Strangles is an infectious disease caused by a bacterium called Streptococcus equi. The disease was first described in veterinary literature in the 1200s. Of all the infectious diseases involving horses, few strike more fear in an owner’s heart than strangles. While most horses do recover, 10 to 20% of horses in any given outbreak may have serious or even life-threatening complications.
Like most infections, the first symptom is a fever, typically to 103?° or higher. The horse will likely also act depressed and may be off feed. This is followed by the appearance of a nasal discharge and enlargement of the lymph nodes under the lower jaw. Other lymph nodes in the area of the throat, the throat itself, and the gutteral pouches are also infected and inflamed.
There may be extensive edema in the throat/upper neck and serum may ooze through the skin. Horses may stand with their heads extended as a result of the pain and to ease breathing. The time period from exposure to the infection and the appearance of the typical swelling is about a week.
Healthy adult horses may show only slight lymph node enlargement (abscesses) which may go down on its own. In young horses or compromised adults, the lymph node abscesses continue to enlarge and eventually break open and drain. They can reach a size sufficient to block the horse’s ability to breathe. This is where the disease got the name strangles.
The infection can spread along lymphatic vessels to involve lymph nodes deep inside the throat, virtually anywhere on the head and even extend to the lymph nodes inside the chest. Infection of the lung tissue itself is rare, however, and most horses with strangles don’t have a cough. This can help distinguish early strangles cases from influenza.
Abscesses will take anywhere from days to weeks to open up and drain. Once all the pus has drained, they begin to heal over.
Bastard Strangles and Other Complications
”Bastard strangles” is a term used to describe horses who have had spread of the bacteria to other areas of their body, including the chest, abdomen and brain. Virtually any organ may be involved, since S. equi spreads along the lymphatics. Deep lymph node abscesses may become very large. The symptoms they cause depend on their location. Colic related to exercise is a common complaint with abdominal abscesses. Rupture of internal abscesses may cause death. Slow leakage from internal abscesses often produces periodic fevers, weight loss and a generally unhealthy appearance.
Purpura hemorrhagica is a serious complication of S. equi infection. It is an inflammation of the blood vessels caused by interactions between circulating antibody and bacteria that have gained access to the blood stream. The antibodies and bacteria form what are called immune complexes. The skin of the legs is the most common site for purpura symptoms to appear, but skin virtually anywhere on the body may be involved. Extensive edema and swelling occurs, followed by oozing of serum through the skin. Laminitis is a common complication of this. Immune complexes can also cause destruction of large areas of muscle.
Antibiotic treatment of S. equi infections remains controversial. In general, once abscesses have formed it is generally considered best not to use antibiotics since they will slow the rate with which the abscesses become mature and drain. However, very ill horses, horses with difficulty breathing and any horse suspected to have rapidly spreading disease should receive antibiotics.
Horses in the early stages, before abscesses begin, should also be treated and treatment may prevent the symptoms from going any further. Several antibiotics have been used but penicillin is still the drug of choice. Horses that don’t respond well to penicillin are unlikely to do any better with another antibiotic. In those cases, it is usually a weak immune response that is the problem, not the actual choice of drugs.
Abscesses can be hot packed to keep the overlying skin soft and hopefully hasten their drainage. Once open, your vet will likely instruct you to flush the abscesses daily with dilute Betadine solution or peroxide followed by Betadine solution.
The skin around the abscesses, as well as the path that drainage will take, should be clipped. The pus can cause severe skin irritation, even damage. Protect skin in the pathway of drainage with a generous layer of petroleum jelly or Desitin.
If purpura develops, the skin will be fragile and susceptible to secondary infections. Follow your vet’s instructions carefully. In general, gentle daily cleaning and application of a protective barrier cream such as Desitin or antibiotic ointment is a common recommendation.
Eating and drinking may be painful but are critical for supporting the immune system and recovery. Soaked hay cubes/pellets or a soaked complete feed will be better tolerated than a regular diet. Use alfalfa and bran to tempt appetite. Water must be freely available at all times, kept clean and at an appealing temperature. Some horses will eat better if hand fed. Don’t expect normal water and food consumption but if you have any question at all about whether the horse is drinking and eating enough be sure to let your vet know.
Nonsteroidal anti-inflammatory drugs may be used to control pain and fever but should be used at the lowest doses possible and with great care. Ill horses that are not eating and drinking normally are at higher risk for gastro-intestinal and kidney complications from these drugs.
An alternative that often works well in controlling acute inflammation is Microlactin. This is a compound isolated from milk that blocks the movement of neutrophils, a form of white blood cell, into inflamed areas. It is available in bulk powder for human use and is the active ingredient in the equine product Duralactin. As with any serious illness, always clear the use of this or any other supplement with your veterinarian first.
How Do They Get It'
Most horses become infected after coming in contact with another horse that is harboring the bacteria.
Contrary to common belief, the bacteria aren’t particularly hardy outside the body. Under laboratory conditions it may survive up to two months, but S. equi is sensitive to substances called bacteriocins produced by bacteria normally present in the environment and soil. Bacteriocins are natural antibiotic-like chemicals that bacteria secrete to kill competing organisms.
Contact with nasal secretions or pus from a horse with an active infection is obviously risky, but many outbreaks likely start from contact with horses that are asymptomatic carriers.
Apparently recovered horses may continue to shed the organism in their nasal secretions for two to three weeks, some as long as six weeks. As many as 10% of horses that have recovered from strangles will continue to shed S. equi for months or even years.
The most common location for S. equi to ”hide” in recovered horses is inside the gutteral pouches. Pus that did not completely drain can become formed into large, partially dehydrated collections called chondroids. Both chondroids and the lining of the pouches will contain the organism.
Other horses continue to have liquid pus in their pouches for several months. Intermittent coughing and periodic draining of pus from the nose, often on o ne side only, are the only symptoms but roughly half of horses harboring this pus in their pouches show no signs at all. Persistent sinus infection may also cause a carrier state.
Vaccinations And resulting immunity
About 75 to 80% of the horses that have recovered from strangles will have a strong natural immunity that lasts five years or longer. Immunity is in the form of both circulating IgG and IgA antibodies along the lining of the upper respiratory tract.
If exposed again, the horse will show either no symptom or only a nasal discharge with no lymph node enlargement. However, horses that do show only the discharge can still infect other horses.
Two types of vaccination are available and there are potential problems with both of them. The intranasal form contains modified live bacteria that should not be capable of actually causing the disease, but some vaccinated horses do show symptoms. This is less of a problem now than it was when the vaccine first came out.
The intranasal vaccine produces strong protection at the organism’s point of entry, but no circulating antibody response so it is not appropriate for boostering pregnant mares to get good antibody levels in the colostrum.
The older form of vaccine is an intramuscular bacterin. A bacterin is a suspension of cell wall proteins from the bacteria. This produces a strong circulating antibody response but poor protection at the mucus membranes and therefore weaker protection from disease.
This vaccine will increase antibodies in the colostrum, but only the IgG antibodies. Colostrum from mares that have had strangles in the past contains both IgA and IgG antibodies. The intramuscular vaccine also carries a risk of injection-site abscesses and systemic reactions, including purpura.
Many factors have to be considered when deciding whether or not to vaccinate and this is a topic you need to discuss with your own veterinarian. Serological testing for antibody titers is available and can be used to help guide the decision.
This is important with respect to risk of purpura reactions. Horses with very high titers, or a history of purpura, should not be vaccinated. Discuss this with your own veterinarian.
Article by Eleanor Kellon, VMD.