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Vaccines protect horses from disease by increasing circulating levels of certain antibodies in the blood. Most vaccines require two doses, 2-4 weeks apart, when given for the first time in a horse’s lifetime. There-after, boosters are required to maintain immunity each year. Below are listed the vaccines most commonly given. We recommend that all horses receive EEE/WEE/tetanus, West Nile Virus and Rabies annually, as bare minimum protection.
EEE/WEE/Tetanus
All horses should receive this 3 in 1 combination vaccine. EEE (Eastern Encephalomyelitis) and WEE (Western Encephalomyelitis) are fatal viral diseases transmitted by mosquitoes, which can affect humans as well as horses. Signs include profound neurologic disease and acute death. Teta-nus is usually associated with wounds, especially punctures. Treatment for encephalitis or tetanus is rarely successful.
West Nile Virus
The West Nile virus outbreak which has affected horses all over the United States in its relentless march westward is still an important disease that requires that our horses be vaccinated on an annual basis. The virus is carried and passed through the bites of mosquitoes, and is amplified in birds, which can harbor the virus for long periods of time.
When mosquitoes feed on the birds, they then become carriers of the virus. Horses sick with West Nile Virus show neurologic signs that can mimic EPM or other diseases. Treatment for affected horses is symp-tomatic and aimed at relieving inflammation of the nervous system, maintaining hydration, and preventing secondary illnesses. Nationally, about 40% of affected horses die or are euthanized if affected by West Nile virus. Although cases can be seen any time that mosquitoes are ac-tive, the majority of cases that we have seen have been in late summer and early fall. The vaccine is safe for use in all age groups, including pregnant mares. Two doses 2-6 weeks apart are required the first year the vaccine is given, followed by a bi-annual booster. We recommend that all horses be given this vaccine in March or April, and again in August or September.
Rabies
All pet animals and livestock need to be vaccinated. In horses, the vac-cine must be administered every year. If a vaccinated animal is bitten by or comes in contact with a rabid animal, they must receive a booster Rabies vaccine within 5 days. If an unvaccinated animal is exposed to a rabid animal, NY State recommends euthanasia. The alternative is a 6 month period of observation with strict isolation. The vaccine is very safe and effective, and we strongly recommend that your animals be protected by annual vaccination.
Potomac Horse Fever:
Potomac Horse Fever is a non-contagious disease that causes de-pression, fever, and loss of interest in feed, followed by diarrhea and commonly by laminitis, especially if early signs are ignored. Treatment includes intravenous fluids, antibiotics and anti-inflammatory medica-tions for 5-7 days. Vaccination for PHF can be effective in reducing the
severity of the disease.
Dedicated researchers led by veterinarian Dr. John Madigan have solved the mystery of what causes Potomac Horse Fever. The agent that causes the disease, Ehrlichia risticii, is found in nature in a complex life cycle involving flukes, freshwater snails, and aquatic insects that are eaten by bats and birds. Consuming the infected adult insects, which harbor the E. risticii in an immature phase of a fluke, infects horses. Flukes are a type of tiny parasitic worm that are common in the intestines of bats, birds and amphibians. The immature phases are present in aquatic insects. Under natural conditions, horses grazing near rivers or streams where hatches occur will ingest adult insects along with grass, or consume adult insects that, attracted by stable lights, accumulate in feed and water. Untangling the complexity of this disease has taken incredible scientific imagination and decades of research.
The wet weather that our area experiences frequently makes conditions ripe for Potomac Horse Fever. In recent years we have treated multiple cases with severe diarrhea, dehydration and toxemia. Several of the horses have foundered as a result. The earliest signs of Potomac Horse
Fever include disinterest in feed, dullness or depression, and quiet or ab-sent gut sounds. Fever may be transient or occur only after several days. If your horse shows any of these signs, please call our office promptly. Treatment with intravenous oxytetracycline usually results in prompt
improvement, but if diarrhea or dehydration has set in, large volumes of intravenous fluids, along with anti-inflammatory medications and correc-tion of electrolyte abnormalities are often required. If laminitis occurs from absorption of toxins, permanent disabling lameness can result. Vaccination for Potomac Horse Fever can be helpful in reducing the incidence and severity of the disease. We usually do not see cases with severe diarrhea in vaccinated horses. Available vaccines are labeled as providing protection for one year, but it is likely that immunity wanes in the latter part of the year. In locations where there is a history of previous Potomac Horse Fever cases, or property is quite marshy or near streams or rivers, vaccination in both spring and early fall is prudent. We recommend annual spring vaccination for all horses in our practice. If the nature of the cases seen in 2006 warrants it, we may recommend an addition fall booster.
Equine Herpes Virus (EHV): Equine Herpes Virus (EHV) most commonly causes a respiratory illness characterized by fever, nasal discharge, and cough. Clinical disease is most commonly seen in horses that travel, show or are otherwise exposed to a changing population of horses, or under stress. In pregnant mares, exposure to EHV (also known as rhinopneumonitis) may cause abortion. An estimated 80% of horses over the age of 2 years are carriers of EHV, which lies latent until activated by stress. When the virus becomes active, the horse sheds virus in his nasal secretions that is contagious to other horses, even if he does not become ill. Because carriers can both become ill repeatedly, and spread the disease to others, future outbreaks are inevitable in the horse population.
Along with abortions and respiratory disease, EHV can also cause a neurologic syndrome with weakness, ataxia (loss of balance), and hind end paralysis. A recent mutation in the virus has occurred, causing an increase in the number and severity of cases of neurologic EHV. The new strain of neurogenic EHV appears to be much more contagious, and causes its devastating symptoms due to attacking endothelial cells throughout the body. In 2003, many horses in Ohio at a Findlay riding school were affected with the neurologic form, and in 2004-2005 there were clusters of cases in Canada, Pennsylvania, and KY. Early in 2006, there were multiple premises under quarantine for the neurologic form of EHV in Maryland, West Virginia, Pennsylvania, Michigan, and Kentucky, and restrictions on travel to many states were in effect. In early 2007, there were 10 stables under quarantine in Florida following the illness of 12 horses, of which 6 died or were euthanized. The outbreak originated with a horse imported from Germany that came through the Newburgh, NY USDA quarantine station prior to shipping to Florida. Several other horses originating in the same shipment also became ill, one in California and one in New Hampshire. The emotional and economic cost of this disease outbreak is extreme, as competitions were cancelled and loved horses succumbed.
Horses that are at risk due to travel, showing or racing should have daily temperatures taken on a routine basis. If a fever develops, nasal swabs and whole blood should be submitted for PCR testing for EHV of the neurogenic strain. In addition, these febrile horses should be isolated from all others, and cared for by people that do not contact the other horses. Some horses develop fevers and shed virus, but do not become terribly sick or develop neurologic problems. However, they do shed high numbers of virus particles in highly contagious nasal secretions. Although the majority of clinical cases recover, 30-50% of horses that develop neurologic signs are ultimately euthanized. Disinfecting surfaces with a 10% bleach solution or a quaternary ammonium disinfectant is effective in killing the virus. Hand washing and laundering clothes is essential.
Vaccination with killed vaccines does NOT provide protection against the neurologic form of the disease. Recently studies by Dr. Klaus Osterrieder at Cornell have shown some convincing evidence that use of the modified live vaccine, Rhinommune, may provide protection against the neurological form of EHV. Dr. Osterreider’s study exposed 15 horses to the strain of EHV isolated from the Findlay outbreak. Five of the horses were not vaccinated, five had been vaccinated with a killed vaccine, and five had been vaccinated with the modified live product. Of the groups, only the horses in the modified live group did not show neurologic signs, and the amount of virus shed in nasal secretions was reduced. In addition, the duration of shedding was reduced to 2 days from the 5-7 days observed with the other groups. We currently recommend vaccination of all horses likely to be exposed to EHV or stressed by their lifestyle, with Rhinommune, a modified live vaccine. Use of the vaccine every 90 days provides maximum protection, but timing of the vaccine to coincide with likely exposure is another good approach. In pregnant mares, vaccine boosters for “Rhino” need to be given at 3, 5, 7, and 9 months of gestation. The use of the modified live product has been shown to be safe and effective in pregnant mares.
Influenza
Horses that travel, show, or are in contact with a changing population of horses should be vaccinated twice a year, as the immunity conferred by these vaccines is not long lived. “Flu” is a highly contagious viral respira- tory disease that causes high fever, depression, cough, and nasal discharge, but is rarely life threatening. However, the inflammation may predispose the horse to chronic coughing and/or the development of COPD/allergic lung
disease (“heaves”).
The modified live intranasal form of influenza vaccine from Heska, called Flu-Avert, has been shown to be very effective. There is also a new intramuscular vaccine from Fort Dodge, Flu-Innovator, which has shown increased effectiveness against the current prevalent strains of influenza. Horses that object to intranasal vaccines now have an effective alternative. Because the intranasal vaccine will not produce colostral antibodies in the pregnant mare, use of the intramuscular vaccine is recommended in this population. The intranasal vaccine is very effective in reducing respiratory disease in weanlings and yearlings, so is recommended for these age groups.
Strangles
Strangles Horses that travel and show, or horses that are in contact with such horses, as well as farms with histories of outbreaks, should be vaccinated annually. Outbreaks of Strangles are not uncommon in our practice area. Strangles is a highly contagious bacterial disease characterized by high fever, thick nasal discharge, and lymph node abscessation. While rarely fatal, horses can require weeks of recuperation and nursing care. Rarely, internal abscessation occurs, and causes death after a prolonged course. The nasal secretions of sick horses contain large numbers of bacteria, and these can be transferred unknowingly by people. Even if your horses don’t travel, if you visit stables or other horses, you could unwittingly infect your horse.
The available intramuscular vaccine is associated with short term marked swelling and painful reactions in the area of administration, so we prefer to avoid its use. The Fort Dodge intranasal vaccine provides effective protection without the painful side effects, but does not provide colostral antibodies in pregnant mares. We recommend its use in all horses which will be shown, contact other horses, travel to different premise, or reside on farms with a history of Strangles outbreaks.
BotulismThe bacteria Clostridium botulinum causes this neurologic disease characterized by ascending paralysis, which can progress to death by suffocation. In 2002, we saw a cluster of cases of botulism associated with contaminated bedding. Treatment with antitoxin plasma is available, but is costly. Supportive care, tube feeding and IV fluids are usually necessary. Foals are particularly susceptible to the effects of the botulinum toxin. Because of the increased prevalence of this disease among both foals and adults in the Mid-Atlantic states (PA, VA, DE) and in KY, we recommend vaccination for horses traveling to or moving to these areas. In addition, because foals are highly susceptible to the toxin, we recommend vaccination of all broodmares on an annual basis 30 days prior to their foaling date, so that their colostrum will provide protection. The first year of vaccination, three doses one month apart must be given. In subsequent years, a single dose booster is given.
Lyme Disease
Borrelia burgdoferi is a spirochete that causes dullness, shifting leg lameness, and behavior changes in some horses exposed through tick bites. There is no vaccine approved for use in horses, but the use of the vaccine marketed for dogs has been shown to have no safety problems associated with its use. Veterinary researchers at Cornell have done studies that have shown effectiveness with a similar experimental vaccine in preventing disease in horses. Their study protocol is most closely imitated by the administration of three 1 ml doses of dog vaccine, the second dose three weeks after the first, and the third dose three months after the first. We will administer the vaccine at the owner’s request, but it should be noted that this is a vaccine not licensed for use in horses. The first year three doses are required, followed by boosters every six months. Recent studies have shown that antibody titers from vaccination begin to decrease after six months, so boosters every six months are needed to maintain adequate levels of protection.
Rotavirus
This virus causes extremely contagious diarrhea among groups of young foals. Intensive fluid therapy and supportive care allows recovery within a few days, but dehydration can lead to death if not recognized and treated aggressively. This vaccine will provide immunity through the colostrum in mares that are vaccinated on an annual basis with three doses one month apart beginning 90 days before the due date. The vaccine is safe and studies in KY have shown it to be effective. We recommend vaccination of broodmare bands where rotavirus has been a farm problem. The risks for single mares’ foals are much reduced, as this disease is most devastating as a herd outbreak.
Leptospirosis This bacterial disease is caused by the spirochete Leptospira, and hundreds of different serovars exist. Populations of deer pass the spirochete in their urine, where it contaminates streams, ponds, and soil. Horses (and dogs and people) can become infected by ingestion of contaminated water or feed. Pregnant mares will often abort their foals several months after becoming infected. About six months after infection, it is not uncommon for horses to develop uveitis, an inflammatory condition of the eyes that can lead to blindness. Animals that are infected pass spirochetes in their urine for a variable period. These are also infective to humans, causing fever, achiness, and flu-like symptoms. A vaccine licensed for use in cattle has been shown to have some effectiveness against this disease, but protects only against the five most common serovars. We recommend vaccination only on breeding farms where abortions caused by Leptospirosis have been a major concern. Horses with recurrent uveitis may also benefit from vaccination. Because of the lack of an approved vaccine for horses, and the low number of serovars in the vaccine, more widespread vaccination is not indicated.
EPM/ Equine Protozoal
Myelitis Equine Protozoal Myelitis (EPM) is a neurologic disease caused by infection with the protozoa Sarcocystis neurona. Horses can be exposed to the protozoa through contact with feed or pasture contaminated with the feces of possums, skunks, raccoons, and other animals which have eaten the muscle tissue of birds infected with the protozoa. Clinical signs include wobbly drunken gait, lip or ear droop, head tilt, muscle atrophy, or odd lameness. These neurologic signs are associated with inflammation of the spinal cord. Approximately 50% of horses in the Northeast have antibodies to Sarcocystis in their blood, which demonstrates that they have been exposed to the organism. Only a small number of these animals will develop clinical disease. The vaccine released in 2001 was given approval after its safety was demonstrated. However, although the vaccine will produce measurable antibodies, it has not been proven that these antibodies will provide any protection against disease. Currently, when we examine a horse with neurologic signs, if that horse tests negative for Sarcocystis antibodies, we can feel confident that EPM is not the cause of his signs. Vaccination for EPM will compromise our ability to distinguish those horses never exposed, making diagnosis of the disease more difficult and costly. For these reasons, we do not recommend use of this vaccine.