The rhinopneumonitis virus causes clinically significant respiratory tract disease in young or immunocompromised horses, abortion in mares and sporadic outbreaks of neurological disease. The neurological form appears to be on the upswing in recent years, and the outbreaks haves resulted in a number of equine deaths.
The rhinopneumonitis virus is a strain of herpes. Like all herpes, the ”rhino” virus has the potential to go into a latent/dormant stage and remain in the body of the horse for life. A study published in the Journal of the American Veterinary Medical Association really drives this point home.
Researchers experimentally exposed weanlings to both respiratory strains and potentially neurological strains of herpes virus. Four to five years later, they biopsied the mandibular (under the lower jaw) lymph nodes and tested for DNA evidence of latent infection. Latent viral DNA was found in 76% of the test horses, with 83.3% of the horses exposed to the potentially neurological strains still carrying the virus. DNA typing confirmed all horses were carrying the same strain they had been exposed to as weanlings.
This prolonged persistence of the virus in exposed horses is how these infections travel easily between barns, racetracks and over state lines. This virus dies rapidly in the environment, but its ability to become dormant and escape immune system recognition keeps it snug, secure and safe inside the carrier horses.
When the carrier horse is stressed, the immune system can become weakened enough to allow the carrier to shed the virus, often without showing any glaring signs or symptoms of having any type of respiratory infection.
Stressors include heavy exercise, shipping, injury, other infections, even vaccination. If the shedding horse is in close enough contact with susceptible horses, particularly if they are also dealing with stressors, the stage is set for an outbreak. In this scenario, it’s not difficult to understand how and why outbreaks typically hit breeding farms, racetracks, large show barns and boarding stables with a lot of in-and-out traffic.
Vaccines do a poor job of completely protecting against the infection, but they do at least reduce the level of virus shed by symptomatic horses. Since most healthy young and middle-aged horses rarely show clinical signs of a herpes/rhino infection, and the wisdom of frequent vaccinations has been questioned by many horseowners and veterinarians, experts stress the importance of management in reducing the risk of herpes outbreaks.
Adult, nonbreeding horses are at greatest risk from the neurological strains of the virus, and the current vaccines are not protective for those strains. (Note: There is some evidence that the modified live vaccine manufactured by Pfizer may offer better protection against the neurological form.) Barn and horse management, on the other hand, are extremely important:
• Broodmares, young stock, older or immunocompromised horses should be strictly isolated from horses that travel on and off the premises.
• All new horses should be isolated for six weeks, including no sharing of any equipment or turnout facilities, with twice daily temperature taking for the first three weeks and veterinary examination and culturing for any horse that spikes a temperature over 101.5?°, even if not showing other clinical signs/symptoms.
If strict isolation isn’t possible, at least observe the temperature taking and put the horse in a stall downwind from the facility’s air flow. Hanging a light sheet across the bars of the stall will reduce the risk of aerosolized virus from a possibly shedding horse reaching other horses in the barn. Get your veterinarian’s advice for more details on reducing spread in your situation.
• Always maintain good ventilation through the barn, even in cold weather (but avoid drafts).
• When away from home, never allow the horse to eat from the ground in areas of high horse traffic.
• If you need to use a stall at shows, on racetracks, at veterinary hospitals or clinics, etc. always muzzle the horse to prevent her from licking/sniffing walls that may be contaminated with the herpes virus.
• Minimize stress and maximize solid nutrition.