Mules' Role as Shedders in California EHV-1 Outbreak

Mules' Role as Shedders in California EHV-1 Outbreak

While equal percentages of horses and mules were exposed to EHV-1 during one outbreak, neurologic disease was only reported in horses.


Outbreaks of the neurologic form of equine herpesvirus-1 (EHV-1) have caused great financial and equine losses in recent years. These cases have solely affected horses, with no reports in the literature of any mules or donkeys showing clinical signs. Some of these equids have, however, shown evidence of antibodies against EHV-1 in their blood, prompting a group of researchers from the University of California (UC), Davis, to evaluate the role mules might play in EHV-1 spread.

Nicola Pusterla, DVM, PhD, Dipl. ACVIM, a professor in the Department of Medicine and Epidemiology at UC Davis, shared the results at the 2012 International Conference on Equine Infectious Diseases, held Oct. 21-26, in Lexington, Ky.

"The aim of this study was to investigate the role of mules as possible silent shedders during an outbreak of EHM (equine herpesvirus-1 myeloencephalopathy) occurring at a packing station in northern California (in September 2011)," he explained.

During that outbreak, veterinarians performed physical and neurologic evaluations and collected blood samples and nasal secretions for quantitative PCR (polymerase chain reaction, a type of DNA test) and serological detection of EHV-1 from 82 horses and 59 mules. Fifty-six of these animals (39.7%) tested PCR positive for EHV-1.

The researchers then separated these animals into three groups--asymptomatic (showing no clinical signs) horses (27); asymptomatic mules (24); and neurologic horses (6)--and compared the viral loads in their blood and nasal secretions. They found that:

  • The viral loads in blood were significantly different between the three groups, with asymptomatic horses having the lowest viral loads and neurologic horses showing the highest viral loads.
  • Asymptomatic mules had significantly higher viral loads in nasal secretions compared to asymptomatic horses.

Overall, 33 of the 141 animals (23.4%) tested seropositive (had antibodies against EHV-1 in their blood serum) for the disease, while 48 out of 82 horses (58.5%) and 32 out of 59 mules (54.2%) had evidence of EHV-1 exposure. These results showed that equal percentages of horses and mules were exposed to EHV-1 during the outbreak; however, neurologic disease was only reported in horses.

"The molecular and serological results support the role of mules as silent shedders, highlighting the need to institute appropriate biosecurity protocols when horses and mules are comingled," the researchers concluded.

About the Author

Alexandra Beckstett, The Horse Managing Editor

Alexandra Beckstett, Managing Editor of The Horse and a native of Houston, Texas, is a lifelong horse owner who has shown successfully on the national hunter/jumper circuit and dabbled in hunter breeding. After graduating from Duke University, she joined Blood-Horse Publications as Assistant Editor of its book division, Eclipse Press, before joining The Horse.

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