Ninety horses in 10 states and 15 in Alberta were infected with equine herpes virus-1 this spring following an outbreak at a cutting horse show in Ogden, Utah.
The herpes virus family causes disease in a large number of animal species, including humans, but most herpes viruses infect only one species. For example, people cannot become infected with equine herpes viruses.
EHV-1, one type of equine herpes virus, can cause several diseases in horses ranging from minor cold-like signs to abortion or early death in foals. The most devastating and economically important is the neurological form, called equine herpes myeloencephalopathy (EHM).
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Most horses will become infected with equine herpes virus as foals. After an upper respiratory infection characterized by fever and nasal discharge, the virus goes into hiding, only to reassert itself during times when the horse is stressed, such as weaning, hauling, mixing groups of horses and being infected with other pathogens.
This is a similar mechanism to people infected with herpes simplex virus who develop cold sores shortly after stressful events such as final exams at school.
This virus is shed in respiratory secretions and initially infects the cells of the upper respiratory system, which results in the typical snotty nose and fever.
From there, white blood cells spread the virus to other tissues in the body, including the cells that line blood vessels in the spinal cord. Inflammation, blood clots and hemorrhage can occur as these virus-infected blood vessels are damaged.
The combination of decreased blood supply, which diminishes oxygen delivery and waste removal from the tissue, and the swelling that accompanies inflammation and hemorrhage, damages the spinal cord.
The spinal cord has limited capacity to heal and regenerate, unlike tissues such as the skin. As a result, it resides in a tight, bony column created by the vertebrae.
Unfortunately, in cases of EHM, the body column provides limited room for swelling, contributing to compression of cord and cell death. When this occurs, the brain can no longer sense or communicate with the feet and the horse loses the ability to stand.
The most common clinical signs of EHM are staggering gait, weakness and urine dribbling followed by collapse.
Why do only 10 percent of horses develop EHM? The most current theory is that some strains of the virus create a greater amount of virus in the blood than others. The more the virus replicates and reproduces, the more likely it is to infect the spinal cord blood vessels.
As well, some horses seem more susceptible than others. However, this explanation is likely oversimplified and further research needs to be done.
An interesting observation is that older horses are more likely to develop EHM rather than just the simpler upper respiratory infection. Older horses’ immune systems respond slightly differently to the virus than younger horses.
Horses that carry and shed the virus without signs of illness are strongly suspected to be critical in spreading the virus.
Risk factors for EHM have been evaluated:
* Summer is the least common season for EHM.
* Although horses of all ages can get EHM, it seems to be most common in horses older than five, while the respiratory infection is most common in horses younger than two.
* Fevers that tend to reoccur after initial infection with EHV-1 are related to more cases of EHM.
* Mares and larger breeds are more often affected.
My next column will discuss testing, treatment options, vaccination and how to prevent infection.
