Considering the recent clinical cases of Equine Herpes Myeloencephalopathy, this article will give you information about Equine Herpes Disease and the virus in its many forms and how to prevent it. The herpes viral infection typically first attacks the horse’s nose and upper respiratory tract. This virus can cause three different clinical conditions. The first is a respiratory disease with high fevers, bronchitis, pneumonia, secondary guttural pouch infections, and potential 6-week recovery and healing time for the damaged tissue of the respiratory tract. The second disease process is late stage pregnancy abortion. The third disease is referred to as Equine Herpes Myeloencephalopathy and is inconsistent, unpredictable, delayed, with permanent spinal cord damage which can result in signs ranging from incontinence and subtle gait changes to severe paralysis which could result in euthanasia. The clinical presentation can be very similar to Equine Protozoal Myeloencephalitis (EPM). This neurologic disease will follow infection with the herpes virus named EHV-1. There are five other equine herpes viruses, EHV-2 through EHV-6 that can all cause disease in the horse. EHV-2 and EHV-4 can also cause respiratory disease and EHV-3 causes a form of superficial venereal disease. EHV-1 is the real bad actor. EHV-5 and EHV-6 have little clinical significance. A new strain of the herpes virus (referred to as the G strain) has been identified that has a change in the DNA sequence that results in a 5-10-fold higher replication rate in the white blood cells which then carry the virus to the endothelial cells of the blood vessels (cells that line the blood vessels) in the nervous tissue of the horse. The damage to the blood vessels occludes the vessels not allowing blood flow to the spinal cord. This functionally causes mini strokes throughout the spinal cord, paralyzing the horse. This enhanced replicative vigor can overcome the antibodies produced by some horses vaccinated with the vaccines currently available. But even though this newly modified strain seems to be associated with the neurologic disease, it has been shown by looking at tissue samples saved over the last 50 years, that even the older identified strains of the EHV-1 virus (referred to as the A strain) can cause neurologic disease in 25-30% of the cases. So, the neurologic disease cannot be solely attributed to the newly identified strain. This presents new challenges to the researchers and vaccine manufacturers, to develop vaccine products that specifically target the new neuropathologic strain of Herpes (EHV-1) in addition to the other strains. The clinical signs to be concerned with, in trying to identify a herpes infection, are a fever of 102-104° F, possible respiratory signs of runny nose, sneezing or coughing, and of course changes in the horse’s gait as in tripping, stumbling, wobbliness, and difficulty getting up and down in the stall or paddock, “dog-sitting” or complete recumbency. The neurologic signs may not become apparent for 3-10 days after the fever or respiratory signs. Not all horse will have an identifiable fever and not all horse will show respiratory symptoms prior to neurologic signs. Multiple states have been reporting outbreaks of EHV-1 causing respiratory signs and neurologic signs. The respiratory disease is very common and, in my opinion, has probably been around Delaware and many tracks for years. According to one estimate as many as 80% of the horses over the age of 2 worldwide may be carrying the virus. In Kentucky a post mortem study found 54% of the horses necropsied had virus in the lymph nodes of their head. Another study showed 3.8% of over 400 horses arriving at a show grounds tested positive for shedding the herpes virus by PCR test upon arrival and were not clinically sick at the time of testing. It may be becoming more prevalent due to genetic changes in the virus allowing it to be more easily replicate in the horse and therefore more easily spread to neighboring horses. The neurologic form, although uncommon, has become more frequently diagnosed in recent years. Diagnosis is based on clinical signs and viral testing of febrile horses (with or without respiratory disease), by nasal swab sampling and EDTA blood samples to identify the DNA with PCR technique. This will rule in or out the A strain (usually NON-neurologic) or the G strain (always neurologic). If identified, it is very important to isolate and quarantine the individual that is sick (index patient) and all horses stable in the same barn to avoid infecting additional horses. This may require quarantine of entire barns or entire race tracks for periods of 14-21 days. Owners and trainers, if suspicious of disease, should stop jogging and training, have the horse examined by a veterinarian. Treatment is possible with supportive care like IV fluids, antipyretics like Bute or Banamine™ and antiviral drugs such as valacyclovir for the non-neurologic forms. Antibiotics would not be beneficial for the viral infection. But may help with secondary bacterial infections. There is limited success in treatment of the neurologic form of the disease at this point in time. Much of the damage to the horse’s body as the result of this virus is the body’s own immune response. The immune response, as described above, can result in severe tissue damage and ultimately death or the need for humane euthanasia. Vaccinate! At least twice each year! All horses are at risk regardless of age, although foals are rarely affected. All horses in a barn or stable area should be vaccinated to prevent the spread of the virus by reducing shedding from the respiratory tract. There are several vaccines on the market. Based on a recent study in the AAEP Proceedings, the products used for preventing herpes viral abortion were the most able to stimulate the horse’s immune system. No vaccine has been shown to protect against the new neurologic strain of the disease. This virus is an intracellular virus, living inside the lining (endothelial) cells of blood vessels and in the lymphocytes in lymph nodes. It is very hard for the horse’s immune system to kill it. In fact, it may not get killed but go into dormancy or latency in the cells only to reappear and cause disease at a later date. Herpes virus stays in the horse’s body for life. Horses that have been vaccinated in the past 6 months need a booster. Revaccinating at 3 or 4-month intervals may provide the best protection. Vaccinating at 2-month intervals may also been shown to have benefits. The 2-month program has been shown to minimize the abortion outbreaks on breeding farms. Horses that have not been vaccinated in the last 6 months need a 2-shot series 30 days apart to get the best possible immune response. The horse will not have protective antibodies until about 10 days after the second shot. One shot of the vaccine in an unvaccinated horse is not enough! They need the series! Control measures include aggressive vaccination programs, isolation of new arrival horses at the farm or stable areas, cleaning feed tubs and water buckets, cleaning work clothes after exposure to new or suspect horses. The virus is easily killed with common household disinfectants like soaps, detergent and bleach (1:100 dilution for surfaces). Avoiding across-the-fence or nose-to-nose contact and sharing of water buckets with new horses is the first line of defense. The virus has been estimated to be aerosolized 35-50 feet and remain viable for 3-4 days in the environment. Cleaning stall walls and cross-ties between new horses will eliminate exposure from saliva and nasal discharge. Cleaning and disinfection of horse trailers and minimizing the hauling of non-resident horses from your farm would be of great benefit. The incubation time can be 3-10 days and sometimes 3-14 days. Maintain an adequate isolation period for all new horses. Re-testing exposed horses on the farm at 7, 14 and 21-day intervals after exposure will be the key to removing the quarantine and minimize infection of additional horses. Unlike kindergarten, do not share your equipment and toys.
References: Equine Herpesvirus Myeloencephalopathy: an update on EHV-1, 13th International Congress of World Equine Veterinary Association, 2013 - Budapest, Hungary
-submitted by Dr. Paul F. Hanebutt, Jr. DVM, Brenford Animal Hospital
Comments