Infectious and Neoplastic Conditions of the Vulva and Perineum

Etiology

In 1977 an outbreak of a transmissible venereal disease of horses, subsequently named contagious equine metritis (CEM), occurred in the Newmarket area of England and in Ireland. An outbreak affecting several Kentucky breeding farms occurred the following year, after the importation of two Thoroughbred carrier stallions from France in the fall of 1977. Since 1978 the disease has been reported only once in the United States, in one Thoroughbred stallion and two mares in 1982. The etiologic agent was identified to be a gram-negative microaerophilic coccobacillus and was initially identified as Haemophilus equigenitalis and later renamed Taylorella equigenitalis.¹

More than 20 distinct genotypes among over 200 strains of T. equigenitalis have been identified through pulsed-field gel electrophoresis.² A recent study using polymerase chain reaction (PCR) assay indicates that Taylorella is endemic to the horse population by the presence of Taylorella DNA in the genital tract of 7 out of 24 horses in the isolated Icelandic horse population.³

In 1997, bacterial isolates of an atypical T. equigenitalis strain were obtained from donkey jacks in Kentucky and California. Although the isolates were phenotypically indistinguishable from T. equigenitalis in culture, comparisons of the DNA sequence revealed that the new isolates were different. Pulsed-field gel electrophoresis showed that the Kentucky and California isolates were also different.⁴ This strain of Taylorella has been named Taylorella asinigenitalis sp. nov.⁵ One study attempted to infect mares with both the California and Kentucky isolates of T. asinigenitalis sp. nov., along with T. equigenitalis. This study found that the two mares inoculated with the California isolate did not become infected, whereas the mares inoculated with the Kentucky isolate of T. asinigenitalis sp. nov. and with T. equigenitalis did develop infections. The Kentucky isolate produced a CEM-like infection that developed several days later and with less intensity than the T. equigenitalis infection, but otherwise showed clinical, serologic, and pathologic changes that closely resemble classic CEM.⁶ Clearly, different isolates of T. equigenitalis are found among U.S.-origin Equidae, and one isolate has been shown to produce CEM-like symptoms in mares. A veterinarian must consider atypical Taylorella sp. infections as a differential diagnosis of equine infertility when examining U.S.-origin mares.⁶ Any veterinarian who suspects that a horse in his care may be affected with T. equigenitalis is required under law to report such suspicions to his or her state veterinarian.

Diagnostics

Clinical Signs

Mares infected for the first time usually exhibit a copious grayish vulvar discharge 8 to 10 days after infection that persists for 13 to 17 days. The discharge typically disappears without treatment. Another clinical sign is a shortened interestrous interval due to acute endometritis. Most mares recover spontaneously; however, some mares become chronic carriers of the bacteria ⁷ and subsequently may transmit the bacteria to stallions. On the other hand, a mare may become infected by a stallion, show no clinical signs, and become an asymptomatic carrier. These mares can become pregnant and carry the pregnancy to term. The organism has been recovered from the placenta of positive mares⁸ and from the genitalia of colts and fillies,⁹ indicating that it can be transferred in utero and/or at parturition. There have been two reported cases of abortion in mares caused by a T. equigenitalis infection. In these cases the bacterium was cultured from several sites in the aborted fetuses.¹⁰ However, the transmission of CEM primarily occurs during coitus or by fomites used in handling and treating infected mares and stallions.

Bacteriologic Cultures

Bacteriologic testing is the most reliable means of diagnosing CEM,¹¹ but a major disadvantage of this method is the extended culture time, resulting in delays in diagnosis.¹²

In one study a group of mares was experimentally infected by intrauterine inoculation with T. equigenitalis and subsequently necropsied at various intervals post inoculation.¹³ During the first 2 weeks after infection the organism was isolated from the uterine and cervical lumen, most commonly, and less frequently from the vaginal vestibule, clitoral fossa, clitoral sinuses, and oviducts. Between 3 and 4 months after infection, T. equigenitalis was only occasionally recovered from the ovarian surface, oviducts, uterus, cervix, and vagina, but more frequently from the clitoral sinuses and fossa.¹³ As a result of this study, the culture sites for screening mares for CEM for import/export regulations are the mucosal surfaces of the clitoral sinuses and clitoral fossa.

All cultures for CEM must be performed at a licensed state facility (Box 25-1), under current U.S. Department of Agriculture (USDA) guidelines. Sets of specimens for culture must be collected from the mucosal surfaces of the clitoral sinuses and fossa on three separate occasions over a 7-day period (including days 1 and 7 of quarantine). The openings to the clitoral sinuses, located on the dorsal aspect of the clitoris, require a small swab. Culture specimen swabs submitted for culture must be placed in Amies transport media with charcoal and maintained at approximately 4° to 6°C for transit. Specimens must be received within 48 hours of collection by the National Veterinary Services Laboratories (NVSL) in Ames, Iowa, or by a diagnostic laboratory that is approved to conduct CEM cultures and tests.¹⁴T. equigenitalis requires chocolate agar plus 10% horse blood as its culture media. The plates are incubated at 37°C, in an atmosphere of 5% to 10% carbon dioxide. Growth is enhanced in an atmosphere of 90% hydrogen gas and 10% carbon dioxide.¹² All culture set plates must be negative and readable by an approved state or federal animal disease diagnostic laboratory. The organism is oxidase, catalase, and phosphatase positive but is otherwise biochemically nonreactive.