Emergency Management and Information Network

Pennsylvania Department of Agriculture
Bureau of Animal Health and Diagnostic Services
John I. Enck, Jr., V.M.D., Director
c-animalem@state.pa.us
Telephone No: 717-783-6677 Fax No: 717-787-1868

Update on Equine Herpes Virus (EHV-1) in Pennsylvania

April 1, 2003

In February, several horses at Penn National Racetrack in Grantville were diagnosed with the neurologic form of EHV-1. The virus was identified in 5 horses from one barn at the racetrack, and 2 of these horses were euthanized after becoming recumbent. The illness appears to have been contained to one barn through a voluntary quarantine, which has now been lifted. Restrictions on shipping to Philadelphia Park have also been lifted. There have been no new cases reported. It is not uncommon for periodic outbreaks of EHV-1 to occur in Pennsylvania, and usually these outbreaks are contained to one or two locations and a small number of horses. There was an outbreak in an equine facility in Ohio several months ago, and Kentucky has recently reported a small outbreak of similar disease at a racetrack there.

Facts About EHV-1

••Many horses are latently infected with EHV-1 and can remain in this state for most of their lives.

Equine herpes is a term that refers to any one of several highly contagious diseases in horses, which may occur as a result of infection by either of two closely related herpes virus, equine herpesvirus-1 and equine herpesvirus -4 (EHV-1 and EHV-4). Both EHV-1 and EHV-4 are worldwide in distribution and constitute a health risk for domestic horses of all ages. Infection with either EHV-1 or EHV-4 is characterized by respiratory tract disease. Severity of clinical signs varies depending on age and general health of the infected horse. EHV-1 is also often referred to as “Equine Rhinopneumonitis”.

Although infection with EHV-4 is often confined to the respiratory tract, infection may spread beyond the respiratory tract to cause more serious disease, characterized by abortions, newborn foal death, or neurological disease. In most cases, following infection with EHV-1 or EHV-4, the virus becomes latent, surviving in the animal but not causing signs of disease, until the animal is stressed. In response to stress (weaning, transport, social disruption, etc.), the virus may be reactivated in latently infected animals and transmitted to other susceptible horses.

Horses infected with EHV-1 may present with a biphasic fever that may spike to 102 –106 degrees F. Signs of respiratory disease, including a cough, may be present. Secondary bacterial infections may occur due to a compromised immune system. Abortions may occur, usually in late gestation (after 7 months), and affected mares do not always show clinical signs of infection before aborting. In utero infections may lead to premature deliveries and unhealthy newborns, or infected newborns may appear normal at birth, but within the first week become weak and lethargic. Clinical signs in these foals may include respiratory problems, pneumonia; tachycardia, and diarrhea. Internal organs, including the liver, may be damaged. The prognosis is poor for these foals.

Infection with EHV-1 may also cause neurological disease in infected adult horses, but this is a less common manifestation. Neurological disease associated with EHV-1 can range from mild incoordination to severe posterior paralysis, with exceptional cases developing quadriplegia. The most severely affected animals may die or may need to be euthanized. Previous respiratory infection, either in the affected horse or in surrounding horses, may or may not be present.

Clinical signs of equine herpes are similar to other respiratory diseases in horses, so it is difficult to make a definitive diagnosis from clinical signs alone. Serologic tests may help establish a diagnosis, but are not considered to be a reliable means of confirming a diagnosis of EHV-1 or EHV-4. Because most horses will possess some level of antibody to EHV-1 and/or EHV-4, the demonstration of specific antibody in the serum collected from a single blood sample is not sufficient for a positive diagnosis of recent, active infection. Paired sera samples from animals suspected of being infected are more useful because they may show a significant rise in antibodies, which would be expected with active infection. Virus isolation can be done using nasal and throat swabs taken during the febrile stage of respiratory tract infection. •Aborted fetal tissues may aid in diagnosis of EHV-1, based on gross characteristics and microscopic lesions in the fetus, and virus isolation on fetal liver, lung, spleen, or thymus tissues.

Treatment of infection with EHV-1 is based on rest and stress reduction of affected horses. Stalls should be well-ventilated and as dust-free as possible. Antipyretics and antibiotics may be indicated in some cases.

Direct contact with virus particles is the most common means of transmitting the disease between horses. Transmission through indirect contact is possible, but less likely. Therefore, horses that are in close nose-to-nose contact with infected horses, infective placentas, or infective aborted materials are more likely to become infected themselves than horses that are kept isolated. Recovered horses may continue to shed virus for up to 3 weeks, and the virus may survive in the environment for up to 2 weeks in certain conditions.

Prevention is based on isolation procedures and implementation of a regular vaccination program. New horses, horses that have left the farm and returned, and sick horses, should be isolated from resident horses, particularly pregnant mares, for up to six weeks. Pregnant mares should be kept in small groups, and new arrivals should not be added to these established groups. These mares should be isolated from all other age groups, and mares carrying their first foals should not be mixed with older mares. All pregnant mares should be maintained on a vaccination program.

Although an adequate immune response usually occurs after vaccination, protection is short-lived, requiring frequent boosters. Vaccinated animals may still become infected, and are able to shed virus to other horses in nasal secretions, but the severity and length of illness are usually shortened by vaccination. Vaccination may not protect against the neurological form of EHV-1. Pregnant mares should be vaccinated at 5, 7, and 9 months of gestation, and foals should be vaccinated at 3-4 months of age, then 4-8 weeks later. All horses on the premises should be vaccinated several times a year, in addition to the recommendations listed above.