EHV-1, facts about the disease and the current outbreak


As many are aware there is a current outbreak of EHV-1 neurologic disease or EHM Equine Herpes Myelitis. This has primarily been a western state outbreak but cases are now reported in Florida as well. Several cases were reported in South Florida several years ago as well and sporadically across the country over the past 20 years.
As always happens with disease outbreaks, the misinformation and fear spreads faster than the disease itself so what I wanted to do is put out there are some facts that we currently know and questions that we dont know the answer to. You can then use this information to make wise decisions for your horse.
This information comes from the consensus statement from the ACVIM ( American College of Veterinary Internal Medicine) .

EHV - Equine Herpes Virus or Rhinopneumonitis . There are 5 known strains of EHV. EHV-1 is associated with viral abortion and neurologic disease. This is a DNA virus and can remain dormant in neural tissue much like human Herpes viruses and become active during periods of stress.

Transmission- This virus is transmitted primarily by respiratory secretions from infected horses
via droplets from coughing or snorting . Typically viral shedding lasts 7-10 days but may persist for several weeks. Also it can be transmitted from contaminated surfaces or aborted fetuses. Virus particles may remain active in the environment for several weeks.

Incubation period- Typically the viral incubation period is 4-6 days but can be as rapid as 24 hours after infection.

Pathogenesis- Once viral infection occurs and viremia develops the virus is carried throughout the body and can infect the cells lining the blood vessel (endothelium). This activates the inflammatory cascade and causes necrosis and thrombosis of theses small blood vessels. This vascular damage ,whether in the uterus or spinal cord then leads to abortion or neurologic disease.

Latency- This is the main problem with this disease. When horses are infected with EHV-1 , usually within the first 2 years of life, the virus takes up residence within the neural tissue and can remain dormant for years, usually in the trigeminal ganglia. The infection rate may be as high as 60-80% of the horse population. During periods of stress the virus can then be activated (recrudescence) and a second viremia occurs leading to respiratory shedding. Reactivation from latency, with shedding and transmission to susceptible hosts, is a defining feature of herpesviruses and is very likely to play an important role in the etiology of EHV-1 disease outbreaks.

The Neuropathologic Strain- This has been identified as the EHV-1 D752 and N752 genotype.
There are 2 alternative scenarios for the origin of ‘‘high risk neuropathogenic’’ (D752 genotype) EHV-1 variants; either reactivation from a horse latently infected with a D752 variant or spontaneous mutation from a ‘‘low risk variant’’ (N752) to the high-risk genotype. It is possible that both events occur and given the rarity of neurologic disease outbreaks, it is very difficult to determine the relative contribution of one or the other event to such outbreaks. It seems likely that horses that are exposed to D752 variants during a neurologic disease outbreak will
become latently infected carriers of that strain, even if they were already latently infected with an N752 variant.

Vaccination- use of a modified live vaccine has been shown to reduce viremia and viral shedding during an outbreak. It has also been shown to increase the mucosal antibodies in the respiratory epithelium and this may be critical to reducing viremia. It must be noted that vaccination does not prevent recrudescence of the latent infection. Vaccination in the face of an outbreak may be beneficial in previously vaccinated horses as there will be a rapid anemestic response with a rapid rise in antibody levels.

Immunostimulants- It is theoretically possible that activation of the immune system could prevent viral reactivation but this is unknown. One study does show some benefit in preventing respiratory disease.

Biosecurity AAEP guidelines

ACVIM EHV consensus statement pdf

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