Determining the cause of an equine respiratory disease is often difficult, and as a result, treatment options are limited. Veterinarians see the horse because of weight loss, fever, coughing, increased respiratory rate and nasal discharge, but are often at a loss as to how to treat the case.
Because of the problems in correctly diagnosing respiratory disease, a number of studies have been conducted into how causative agents work and better diagnostic procedures.
Dr. Stephanie Bell of U. C. Davis Equine Viral Disease Laboratory at
UC Davis School of Veterinary Medicine recently conducted a study at Santa Anita and Hollywood Park racetracks that involved the collection of nasopharyngeal swabs from horses 2 through 6 years of age from three different stables at both tracks. The swabs were collected three times at one-month intervals from horses that were known to be free of clinical respiratory disease.
The idea behind the study was that if it could be determined what types of viruses and virus subtypes were present in horses at these training facilities when disease was not present, then specific causative viral agents could more accurately be determined when disease occurred.
The results of the study showed that equine rhinitis A or B1 virus, equine herpes virus (EHV) 3 and 4, equine adenoviruses 1 and 2, equine arteritis virus (EAV), and influenza were not present in any of these horses.
However, positive results for EHV 1, 2 and 5 were obtained. In the first sample, 1 horse was positive for EHV-1, 8 were positive for EHV-2, and 11 for EHV-5. One month later, 4 horses were positive for EHV-1, 12 for EHV-2, and 11 for EHV-5. At the final sampling at two months, none was positive for EHV-1, but 6 were positive for EHV-2 and 10 for EHV-5.
The identification of herpesviruses, which are well known for their ability to establish latent infections, is consistent with the respiratory disease-free status of the cohort of horses tested. The percentage of horses in this study infected with EHV-2 (75%) is comparable to the results obtained in other studies.
However, the proportion of horses in Dr. Bell’s racetrack study infected with EHV-5 was higher than previously reported.The consistent detection of EHV-2 and 5 in nasopharyngeal swabs collected from this population of horses, as well as the sporadic identification of EHV-1, illustrates the dilemma often confronted by clinicians who are faced with interpreting the significance of such positive results.
While EHV-1 is universally recognized as a cause of febrile upper respiratory disease, and occasionally pneumonia, the pathogenic importance of EHV-2 and 5 is hotly debated. While some studies suggest that these cell-associated herpesviruses may be causative agents of respiratory disease in horses, their ubiquitous presence in the equine population precludes accurate determination of the significance of a single positive PCR result.
EHV-2 and 5 are gamma-herpesviruses, members of the same herpes subfamily as Epstein-Barr virus, the cause of infectious mononucleosis in humans. The symptoms of mononucleosis, which include pharyngitis, fever, enlarged lymph nodes, and malaise, are strikingly similar to clinical signs in horses that are purportedly associated with EHV-2 infection.
Additional studies are clearly needed to define the significance of the gamma-herpesviruses in horses.The sporadic presence of EHV-1 in the nasal secretions of the clinically normal horses that were sampled in this limited study is also noteworthy, as it further indicates that identification of this virus is not invariably associated with disease.
The specificity of molecular testing for the other respiratory viruses examined in this study is confirmed by the negative results for these viruses in this group of clinically unaffected racehorses.
Dr. Bell expects, then, that these assays should be useful for the rapid diagnosis of outbreaks of respiratory disease in horses caused by equine rhinitis viruses, influenza, and EHV-4.
While this initial study has provided new and useful information, it raises almost as many questions as it answers for the veterinary practitioner and client trainers. It does establish the credibility of molecular testing as a viable tool for the analysis of respiratory disease outbreaks and provides a basis upon which further diagnostic progress can be made.