When a veterinarian suspects a horse has the neurologic form of equine herpesvirus-1 (EHV-1), it’s imperative he or she take the appropriate diagnostic and control measures to prevent an outbreak of this often fatal infectious disease. This is because EHV-1 can spread between horses before they show any signs of infection, creating a “potential perfect storm,” said Richard Newton, BVSc, MSc, PhD, FRCVS, during the 2018 British Equine Veterinary Association Congress, held Sept. 12-15, in Birmingham, U.K.
Newton is the director of Epidemiology and Disease Surveillance at the Animal Health Trust, in Newmarket, U.K. In his presentation he described how to manage EHV-1 cases and outbreaks.
Neurologic EHV-1 101
Neurologic EHV-1 is a contagious viral disease that can cause signs of ataxia (incoordination), weakness, paralysis, difficulty urinating and defecation, and recumbency (an inability to rise). It spreads via horse-to-horse contact, contaminated hands and equipment, and possibly through aerosolization, said Newton.
Horses can be infected latently without showing clinical signs (i.e., subclinically infected)—“the cornerstone of EHV-1’s continued success,” he said, particularly among groups of commingling horses at events.
“Clinical neurological signs often occur about 10 days after infection, as antibody levels (against the pathogen) are rising,” he explains.
Controlling EHV-1 Spread
With these challenges in mind, Newton listed his general principles for controlling disease spread:
- Act on suspicion, and make a diagnosis as soon as possible. Newton said the AHT, for instance, becomes involved with these cases when called in by a veterinarian with a suspected case. To confirm diagnoses they use blood testing, qPCR tests on nasopharyngeal swabs, histopathology on affected tissues collected postmortem, and/or the complement fixation test (CFT) to measure antibodies in the blood against the virus;
- Isolate affected horses or groups of horses. Newton suggests categorizing groups of horses by their risk of circulating virus;
- Stop all mixing of horses and horse movement on and off the property, and implement increased biosecurity measures such as use of dedicated equipment, foot baths, and hand disinfection when moving between horses;
- Heighten clinical monitoring of horses to observe for clinical signs that might be associated with early viral infection, such as taking temperatures twice daily to look for fever spikes and assessing whether animals have nasal discharge, are coughing, or might not be finishing their meals;
- Perform extensive, repeated laboratory testing to evaluate subclinical infection; and
- Establish freedom of active infection and return horses to normal activity. Newton said he considers horses to be EHV-1-free when both their CFT and qPCR are negative. As a result, “You end up with a safer population that you can prove is not infected,” he said.
The Bottom Line
“There’s no quick, easy fix for getting through these outbreaks,” he said, “But the basic premise is to segregate, sample, test, and observe for at least three to four weeks.”
While a vaccine against EHV-1 does exist, it’s not licensed to prevent the neurologic form. Newton says he advises against vaccinating horses in the face of an outbreak because the vaccine can interfere with blood screening and might create a false sense of security among horse owners.
“Appropriate management of index cases can prevent major outbreaks,” he said. “But disaster can happen even with good management.”
Thus, he urged veterinarians to remain vigilant, particularly because the prevalence and burden of neurologic EHV-1 varies from year to year. “There’s never zero risk with this particular disease.”