Both voluntary and official quarantines are used to limit the spread of equine herpesvirus-1 (EHV-1). For a month beginning the week prior to Thanksgiving 2019, both types of movement restriction were implemented in several parts of the country. Case reports can be found on the disease outbreak alert page maintained by the Equine Disease Communication Center (EDCC).
The Impacts of EHVs on Horses
Equine herpesviruses are associated with respiratory disease, abortions, neonatal death, and neurologic disease in horses. Because of these different manifestations of disease, the virus is also referred to as equine rhinopneumonitis (because it attacks the linings of the nose and lungs) or viral abortion (for causing abortion). The neurologic disease associated with EHV-1 infection is called equine herpesvirus myeloencephalopathy (EHM). The neurologic form is the most devastating because affected horses are often euthanized. However, any type of infection with EHV may lead to movement restrictions.
The increasing number of outbreaks with high fatality rates and the identification of more virulent strains of EHV-1 in the latter half of the 2000s led to considering EHM as a potentially emerging disease (AVMA, 2007). In every state, the neurologic form of EHV-1 is reportable[JMS2] to the state animal health authority (EDCC). When a quarantine is placed on a premises, release typically requires all horses test negative for the virus twice a week apart and a full 14 days has passed since the last exposure to an infected horse with no clinical signs observed.
Transmission and Clinical Signs of EHV-1
Older horses are frequently infected with equine herpesviruses without showing signs of illness, but can still transmit the virus to other horses. Horses can become latent carriers that have not fully cleared the virus after infection and are not showing signs of illness. In fact, 80 to 85 percent of the global equine population is latently infected with EHV-1 or EHV-4 (CFSPH, 2014). Reactivation of the virus by stresses such as transportation or illness can lead to viremia meaning the virus can be found in the bloodstream. A horse that is viremic after being latent or after an initial infection can have an abortion or develop neurologic signs.
The clinical signs associated with EHV infection include (AAEP, 2017):
- Nasal discharge
- Lack of coordination
- Difficulty urinating
- Hind limb weakness
- Loss of tail tone
- Head tilt
- Inability to rise
EHV-1 has an incubation period of up to 14 days, but usually disease will be evident within 8 to 12 days of exposure. Nose-to-nose contact spreads EHV-1 through nasal and respiratory secretions. It can also be spread through fomites, which are objects that came into contact with infected sources. These objects can include hands, clothes, equipment, vehicles, and feed and water buckets. The virus can survive in the environment up to seven days.
At present, there is no specific treatment for EHM; only supportive care can be given. However, there is a novel therapy being studied involving heparin (Bentz, 2018). Horses that are down are usually euthanized. Vaccination is not guaranteed to protect against EHM, although it may reduce shedding. So how can EHV-1 be prevented?
Preventing EHV-1 Infection
The best way to prevent EHV-1 is to actively implement biosecurity measures. The following recommendations can reduce the chance of your horse acquiring EHV infection or many other diseases (Munson, 2019):
- Isolate animals upon returning to a premises after traveling.
- Disinfect and sanitize vehicles and equipment used off the premises.
- Never share equipment between farms without sanitizing.
- Start with healthy horses and end with ill and isolated horses when completing daily chores and feedings.
And, of course, always monitor your horse for clinical signs of disease.
Through biosecurity practices and movement restrictions, the spread of EHV-1 can be minimized. Reporting any clinical signs or potential contact with an infected animal to a veterinarian can increase awareness of the disease. Increased awareness can allow producers to take initiative and protect the horses on their premises.
Nevada Department of Agriculture. [JMS3] May 2019. All EHV-1 Quarantines and Restricted Travel Recommendations Lifted. Retrieved from http://agri.nv.gov/News/2019/All_EHV-1_quarantines_and_restricted_travel_recommendations_lifted/
American Association of Equine Practitioners (AAEP). 2017. FAQ: Equine Herpesvirus (EHV). Retrieved from https://aaep.org/horsehealth/faq-equine-herpesvirus-ehv
United States Department of Agriculture, Animal and Plant Health Inspection Service (USDA, APHIS). December 2018. Equine Herpesvirus (EHV). Retrieved from https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/equine/ehv
Bentz, Amy I. April 8, 2018. A promising novel treatment for equine herpesvirus myeloencephalopathy, American Veterinarian. Retrieved from https://www.dvm360.com/view/a-promising-novel-treatment-for-equine-herpesvirus-myeloencephalopathy
Equine Disease Communication Center (n.d.) Reportable Diseases. Retrieved from http://www.equinediseasecc.org/reportable-diseases
American Veterinary Medical Association (AVMA). March 1, 2007. Neurologic form of EHV-1 a potentially emerging disease. JAVMA News. Retrieved from https://www.avma.org/javma-news/2007-03-15/neurologic-form-ehv-1-potentially-emerging-disease
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