Rhinopneumonitis (“rhino”) in horses is roughly equivalent to the common cold in people---it is a highly contagious respiratory infection spread by virus that passes readily from horse to horse. An outbreak can spread quickly and easily through a herd. Signs range from mild to severe and might include fever, lethargy, cough and nasal discharge. There is no specific treatment, but most horses recover completely within a week or two with rest and supportive care, which might include drugs to lower a high fever as well as antibiotics if secondary infections set in.
But unlike the common cold, rhino has a dark side: It is caused by one of two types of equine herpesviruses, EHV-1 and EHV-4. Some strains of each organism can pass beyond the respiratory system and affect other parts of the body. If the virus infects the epithelium that lines the uterus, it can trigger abortion. The fluids from a lost pregnancy contain a large quantity of virus, which can easily be picked up by other mares in a field, potentially leading to “abortion storms” that sweep quickly through a herd.
Some strains of EHV-1 and -4 can also cause equine herpesvirus myeloencephalopathy (EHM), an inflammation of the blood vessels that supply the brain and spinal cord. Affected horses develop neurological signs, including altered gaits, ataxia, a floppy tail and urinary leakage. With supportive care, most horses who are able to remain on their feet are likely to make a full recovery from EHM. Outcomes are generally worse for those who become recumbent, although horses may recover with intensive care, which can include the use of support slings.
Most horses are first infected with EHV as foals, and serious signs of res-piratory illness are more likely to develop in those younger than 5 years old. EHV, like herpesviruses in other species, is never completely cleared from the body. Instead, the virus goes “latent” and hides from the immune system. However, when a horse is stressed, the viruses may reactivate and cause a new respiratory infection. These later infections are often less severe, and any outward signs may easily go unnoticed. However, the horse will be shedding the virus in his nasal secretions, and he can spread the illness to others. This is how the disease sometimes appears in herds that are not exposed to outsiders.
Older horses, who have already developed natural immunity from previous exposures to EHV, are less likely to develop serious respiratory illness when they encounter a new strain of the virus, but they are more likely than younger horses to develop EHM.
Vaccinate “at-risk” horses
The American Association of Equine Practitioners includes rhinopneumonitis on its list of “risk-based” diseases, which means that vaccination is recommended only for horses who are more likely to be exposed to, or suffer more serious effects from, the virus. Candidates might include broodmares, other horses who come in contact with mares on breeding farms, foals, racehorses and others who travel frequently to competitions, events and shows.
Vaccination will not necessarily prevent an exposed horse from developing rhinopneumonitis, but it can reduce the severity of the disease if it does occur, and it can also limit the amount of the virus the infected horse will shed through nasal secretions, which may limit contagion.
Several products are available, which include either inactivated or modified live vaccines, and two are labeled for the prevention of abortion. No vaccines are labeled to prevent EHM, but in the case of an outbreak, veterinarians may vaccinate nearby horses who are not yet showing signs of illness to limit shedding of the virus and help control the spread of the illness.
For foals, the recommendation is to vaccinate at 4 to 6 months of age, with one booster administered four to six weeks later, followed by a second booster at the age of 10 to 12 months. Thereafter, boosters are suggested at six-month intervals for at-risk horses. Pregnant mares may be vaccinated during the fifth, seventh and ninth month of pregnancy, both to prevent abortion and to confer immunity to the foal.
Your veterinarian will help you determine whether vaccines against EHV are appropriate for your horse.
EHV spreads readily via direct nose-to-nose contact, from contact with infected surfaces, through shared feed and water, and from airborne droplets exhaled by an infected horse. The virus can also be carried on a person’s hands and clothes. Recent studies show that under the right conditions, the virus can survive up to 21 days or more in water, and it can persist for hours or days on hard surfaces. EHV is so common among horses that preventing exposure to it entirely may not be possible, but you can take basic steps to limit your horse’s risks and control the spread of an outbreak:
• Prevent close contact with new horses. When at shows or on trails, avoid letting your horse greet others nose to nose. EHV can also travel via airborne droplets, so a distance of eight to 10 feet is safer.
• Do not share water buckets or equipment. Keep a separate set of grooming tools, buckets and other equipment for each horse in your care, and avoid borrowing or sharing these items at horse shows. One handy way to organize your tools is to assign each horse a color and use multicolored electrical tapes to tag his gear.
• Clean your hands after working with each horse. Make it a habit to wash with soap or use a hand sanitizer before moving on to the next horse. A few conveniently placed bottles of sanitizer around the barn can make this habit easier.
• Isolate newcomers. A horse who has been exposed to EHV may incubate the virus for up to two weeks before showing signs of illness, and he might be shedding the virus during that time. Keep any new horse separate from the resident herd for at least two weeks. A separate turnout area that does not share a fence line with your other horses would be ideal, but if that’s not an option, consider using temporary fencing to cordon off a section of your main pasture for the newcomer. You’ll need to use a double line, spaced 8 to 10 feet apart, to prevent nose-to-nose contact. In the barn, place the new horse in an end stall, and position fans to direct airflow out a door or window rather than toward other horses.
• Check for fevers. If you frequently travel with your horse, check his temperature twice daily for three days before each trip and continuing up to five days afterward. If you detect a fever, isolate the horse from others and contact your veterinarian.
This article first appeared in the September 2017 issue of EQUUS (Volume #480)