The Continuing Threat to the US Horse Population Posed by CEM

 

 

Peter J. Timoney, University of Kentucky, Department of Veterinary Science, 108 Gluck Equine Research Center

 

 

Of the range of equine venereal infections, contagious equine metritis (CEM) has had the most significant impact on the international trade in equids and semen since the disease was first discovered in 1977.  Much of the concern over CEM is based on the ease with which this disease can be spread through the movement of carrier stallions or mares.

 

Caused by Taylorella equigenitalis, CEM is characterized by infertility and, very rarely, abortion in mares and infertility in stallions.  Considerable economic loss can result from the introduction of the disease into a previously unexposed and fully susceptible breeding population.  Categorized by the OIE as a List B disease, CEM continues to be the subject of stringent import regulations by all the major horse breeding countries.  Despite such measures, however, the risk of spread of CEM is a very real threat to the horse industries worldwide.  The seriousness of this threat is well illustrated by the relatively large number of stallions and mares detected carriers of T. equigenitalis on post-entry quarantine and testing in the USA and Canada over the past 5 years.

 

Since September 1997, a total of 21 carrier stallions and mares were imported into the USA and 5 into Canada.  All of these importations involved various Warmblood breeds.  In every case, the carrier animal had been certified by a representative of the national regulatory authority of the exporting country as being free of T. equigenitalis in accordance with internationally accepted testing procedures.  The imported carrier animals originated from a number of European countries, all member states of the European Union.  A significant percentage came from one country, Germany.

 

Of the 21 carrier animals detected on post-entry testing in the USA, 16 were stallions and 5 mares.  In conformity with USDA regulations governing importation of stallions and mares from CEM affected countries, these horses were bacteriologically cultured for T. equigenitalis.  Stallions were also required to be bred to two test mares and the mares monitored clinically, bacteriologically and serologically for evidence of infection.

 

In the case of the imported carrier mares, there was no problem in isolating T. equigenitalis from the sites of persistence in the distal reproductive tract.  This raised the question, why were these animals not readily detected on pre-export screening for this infection if indeed the appropriate swabbing, handling and culturing protocols were being followed?  In comparison, the carrier state was not so easily detected in stallions.  Only 8 (50%) of the 16 stallions detected yielded positive cultures for T. equigenitalis on initial bacteriological examination.  In the case of the remainder, the carrier state was confirmed based on transmission of infection to one or both test mares.  Experience gained over the past 5 years has clearly shown the value of test breeding a putative carrier stallion as a highly reliable, albeit time consuming means of detection of the carrier state.

 

The relatively large number of stallions and mares detected carriers of T. equigenitalis on post-entry quarantine and testing since 1997 underscores the continued significant risk of spread of CEM through international trade.  It is evident that stallions present a greater risk than mares.

 

To this point, the post-entry control measures implemented at the federal and state level have proven effective in preventing the reintroduction of CEM into the USA.  It would be unwise, however, to assume, based on past experience, that these are foolproof.  The continued increase in the number of stallions and mares imported into the USA enhances the risk of CEM being reintroduced into the country at some point in the future.  Appropriate steps need to be taken at the national and international level to ensure that this does not happen. 

 

It is clearly evident from the data gleaned over the past five years, that the reliability of much of the pre-export testing for CEM carried out by certain countries is subject to question.  It remains to be established whether the primary problem is one of inadequate sampling, specimens inappropriately handled in transit to the laboratory or short-comings in how swabs are cultured and screened for T. equigenitalis.  Clearly, there is a pressing need to have the problem of discrepant results raised with the appropriate authorities in the countries frequently implicated in exporting CEM carrier animals.  The problem experienced over the past five years over Warmblood importations from these countries must be remedied.  Furthermore, in light of the significance attributed to CEM with respect to international trade, it would be prudent and timely to request the OIE to review the organization’s published protocols for the diagnosis and control of CEM and, where appropriate, strengthen these so as to minimize the risk of spread of the disease internationally.