Is Your Horse at Risk?

Like colds and the flu in humans, horses are at risk for contracting a number of common diseases such as mild respiratory ailments (colds) and flu, along with others that can have mild to devastating consequences. Fortunately, most of these diseases can be prevented or the symptoms reduced through preventive management. Here are the diseases you should watch out for.

Respiratory Infections

By far, the most likely infections a horse might get are respiratory diseases. "They are still the number one problem as far as contagious diseases go," says Robert Hertzog, DVM, of Lee's Summit Animal Clinic in Lee's Summit, Mo.; American Veterinary Medical Association executive board member; and veterinarian for the American Royal Horse Show in Kansas City, Kan. "It's an ongoing, year-round problem."

Of these, influenza and Streptococcus equi (strangles) are frequent assailants. Horses most at risk are those exposed to transient populations, those stressed by shipping or heavy training schedules, youngsters, and seniors.

Although not usually life-threatening, influenza makes a horse uncomfortable and vulnerable to other diseases. Clinical signs include fever, lethargy, cough, nasal discharge, muscle aches, and inappetence. Treatment includes a minimum of three to four weeks of rest to avoid development of long-term respiratory problems (for every day of fever, stall-rest the horse for one week; see Ask the Vet on page 76), antibiotics if secondary bacterial infection is suspected, drugs to control fever and aid in airway clearance, palatable feed, and housing in a low-dust environment.

Josie L. Traub-Dargatz, DVM, MS, Dipl. ACVIM, professor of equine medicine at Colorado State University, explains that influenza is spread by horses shedding the virus by aerosol or by fomites (any objects capable of mechanically transporting an infectious agent). Keep in mind that an infected horse can in some cases continue to shed the virus and be contagious for more than a week after the fever has broken. She notes that exposure needs to be fairly immediate, as the virus is short-lived in the environment. Once the horse is over the disease, he no longer sheds the organism.

It's a different story for strangles. This bacterium can live in the environment for weeks under ideal conditions and can be shed for months, even a year, by a small number of recovered horses. Says Traub-Dargatz, "Horses can look normal, but still shed this bacterium. It's been hypothesized that's how this disease moves around."

Clinical signs include cough, purulent (pus-like) nasal discharge, fever, decreased appetite, and abscesses in associated lymph glands, reports Stacy B. Smith, DVM, of Smith Equine Service in Canyon, Texas. In more serious cases, internal abscesses can occur in other areas of the body.

"Diagnosis is confirmed by bacterial culture (or polymerase chain reaction, or PCR, examination) of nasal swabs, transtracheal washes, and swabs of abscesses," says Smith. "Treatment consists of penicillin and non-steroidal anti-inflammatory drugs (NSAIDs) to control fever; occasionally a tracheotomy is necessary if severe swelling of submandibular lymph nodes causes respiratory distress. All halters, leads, waterers, feed buckets, etc., need to be disinfected to prevent contamination and spread of the disease."

Note that treating strangles with antibiotics is controversial, since it is debated whether antibiotics just keep the disease condition at the status quo so that abscesses don't mature and burst and don't shrink and go away. Also, the form that the bacterium takes on in an abscess might make it difficult for the antibiotic to have an antimicrobial effect. Traub-Dargatz notes, "Some clinicians do not treat with penicillin if the abscess(es) are limited to the head region and are close to rupturing when the disease is recognized in a given horse."

Reduce the spread of strangles by isolating affected horses from healthy ones for several weeks after all clinical signs of the disease have resolved; quarantine new or traveling horses from resident populations (those that don't travel); screen bacteriologically or by PCR examination to determine any asymptomatic carriers, and keep at-risk horses current on strangles and influenza vaccinations. Adds Traub-Dargatz, "It may be appropriate to test that shedding of Streptoccocus equi has resolved prior to moving or introducing new horses."

Skin Irritation

Ringworm and rain rot are skin diseases transmitted by direct contact with an infected horse, often among horses which share grooming equipment, tack, and blankets. The diseases are enhanced by broken or damaged skin (from insect bites, trauma, poor grooming, etc).

Ringworm (dermatophytosis, also called dermatomycosis) is a fungal disease typified by circular patches of hair loss, reddening, scaling, crusting, and itching. Lesions commonly and initially affect areas where tack contacts the skin, then can spread to the head, chest, flanks, and croup. Spontaneous recovery can occur in five or six weeks, although lesions could be prolonged if secondary infection is present. Anti-fungal topical treatments can aid recovery. Infected stalls and woodwork should be treated by pressure-hosing or using bleach, and you should disinfect tack, grooming tools, and other objects that have come into contact with the infected horse.

Moisture and damaged skin are the two agents needed to produce rain rot or rain scald (dermatophilosis, streptothricosis). Clinical signs include crusty or scabby skin, matted hair, hair loss across the back and trunk, reddened skin beneath scabs, and pus. Treatment depends on severity; mild cases can resolve spontaneously in drier weather, while severe cases might require systemic antibiotics. Horses can be treated topically with 1% povidone iodine shampoo for 10 days to aid healing. In all cases, affected horses should be protected from rain, biting insects, and prickly vegetation.

"For skin diseases such as ringworm or rain rot, horses that are parasitized or receive poor nutrition seem to succumb first," Hertzog states. "Their immune systems are not the strongest."

Minimize risk by isolating affected animals, reducing the horse's exposure to moisture, maintaining healthy skin via fly control and good grooming, providing good nutrition and regular deworming, and avoiding shared equipment.

Little Bites Can Cause Big Problems

The mosquito-borne disease making big news is West Nile virus (WNV), which can cause serious and frequently fatal inflammation of the spinal cord and brain in up to a third of infected horses. Says William Saville, DVM, PhD, Dipl. ACVIM, extension epidemiologist/large animal internist in the Department of Veterinary Preventive Medicine at The Ohio State University, "Based on data from 2001, the greatest risk of disease is in horses aged 25 years and older, probably due to immune suppression.

"Neurologic signs include weakness, depression, and loss of coordination (ataxia)," says Saville. "No specific cure exists. Treatment is supportive therapy."

Anti-inflammatories can be administered, but prevention is the best protection. "Utilize an integrated mosquito control program on your farm," Saville recommends. "Keep horses in during times of highest mosquito activity (dawn and dusk), use fans on them to keep mosquitoes off of them, and remove standing water on your property--buckets, barrels, wagons, tires--so mosquitoes don't have a place to breed."

Vaccinating against WNV is usually recommended; the challenge study done by Fort Dodge Animal Health, makers of the vaccine, showed that the vaccine was efficacious (up to 94%). Current vaccination protocol, says Smith, consists of an initial series of two vaccinations with follow-up boosters as recommended by your veterinarian.

"The horse is not considered protected for at least two weeks after the second shot," she says.

Touchy Tummies

Salmonellosis is one of the most common infectious causes of diarrhea in mature horses.

"Salmonella (bacteria) are passed from animal to animal or through environmental contamination," says Traub-Dargatz. "Horses can acquire the infection by ingesting the organism in feed or water or by contact with surfaces such as a caretaker's hand that is contaminated by infected fecal material. The bacterium may be shed in feces by affected and recovering horses for as little as a few days or up to a few months."

Clinical signs are severe watery or bloody diarrhea, weight loss, inappetence, fever, or lethargy. Treatment consists of supportive therapy to replace fluids and electrolytes and isolation for up to six weeks. Contaminated stalls should be disinfected.

Salmonellosis in horses occurs sporadically or in outbreaks, Traub-Dargatz says. Salmonellosis is the leading cause of hospital-acquired infections in North American veterinary teaching hospitals.

These are some of the most common ailments. Keep an eye out for them so that if and when they occur, you'll recognize and begin treating them early to minimize your horse's distress.

PREVENTIVE MEASURES

Protecting Horses

Vaccination remains the best defense against many infectious diseases. However, risks are not equal. Where the horse lives, what he's used for (lawn ornament versus show), and his exposure to new populations of horses affect the decisions on what he should be vaccinated against. In other words, a vaccination program should be tailored to the individual.

Rob Keene, DVM, field service veterinarian for Fort Dodge, suggests that owners ask:

How is the disease spread?--Some diseases, such as influenza, rhinopneumonitis, and strangles, require direct horse-to-horse contact. If your horse does not travel or see new populations of horses, then he is at low risk; but if you're campaigning a horse or your horse is at a facility where other horses come and go, then his risk to exposure increases. Diseases such as tetanus, equine protozoal myeloencephalitis, West Nile virus (WNV), Eastern equine encephalomyelitis (EEE), and Western equine encephalomyelitis (WEE) can afflict a horse without him ever leaving the property or being exposed to another horse; in areas of the country where the disease is present, all horses are at risk.

Is the disease life-threatening or does it cause permanent damage?--Strangles can be fatal, as can tetanus, rabies, and the encephalomyelitides. Foregoing vaccinating against serious and fatal diseases is gambling with your horse's health and his life.

Is the disease difficult or expensive to treat?--"If you have a disease that is 100% treatable, inexpensively, and doesn't result in any long-term consequences for the horse, there may be a vaccine that you want to eliminate from the protocol," Keene says. Other diseases, like EPM, are expensive to treat.

Because not all diseases are present in every part of the country, along with individual factors that will influence exposure, owners should work with their local veterinarians to devise appropriate vaccination programs. That said, Keene recommends a preventive program that at least includes vaccinating against tetanus, WEE, EEE, WNV, and strangles. Depending on the area of the country, some veterinarians might add the rabies vaccine to the protocol.

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OPERATIONS' VACCINATION OF HORSES Numbers show the percentage of operations vaccinating against these diseases.
VACCINATION AGAINST	12 MONTHS OLD OR LESS	BROODMARES	OVER 12 MONTHS
Influenza	46.5%	61.2%	63.0%
Streptococcus equi (strangles)	13.0	14.0	13.3
Herpesvirus (rhino)	28.0	54.9	42.8
Potomac Horse Fever (PHF)	4.0	11.0	18.0
Rabies	10.3	20.3	24.5
Encephalitis	46.3	57.2	63.2
Tetanus	63.0	69.7	70.4
Clostridium perfringens (C & D)	0.1	0.8	1.0
Equine viral arteritis (EVA)	0.4	2.5	1.8
Rotavirus	0.1	4.8	2.3
Leptospirosis	0.9	2.8	2.5
Botulism	0.5	0.5	0.6
Other	0.0	0.0	0.3
Any	64.1	73.5	74.7

Source: National Animal Health Monitoring System '98 Equine Study

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One night you hear kicking and crashing coming from the barn and think you have a cast horse. You run to see if you can help, and what you find is a horse down on his side paddling as if galloping on the ground. His head and neck are extended and his eyes are slightly rolled back in their sockets, showing more white than normal and moving back and forth in a quick side-to-side motion. The horse does not respond to touch or sound and is oblivious to his surroundings. You can do nothing to help except to stay out of the way and ensure your own safety. The thrashing stops after 10-15 minutes that seem like hours. The horse stays down, is depressed, has a decreased response to stimulation, and acts somewhat blind. You call your veterinarian, who arrives and confirms that your horse had a seizure.

What I have described is the classic "grand-mal seizure." This article will discuss the various types of seizures and the disease states that can cause them.

What's a Seizure?

The most common seizure disorder is epilepsy, which is fortunately rare in the horse (it is actually controversial if "true" epilepsy actually occurs in the horse). There is little that can safely be done to help a 1,200-pound animal having a seizure.

To understand seizures, we need to review anatomy and physiology. Seizures occur when specific anatomical locations of the brain (forebrain and thalamus) are affected by some type of pathology. By definition, a seizure (also known as a fit, ictus, or convulsion) is considered abnormal behavior; seizures are physical expressions of abnormal electrical discharges in the forebrain neurons that initiate spontaneous, paroxysmal (spastic), involuntary movements.

The seizure event is broken into three phases--the pre-ictal, ictal, and post-ictal periods. Many people who suffer from epilepsy know they are going to have a seizure just before it happens. There might be a strange feeling or focal muscle twitching. The ictal phase or the seizure can be as mild (only muscle twitching), or it can be a full grand-mal seizure. Typically the ictal phase only lasts 10-20 minutes unless there is an ongoing factor (such as a seizure occurring as a result of a severe allergic reaction) that prolongs the event. The post-ictal phase can last minutes to hours and generally consists of depression, an out-of-touch feeling with your surroundings (often called the "aura" in people), and blindness. The blindness is temporary and can last from a few hours in adults to a few days in babies.

What Causes a Seizure?

The pathology that causes the abnormal electrical discharges of a seizure can be varied and includes genetic abnormalities in the brain tissue (epilepsy), structural defects (tumors), traumatic insults, metabolic insults, and toxic insults.

The syndrome of inherited, recurrent seizures that continue throughout life without an underlying cause (disease process) is "true" epilepsy. Technically this has not been demonstrated in the horse, but it can be difficult to follow genetic lines in some cases and to diagnostically rule out all other potential causes in older horses.

What has been called "benign" epilepsy is a seizure condition of foals up to 12 months old and occurs in all breeds, but has a higher incidence in Arabians. If a seizure occurs in very young foals, it is very important to rule out other causes, as seizures in foals can be caused by various diseases (such as low blood sugar, septicemia, and hepatitis) that require specialized treatment. If there are no other abnormalities and the diagnosis is "benign seizure disorder of foals," the news is good as they typically outgrow the problem.

Adult horses have been reported to develop seizure disorders following a lightning strike.

Any structural abnormality, i.e. tumor, can cause seizures if it's in the correct location within the brain. A common tumor that grows in the forebrain (front section of the brain) of the horse is called a cholesteatoma. These are slow-growing tumors of the older horse and are frequently found incidentally at necropsy in horses that were neurologically normal, although they can cause seizures and other neurologic abnormalities. Unfortunately, the diagnosis can be difficult and expensive to confirm, requiring an MRI or CAT scan.

Another common brain tumor affecting the horse is a pituitary adenoma, which in addition to the familiar signs of Cushings (long, non-shedding hair coat, founder, excessive thirst and urination, etc.) can occasionally cause blindness, but not typically seizures because of the location of the pituitary gland.

Another cause of seizure is the accidental injection of various drugs into the carotid artery in the neck when attempting an intravenous (IV) injection. This is one reason why it can be very dangerous to attempt IV injections unless you have been trained by a veterinarian in the correct procedure. Within the neck just beneath the jugular vein is the carotid artery, which carries oxygen-rich blood from the heart directly to the brain. Therefore, anything injected into the carotid goes directly and relatively undiluted to the brain. The intensity of the reaction can vary from a mild seizure to instantaneous death or prolonged coma, depending on the type of drug and how much of it went into the artery.

Another reason for untrained people to stay away from IV injections is that in addition to the vein and artery, there is a nerve bundle in close proximity. If caustic drugs such as phenylbutazone are injected outside the vein, they can cause nerve damage that can lead to paralysis of the larynx on the right side of the neck. For most situations involving drug injection into the carotid artery, there is no specific treatment. So, it is best to avoid the situation altogether and leave IV injections to your veterinarian.

There is also a variety of metabolic disturbances that can cause seizures. For example, altered concentrations of sodium in the blood can affect the electrical activity of the brain. A situation leading to this is when a horse has not had access to water for a few days--be sure to check the heated water tanks and automatic waterers in winter. Also, as the horse becomes progressively more dehydrated without loss of electrolytes (as when sweating), the blood concentration of sodium increases (hypernatremia).

The real trouble starts when the dehydrated horse is discovered and allowed free access to water--you do not want this horse to be rehydrated quickly. In this case, the brain tissue is slowly compensating for the increasing sodium concentration in the blood to maintain normal "electrical" function. If the dehydration is rapidly corrected, the blood concentration of sodium is rapidly lowered, but it takes more time for the brain to adjust. Now the brain has a higher sodium content than the blood.

Going back to a little basic chemistry, the osmotic (water) balance is mainly determined by the sodium concentration, with water typically going where sodium concentration is higher along the osmotic gradient. So water moves into the brain, causing cerebral edema (fluid swelling) and potentially seizures.

Very low blood concentrations of sodium (hyponatremia) can also cause seizures. This is most commonly observed in foals with severe kidney (renal) disease or water overload (water toxicity). Sometimes very young foals discover water and for whatever reason decide to drink enormous quantities, which can affect their electrolyte/water balance.

Low blood concentrations of calcium (hypocalcemia) can be associated with seizures. If your horse's diet is not properly balanced with an adequate amount of calcium, he can go into a hypocalcemic state. Equine diets with little roughage and a lot of grain are typically low in calcium.

Another cause of seizures to consider is toxins, including organophosphates, chlorinated hydrocarbons, strychnine, lead, bracken fern, locoweed, and tetanus. Salt can also be a toxic element when eaten in extremes. Free-choice salt is mandatory for the good health of your horse, but every once in a while you have a horse that digs right in and polishes off a salt block in a few minutes. You have to take note and limit salt block access with such a horse. This situation could lead to a hypernatremic state and cause seizures by affecting the osmotic/water balance as previously discussed.

Treatments

If the seizures are being caused by a specific disease or toxicity state, then treatment will focus on that and the seizures will resolve with the problem. If they are mild and/or infrequent, your veterinarian might elect not to directly treat them unless they become more frequent or dramatically worsen.

The common treatment for seizures when the primary cause cannot be treated is the drug phenobarbitol (a powerful sedative). The response to this drug is variable; the goal is to figure out a daily dose that will prevent the seizures without sedating the horse to a point where he has a poor quality of life (a very fine line in some cases). With over-sedation, a horse can become so lethargic that it is hard to get him to leave the stall. Another difficulty with therapy is that the drugs become less effective over time, requiring adjustment of the dosage.

Other human anti-seizure drugs have been used on horses, but have difficulties associated with expense, maintaining an effective dose, and toxicity. In addition, these drugs and phenobarbitol can cause liver disease, so frequent monitoring of liver function is necessary.

Dangers to Humans

Another aspect of having a horse that seizes in the barn is that they can be extremely dangerous to be around. They can become quite violent and have no control over a flailing 1,200-pound body, and neither do you. The safest thing to do is to get out of the stall and shut the door. Depending on where they are when having a seizure, the horse can bang himself up quite badly, and it can be very alarming to watch.

Some owners maintaining a horse who has seizures will heavily pad the stall in an effort to limit self-injury. The seizures typically happen unpredictably, but some owners can tell when one is approaching by subtle behavioral changes in the pre-ictal phase (often only minutes before the actual seizure). Every case is different, but caring for a horse that has seizures can be very taxing and place you in a situation with some liability should he hurt someone while having a seizure.

Take-Home Message

Seizures are not normal--if your horse has one, contact your veterinarian immediately. There are causes that can be reversed, and disease processes that can be treated. Above all, try to protect yourself and your horse from injury during and after the seizure.

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I’m interested in purchasing a horse which has a history of seizures. What could they be caused by, are there different forms, and is there anything I can do to prevent them from happening again?

First of all, you would need a veterinarian’s assessment of the horse and the predisposition to seizures. The history of the individual horse is extremely important. A horse, like any other animal, can have a true or a false seizure. For example, a dog with an ear infection can be put on a table, and the animal might fall over, presenting what looks like a seizure. Horses with an inner ear infection can look the same, with possibly a loss of balance, while retaining their normal strength. Narcolepsy, characterized by uncontrolled episodes of sleep and loss of muscle tone (cataplexy), can look like a seizure as well. The horse appears as if it is passing out and falling down. A true seizure deals with electrical activity within the cerebral cortex of the brain.

The problem with diagnosing seizures is that a veterinarian usually isn’t examining the horse while it is having a seizure. It is rare that the veterinarian actually sees the seizures, and this usually happens only if the onsets are frequent or perpetual.

The causes of seizures are quite varied. Horses do get epilepsy. Some seizures are a metabolic reaction, characterized by convulsions, which are of a short duration. The horse needs oxygen and glucose to conduct electrical activity in the brain correctly. Neither is stored in the brain. The horse might be hypoglycemic (low blood glucose), resulting in a seizure.

It is not entirely uncommon for foals to have seizures from metabolic imbalances. The foal can become hypoxemic, suffering lack of oxygen to the brain, and respond with a seizure.

Kidneys are the filters of the body. There might be a kidney disorder in the horse that prevents the animal from retaining the electrolytes for the brain. Other causes of seizures might be an invasion of infections in the brain (meningitis, for example), bacteria, viruses, or protozoal myelitis. Head trauma might cause excessive hemorrhage to penetrate the brain, thus spurring a seizure.

Masses in the brain, such as tumors, disrupt the normal electrical activity of the brain and can trigger seizures. Abscesses have been known to form in the brain as a result of strangles (Streptococcus equi) and have the same potential effect.

Toxicities and reactions to drugs can cause seizures. For example, a reaction to procaine penicillin, lead poisoning, or other heavy-metal poisonings can trigger seizures. Occasionally you will hear about a horse which, upon receiving an injection, responded with a seizure. This usually is caused by the injectable’s mistakenly entering the carotid artery. Moldy corn and locoweed toxicity also can cause seizures.

The seizure itself can be perpetual, sustained, or intermittent. Seizures are predominantly intermittent, and rarely sustained. A spinal fluid sample or bloodwork will alert the veterinarian as to whether there is inflammation, infection, or electrolyte imbalances.

Radiographs give practitioners the ability to see old skull fractures. The majority of epilepsy cases stems from previous trauma to the brain. MRI scans and CT scans for the head are largely unavailable because of the sheer size of the animal’s head and the costs involved in these procedures; however, foals are small enough to undergo these diagnostic tests.

An electrical encephalogram (EEG) can be used to view the brainwaves or electrical impulses of the brain. While not used to diagnose the cause of a seizure, EEGs readily can confirm activity that causes seizures. The Ohio State University is one of the few places a horse can undergo an EEG exam.

The prognosis of the horse with seizures will vary, depending on the cause of the seizure. A young foal’s prognosis normally is good, as the foal usually will outgrow the symptoms and go on to perform normally. There are many things to consider in the prognosis of an adult horse affected by seizures. You must consider the danger to the people working with the horse; caution should be taken, as a 1,000-pound animal in a seizure can inflict serious harm to his handlers and himself.

Prognosis for horses which have seizures as a result of viral infections, such as herpes, is dependent upon the severity of the infection. Treatment only allows the owner to maintain the horse, not "cure" him.

A horse affected by a bacterial cause of seizures can be treated with antibiotics. The bacteria are detectable in the blood/spinal samples, and the respective medication can clear up the infection, whether it be bacterial meningitis or something of a protozoal nature. Fungi within the brain are rare. Prognosis is normally poor, as it is very difficult for anti-fungal drugs to reach adequate levels in the deep crevices where the fungi reside in the brain. Anticonvulsant medications might manage seizures in some horses.

Surgery, whether to drain the abscess or remove a mass in the brain, is very uncommon. Although skull trauma might fracture and dislodge a piece of bone in an area where a surgeon can remove the chip, fractured parts usually are not accessible for the surgeon to reach.

Once again, diagnosis and prognosis of the horse with seizures are highly dependent on the background of the horse. Delve deep into the horse’s history--daily habits, previous injuries, and living situations—and make sure an experienced veterinarian knows these facts. It remains unnerving to witness a seizure in a horse, but there is hope in understanding, managing, and solving the problem.

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