Independent Secondary Isolation of CEM in Kentucky

In a separate and unrelated incident, a second contagious equine metritis (CEM)-like organism was recovered from a jack in Kentucky which has no known association or relation to the California jack. Two Standardbred mares which had no clinical signs and which were not suspected of the disease were undergoing examination to qualify as post-import breeding test mares. These mares were part of a group of five which were used as nurse mares and which were domestic horses with no known exposure to imported horses. On Jan. 12, 1998, when both mares were unexpectedly diagnosed positive for CEM by complement fixation (CF) tests at the Kentucky Animal Disease Diagnostic Center in Lexington, the serologic tests were repeated to rule out tasting error and bacteriologic specimens were additionally collected. A donkey jack which had recently been used to cover the mares was also cultured. On Jan. 20, 1998, the second serologic tests were positive and were supported by microbiologic isolation of a CEM-like organism from the mares and the jack. The Kentucky CEM-like organism, in similarity to the California isolate, displayed cultural characteristics and biochemical reactions which resembled but were not identical to reactions of the CEM organism associated with the 1978 incident in Thoroughbred horses in Central Kentucky. The National Veterinary Services Laboratory confirmed the isolation of a CEM-like organism on Jan. 21.

Epidemiologic Survey and Action (Kentucky)

The jack arrived on the Fayette County, Kentucky, premises on Dec. 12, 1997, from Rowan County, Kentucky, for the purpose of breeding three nurse mares. A fourth mare on the Rowan County premises is pregnant by the jack and is scheduled to foal in March 1998. Examination of the four mares revealed no visible vaginal discharge or other signs of CEM. The premises was additionally populated by 39 breeding-age mares, 34 broodmares, 22 yearlings, and five horses which had recently arrived from training. No stallions were present since all breeding was performed by artificial insemination. On Jan. 21, the premise was quarantined and broodmares on the original premises underwent blood collection for CF testing. The premise on which the jack was located prior to Dec. 12, 1997, also was quarantined and blood was drawn for CF testing from those horses as well. Currently, 211 animals in addition to the four original nurse mares, have been identified, quarantined, and tested. Of these, 213 are CF negative, but the jack is culture positive and one of the four original broodmares was positive by culture as reported by NVSL on Feb. 4, 1998.

Current Areas of Activity: Epidemiologic action In both California and Kentucky, efforts to locate additional or potential contacts (tracebacks) of the affected jacks continues. Jacks and mares with positive CF or culture results are under quarantine, as are premises containing high and low risk exposed animals.

Research Activity

This strain of CEM-like bacteria has not apparently been previously isolated. Emergency Programs staff is therefore currently collecting data and coordinating recent and proposed research. It is hoped that he recent isolates in both California and Kentucky will be compared to each other and to the historical CEM bacteria to determine their genetic relationship and relative pathogenicity (see following article). Currently, the NVSL, the University of California-Davis, the Gluck Equine Research Center in Kentucky, and Cornell University are contributing data and suggestions to this investigation.

Continuing Issues and Questions

  • Isolation of the CEM-like organism has not been associated with clinical disease, thus its impact on domestic equine production and international movement of horses cannot be accurately stated. However, of the few mares with positive CF results, none have clinical signs traditionally associated with CEM, thus the CEM-like organism has also been labeled "nonpathogenic CEM" by the NVSL. The genetic relationship between the two current isolates (CA and KY) has not yet been established.
  • The genetic relationship of the two current isolates and the 1978 strain also has not been well established. Informally, 96% identity has been reported between the CA and 1978 strains, enough of a difference to consider the current CA strain significantly different than the 1978 strain.
  • The prevalence of CEM-like organism colonization of normal donkeys has not been established, raising the possibility that additional infections may be found.
  • The transmissibility of the current isolates is not well established, although one mare in Kentucky has developed a 1:4 titer by CF after exposure to the KY jack, and one mare on the California premises where the CA jack was formerly stables has a 1:4 CF titer, and a mare in Oregon bred to the CA jack is also CF positive at 1:4. The current CF test results do not seem by inspection to correlate perfectly with culture recoveries of the organism.
  • In some instances, conversion from negative to positive status by complement fixation has occurred later than the 15 days after breeding, when the specimen must be taken as currently required by 9 CRF 92.301(e)(3)(B). If verified, this fact may force a change in the testing requirement.


A CEM-like organism was recovered from an asymptotic jack in California, and the recovery was confirmed by NVSL on Dec. 24, 1997. In a separate incident, two mares in Kentucky which were positive for CEM by CF test on Jan. 12, 1998, were known to have been recently exposed to a jack which was subsequently cultured and shown on Jan. 20, 1998, to carry a CEM-like organism. One of the CF positive mares which was originally cultured negative became culture positive on Feb. 4, 1998. Because of the bacteriologic cultural differences, biochemical differences, diminished fluorescence on fluorescent antibody test, and lack of virulence, the accurate characterization of both CEM-like organisms remains obscure. The impact on the domestic donkey population is uncertain, but currently considered low. However, efforts proceed in characterization of the organism. All culture positive animals are quarantined and will be cleaned and tested until negative. Investigation of the genetic identity, transmissibility, and possible pathogenicity of the two current isolates is underway as a cooperative effort between the National Veterinary Services Laboratory, the University of California-Davis Veterinary Medical Teaching Hospital, Cornell University, the Kentucky Animal Disease Diagnostic Center-Lexington, Veterinary Services Field Staff in California and Kentucky, and the Emergency Programs staff.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More