With the recent outbreak of strangles in Canada—and the coming of spring show season when our horses are exposed to many others--we turn our attention to this highly contagious disease. Jeff Cook, DVM, of Rood and Riddle Equine Hospital near Lexington, Kentucky, walks us through the basics of understanding strangles.
The infection caused by the bacteria Streptococcus equi, commonly known as strangles, has been described in horses for almost 800 years. The name strangles describes the condition in which an affected horse is suffocated as lymph nodes in the throat region become enlarged and obstruct the airway, but thankfully this does not happen in all cases.
Many misunderstandings exist regarding strangles, most likely due to improperly informed horse people passing on tales regarding the infection. Common myths surrounding strangles refer to clinical signs, transmission, treatment, persistence of the disease in the environment and prevention.
Symptoms of Strangles
Strangles is typically characterized by a sudden on set of fever, with subsequent formation of abscesses of lymph nodes of the head and neck 7-10 days following exposure. The most common lymph nodes affected are those under the jaw.
These abscesses may open and produce a thick, yellow drainage that might also be seen as a nasal discharge. Misunderstandings regarding the clinical signs of strangles can lead to a false sense of security among horse owners.
It has been said that all horses with strangles have several large lymph nodes under their jaws with copious amounts of yellow drainage. This isn’t true. The severity of clinical signs varies depending on the immune response on the individual horse.
Younger horses, or horses that have never been exposed to the bacteria, will have a more severe form of the disease. Horses that have been exposed in the past and have developed some level of immunity, often exhibit a mild form of the disease seen as slight nasal discharge with little to no enlargement of the lymph nodes under the jaw.
The symptoms of the disease following exposure to a group of horses can vary from severe lymph node enlargement with difficulty breathing to no outward signs with a slight nasal discharge.
How Strangles is Transmitted
Misunderstandings regarding the transmission of the bacteria causing strangles, Streptococcus equi, also exist. It is often said that once a farm has had an outbreak of strangles the problem will always be on the farm and can show up at any time. A fact that needs to be understood is that the source of infection from year-to-year and farm-to-farm is the horse, not any part of a barn, pasture, fence, etc., or other animals besides a horse.
After outward clinical signs of strangles have ceased, the majority of horses clear the bacteria and no longer pose a threat for infecting others by 2-3 weeks. However, following an outbreak, a number of horses (this can be as high as 10%) cannot clear the bacteria and become persistently infected.
The bacteria can survive in the guttural pouches, which are located in the pharyngeal region, for years. These persistently infected horses that might not be showing any outward signs are known as asymptomatic carriers.
A carrier horse that undergoes some form of stress--such as foaling, weaning, competing at a show, or a simple change in routine--can begin to shed the bacteria and serve as a source of infection in its herd.
These carrier horses can be the source of new outbreaks when introduced to a new herd.
Transmission of Strep. equi occurs by either direct or indirect contact. Direct transmission occurs during horse-to-horse contact through everyday social behavior. The indirect transmission can be more difficult to control and occurs through the sharing of recently contaminated stalls, water buckets and troughs, feed tubs and bits, as well as the tools and clothing of farriers, veterinarians and dentists unless appropriate precautions are taken.
Water sources, either in shared stalls or in field settings with a common water supply, are the most common culprit when it comes to infecting a herd during an outbreak. When a horse is shedding the bacteria and dips his nose into a water source, the water serves as a reservoir for the bacteria to be passed to every horse that comes in contact with the water. If the water is not disinfected regularly, the infected horse will continue to contaminate and infect the herd.
There is a misunderstanding regarding the persistence of Strep. equi in the environment. With the exception of in a water source, the bacteria will not survive for prolonged periods in the environment. This means horses do not become infected with the bacteria from the soil, grass or fences unless a horse currently shedding bacteria is present.
Strangles is often diagnosed by clinical signs, but it takes a positive culture with or without a positive PCR test to confirm the presence of Strep. equi . There are other bacteria that can cause similar symptoms to strangles, but they are not nearly as infectious. It is important to determine the causative agent of a swollen or draining lymph node immediately to prevent a possible out break. Both tests utilize a sample from a nasal wash or direct swab from an enlarged lymph node to detect the bacteria. Each test has its limitations, but when used in conjunction can be very effective in detecting the bacteria in a horse actively showing clinical signs and a carrier horse that might appear outwardly healthy.
Treatment of Horses With Strangles
Common mistakes are made in the treatment of an individual horse or entire herd during an outbreak of strangles. Once a horse is confirmed to have strangles, different opinions exist regarding the treatment of the individual. Unless a horse is in distress, such as difficulty breathing or severely depressed with a fever above 103 F, they should be isolated from other horses and monitored with no medications.
As far as herd management during an outbreak, the use of vaccines should be avoided. Vaccinating during an outbreak can actually cause more harm than good. Horses are at different stages of the disease during a herd outbreak, and if vaccinated following exposure to the bacteria, a horse may have an immune system response to the bacteria that can be more severe than the actual disease.
If not used correctly, antibiotics can also lead to problems during an outbreak. Often when a horse that has been exposed and is not yet showing clinical signs is placed on antibiotics, the disease process is simply suspended. When the antibiotics are stopped, the horse might continue to progress through the stages of the disease with no benefit from the antibiotics.
The best management during an outbreak is to segregate the horses showing clinical signs, giving them supportive care, and monitoring the temperatures of the healthy horses for 2-3 weeks after the last horse with clinical signs was removed. Steps can be taken to prevent the exposure of your horse to strangles.
It is important to remember that a horse does not have to be showing active clinical signs of strangles to be capable of infecting others. Care should be taken to minimize exposure to other horses, particularly at shows and stables with a changing population. Particular attention should be given to the water source you use. When traveling to shows, water buckets should be brought and not shared with other horses. Do not permit direct or indirect contact with other horses while at the show. This includes nose-to-nose contact as well as sharing such things as stalls, water buckets, feed tubs, grooming tools, tack and trailers.
In a stable or herd situation, a few simple prevention methods can be used to decrease the likelihood of exposure to strangles. Isolation of all horses for 2-3 weeks before they come in contact with others can decrease the potential exposure. Testing incoming horses for Strep. equi can be an effective tool in limiting the introduction of strangles into a herd or stable.
Strangles is a preventable disease and with the proper steps, the risk of exposure can be minimized.