEM?

Contagious equine metritis (CEM) is a highly contagious sexually transmitted disease caused by the bacterium Taylorella equigenitalis. The disease is spread through breeding or contaminated instruments and causes temporary infertility in mares, and a non-symptomatic carrier state in stallions. In addition to costs associated with infertility of the mares, infected stallions need to be treated and rigorously tested free from the disease before they can breed again. The disease can be catastrophic to the horse industry if it goes undetected. Stallions show no clinical signs, but can carry the CEM bacteria on their genitalia for months or even years. If tested positive, they need to be taken out of breeding for several weeks or even months.

Shaila Sigsgaard

CEM has a devastating effect on fertility; it is a very costly and serious disease in horse operations that practice natural cover. The United States has largely been considered CEM-free since 1978, when an outbreak occurred in Kentucky with significant costs to the Thoroughbred industry. Importation regulations currently apply to horses imported from countries considered endemic for the disease.

“Although there is no current threat to the U.S. horse population, it always raises concerns that imported horses may bring the disease into the U.S.,” said Mats Troedsson, DVM, PhD, Dipl. ACT, director of the Gluck Center and chair and of the Department of Veterinary Science.

In 2006, three imported stallions in Wisconsin tested positive for T. equigenitalis. All stallions had tested negative prior to exportation to the U.S. and again upon arrival. They had bred several mares resulting in normal fertility and no clinical signs of disease, Troedsson said. In 2008, a Quarter Horse stallion tested positive for T. equigenitalis upon routine testing required for frozen semen to be exported to Brazil. However, no clinical observations of disease or infertility had been reported in mares bred to this stallion.

The Study


In the study, the researchers compared T. equigenitalis growth in raw semen from a CEM-positive stallion and the same semen extended in media containing antibiotics. They also assessed the outcome in mares inseminated with raw, extended, or frozen semen from the same stallion.

The team chose at random 21 adult mares and assigned each one into three different groups to be inseminated with either raw, extended, or frozen/thawed and extended CEM-positive semen or insemination with CEM-negative semen (control group). Additionally, the researchers cultured semen from an experimentally infected stallion to test the effect of antibiotics in extenders on the growth of T. equigenitalis.

Upon analyzing the data, the researchers found the following:

  • Commercial semen extender (EquiPro with amikacin and penicillin) contained enough antibiotics to prevent T. equigenitalis transmission to any of the mares in the study through artificial insemination with fresh, cooled, and frozen semen.
  • Six mares inseminated with raw CEM-positive semen had clinical signs of CEM (e.g., vaginal discharge) at each sampling point after artificial insemination.
  • In contrast, none of the mares inseminated with extended or frozen semen from the CEM-positive stallion or mares in the control group inseminated with raw CEM-negative semen developed clinical signs of vaginitis.

Shaila Sigsgaard

Following a subsequent national disease investigation, 22 stallions tested positive for T. equigenitalis, resulting in 715 mares having been exposed, Troedsson said, while only five of these mares (0.7%) tested positive for T. equigenitalis. This is in sharp contrast to previous outbreaks in Thoroughbred populations in the United States, France, Great Britain, etc., where natural breeding is practiced exclusively.

Troedsson said a possible explanation for the low CEM transmission during these outbreaks in 2006 and 2008 was that all mares were bred by artificial insemination with the semen extended in media containing antibiotics. Semen extenders with antibiotics are routinely used for artificial insemination in order to minimize any contaminating bacteria's effect on semen quality. However, CEM can potentially go undetected when semen is extended in media with antibiotics. If CEM becomes endemic in breeds that practice artificial insemination, it could potentially spread with devastating consequences and even enter Thoroughbreds through cross breeding using natural breeding.

Therefore, the rationale for conducting the present study was the low CEM transmission in mares bred via artificial insemination in an extender containing antibiotics during these outbreaks.

“We hypothesized that the inclusion of antibiotics in semen extender prevents growth of T. equigenitalis in extended semen and therefore reduces the risk of CEM transmission. It also makes it difficult to diagnose CEM in horses that are bred exclusively with artificial insemination,” Troedsson said. “However, the purpose of the present study was not to show that it is possible to treat semen from infected carrier stallions. That could potentially make the disease endemic in the U.S., largely because of the difficulties to diagnose CEM in mares bred to a stallion.”

Shaila Sigsgaard is an editorial assistant for the Bluegrass Equine Digest.

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