Vaccination easily ranks as one one of the single most important things you do to protect your horse’s health. In fact, vaccines have been so successful that it’s rare to even hear of horses contracting several dreadful diseases that once loomed as a constant threat.
It is worthwhile, though, to remember what those injections are doing—especially the four “core” vaccines the American Association of Equine Practitioners (AAEP) recommends for every horse. The AAEP guidelines distinguish between two categories of equine vaccines: Nine are included in the “risk-based” group, which your veterinarian might recommend only for those horses most in need of protection against certain illnesses (anthrax, botulism, equine herpesvirus, equine viral arteritis, equine influenza, Potomac horse fever, rotaviral diarrhea, snakebite and strangles). Risk may be based on diseases endemic to a particular region, an outbreak or epidemic in a specific area, and/or whether a horse’s “lifestyle”—as a breeding animal or equine athlete—increases the risk of exposure to particular pathogens.
In contrast, the four core vaccines—eastern/western equine encephalomyelitis (EEE/WEE), rabies, tetanus and West Nile virus (WNV)—have several characteristics:
• They protect against diseases that occur year after year—by way of disease carriers in the soil, carried by insects or local wildlife—so that every horse is at risk, regardless of location or lifestyle.
• They prevent diseases that have a high mortality rate and/or have no effective treatment.
• They are safe, effective and widely available.
• In the case of rabies, the vaccine protects human health and lives.
Here is a brief overview of the four diseases the core vaccines protect against.
Core vaccine: EEE/WEE
Definition: Eastern and western equine encephalomyelitis are a pair of closely related viral diseases that affect the horse’s central nervous system. Eastern equine encephalomyelitis (EEE) is more deadly than western equine encephalomyelitis (WEE). (A related disease, Venezuelan equine encephalomyelitis [VEE], occurs in Central and South America. The last recorded cases in the United States occurred in southern Texas in 1971.)
Transmission: EEE and WEE are caused by alphaviruses of the family Togaviridae. The two forms are separated by geography. EEE occurs in the Southern and Eastern United States, and WEE in the West. However, outbreaks of both viruses have occurred outside of their normal ranges. The viruses that cause EEE and WEE are carried by birds and spread by mosquitoes. Birds that carry the viruses do not become seriously ill. However, a mosquito who feeds on an infected bird can transmit the virus to its next host. Horses are considered dead-end hosts, meaning that they cannot pass the virus on to mosquitoes or other animals once infected.
Signs: Five to 10 days after a horse is bitten by an infected mosquito, the virus passes through the blood-brain barrier to infect the central nervous system—the brain and spinal cord—where it multiplies and begins to kill nerve cells. The earliest signs of EEE are listlessness, fever and loss of appetite, but within the next 24 hours the horse will develop more significant neurological signs, including incoordination, sensitivity to sound and touch, muscle twitching in the shoulder and flank, head pressing and seizures. Within another day the horse will develop paralysis and become recumbent; coma and death follow. The signs of WEE are similar—including fever, depression, ataxia and head pressing—but tend to be milder.
Treatment: The only treatment is supportive therapy. The horse might receive intravenous fluids and corticosteroids to help reduce edema in the brain. Even with treatment, about 90 percent of horses who develop EEE will die within two to four days; those who survive are likely to have lifelong neurological impairment. The mortality rate for WEE is estimated to range from 20 to 40 percent if the case progresses to full-blown encephalomyelitis, but milder cases may never even be diagnosed if the horse recovers before developing serious signs of infection.
The vaccine: A single bivalent vaccine protects against both EEE and WEE (and provides some immunity against VEE as well). The vaccine is an inactivated adjuvanted whole virus product— that is, it contains both whole viruses, which have been rendered inactive by mixing them with a formaldehyde solution called formalin, combined with an adjuvant—a substance that stimulates a greater immune response to encourage the production of more antibodies against the viruses.
The first time a mature horse is vaccinated, or for a horse whose history is unknown, the recommended schedule is two doses of vaccine spaced four to six weeks apart. Thereafter, mature horses can be vaccinated once per year, prior to the start of mosquito season. In areas where the mosquitoes remain active year- round, your veterinarian may recommend booster vaccines every six months, especially for horses with compromised immunity.
The AAEP also suggests that pregnant mares receive a booster four to six weeks before they are scheduled to deliver, and that foals of vaccinated mares receive a three-dose series at four- to six-week intervals, beginning at 4 to 6 months of age. Your veterinarian will be able to make more specific recommendations based on your local climate and conditions and your horse’s own health needs and risks.
Additional preventive measures: EEE, WEE and WNV
• Limit mosquito populations. Mosquitoes lay their eggs in calm, stag-nant water, so take steps to close the breeding grounds. That means picking up old tires and other debris that can catch rainwater, overturning unused wheel-barrows, and keeping drainage ditches and rain gutters flowing freely. Also repair dripping faucets, and clean water troughs and buckets regularly.
• Maintain healthy ponds and streams. Fish and other predators will feed on mosquitoes and their larvae and help to keep their numbers down. Your local extension agent can advise you on how to properly manage natural water sources on your property.
• Use fly sprays. Check that your fly sprays are also effective against mosquitoes, and apply them to your horses prior to turnout and before riding on trails that go near wetlands.
• Bring horses inside at dawn and dusk, the hours when mosquitoes are most active. Mosquitoes are weak fliers and avoid breezes, so a few well-placed fans will also deter them from approaching your horses.
Core Vaccine: Rabies
Definition: an acute viral disease that affects the central nervous system.
Transmission: Rabies is caused by a lyssavirus, a neurotropic rhabdovirus that infects neural tissue. Transmitted via the saliva of an infected animal, the virus does not travel through the bloodstream; instead, it enters the neural tissue at the site of the bite and then migrates from nerve cell to nerve cell until it reaches the spinal cord and brain. Upon entering the central nervous system, the rabies virus reproduces rapidly, spreading back out through the peripheral nerves and into the salivary glands ready to be passed to a new host.
Raccoons, bats, foxes and skunks are the most common carriers of rabies in North America, but other mammals can transmit the disease as well. Horses are most often bitten on the lower legs or muzzle. A horse shows no outward signs of illness during the incubation period—the days, weeks or months it takes for the virus to migrate from the bite wound to the brain. The length of the incubation period depends on the location of the bite on the horse’s body. After a bite to the hind leg, it can take weeks or months for the virus to reach the brain, but if a horse is bitten on the muzzle or head, the incubation period can be as short as a few days.
Signs: Rabies is difficult to diagnose because the signs are variable and can mimic those of other diseases or problems. After the virus reaches the brain, a horse may have a low-grade fever, mild depression, loss of appetite and slight weakness or incoordination. Within four or five days, however, the signs can progress rapidly to convulsions, excessive salivation and more severe neurological impairment. Pain and sensitivity can recur at the initial site of infection; if the horse was bitten in the leg he may violently gnaw at the affected limb.
Acute rabies develops into two forms: The furious form (mad dog syndrome) is the type most often dramatized in books and movies, characterized by agitation, aggression, hyperactivity and paralysis of the face and tongue. This form is more commonly seen in horses who were bitten on the head. The paralytic (dumb) form causes depression, excessive salivation, ataxia, paralysis and eventually recumbency. Some horses may even show a combination of signs from both forms of the disease. A definitive diagnosis of rabies can be made only with a postmortem examination of the brain tissue.
Treatment: There is no treatment, and rabies is invariably fatal, usually within one to five days after the first signs of illness appear. Because of the public health risk, any horse suspected of having rabies must be quarantined or destroyed, and everyone who had been in contact with that animal must seek immediate medical care—they may need to receive a series of shots to protect them against the disease. The current postexposure prophylaxis (PEP) for people exposed to rabies is a series of five shots given over a 14-day period. If a horse had been inoculated against rabies and then is bitten by a rabid animal, the current protocol is to revaccinate him immediately and then observe him for 45 days for clinical signs of disease.
The vaccine: Three rabies vaccines are available for horses. All are inactivated tissue-culture-derived products where the virus is grown in cell cultures and then killed to make the vaccine. A single dose is enough to induce strong immunity, and the current recommendation is that mature horses be vaccinated once a year.
According to the AAEP, mares vaccinated prior to breeding develop enough immunity to produce sufficient levels of antibodies in their colostrum to protect the newborn. Alternatively, a pregnant mare can be vaccinated four to six weeks prior to foaling. If the mare was vaccinated, a foal won’t need his first rabies shot until he is 6 months old. Foals get a two-dose series, spaced four to six weeks apart, and thereafter are vaccinated once annually as mature horses.
Additional preventive measures: rabies
• Learn which wild species pose the greatest threat in your area. Any mammal can host rabies, but only a few species are both highly susceptible to the disease and likely to bite a horse. In 2010 (the last year for which data is available), 6,154 cases of rabid animals were reported to the Centers for Disease Control and Prevention. The most common wild species reported were raccoons (36.5 percent), skunks (23.5 percent), bats (23.2 percent) and foxes (6.9 percent). But the occurrences were not evenly distributed: Raccoons were the most frequent carriers of rabies on the East Coast; skunks in the middle portions of the country from the Dakotas to Texas plus California; and foxes in Alaska and the Southwest. Cases reported in bats were scattered all around the country.
• Discourage wild animals from hanging around your farm. You can’t, of course, avoid all encounters with local wildlife, but do take measures to avoid attracting high-risk species. That may mean storing grains and feeds in tightly fastened bins, blocking off access to crawlspaces under storage sheds and other buildings, and keeping your garbage in sealed cans. • Avoid and report ill wildlife. If you notice unusual behavior in wild animals—a nocturnal animal ranging about in daylight, for example, or acting unusually aggressive—call Animal Control or your local police. Do not approach the animal or allow it to come near you or your animals.
• Vaccinate all your domestic animals. In 2010, 8 percent of rabies cases reported were among domestic animals, mainly cats (4.9 percent) but also dogs (1.1 percent) and cattle (1.1 percent). Vaccinate all dogs and cats on your property, including any feral or semi- feral barn cats. Currently, approved rabies vaccines are also available for cattle and sheep. Talk to your veterinarian about the risks of off-label use to vaccinate goats or other species if rabies poses a significant threat in your region.
Core Vaccine: Tetanus
Definition: a paralytic disease caused by toxins of the bacterium Clostridium tetani
Transmission: C. tetani is an anaerobic bacterium that normally inhabits the equine digestive tract. But when the bacteria pass into the soil, they form spores that can survive for years. When the spores encounter an anaerobic environment—such as a puncture wound that heals over on the surface—the bacteria reactivate, grow and multiply, producing a potent neurotoxin.
The toxins enter the motor nerves and travel through them to the spinal cord. As they spread through the nervous system, the toxins interfere with the release of neurotransmitters, causing the skeletal muscles to lock into rigid spasms. In some cases, the spasms may be strong enough to fracture bones. Paralysis of the muscles of the head and mouth (called “lockjaw”) make it impossible for the horse to eat or drink. If the spasms affect the larynx and diaphragm, the horse will suffocate. The incubation period may vary, but tetanus usually develops within 10 to 14 days after the initial wound, which can be so minor it was never even noticed.
Signs: The first sign of tetanus is localized muscle stiffness in the vicinity of the wound where the bacteria entered as well as in the jaw, neck and hind limbs. The horse’s tail will be rigidly elevated and he will move with a stiff gait. Within another day, the horse will experience generalized stiffness throughout his body as well as hyperesthesia—a hypersensitivity to sensory stimuli—which causes him to be extremely reactive to sounds and touches. He may be unable to walk and will stand with a characteristic “sawhorse” stance with stiff legs, an arched back and a backwardly arched neck. The horse may develop the characteristic facial expression, called “risus sardonicus” or the “sardonic grin,” which consists of drawn back lips exposing the teeth, nostrils rigidly flared, erect ears and prolapse of the third eyelid.
Treatment: Early intervention is critical to successful treatment of tetanus. An antitoxin is available, which is injected directly into the fluid surrounding the brain, but it neutralizes only those toxins that have not yet bonded to neural tissue. Other treatments include the administration of antibiotics and various muscle-relaxing drugs and pain medications, along with intravenous fluids. Infected wounds also need to be drained and cleaned. If the affected horse cannot eat he needs to be fed via nasogastric tube. Even with aggressive treatment, the mortality rate of tetanus is 50 to 75 percent.
The vaccine: The vaccine against tetanus is a formalin-inactivated adjuvanted toxoid—the toxin is chemically rendered harmless, mixed with an agent that stimulates the immune system and injected into the horse. Unvaccinated mature horses receive a two-dose series, spaced four to six weeks apart. Thereafter, a single annual booster is recommended for all mature horses. An additional booster is recommended if a horse sustains a wound or undergoes surgery more than six months after his last vaccination. The AAEP recommends vaccinating a pregnant mare four to six weeks prior to delivery, both to increase the antibodies in her colostrum and to protect her if the delivery is difficult. Foals get a three-dose series starting at 4 to 6 months of age, with four to six weeks between the first and second inoculation, and then a third administered at 10 to 12 months. Because C. tetani spores are present in soils, especially on farms with large animals, it’s a good idea for you to stay up-to-date on your tetanus shots, too.
Additional preventive measures: tetanus
• Watch for puncture wounds. Deep wounds that close over, trapping dirt and bacteria under the skin, are perfect incubators for tetanus. Inspect wounds carefully to determine how deep they go, and call your veterinarian if you’re unsure. Be especially attentive to wounds on the hooves and lower legs, which are more likely to be exposed to manure and dirt.
• Clean all wounds. The bacteria that cause tetanus can take hold even in small wounds. Cleanse and disinfect any wound you find on your horse. A quick rinse with a topical antiseptic will suffice for most minor wounds.
• Clear up clutter. Be vigilant about cleaning up broken glass, loose nails, abandoned farm tools, barbed wire and other debris that can injure horses.
Core Vaccine: West Nile Virus
Definition: viral disease that affects the central nervous system. Technically, “West Nile virus” (WNV) is the agent that causes the disease “West Nile encephalomyelitis,” but people commonly use the name of the virus to refer to the illness.
Transmission: WNV is a flavivirus carried by birds and spread by mosquitoes. Most wild birds are asymptomatic carriers of the virus with the exception of ravens, crows and other corvids, for which WNV is often fatal. Dead ravens and crows are often an indicator of an outbreak in a local area. Horses are considered dead-end hosts; that is, once infected they do not directly infect others. WNV is a relative newcomer among U.S. diseases.
The virus was first identified in the West Nile river region of Uganda in 1937, and for more than 50 years its range seemed to be limited to Africa, the Middle East and parts of Europe and western Asia as well as Australia. In 1999, however, the first North American cases appeared in the New York City metropolitan area, and by 2002, the virus had reached California. WNV is now well established in all 48 contiguous states as well as Canada, Mexico and several Central and South American countries.
Signs: Most horses bitten by mosquitoes carrying WNV will show few, if any, signs of any illness. Horses newly exposed to the virus might develop a low fever and listlessness for a few days, but most are able to fight off the infection and recover fully. However, in roughly 10 percent of cases, WNV crosses the blood-brain barrier to infect the central nervous system. In those cases, within five to 15 days horses will begin to show a number of more serious signs of illness. These include elevated fever, muscle weakness and incoordination, loss of appetite, muscle twitching of the face, behavioral changes and paralysis and recumbency. The most striking signs are incoordi- nation, constant waves of muscle twitching and major changes in personality—most often with exaggerated fear responses.
Treatment: The only treatment is supportive therapy, including intravenous fluids and anti-inflammatory medications. Recumbent horses may benefit from being supported in a sling until they recover well enough to stand. Neurological impairment will generally begin to diminish after five to seven days, but some cases may last for several weeks. Roughly a third of horses who develop neurological disease from WNV will die or be put down. Those who recover may take up to a year to return to their previous level of work.
The vaccine: Currently, three types of WNV vaccines are available:
• One is an inactivated whole virus formula with an adjuvant. To produce the vaccine the manufacturer grows the virus in culture, kills it so it can no longer cause disease, and then mixes it with a substance that encour-ages an immune response to the virus. The recommended protocol for this type of vaccine is a primary dose followed by a second dose three to six weeks later and then a yearly booster.
• Another WNV vaccine type is a nonreplicating live canary pox recombinant vector vaccine. More simply put, bits of the WNV DNA are spliced into a carrier virus that does not cause disease in horses. When this combined virus is injected into the horse, he develops antibodies to the WNV without risk of infection. This vaccine requires a primary series of two injections four to six weeks apart and then a revaccination every 12 months.
• The third type is an inactivated flavivirus chimera vaccine. For this one, a hybrid of WNV and another flavivirus, yellow fever, is created, killed and mixed with an adjuvant to stimulate the production of antibodies against WNV antigens. This vaccine requires a series of two injections three to four weeks apart with yearly boosters thereafter.
None of the WNV vaccines are labeled for use in pregnant mares. However, experience has shown the vaccine to be safe for pregnant mares. Foals can receive any of the vaccinations, which are administered in a three-dose series—two at 4 to 6 months of age, with a four- to six-week interval, and then the third at 10 to 12 months of age.
The AAEP recommends that mature horses be vaccinated against WNV each spring, prior to the start of mosquito season. Your veterinarian may recommend more frequent boosters if your horse is at high risk of exposure or has compromised immunity.
This article first appeared in EQUUS issue #444.
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