In This Chapter
- Recognizing symptoms of infectious diseases
- Understanding diagnosis and treatment
- Preventing infectious diseases in your horse
Horses are social creatures, and they have a host of infectious diseases that plague them to prove it! Passed from horse to horse or by a parasitic host, these diseases are dangerous and can be fatal.
In this chapter, we give you the
basics on a number of common infectious diseases that can affect your horse. As
soon as you see any suspicious symptoms, be sure to contact your veterinarian
immediately for diagnosis and treatment. The good news is that vaccines and
other prevention methods are available for many of these problems. After
reading this chapter, you’ll be motivated to keep your horse inoculated. (Flip
to Chapter Getting
Up to Speed on Routine Care for the basics of equine vaccinations.)
A Toxin Produced by Bacteria: Botulism
Most people have heard of
botulism, the deadly disease that infects food and causes severe illness and
often death. Although botulism is most often thought of as something that can
show up in canned food, the disease is actually a problem for horses as well.
Horses are affected by the toxin
that’s produced by the bacterium Clostridium botulinum. These bacterial
organisms are strict anaerobes, which means that they can’t live in the
presence of air. These organisms produce seven different types of toxins, and
these neurotoxins cause very distinct symptoms. Protection from one type
doesn’t cross-protect for another.
Horses can be infected with the
botulism toxin in three different ways:
- With any deep puncture wound or when a wound results in considerable dead tissue.
- When a rodent or other animal dies in a field where hay is cut and processed. The tightly compressed hay bales limit the amount of oxygen that comes in contact with the dead animal, creating a perfect growth environment for the oxygen-hating Clostridium botulinum. The horse accidentally ingests the bacteria when eating the hay or hay cubes.
- When the horse ingests bacteria that’s living in pasture soil. After the horse accidentally swallows it, the bacteria begins to grow and produce toxins in the gastrointestinal tract. These toxins are absorbed from the gastrointestinal tract into the bloodstream. From there, it goes to the nerves throughout the body.
After a horse absorbs the
bacteria, production of the toxin takes place within the horse’s gastrointestinal
tract.
Symptoms
Botulism causes weakness and
paralysis. Muscles that are involved with standing and moving, as well as
swallowing, are often involved first.
The first sign you may notice in
your horse is the inability to swallow or a lot of drooling. The horse may not
be able to get up after lying down, and he may begin to lose control of his
muscles. In the early stages, this loss of control may cause extreme anxiety in
the horse. You may also see his muscles quiver and tremble.
Affected horses may also fall
abruptly when trying to lie down. The third eyelid begins to protrude, and the
horse can’t control his tongue. The internal muscles are also affected, and the
horse can’t control his bladder. His digestive system stops working properly.
Finally, the diaphragm, which is the largest muscle involved with breathing,
stops functioning, resulting in the horse’s death.
Diagnosis and treatment
No specific diagnostic test for
botulism exists, and blood work on affected horses is typically normal.
Veterinarians make a diagnosis based on clinical signs. Although serum and
feces can be submitted to a lab for toxin analysis, the vet will probably begin
treatment immediately because time is of the essence when you’re combating this
disease.
Treatment involves giving the horse hyper immune plasma that’s very high in antitoxin titers. However, because botulism types vary, the vet has to use plasma that’s specific to the particular toxin that affected the sick horse.
Treatment is costly, and
unfortunately, after the disease progresses and the toxin adheres to nerve
cells, the antibodies are ineffective. Treatment is then aimed at providing
intense nursing care for the horse until his body can make new neuromuscular
transmissions and restore nerve function. This can take seven to ten days, and
all the horse’s bodily functions must be supported in the meantime. This
support involves providing fluids, feeding the horse through a nasogastric
tube, and giving him antibiotics to combat secondary infections.
Prevention
To help prevent botulism, inspect
your horse’s hay regularly for animal carcasses that may have been accidentally
baled into it. If you find animal parts, don’t feed the rest of the bale to
your horse, and keep an eye on him for possible symptoms. Wound botulism can be
prevented by calling a veterinarian right away if your horse seriously injures
himself. Immediate treatment of the wound can help ward off botulism.
Affecting the Nervous System: Encephalomyelitis
Encephalomyelitis is a viral
disease of the nervous system. Three strains of the same disease can infect
horses. Called Western equine encephalomyelitis (WEE), Eastern equine encephalomyelitis
(EEE), and Venezuelan equine encephalomyelitis (VEE), the illness can cause
severe neurological symptoms and often death. EEE and WEE are most prevalent in
the U.S., but VEE is becoming more common, especially in the South.
The virus lives in hosts like
birds and rodents, and all strains of equine encephalomyelitis viruses are
spread by blood-sucking insects, specifically the mosquito. A mosquito bites
the host and then transfers the virus from the host to the horse by biting.
Symptoms
Symptoms of equine
encephalomyelitis viruses can be lethargy, lack of appetite, and persistent
fever. If the virus isn’t successfully cleared by the horse’s immune system
after the initial infection, the disease can cause neurological behaviors such
as circling, seizures, stumbling, and lethargy. As the disease progresses, the
horse may begin head pressing (pressing the head against a wall or other
Warning!
EEE and VEE are especially deadly, and in severe cases that don’t respond to treatment, the horse must be euthanized.
Diagnosis and treatment
The encephalomyelitis viruses may
be diagnosed by blood tests, which isolate antibodies to the disease, along
with clinical symptoms, or on postmortem testing.
No cure is known for equine viral
encephalomyelitis, but veterinarians can provide supportive treatment for
affected horses in the form of intravenous fluids, anti-inflammatories, and
anti-diarrhea medications.
Prevention
Remember
Vaccinating your horse is the best way to prevent serious infection with encephalomyelitis. Your veterinarian can recommend a vaccine protocol that is appropriate for your particular area of the country.
Tip
Eliminating areas where mosquitoes can breed is another way to help control encephalomyelitis. Here are a few tips:
- Remove standing water from your property. Mosquitoes lay their eggs in standing water, and the larvae hatch and grow there. Eliminating water where the insects can lay their eggs can help reduce mosquito populations.
- If you have a water trough for your horses, stock it with mosquitofish. The fish will eat the mosquito larvae and reduce the number of mosquitoes that may bite your horse. Mosquitofish are often available at no charge from county vector control agencies.
- Spray your horse with mosquito repellent during mosquito season, and try to keep her indoors at night (or during the dusk and dawn mosquito feeding times) if at all possible. You can buy mosquito repellent designed specifically for use on horses at your local tack and feed store, in equine product catalogs, or on the Internet.
A Disease in Three Forms: Equine Herpes Virus (EHV)
The viral disease equine herpes
virus (EHV), also known as rhinopneumonitis, has gotten more publicity lately
because highly contagious neurological forms of the disease have become more
prevalent. Horses transmit EHV to one another through direct contact or by
coughing or snorting and releasing the virus into the air.
Symptoms
The symptoms of EHV depend on the
form:
- The neurological form of herpes can start with mild fever, slight lethargy, and mild respiratory signs such as a cough.
- The respiratory form usually begins with a cough, fever, and nasal discharge. Many horses also develop a loss of appetite and lethargy, while some horses aren’t affected much at all.
- The abortion form of EHV can be a silent and deadly killer. A mare infected with the virus in the last trimester of pregnancy can abort her fetus two weeks to several months after infection. Some foals may escape abortion but are weak and sickly and die soon after birth. Some mares show a respiratory infection, but many don’t.
Diagnosis and treatment
Neurological EHV can show the
same symptoms as many other neurological diseases discussed in this chapter,
such as WEE, EPM, and rabies. You can help your vet make the proper diagnosis
by maintaining good vaccine records and allowing him or her to perform the
appropriate diagnostic tests. These include microscopic examinations of blood,
nasal discharge, and tissue samples.
As with most viral diseases, no specific treatment exists for EHV. Most horses recover with good nursing care, and antibiotics may help treat secondary bacterial infections. Unless the horse’s fever is very high, coauthor Dr. Kate prefers to let the disease run its course because the cyclical fever can be used to help monitor the disease’s progress. Also, fever is part of the immune system’s defense.
Prevention
Remember
The vaccines currently on the market are labeled to protect against the abortion and the respiratory forms of EHV, not the neurological form. However, it’s important to vaccinate against EHV because cross-protection may occur. The vaccine will also stimulate your horse’s immune system.
As with other diseases, horses
involved in activities such as racing, training, showing, or transport are at
an increased risk, not only due to exposure, but also because of stress on the
immune system. Do whatever you can to reduce your horse’s stress level (see
Chapter Connecting
Your Horse’s Behavior to Health for help), and limit your horse’s exposure to common water buckets at
shows.
If you decide to breed your mare,
be sure to ask your vet about proper vaccine protocols, and be diligent about
protection so that your mare doesn’t abort as a result of EHV. Chapter Breeding
Your Horse has
more information on breeding horses.
Equine Infectious Anemia (EIA)
Equine infectious anemia (EIA) is
a disease that affects the blood. It’s caused by a virus that’s spread between
horses by biting horseflies. It can also be spread by using needles, dental
floats, and other contaminated equipment between horses. Infected pregnant
mares can pass the disease on to their babies if they don’t abort them first.
Symptoms
Horses with a serious case of EIA
have a high fever, are lethargic, and develop anemia. They often develop thrombocytopenia
(lack of platelets), which causes hemorrhages (heavy bleeding) to
occur on the gums and elsewhere. They may also exhibit stocking up (swelling)
of the lower legs and along the bottom of the abdomen.
The signs of EIA develop 7 to 30
days after exposure to the virus. Horses can die from EIA or become chronic
carriers of the disease. These horses will have an intermittent fever and
weight loss.
Diagnosis and treatment
The best test for EIA is
something called the Coggins test. Because EIA is so serious and must be
reported by law to state health authorities, blood for the
If a horse tests positive for
EIA, he must be euthanized or quarantined for life because this disease has no
treatment, and infection is permanent. The infected horse must be kept a
minimum of 200 yards from any other equine. The virus lives for only 15 to 30
minutes in the horsefly, so keeping horses this far apart means that the virus
usually dies before the infected fly can travel to get another blood meal.
After a horse is deemed positive
for EIA, most states also require an obvious brand on the animal. Check with
your veterinarian to find out your state’s requirements for frequency of the
Coggins test.
Tip
Before transporting your horse across state lines, check out the health requirements for transporting. Many large horse show and competitive venues may require a current Coggins test prior to competition. Some venues even want to see the paperwork before you can bring your horse on the grounds and unload. You may be asked to produce current negative Coggins paperwork at any time during any horse-related activity, so don’t leave home without it.
Prevention
Remember
You’d think that with all this testing, EIA would have been eradicated by now. But unfortunately, that’s not the case. To help protect your horse from EIA, follow these guidelines:
- Board your horse only at a facility that’s vigilant about requiring proof of negative EIA status for all horses. (See Chapter Getting Up to Speed on Routine Care for more information about boarding your horse safely.)
- Never use any needle syringe more than once, and be diligent that equine professionals clean all instruments before using them on your horse.
- Make certain you know the EIA status of any horse you purchase.
Hitting the Respiratory System: Equine Influenza
Equine influenza is one of the
most debilitating and highly contagious viral respiratory diseases to affect
horses. Equine influenza is transmitted from horse to horse and through the air
in particles that come from discharge released from the horse’s nose. All
horses can contract equine influenza, but younger animals are most susceptible.
Symptoms
Equine influenza can cause a high
fever, lethargy, and a nasal discharge that starts out thin and clear, and can
quickly develop into a thick copious discharge (see Figure 12-1). Affected
horses often develop a cough.
Figure 12-1: Significant nasal
discharge is a possible sign of equine influenza. (Dr. Janice Sojka, Purdue
University)
Warning!
Uncomplicated cases clear up on their own within a week, although a persistent cough may last several weeks. Complications and secondary infections can lead to pneumonia and persistent poor performance, especially if the horse is continually stressed and not allowed to recover completely.
Diagnosis and treatment
Veterinarians often diagnose
equine influenza based on clinical symptoms. No cure exists for this illness,
but good supportive care — such as intravenous fluids and drugs to control
fever — can keep a horse from developing serious complications. Sometimes your
veterinarian may recommend antibiotic treatment to deal with a possible
secondary bacterial infection.
Prevention
Remember
The best way to prevent equine influenza is to vaccinate. Although not foolproof, the vaccine for this virus can go a long way toward keeping your horse from contracting this illness. The number of times per year that you should vaccinate depends on the amount of exposure your horse has to other horses as well as her age. Your veterinarian can tell you how often you should vaccinate.
A Common Neurological Disease: Equine Protozoal Myeloencephalitis (EPM)
Equine protozoal
myeloencephalitis, or EPM, is caused by a protozoan called Sarcocystis
nerona and is the most commonly diagnosed neurological disease in horses
today. Many horses have been exposed to EPM, but not all horses develop the
disease. Younger and older horses are more commonly affected, and horses who
have never been exposed to the disease are more susceptible. In horses who have
already been exposed, stress is thought to play an important role in developing
clinical signs of this disease.
The opossum is the host of this
disease. Birds can carry this nasty little parasite in their muscles. The
opossums eat the birds and pass the organism out through their stool. Horses
then get the disease from eating food or water that’s contaminated with the
opossum’s feces.
Symptoms
The EPM protozoa affects the
spinal cord and brain, resulting in a variety of symptoms, including
incoordination of one or both rear limbs, stumbling, muscle atrophy, and other
subtle to severe neurological signs, such as bucking under saddle, an inability
to execute lead changes, and unequal stride length. Any signs of brain and
spinal cord problems can also signal EPM, from paralysis to loss of bladder
control and urine dribbling.
Diagnosis and treatment
At this time, the most definitive
test for EPM requires examination of the cerebrospinal fluid (drawn from a
spinal tap) in conjunction with clinical signs. Other tests are currently being
developed.
Only a few approved products are on the market to treat this disease. The success of treatment depends on the individual horse. Some horses recover completely, while others retain some neurological damage for the rest of their lives. Recurrence of the disease is also a possibility.
Prevention
Warning!
A vaccine is available for EPM, but it’s somewhat controversial because it can give a false positive on tests for EPM in horses who are suspected of having the disease. Ask your veterinarian if this vaccine should be a regular part of your horse’s protocol.
Tip
To help prevent EPM, keep water and feed sources clean and as free from contamination as possible. Keep trash covered in secure containers to avoid attracting opossums to your property. Remember though that any horse kept outdoors anywhere can be exposed.
Beware of Ticks! Lyme Disease
Lyme disease is more common in
people and dogs than in horses, but equine veterinarians are seeing more cases
in some areas — most frequently in the Northeastern U.S. This tick-borne
disease affects different areas of the body. Ticks that are infected with an
organism called Borrelia burgdorferi spread the disease to horses
through their bite.
Symptoms
The symptoms of Lyme disease may
include a stiff gait, shifting-leg lameness, fever, swollen joints, lethargy,
and unwillingness to work. Incidents of equine recurrent uveitis (a chronic eye
disease) and neurological problems have also been reported with Lyme disease.
Diagnosis and treatment
Warning!
Lyme disease is tricky to diagnose because current tests can only indicate whether your horse has been exposed to the causative organism, not whether the horse has an active infection.
Because of the difficulty of this
test, Lyme disease is usually diagnosed based on clinical signs and after other
diseases with similar symptoms have been ruled out.
Treatment for Lyme disease is a
regime of tetracycline, possibly given orally and by injection. In some cases,
the disease is never completely eradicated from the body.
Prevention
Tip
As yet, no vaccine for Lyme disease is available. The best prevention is to keep your horse as tick-free as possible by keeping him stabled indoors if Lyme disease is a problem in your area; you can find out by asking your veterinarian. Groom your horse every day as well, keeping an eye out for embedded ticks; we provide details on ridding your horse of pests in Chapter Getting Up to Speed on Routine Care.
A Fatal Neurological Disease: Rabies
Nearly everyone has heard of
rabies, but most people don’t know that horses can contract this disease.
Rabies, which is alive and well all around the U.S., is caused by a virus that
affects the nervous system. It’s transmitted through the bite of an infected
animal. When an open wound comes into contact with infected saliva, the virus
gains entry into the body. Just about any type of wild mammal can transmit the
disease to a horse, including a coyote, bat, or raccoon.
Symptoms
Infected horses may show classic
symptoms such as hypersalivation, seizures, and other neurological problems,
but they can just as often experience lethargy, colic, lameness, muscle
twitching, or even abortion.
Diagnosis and treatment
The symptoms of rabies are
similar to other neurological diseases, such as EPM (which we discuss earlier
in this chapter), the equine encephalitis diseases (discussed earlier), equine
herpes virus (also discussed earlier), and tetanus (which we discuss later in
this chapter), so this disease can be hard to diagnose. In most cases, ruling
out other possibilities is the best way to determine whether rabies is the
culprit in a horse who’s showing neurological symptoms. (A horse who’s
suspected of having rabies must be quarantined.)
Warning!
No treatment exists for rabies in horses. The disease is almost always fatal, and can be definitely diagnosed only with a necropsy (an animal autopsy).
Prevention
Remember
To minimize the possibility of your horse contracting rabies, avoid attracting wildlife to your property by keeping trash containers well secured and keeping grain stores under tight control. But the best way to protect your horse against rabies is to vaccinate her. Talk to your veterinarian about the possibility of adding rabies to your vaccination program. Although rabies may be a relatively infrequent occurrence, the disease’s frequency is increasing. Vaccination is a small price to pay for huge peace of mind.
A Nasty Bacterial Disease: Strangles
Strangles sounds nasty, and it
is. Caused by the Streptococcus equi bacterium, this disease affects the
lymph nodes. Strangles commonly occurs in younger horses between the ages of
four months and five years. Older horses seem to be less susceptible, probably
because their immune systems are more developed.
Strangles is transmitted from
horse to horse, either by direct or indirect contact. A horse with strangles
can contaminate a drinking trough, exposing all other horses who drink from
that same trough. Horses can also pick up the disease from contaminated stalls.
Warning!
The strep bug is a tenacious fellow! The bacterium can survive cold temperatures. In fact, freezing preserves it. It can live for three to four weeks in water that’s been contaminated by nasal discharge. It can also live on wood, glass, and other surfaces for weeks.
Symptoms
Strangles causes fever, lethargy,
and a thin, watery nasal discharge that eventually becomes very thick, and
usually yellow. Horses usually start showing signs of the disease 7 to 12 days
after exposure.
As strangles progresses, it often
causes swelling at the throat area, where lymph nodes are situated between the
jawbone and throat area. Affected horses may develop a harsh cough or noisy
respiration, and a large abscess may form in the lymph nodes and then break
open, draining the thick, ugly pus. Most horses recover, but some develop
secondary infections or complications.
Approximately 15 to 20 percent of horses experience complications of strep infection, such as laryngeal inflammation and paralysis, and chronic infection of the guttural pouches (see Figure 12-2). These horses carry and shed the strep bacteria and may have recurrent episodes of coughing and nasal discharge. Other horses just carry
the bacteria and aren’t sick themselves, but can infect other horses.
Figure 12-2: A horse with an
advanced case of strangles may have a swollen guttural pouch. (Dr. Janice
Sojka, Purdue University)
Warning!
Horses can also experience two very serious complications of strangles.
- One is purpura hemorrhagica, an immune system problem that causes severe inflammation of the blood vessels. Signs include swelling of the legs and hemorrhages on the gums and mucous membranes, and it may occur about three to four weeks after the initial infection.
- The other serious complication is “bastard” strangles, where the strep bug goes to the lungs, liver, kidneys, brain, or any other lymph node, and causes abscesses in the tissues of these areas. If these abscesses enlarge and rupture, the resulting infection and inflammation can be life-threatening.
Diagnosis and treatment
If your vet suspects that your
horse has a guttural pouch infection, he or she will use an endoscope to view
the guttural pouches, take a culture by doing a swab, and flush the pouches
with antibiotics.
Treatment is supportive with
isolated stall rest. The disease will run its course if the infection is mild.
Severe cases with high fever and severe respiratory impairment from the abscess
may require antibiotics, and even tracheostomy (a hole is cut in the trachea
and a tube is inserted so that the horse can breathe through it). Vets disagree
about how to treat the disease in its early stages, however. Using antibiotics
as soon as the disease is diagnosed can prevent the formation of abscess, but
prevent the horse from building a stronger
The serious complication of
purpura hemorrhagica requires an aggressive approach and is considered a
veterinary emergency. Treatment can include antibiotics, steroids to quiet the
immune over-response, diuretics, leg wraps if necessary, and hydrotherapy,
possibly in a hospital.
Prevention
Try to prevent strangles by
isolating all new horses who come to your facility for at least two weeks.
Before you vaccinate your horse,
ask your vet whether he or she recommends doing so based on your horse’s level
of exposure. Although vaccination may not prevent the disease completely, it
may lessen the severity.
Warning!
Never vaccinate a horse who is already showing signs of the disease, or any horse who has had strangles within the previous 12 months. These horses already have a high antibody concentration, and a vaccine could cause the horse to develop purpura hemorrhagica. If you have any question at all, ask your vet if taking a blood sample to check for a vaccine titer, or possibly a nasal culture, is in order.
Warning!
You’d think that with modern science, strangles could be eradicated, but even the cleanest, best-run facilities have strangles outbreaks. Part of the problem is that even though vaccines are available, and horses who have had strangles infections develop some immunity, this resistance diminishes over time.
A Lockup of the Muscles: Tetanus
Tetanus, also called “lockjaw,”
is a serious and scary disease — and horses are particularly prone to it.
Tetanus infection is caused by a toxin produced by the bacterium Clostridium
tetani. This bacterium is commonly found in the intestinal tract of
animals, and the spores can live in the soil for years. That means that it can
be found everywhere in your horse’s environment. Horses are also the most
susceptible of all domestic animals to the tetanus toxin.
So how is tetanus transmitted?
Horses are known for their predilection for injuries, even in the safest
environment. Cuts and puncture wounds allow the bacterium to enter the tissues.
Here, the bacterium releases its potent toxin. These toxins affect the
neurotransmitters in the horse’s spinal cord and brain, and the animal develops
muscle spasms and can’t relax.
Symptoms
The symptoms of tetanus include a
stiff gait, muscle spasms, and protrusion of the third eyelid. Affected horses
often jump violently when touched.
Diagnosis and treatment
A tetanus diagnosis is based on
observing the horse’s symptoms. Depending upon the severity of clinical signs
when the horse develops symptoms, treatment focuses on locating and treating
the wound, neutralizing residual toxin by administering an antitoxin, and
providing nutritional and airway support if the horse’s respiratory system is
affected. Medication is also given to control muscle spasms.
Sadly, even with all possible
intervention, tetanus in unvaccinated horses is always a very guarded to poor
prognosis.
Prevention
Tetanus vaccine is one of the
safest and most effective of equine vaccines. Every horse should be vaccinated
against this disease.
Tip
Horses who are fortunate enough to recover from tetanus don’t acquire future immunity and still require the vaccine yearly.
Attacking the Brain: West Nile Virus
West Nile virus has gotten a lot
of press lately. The disease usually has serious effects on only two mammals:
humans and horses. It attacks the body, infecting the brain and nervous system.
The virus was first detected in
the U.S. in 1999 in Connecticut, and has since spread throughout the country.
It can be an especially devastating disease, with up to 30 percent of infected
horses dying or requiring euthanasia from complications of the disease. Of the
survivors, approximately 17 percent have permanent neurological deficits.
Horses contract West Nile virus
after being bitten by an infected mosquito. Birds are the main host of the
disease. Mosquitoes bite infected birds, and then bite horses, transmitting the
infection to them.
Symptoms
Horses infected with West Nile
virus may experience either mild or severe neurological symptoms, including
- Lack of muscle coordination, stumbling, and weak limbs
- Partial paralysis
- Muscle twitching (especially around the muzzle)
- Hypersensitivity to sight or sound
- Head drooping
- Lethargy
- Falling asleep at inappropriate times, such as while eating
Symptoms usually appear seven to
ten days after the horse is infected. Some horses also run a fever when they
first start showing signs of the disease.
Diagnosis and treatment
Veterinarians diagnose West Nile
virus by testing the horse’s blood serum for antibodies against the virus. No
cure exists, so treatment involves supportive care, such as intravenous fluids
and anti-inflammatory drugs, to help the horse’s body battle the effects of the
organism.
Though not all horses who are
bitten by an infected mosquito develop the disease, after a horse begins to
show neurological problems, the prognosis for recovery is guarded.
Prevention
Remember
Regular vaccination is the best way to protect against West Nile virus. Most veterinarians recommend vaccinating twice a year for optimum protection. Vaccination protocol may depend on the climate and epidemiological status of the disease in different parts of the country.
Discourage mosquitoes from
breeding on your property by getting rid of standing water and by adding
mosquitofish to large water troughs and ponds.
by Audrey Pavia with Kate Gentry-Running,DVM,CVA
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