Strangles is a bacterial infection of the respiratory tract that has been with us for close to 800 years by historical reports, likely longer. It is caused by a bacterium called Streptococcus equi.
If you are a lover of horse books, or were fortunate enough to have The Red Pony included in your required middle school reading, you're familiar with John Steinbeck's heart-wrenching description of how this disease can literally strangle the life from a horse. Only very severe cases pose this kind of immediate threat to the horse's life. But, at best, strangles can rob you of several weeks of work, and, at worst, it can lead to complications that may claim your horse over the weeks, months or even years following the infection.
The rather descriptive name, strangles, comes from the fact that after invasion of the upper respiratory tract (nasal passages), the infection next seats itself in the lymph nodes of the head, where it causes abscesses to develop in the nodes between the lower jawbones and in nodes located in the back of the horse's throat. The abscessed lymph nodes can become large enough to interfere with the free movement of air into the lungs.
Strategies for Controlling Strangles
- An internasal vaccine that protects against strangles is effective and can help horses at high risk, but beware of possible side effects.
- Watch for fever followed by a thin, clear nasal discharge, which are the first signs of strangles.
- Quickly isolate infected horses to avoid spread of the bacteria.
- Keep your vet involved at every stage.
- Antibiotics, usually intramuscular penicillin, should be administered at the first sign of fever.
The first symptom of infection is a high fever, which appears a few days to two weeks after the horse picks up the organism. This can easily be missed because no other symptoms have yet appeared.
A thin, clear nasal discharge usually develops next, which changes rapidly to a thick, white/yellow consistency characteristic of pus. Swelling/edema starts to develop under the jaw and around the back of the jawbones. Lymph nodes enlarge to the point of obvious ball-shaped lumps between the lower jawbones. The area from the base of the ear and along the back of the jawbones also often swells.
The interior of the horse's nose and throat are reddened from inflammation, and swallowing may be painful. The eyes may also be red and inflamed, with a yellow discharge. Food or water may come out the nose when the horse tries to eat or drink. Since swallowing is painful and difficult, many horses show a sharp drop in appetite and are, understandably, obviously depressed.
The infection in the lymph nodes causes abscesses to develop inside these glands. The abscesses eventually rupture and drain, which can take anywhere from a few days to a few weeks. There is usually little to no cough, although a cough can often be induced by squeezing the horse's larynx ("voice box") or upper trachea ("windpipe").
A strangles infection may find its way into the lungs and cause pneumonia. This happens either when pus from the throat gets into the trachea, or if internal lymph nodes in the lungs rupture into the chest.
If a cough develops, the infection may have spread to the lungs. Otherwise, symptoms of pneumonia include difficulty breathing, noisy breathing, and very rapid, shallow breathing. You'll see similar symptoms if the horse is having trouble breathing because of large lymph nodes and swelling interfering with airflow. A horse that develops any trouble breathing needs to be checked by your vet right away.
Another common complication involves the guttural pouches. These two large sacs, one on each side of the head, are outpouchings of the Eustachian tube, which runs between the ear and the throat. They usually become infected when a nearby internal lymph node abscess opens and drains into them.
Persistent guttural pouch infections, which can occur in about 10% to 20% of strangles cases, can be an ongoing source of bacteria for both the horse and the environment. Pus that does not effectively drain from the pouches becomes hard and dried over time, forming chondroids, which usually contain live organisms. Liquid pus or chondroids in the guttural pouches often leads to a chronic nasal discharge that will persist after other infected horses have stopped showing symptoms.
The organism may also spread through the lymphatic system and cause internal abscesses, so-called "bastard strangles." If it reaches lymph nodes deep within the chest, abscess formation can compress the airways. Abscesses may form in any part of the body and in any organ, even the brain.
Horses with internal abscesses may seem normal except for periods of unexplained symptoms (such as colic with abdominal abscesses) or may be "poor doers." Some recent cases directly involved the skeletal muscles and caused extensive damage.
A very serious complication is called purpura hemorrhagica, caused by antibodies bound to portions of the bacteria clogging small blood vessels. The signs are small hemorrhages along the gums and edema of the legs. Skin swellings may also appear elsewhere on the body, and the joints may be involved. The skin may ooze serum or even break open. Laminitis may occur. Large areas of muscle, intestine or other organs may be damaged because their blood supply is interrupted.
Purpura, in some outbreaks, appears only in a few animals, but in other outbreaks it may affect almost half the horses to some extent. This is a very serious complication and can lead to death or to the horse having to be euthanized.
Strangles can also damage critical nerves in the head. The recurrent laryngeal nerve supplies the muscles of the larynx, which control opening and closing of the vocal cords. This nerve can be damaged by pressure or inflammation from nearby abscesses. Other neurological complications, such as facial nerve paralysis, have also been seen.
How Common is Strangles?
In 2000, the National Animal Health Monitoring System, a division of the USDA, published the results of a year-long survey of 1,034 non-racing equine operations housing three or more horses. Overall, an average of 4.6% reported having at least one horse with strangles over the monitoring period, and about 20% of all upper respiratory tract infections were strangles. Cases were more common in the spring, likely a reflection of horses and riders being more out and about, in contact with other horses.
These figures will be modified greatly by your individual circumstances. If your horses never have contact with other horses, your risk drops to near zero, while if you keep your horse in a busy barn with a lot of traffic on and off the premises, your risk of exposure is much higher.
Although strangles is a bacterial disease, traditional wisdom was to forego the use of antibiotics for fear that the antibiotics would slow the maturation of the abscesses and increase the risk of spreading the infection beyond the head.
However, there is really no proof that these concerns are valid. Given the serious nature of potential complications from strangles, more and more vets have decided that it makes more sense to give antibiotics than to withhold them.
The American College of Veterinary Internal Medicine agrees and has issued a consensus statement that recommends antibiotic treatment for exposed horses at the first sign of fever and any horse obviously ill from strangles or with complications. According to the recommendation, the only time antibiotics may not be needed is in horses that do not appear to be systemically sick (such that they're bright and eating well) but are discovered to be bringing up abscesses. In those cases, it's reasonable to allow the abscesses to mature and drain without antibiotics.
Intramuscular penicillin is still the antibiotic of choice for strangles. Some newer, much more expensive antibiotics can also work, though not better. Trimethoprim sulfa combinations (Tribrissen, Uniprim, Bactrim) are popular because they can be given orally, but they can't measure up to penicillin.
Horses with early infections and only showing fever are usually treated for five to seven days. If strangles is caught this early, you may be able to completely prevent the development of abscesses. Horses with more advanced disease usually receive treatment for at least two weeks, and for several days after their temperatures have returned to normal.
If the guttural pouches have been infected, treatment by flushing the pouches through catheters placed in the pouches endoscopically will be needed. In some cases, only surgery can remove the material trapped in the pouches.
If the horse develops purpura hemorrhagica, corticosteroids (e.g., Dexamethasone) are also given. Corticosteroids also interfere with the immune system, but, in this case, that interference (and control of inflammation) is needed to try to protect the horse from purpura complications. Your vet will work hard to find a dosage and dosing schedule of corticosteroids that controls the complication with as little negative effect on the immune system as possible. Penicillin must be continued while the horse is on corticosteroids.
Laminitis can result from purpura, in some cases so severe that the horse is eventually euthanized. Be proactive. A trim may be the last thing on your mind if your horse is battling strangles, but if your horse's feet need attention and are not at their mechanical best, the consequences of the foot inflammation will be more severe.
Ask your vet to show you how to check for the strength of pulsations in the arteries supplying the feet because a change is an early sign of foot inflammation. It's also a good idea to have the horse on a deep, supportive footing, such as peat or sand, or have a supply of construction-strength Styrofoam on hand to use as a support for the bottom of the feet if laminitis starts to develop.
A common misconception is that the strangles bacteria can survive for a long time in the soil or on surfaces like walls or fencing. Although the bacteria have been found to survive for up to about two months under cold conditions in a laboratory setting, there is no evidence they can do so in stalls or in fields, where competition from other bacteria may do away with them in short order.
Horses become infected by coming into contact with other horses that are shedding the organism in their nasal secretions or through ruptured abscesses, or by contact with things that have been recently contaminated by those secretions.
While it takes from a few days to two weeks after a horse is first exposed for the bacteria to cause a fever, this is also the start of the horse's immune system response. At this point, the newly infected horse is not yet shedding any bacteria. It's therefore very important during an outbreak to frequently check the temperature of all the horses so that new cases can be isolated from horses with normal temperatures.
Once horses begin having bacteria in their secretions, they will continue to shed organisms for two to three weeks. This is also how long it takes their immune system to mount a full response. Thus, the horse may seem to be recovered (especially if on antibiotics) but may still be shedding bacteria. This is why it is usually recommended that nasal cultures be done before a horse is allowed to come into contact with uninfected horses again.
An estimated 10% of strangles cases become chronic carriers, shedding the bacteria either constantly or intermittently. This is most likely to occur if infection becomes seated in the guttural pouches. Farms that have a reputation for repeated problems with strangles probably have one or more chronic carriers on the premises.
Any horse that is taking longer than three weeks to clear up or seems to recover but later has periods of nasal discharge should be suspected of a persistent infection in the guttural pouches. However, it's also important to realize that a horse can be a chronic carrier and show no outward symptoms at all, including no fever.
Strangles outbreaks usually don't involve every horse, and not all infected horses will show the same level of symptoms. Dosage of organisms received has something to do with this. Horses with direct, nose-to-nose contact with another horse shedding the organisms is going to get a higher dose than a horse with a less concentrated exposure from organisms contaminating equipment or turnouts.
Young horses with no prior exposure that have had no priming of their immune system are most likely to get strangles and to have the most severe symptoms. Horses that have had it before, or at least have been exposed to it, will usually have some immunity. How well a horse's immune system will respond to an exposure also depends on his nutritional status and level of stress.
Natural Immunity and Vaccination
Following a natural infection, at least 75% of horses develop a strong immunity that can last for at least 10 years. Immunity does wane with time, and can be lost in older horses.
Stress, such as long transports, heavy exercise, change in location, introduction to new horses, and other factors, can temporarily weaken a horse's immune response, making it especially important to guard the horse from close contact with others during those times. Inadequate intake of nutrients critical to strong immune system function (e.g., protein, vitamin E, zinc, copper) may also compromise the horse's immunity. These factors likely explain why 25% of horses don't develop strong immunity after an infection, and why previously infected horses can sometimes be infected again.
Two types of vaccination are available-intramuscular injection and intranasal.
The intramuscular vaccines produce circulating antibodies, but do not reliably protect from disease because they do not generate antibody protection where it counts the most-at the level of the respiratory tract lining. Purpura hemorrhagica reactions can occur after vaccination in horses that have high circulating antibody levels from a prior exposure. Severe vaccine site reactions can also occur.
Because of the side effects and poor protection, intramuscular vaccines are no longer widely used. Sometimes they are recommended for broodmares that will be foaling in high-risk situations because the vaccines will produce good antibody levels in the mare's milk and these colostral antibodies do protect young foals.
The intranasal vaccine uses live bacteria that have been modified to make them less dangerous. These vaccines, which provide superior protection, mimic a natural infection but with few symptoms. There have been rare reports of horses actually developing respiratory tract abscesses after receiving this vaccine, but most horses show only a slight clear nasal discharge or no reaction at all.
However, the organism can cause abscesses elsewhere on the body if there is any break in the skin, including a hole from a needle. So it is important never to give any injections to a horse receiving intranasal strangles vaccine. The vaccine should also not be used on horses with open wounds. There is also a risk of a purpura hemorrhagica reaction to the vaccine if the horse already has high levels of circulating antibody.
Because of the potential for reactions, strangles is usually not on the list of "routine" vaccinations. Your vet is the best person to help you weigh risks and benefits in your particular situation.
Some general vaccination guidelines are:
- Do not vaccinate during an outbreak.
- Do not vaccinate animals that are in poor health, running a fever, or showing any respiratory symptoms.
- Do not vaccinate horses with a history of having had strangles, or exposure to strangles, within the past two years without checking antibody titers, specifically antibody to the M-protein (ELISA antibody test). Horses with titers over 1:3,200 should not be vaccinated because of a high risk of purpura.
Tips for Avoiding Infection and Spread
- Do not allow new horses to come in contact with the others for at least two weeks. This includes direct contact on turnout as well as using the same paddocks and equipment.
- Take daily temperatures on new horses for two weeks.
- Reserve one or more stalls or paddocks for use only by new horses.
- Have a plan in case a horse gets strangles and you need to isolate him. You'll need a paddock as far as possible from the other horses or a separate building. If that isn't possible, at least put sick horses at one end of the barn (downwind from ventilation) and hang a blanket curtain.
- It's desirable to also have a separate location for horses that spike a fever but don't yet have any other symptoms. For the first 24 to 48 hours after fever starts, these horses will not be shedding any organisms. Identifying them quickly and keeping them out of contact with horses that have normal temperatures will prevent further spread. If discharges develop, move them to the obviously sick group.
- Never close up the barn tightly. This concentrates infectious organisms in the air and leads to buildup of dusts, molds and fumes that irritate the respiratory tract of sick horses and can increase the infection risk for others.
- Before you send your horse to a trainer, ask what infectious disease precautions are in place. Requiring vaccinations for incoming horses isn't enough. No vaccine is 100% effective. If the facility doesn't have routine isolation and monitoring procedures for new horses, look elsewhere.
- If stabling away from home, inquire about whether stalls are routinely disinfected between horses. Same if shipping your horse with a commercial shipper.
- Don't let your horse eat from the ground at show grounds, campgrounds, vet clinics, ride rest stops, or alongside heavily traveled trails. Never use public/community watering troughs.
- Don't let your horse touch noses with strange horses.
Care of Infected Horses
Your veterinarian needs to be actively involved from the start and every step along the way, beginning when you even suspect that your horse has strangles or has been exposed. Your vet will prescribe the correct medications and instruct you on how to use them.
There are other things you can do to maximize your horse's comfort and chances for rapid recovery.
Diet: Sick horses usually have poor appetites. When you combine this with a swollen, painful throat, it only gets worse. Even if the horse seems hungry initially, the pain of trying to swallow dry and/or hard meals may cause him to stop eating. However, good nutrition and adequate fluid intake are critical for fighting the infection and recovery.
Grass is the easiest thing to chew and swallow, and also the most appealing. Grazing is fine, but remember that anywhere the horse goes he may be dropping organisms. So only graze in areas with no risk of another horse coming along and being infected. An option is to pick/cut grass for the horse.
If grass isn't available, soaking hay in hot water will make it softer and easier for a horse to chew. You can also substitute soaked hay pellets or cubes for loose hay. Experiment to find the consistency the horse likes best.
Sick horses often refuse grain, but may accept pelleted feeds soaked to a mash consistency. Other mash possibilities are 50:50 beet pulp and oats or beet pulp with either 2 ounces of rice bran or 4 ounces of ground stabilized flax or wheat bran added per pound of beet pulp. These choices are all fairly well balanced for major minerals. You could also try soft meals made from pelleted complete feed, but try to keep to products with ingredient lists similar to what the horse usually gets.
It's also important to get 2 tablespoons of salt into the horse every day so that he will drink normally. Divide the total daily amount between feeds.
Feeding at ground level encourages drainage of pus in the head, but horses with extensive lymph node swelling may be uncomfortable putting their heads all the way down. Offer meals at ground level first, but if the horse does not readily reach down to eat, try holding his tub at different levels to see if one seems more comfortable.
Abscess care: Warm compresses applied to areas of obvious swelling can help them come to a head and drain. You can either hold a towel soaked in hot Epsom salt solution against the area for 10 to 15 minutes at a time, or hold it in place using a polo wrap looped around the head and throat area, running both in front of and behind the ears, crossing under the jaw.
Nasal secretions and abscess drainage: These fluids irritate the skin and should be cleaned off using warm water and cotton or gauze sponges at least twice a day. Applying petroleum jelly around the nostrils/muzzle and on skin under any open abscesses will help protect the skin.
Because these fluids contain infectious organisms, always wear disposable gloves and put the cotton or gauze as well as the gloves into a sealable plastic bag when you are finished. At least one trashcan in the barn should be designated for disposing contaminated materials like this. Mark the can with a large "X" in brightly colored tape.
TLC: Some horses prefer to be left alone when not feeling well; others appreciate a little attention. If the horse seems to enjoy it, there's no reason you can't groom him.
Protection from wind, drafts, rain or snow is a must, but the horse doesn't necessarily have to be kept in a stall if there is suitable shelter outside. Use common sense with regard to blanketing. If the horse is shivering, blanket. If he's sweating under a blanket, take it off (and towel him dry to prevent chilling). Just like we do with flu or other infections, a horse may very well go back and forth between feeling feverish and feeling chilled.
Vicks, or a generic equivalent product, can help ease your horse's breathing by decreasing swelling/inflammation and helping to keep the nasal discharge thinner. Use it liberally.
Use a halter only when needed to work on the horse. Swellings can develop rapidly and cause the halter to fit too tightly.
Monitoring: Keep a running record of how the horse is doing to help your vet spot trends and for you to catch potential complications early. Twice a day, take the horse's rectal temperature and his respiratory/breathing rate and write them down. Record the color, consistency and amount of nasal discharge or abscess drainage.
Note how much your horse is drinking and eating, and what he is eating. Note any changes in behavior/attitude. Look at, and run your hands over, his legs and feet twice a day, checking for any change in temperature and any puffiness. Check the gums at least once a day for the appearance of any tiny red spots, which could be small hemorrhages that indicate the development of purpura.