Strangles ("distemper") is caused by a bacteria (Streptococcus equi) that is characterized by inflammation of the upper respiratory tract and most often by abscess in the adjacent lymph nodes. In the worst cases, the glands around the throat get so swollen, the horse looks almost like it's going to be "strangled," but in fact it rarely causes asphyxiation. In some regards, strangles is similar to strep throat, in that it is very contagious and causes painful soreness in the throat of affected horses. The bacterial pathogen that causes strangles rarely affects any other species.
All horses are potentially susceptible to this disease unless, as a result of previous exposure, the individual horse has acquired immunity. Although strangles does sometimes afflict newborn foals and old horses, it is most typically seen in older foals and young adult horses.
The causative agent of strangles, Streptococcus equi equi, is disseminated into the environment by clinically sick horses infected with strangles and by clinically recovered horses, or carriers, that act as an unseen source of infection. It is difficult to identify carrier horses because they do not exhibit any signs of infection. The bacterial pathogen is shed in the nasal secretions of infected horses and in the thick yellow exudate that is expelled in nasal discharges and in fluids drained from abscesses that develop in infected horses.
The disease may be transmitted directly from horse to horse if close contact between infected and non-infected horses occurs. However, unlike viral pathogens, the causative agent for strangles is not spread appreciably in aerosolized fluids released from the airway of infected horses. The most important route of transmission involves indirect contact with Streptococcus equi, through contamination of environmental objects (known as "fomites"), most commonly people. Insects (barn flies) may also play a role in disseminating this bacterial pathogen in the warmer times of the year.
Human contact between infected and non-infected horses (especially at the horse's nostrils) is a common method of transmission. Therefore, veterinarians are very careful as they move among horses, especially between different stables, in order to ensure that they are not inadvertently transmitting this bacterial pathogen on their hands, clothes or equipment. Other common fomites include: food and water troughs, stocks, twitches, dental equipment, bridles, horse trailers, pastures, stable walls and fences.
Following exposure to an infected horse or a contaminated fomite, the earliest clinical manifestations of strangles include a sharp rise in body temperature, lethargy, disinterest in food and reluctance to work. These early clinical signs are nonspecific, however, and could occur with numerous other diseases.
As the disease progresses, the following may occur:
Horses tend to lose weight while they are fighting a strangles infection but are able to recover lost weight within a few weeks of resolution of this disease.
Although a majority of horses eliminate Streptococcus equi from their system by the time the symptoms have resolved, a small number of recovered horses continue to harbor Streptococcus equi equi, but do not exhibit any clinical evidence of disease.
Infection of Sinuses and Guttural Pouches
Two areas commonly infected secondary to the initial throat and gland infection include the sinuses and guttural pouches. If you have a horse with a persistent nasal discharge after strangles in the barn, you might be looking at one of these two complications. Sinusitis produces a one-sided (unilateral) nasal discharge, and guttural pouch infections can cause nasal discharge from one or both sides. The key fact is that the nasal discharge is thick and viscous, often white or yellow-white in color. The infection can collect in these regions for long times, so sinusitis or guttural pouch infections can crop up later. The guttural pouch is an area where the strangles organism can survive for a long time, and may explain the presence of some carriers.
Atypical Clinical Presentations
Less commonly, the clinical results of infection may be atypical. The disease leads to early onset extensive swelling under the skin in some infected horses, especially at the head. In other horses, the development of an abscess leads to rupture of the abscess into the throat (instead of rupturing through the skin), which causes a thick yellow nasal discharge. In some horses, a large abscess develops deep in the throat where it interferes with both breathing and swallowing – in those cases, it may be necessary to provide a tracheotomy (hole in the wind pipe) to allow the horse to breathe. Internal rupture of an abscess into the throat sometimes leads to infection of the horse's guttural pouches and the horse may need surgical treatment.
The immune system appears to be relatively slow in eliminating the organism from some horses. In these cases, more than one abscess may develop in the throat area or under the jaw before the infection is finally eliminated.
Another frustrating and difficult manifestation of infection is the rare development of abscesses at other locations in the body. Although abscess development is localized to the throat in most infected horses, strangles abscess formation can arise in remote areas of the body such as the brain, the spinal cord, the lungs, the intestinal system, the udder or in the kidneys.
Additional clinical symptoms are determined by the specific location of these additional abscesses:
The development of abscesses at an area remote to the upper respiratory tract (throat) is commonly referred to as metastatic strangles or bastard strangles. The prognosis for most horses affected with metastatic strangles is unfavorable. Fortunately, metastatic strangles is a rare complication of this disease.
Purpura hemorrhagica is another uncommon complication of strangles. In these horses, swelling in the limbs or under the trunk occurs several days or weeks following resolution of a typical strangles infection. This swelling is a result of an immune reaction against the horse's blood vessels that has been provoked by the development of immunity against the streptococcus organism. In rare instances, purpura hemorrhagica may affect the intestinal tract and cause severe colic or diarrhea.
Other rare clinical effects of strangles infection in horses include immune-mediated damage to the heart, the skeletal muscles and the kidney. Heart damage leads to irregularity in the heartbeat, muscle damage leads to weakness and muscle destruction, and kidney damage may lead to weight loss, increased urine loss and enhanced thirst.
Strangles is primarily diagnosed based on the characteristic clinical signs. Strangles is a very common disease in certain specific locations. For example, some horse barns are known to "harbor" a risk for this disease, and whenever the condition has already occurred in one horse, strangles should be highly suspected if another horse develops similar signs.
New cases of strangles are common following introduction of a new horse (possibly a carrier of strangles) onto a farm on which strangles has not been previously or recently identified. On the other hand, cases of strangles sometimes arise in new horses that are brought onto a farm where the disease is known to occur. Strangles is commonly diagnosed in horses that have been acquired at a horse sale barn.
The diagnosis of strangles is specifically corroborated by demonstrating Streptococcus equi through bacteriological culturing from fluids draining from abscesses or from the throat. In most horses, the pathogen disappears quickly following drainage of an abscess, so failure to identify the pathogen several days following abscess drainage should not rule out that strangles was present.
The most likely location from which a positive culture can be obtained in most horses is the guttural pouch. Culture of the guttural pouch is used to identify asymptomatic carriers. However, these carrier horses represent an important source for future infections of new horses and for contamination of the environment.
Affected horses should be isolated as soon as this disease is recognized, and specific protocols should be instituted in order to minimize the risk of transmitting the disease to other horses. Other horses at risk should be carefully monitored for signs of strangles (by measuring the rectal temperature on a regular basis).
Currently, it is strongly recommended that antibiotics (such as penicillin) should NOT be used in most typical cases of strangles. Rather, the affected horse should be monitored carefully and treated symptomatically. The use of antibiotics commonly leads to increased risk for complications of strangles and interferes with the development of immunity, thus prolonging the course of the infection.
Symptomatic treatments include strict rest, the provision of a soft ration (increased comfort during swallowing), vitamin supplementation and the use of non-steroidal anti-inflammatory agents, as needed, to increase comfort and to enhance water consumption and appetite.
It is important to encourage development and maturation of abscesses that are identified in the throat and under the jaw. A common misconception suggests that abscess development should be inhibited by the use of antibiotics. The fastest recovery can be anticipated for those horses in which an abscess develops and is ruptured to drain through the skin (or internally into the throat).
Although some horses develop more than one abscess before they resolve the infection (by the development of immunity), in most instances, rupture of a single abscess is needed before the recovery phase ensues. Certainly, the use of antibiotics interferes with both the maturation of abscesses and the development of immunity. Abscess maturation can best be accomplished using hot compresses held against the affected area. Your veterinarian knows when to facilitate drainage of an abscess (lancing) and may also base his/her decision on the ultrasonographic appearance of the maturing abscess.
In complicated or atypical strangles, the use of penicillin is usually indicated. Internal abscesses (metastatic strangles) at remote sites carry an unfavorable prognosis and must be treated with long-term antibiotic injections.
Although vaccines against strangles have been available for many years, vaccination has never been shown to be effective for the prevention of this disease. Some vaccines are administered by intramuscular injection and others are administered via intranasal spray. Intramuscular strangles vaccines often cause swelling at the site of injection.
All new horses should be regarded as potential strangles carriers. If it is intended to introduce a new horse onto a premises on which strangles has never been reported in the horses, the new horse should be examined carefully for evidence of strangles and kept separate from the herd until risk of strangles has been nullified. Quarantine is especially important if the new horse is being introduced from a sale barn or from a barn of unknown or uncertain strangles history. Your veterinarian will either examine the new horse's guttural pouches (using an endoscope) or culture the throat using a long bacteriological culture swab.
If a horse is to be taken to a new barn where the risk of strangles is unknown, that horse might be vaccinated ahead of time. It is important to note that all vaccination protocols require several weeks before any immunity will be evident. However, as a rule, vaccination against strangles is notoriously ineffective.
It might not be possible to provide freedom from risk under this circumstance. The horse owner should recognize that strangles rarely causes permanent injury or death. Although the disease is inconvenient because it necessitates rest and special attention, severe complications are uncommon.
There is nothing known to prevent strangles when a non-immune horse is challenged by a contaminated environment. Similarly, there is nothing (not even antibiotics) known to prevent severe complications or the development of atypical strangles under the same conditions. The new horse in a potentially contaminated environment should be monitored carefully and, if any symptoms arise, veterinary attention should be sought immediately.
When contaminated by an infected horse, the environment must be scrupulously cleaned. The most effective disinfectants for this purpose include: phenol (1:200), povidone iodine, chlorhexidine, gluconate and gluteraldehyde. Contrary to popular belief, survival of Streptococcus equi equi in the environment is not long-lived. The organism may survive better if allowed to persist in pus and discharges, but generally it dies when it dries out.