Strangles is the common name for a bacterial disease affecting horses. It is caused by an organism called Streptococcus equi ssp equi (S. equi). This organism has been around for ages, and is found throughout the United States. Periodically, large outbreaks of disease affect certain regions of the country, making it appear to be occurring with an increased incidence. Five years ago, the Mid-Atlantic States were a hot spot for infections, and today, the Midwest is currently experiencing resurgence in the number of horses affected.
Existing around most cases of Strangles are social stigma concerns and controversial recommendations for treating and preventing the disease. Veterinarians do not universally agree on a single treatment regime, and this has led to some confusion amongst horse owners. Here we will try to introduce the reader to the signs and symptoms of a horse infected with S. equi, what treatment options are available, and what can be done to try and prevent horses form contracting the disease.
An intracellular bacteria
S. equi primarily affects the upper respiratory tract of horses. It is an intracellular bacteria, meaning it likes to live and multiply within cells, and particularly inhabits cells of the lymph nodes. Lymph nodes are part of a mammals immune system, and are found throughout the body. When a person has 'swollen glands' associated with a cold or a sore throat, this refers to enlargement of the lymph nodes in the throat region caused by a reaction to an infection in that area of the body. Lymph nodes are composed primarily of a type of white blood cell called the lymphocyte. Lymphocytes come in two main categories - T-cell lymphocytes and B-cell lymphocytes. The B-cells are responsible for antibody production, and the T-cells are responsible for regulating cellular immune responses in the body. Lymph nodes throughout the body are connected to each other by tiny vessels that traffic cells and lymphatic fluid from node to node. The S. equi organism prefers to live within cells and often sets up infection in the lymph nodes around the head and throat. It can survive for long periods of time out of the body, and horses can become infected if exposed to the organism months after an infection had been noted on the property. It is transmitted by inhalation of dust or mucous that contains the organism.
When a horse is infected with S. equi, the first signs of disease are fever, inappetence, and a sore throat. As the infection sets up house in the lymph nodes that drain the head, pain and enlargement of these nodes is noted. Most frequently affected are the submandibular lymph nodes between the lower jaw bones, and the retropharyngeal lymph nodes that lie in the throat latch region of the horse. The disease was named Strangles because extensive swelling in this region can affect airflow causing respiratory sounds similar to one who is being strangled. As infection progresses inside the lymph nodes, abcessation occurs, and the lymph nodes soften and then break opened draining puss containing the bacteria. Horses frequently demonstrate a nasal discharge, and may also develop a cough. Once the enlarged lymph nodes are soft and ready to drain, veterinarians will often lance these to expedite drainage and subsequent healing.
The rule of thumb for S. equi infections in horses is similar to that which occurs with people exposed to the virus causing chicken pox. S. equi is usually a childhood disease, affecting younger horses. Once infected by the bacteria, roughly 80% of horses do not ever contract the disease again. The remaining 20% can become ill again later in life. Older horses that have never been exposed to the disease are equally at risk as a young horse. Also as with chicken pox infections, most individuals recover uneventfully from the infection. Horses are usually sick for about two weeks and then once the lymph nodes drain, they begin to feel much better.
Treatment of S. equi depends on a number of factors, and veterinarians occasionally differ on how they like to manage these infections. Of primary importance is to ensure that the horse is comfortable and able to eat, drink, and breath easily. Fevers are controlled with antiinflammatory agents like phenylbutazone and Banamineâ. Warm compresses applied to affected lymph nodes are frequently prescribed because they provide comfort and expedite maturation and drainage of abscessed lymph nodes. Penicillin is an antibiotic which kills the S. equi organism, but its use in a strangles case is dependent on where the horse is in the course of disease. Horses infected with S. equi that are not able to eat, drink, or breath properly, or are experiencing other severe complications of the disease are treated with penicillin. Horses that have signs of lymph node enlargement but are handling the illness with only fever, nasal discharge, and general malaise are best left off penicillin. These horses are given supportive therapies including antiinflammatory agents and hot-packing of lymph nodes. These horses usually do fine once the lymph nodes drain. Treatment with penicillin will reduce the signs of illness in these horses but when treatment is stopped, the disease frequently recurs until the lymph nodes have the opportunity to enlarge again and drain. Hence, penicillin treatment in typical cases actually prolongs the course of disease. Penicillin may effectively prevent full-blown disease in horses that were exposed if treatment is initiated prior to any lymph node involvement. Usually, though, penicillin is reserved for the complicated cases where reduction in the size of lymph nodes is required for the safety of the horse.
Complications associated with S. equi infection can occur but mortality rates from infection are less than 10%, particularly when veterinary care is sought. Spread of the infection to lymph nodes elsewhere in the body is referred to as bastard strangles, and can cause a variety of problems even months after infection. Brain abscesses, abdominal abscesses, and heart valve infections are all possible sequelae to bastard strangles. The infection can also inhabit the guttural pouches horses exposed to the organism. These horses may become silent carriers of the disease and may not show any illness. These individuals are identified by throat cultures and endoscopic inspection of the guttural pouches. Another complication encountered is called purpura hemorrhagica. This is an immune-mediated condition where antibodies directed against an organism combine with components of the organism leading to inflammation. Signs of purpura hemorrhagica include swelling of the legs and muzzle, fever, laminitis, and little red spots or bruising visible on the lips, gums, and nostrils of the affected animal. These horses need to be treated by a veterinarian and this is considered a complication with a potentially guarded prognosis.
Prevention of infection with S. equi involves eliminating exposure to infected horses or facilities where infection was recently present. Remembering that the organism is spread by inhalation, careful sanitization of objects in contact with discharge from nostrils or abscesses is important. This includes water and feed buckets, bedding, fencing, stalls, trailers, clothing, hands, and boots. Isolating horses with early signs of infection will limit spread of the disease within a barn. Remember that the organism can survive in the environment for months and still cause disease. Suspect carriers of the disease should be evaluated by a veterianrian before exposing them to horse shows, events, or new stablemates. Always quarantine newcomers to a facility if they have a history of disease or more importantly no known history at all.
There are two strangles vaccines available for horses. The first is a traditional intramuscular vaccine that has been available for years. Associated with it are reports of local injection-site abscesses and anecdotal reports of a higher incidence of disease complications if the horse is later exposed to S. equi. The second is a newer product administered intranasally. It was proven to diminish severity of clinical signs in horses given the infection in controlled studies, and is widely used around the country. This product is an avirulent form of S. equi and is produced by chemically treating the bacteria so that it does not cause signs of disease but does induce immunity. Strangles infections have affected horses for generations, and will continue to do so for a long time. Knowing the early signs of illness, isolating affected individuals, and choosing how to manage or treat each case is the key to minimizing complications.