The equine herpesvirus (EHV) cases diagnosed in Ocala, Fla., in February and March of this year remind us of the importance of biosecurity precautions and timely, accurate communication during infectious disease outbreaks. The state veterinarian's office worked closely with horse show veterinarians and management to address the issue in a thorough and rapid manner. Although we all appreciate the prompt release of pertinent information, this can be challenging in the face of a disease outbreak investigation.

In this case, the index horse presented to the University of Florida Large Animal Hospital (UF LAH) isolation unit on Wednesday, Feb. 20. Veterinarians confirmed the suspected diagnosis of equine herpesvirus myeloencephalopathy (EHM, the neurologic form of EHV-1) late Thursday, Feb. 21. The UF LAH contacted the state veterinarian's office and released an official statement on Feb. 22. The state initiated their investigation shortly thereafter. The tent on Horse Shows in the Sun (HITS) showgrounds that previously housed the index horse was quarantined while veterinarians and officials performed further testing and trace-back of other potential cases. Initial evaluation was complicated by the fact that numerous horses presented with fever, but subsequently tested positive for influenza and not equine herpesvirus.

Biosecurity and good communication are critical in the face of an infectious disease outbreak.

Photo: Alexandra Beckstett

As the initial investigation progressed, additional horses from adjacent tents tested positive for EHV-1. This prompted quarantine of the entire showgrounds, and movement on and off the premises was prohibited beginning Feb. 27. The state managed a comprehensive quarantine of the show, as well as 17 additional off-site premises that horses had been moved to. Fortunately, only one additional horse developed EHM and was referred to the UF LAH for further evaluation and -treatment.

On Feb. 26, UF LAH and state veterinarians organized an informational presentation in Ocala. Here, horse owners, managers, and trainers from HITS and the community had the opportunity to learn more about the disease and ask questions about how the outbreak was being managed. An interesting point of discussion was regulatory management of the so-called wild-type versus neurotropic strains of equine herpesvirus-1.

At this meeting state veterinarians explained some of the factors that impacted the original decision not to quarantine the entire HITS showgrounds: The index horse (and subsequent horses) associated with the outbreak were determined to have the wild-type strain. The neurotropic strain of the virus is more likely to cause EHM; however, current research does not support treating the strains differently from an isolation and biosecurity standpoint. Additionally, both strains are similar with regards to virus shedding, suggesting they have a similar capacity to spread in a horse population. Fortunately, the EHV-1 cases associated with the HITS outbreak were identified and contained, and large numbers of horses were not infected.

Nationwide, EHM is a reportable disease. State animal health officials are specifically trained in surveillance, prevention, and control programs for reportable diseases, and they ultimately determine the need for quarantines and restricted movement. They also determine the length of quarantine and appropriate release criteria (based on disease pathophysiology, organism shedding, and previously published precedents). For release of Florida quarantines, all exposed horses were required to undergo 21 days of quarantine from the last known exposure to a positive case and then test negative for EHV-1. Alternatively, exposed horses could be quarantined for 28 days from the last exposure and be released as long as they had no fever or other clinical signs of disease in that period. The final premises was released from quarantine on April 10.

This equine herpesvirus outbreak serves as an excellent reminder of the importance of good biosecurity practices. Always monitor horses with fever closely and treat them as suspect infectious disease cases. Ideally, monitor horses returning home from large events twice a day for fever development. Hand-washing and disinfecting equipment between animals is also of paramount importance. Showing and traveling with horses should be enjoyable, and the entire equine community benefits when good disease prevention measures and biosecurity practices are in place.

Amanda House, DVM, Dipl. ACVIM, is a clinical assistant professor in the University of Florida's College of Veterinary Medicine Department of Large Animal Clinical Sciences.

Originally published in the June 2013 issue of The Horse: Your Guide To Equine Health Care.