One of the most difficult problems in diagnosing contagious equine metritis (CEM) is isolating the organism from heavily contaminated sites in the stallion and mare. CEM is caused by Taylorella equigenitalis, a Gram-negative bacterium, which is found on the genitalia of mares and stallions, as are many other species of bacteria (and some fungi). When you take a swab from a mare or stallion to try and culture T. equigenitalis, these other organisms can (and probably will) contaminate your swab.

If the laboratory doesn’t culture the swab sample properly (on the correct medium, under the proper conditions, and for required amount of time), you are very likely to get considerable growth of organisms other than T. equigenitalis, which can obscure the presence of this bacterium (this is called overgrowth). As a result, you will be unable to tell if the horse has CEM .

CEM has been considered a foreign animal disease in the United States for many years, although there have been cases found in this country in the past. Currently there is a nationwide investigation ongoing to trace and test stallions and mares that might have been exposed to T. equigenitalis, which can cause temporary infertility and very rarely, abortion in mares.

According to the federal government’s Animal Plant Health Inspection Service (APHIS); Veterinary Sciences:

• Four positive stallions are in Kentucky.  
  
• Three positive stallions are in Indiana.  
  
• In addition to the seven positive stallions, authorities have located 265 exposed (to be considered “exposed,” the horse was on the premises in Kentucky where the first case was discovered and/or was bred to a CEM-positive horse) horses, including 30 stallions and 242 mares. All CEM-positive horses, and all exposed horses that have been located, are currently under quarantine or hold order. Testing and/or treatment protocols are being put into action for all located horses.
  
• At least 94 additional horses are actively being traced.
  
• Owners of these horses are located in at least 38 states.

In the Lab

Swabs from stallions or mares being tested for CEM must be handled properly and tested at a USDA-approved laboratory, of which there are 16 currently in the United States besides the National Veterinary Sciences Laboratory in Ames, Iowa.

A special selective medium was developed in the United States in the early 1980s that includes a number of antimicrobials that reduce or eliminate the growth of most organisms except T. equigenitalis. The great value of this special medium is that it allows the growth of streptomycin-sensitive, as well as streptomycin-resistant, strains of the CEM organism (see image below).

One of the most common organisms that can be cultured from the reproductive tracts of stallions and mares is Proteus mirabilis. It grows quickly in an aerobic (with oxygen) environment. CEM, on the other hand, grows slowly in the laboratory and needs exposure to 5-10% carbon dioxide for growth to take place. So, if the appropriate medium is not used when attempting isolation of T. equigenitalis, you will likely end up with a petri dish that is overgrown with Proteus or some other organism that grow much more rapidly than the colonies of CEM (see image).

Read more articles on CEM at TheHorse.com.  

Contributing to this article was Peter Timoney, PhD, FRCVS, of the University of Kentucky’s Gluck Equine Research Center and an expert on CEM.

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Which of these inoculated culture plates show that a swab from a stallion (or mare) has T. equigenitalis? In the case of Plates A and B, neither medium contains any antimicrobials, with the result that there is major overgrowth with other organisms that contaminated the swab sample. Gram-positive/negative bacterial colonies take up two-thirds to three-quarters of the plates. Someone very familiar with CEM growth might note a few suspicious colonies at the top of the Plate A and at about 5 o’clock on Plate B, but these might not be recognized by personnel in most laboratories.

Plate C contains streptomycin, which controls the growth of contaminating gram-negative bacteria, enabling strep-resistant bacteria to grow out. This plate has very faint bacterial growth in upper part of the plate with some contamination in the lower half.

Plate D is the selective medium that has been used for 20-25 years to enable detection of both strep-resistant and -sensitive strains of T. equigenitalis. The medium contains several antimicrobials that allow the isolation of T. equigenitalis with little or no growth of other organisms. This medium, which was developed in early 1980s, is now widely recommended when attempting isolation of this bacterium. (For more information see the chapter on CEM in the OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, 6th ed.) If you don’t use the proper media, under the appropriate conditions of cultivation, and for the requisite amount of time, T