Equine Herpes Virus type 1
The following information on EHV-1 events in 2005 is an excerpt from the 2005 U.S. Animal Health Report, Chapter 7, pages 78-79.
Equine Herpesvirus Type 1 (EHV–1)
Although chiefly a respiratory pathogen, EHV–1 is associated with a variety of clinical manifestations in equids, including abortion and paralysis. The virus is enzootic throughout the world, and almost all horses
over 2 years of age have been exposed. After an equid’s initial exposure, EHV–1 can cause a latent infection, which provides a reservoir of virus for continual transmission. Nationally, reports of neurologic EHV–1 have increased in recent years, which might be attributable in part to a strain of virus that encodes for a particularly robust replicase enzyme. This strain of virus can reproduce rapidly and has a predilection for the blood vessels of tissue of the nervous system. During 2005 and early 2006, seven episodes of neurological EHV–1 in the United States were reported by State animal health officials. Five of the disease
events involved racing venues in Kentucky, Michigan, Pennsylvania, and Maryland, and two occurred in boarding facilities in New York and Maryland.
- Starting in December 2004 and continuing through February 2005, an outbreak of neurologic EHV–1 occurred at the Northville Downs Standardbred racetrack in Michigan. Of four horses considered affected, three were euthanized. Additional information indicated that 12 horses with contact to the case horses were vaccinated against EHV–1 in December 2004 as a precautionary measure.
- In February 2005, a mare at the Meadows racetrack in Pennsylvania was euthanized after being diagnosed with neurologic EHV–1.
- During March 2005, 10 cases of EHV–1 paralysis were reported from the Columbia Horse Center in Columbia, MD. Five animals either died or were euthanized due to
complications of their clinical conditions.
- In early March 2005, three horses at a boarding facility in Tioga County, NY, died or were euthanized after being diagnosed with a combination of neurologic and respiratory forms of EHV–1 infection. Three additional horses that showed clinical signs consistent with EHV–1 infection fully recovered.
- Another outbreak of EHV–1 neurological disease began in May 2005 at Churchill Downs in Louisville, KY. Two horses housed in separate barns were euthanized after they developed progressive paralytic disease associated with EHV–1 infection. Movement restrictions were placed on 3 barns, but the outbreak was mainly confined to 1 stable, where 9 of 37 horses developed signs consistent with neurologic EHV–1. For this outbreak, a relatively new nested polymerase chain reaction (PCR) assay for detecting viral
deoxyribonucleic acid (DNA) was used to determine the extent of viral spread and to help manage the outbreak. By mid-June, the quarantine on all three barns was lifted following a period of 27 days without evidence of clinical disease.
- In December 2005, a 3-year-old filly at Turfway Park in Florence, KY, developed progressive rear-limb ataxia. Following confirmation of EHV–1 infection, regulatory and testing measures were instituted for exposed animals at the racetrack. The investigation documented the occurrence of EHV–1 in horses housed in three barns at Turfway Park and an additional training facility in Henderson, KY. During the investigation, more than 132 horses considered at risk were tested using the nested PCR assay for EHV–1. Of these, positive test results were obtained on buffy coat specimens for approximately 42 animals. Of three horses diagnosed with the neurologic form of EHV–1, two were euthanized.
In all instances, regulatory authorities used movement controls and a variety of biosecurity measures to prevent viral spread. In some cases, races were cancelled because
of continuing transmission of virus within the exposed population and insufficient numbers of nonexposed horses to compete. Overall, use of a quarantine period of at least 21 days appeared to prevent further spread of virus; however, in most of these situations, the criteria used to determine the beginning timeframe were not defined.
From a regulatory perspective, State agencies vary in their requirements for veterinary practitioners to report cases of EHV–1 to State animal health authorities. Most States encourage reporting under general regulations for reporting of communicable diseases, yet few specifically designate cases of EHV–1 as a reportable disease. With the exceptions of required statements of disease-free status of horses intended for export and of the condition for States to participate in the National Animal Health Reporting System, there is no federally mandated reporting of disease conditions attributable to equine herpesvirus.
From a diagnostic perspective, EHV–1 is difficult to isolate, and the most commonly requested serologic tests indicate only prior exposure to viral antigen without differentiating antibody response attributable to vaccination from that associated with disease exposure. Likely, the numbers of cases of neurologic EHV–1 are underreported nationwide, and the cases that reach the attention of animal health authorities are those that occur in public venues or settings where large numbers of horses are stabled.
U.S. Animal Health Report, Chapter 7: Animal Health Events in 2005, pages 78-79