On March 30, Maryland’s state veterinarian confirmed that on March 22, three horses from one barn at Laurel Park Racetrack in Anne Arundel County and one horse at Pimlico Racetrack in adjacent Baltimore County developed fevers and were tested for equine herpesvirus (EHV), the virus that causes EHM. One of the horses at Pimlico tested positive.

The positive horse was isolated at a newly designated quarantine facility at Bowie Racetrack in Prince George’s County, and the quarantine timeline at Laurel Park was reset.

On March 24, testing began for all (approximately 120) horses in the Laurel Park barn, and 26 tested positive. The second round of testing on the 14 asymptomatic horses in Laurel Park’s index barn also began. One of these horses tested positive and was moved to the Bowie facility where the seven surviving horses that had previously tested positive remained quarantined as well.

Although the horse at Pimlico initially tested negative, upon retest it tested positive and was moved to Bowie.

Two additional horses from another Laurel Park barn developed fevers and were tested. One tested positive and was also sent to Bowie.

As of Sunday, March 28, Bowie’s isolation facility housed 36 EHV-positive horses.

Officials decided to enact separate 21-day quarantines for each of the three facilities and discontinue testing asymptomatic horses, because testing them was not leading to a resolution.

Projected quarantine release dates are April 12 for Pimlico; April 17 for Bowie; and April 18 for Laurel Park, contingent upon no horse with clinical signs testing positive on the initial testing during the 21-day period.

Maryland Department of Agriculture (MDA) officials continue to work with racetrack officials to monitor the outbreak and adjust strategies accordingly.

EHV 101

Herpesvirus is highly contagious among horses and can cause a variety of ailments in equids, including rhinopneumonitis (a respiratory disease usually found in young horses), abortion in broodmares, and equine herpesvirus myeloencephalitis (EHM, the neurologic form).

In many horses, the first or only sign of EHV-1 infection is fever, which can go undetected. In addition to fever, other common signs of EHV-1 infection in young horses include cough, decreased appetite, depression, and a nasal discharge. Pregnant mares typically show no signs of infection before they abort, and abortions usually occur late in gestation (around eight months) but can be earlier. Abortions can occur anywhere from two weeks to several months following infection with EHV-1.

Horses with EHM usually have a fever at the onset of the disease and might show signs of a respiratory infection. A few days later, neurologic signs such as ataxia (incoordination), weakness or paralysis of the fore- and hind limbs, urine retention and dribbling, loss of tail tone, and recumbency (inability to rise) develop.

Herpesvirus is easily spread by nose-to-nose or close contact with an infectious horse; sharing contaminated equipment including bits, buckets, and towels; or clothing, hands, or equipment of people who have recently had contact with an infectious horse. Routine biosecurity measures, including hygiene and basic cleaning and disinfection practices, should be in place at all times to help prevent disease spread.

Current EHV-1 vaccines might reduce viral shedding but are not protective against the neurologic form of the disease. Implementing routine biosecurity practices is the best way to minimize viral spread, and the best method of disease control is disease prevention.