Ann Chapman, DVM, Dipl. ACVIM
Equine Medicine Fellow
Equine Health Studies Program
Louisiana State University
School of Veterinary Medicine
Baton Rouge, LA
(225)-578-9500
Strangles is a highly contagious disease of the upper respiratory tract caused by the bacteria Streptococcus equi. It can be a very frustrating and persistent disease for horse owners worldwide. This organism invades the lymph nodes and respiratory tissues of the head and neck. The early clinical signs of the disease are fever, nasal discharge and lymph node swelling. Initially the nasal discharge may be clear, but as the disease progresses, it becomes thick and tenacious. The lymph nodes below the jaw and neck become enlarged, hot and painful. As these lymph nodes abscess, they may begin to ooze and eventually rupture and drain externally or internally. Internal lymph nodes may drain into the guttural pouches of the head leading to pus accumulation. Chronic infection of the guttural pouches can lead to drying of the pus and formation of solid, concretions called chondroids. In complicated cases of Strangles, the lymph node swelling may exert pressure on the esophagus and pharynx, which results in difficulty swallowing. If the lymph node swelling obstructs the airway, the horse will develop respiratory difficulty and require emergency attention, hence the origin of the term “strangles”. Any horse lacking sufficient immunity to the organism is vulnerable, however the very young (1 to 5 years of age) and very old are predominantly susceptible.
Deaths attributed to Streptococcus equi are rare; however the disease can rapidly spread from horse-to-horse resulting in a local outbreak. The bacteria can be transmitted from contaminated buckets, feeding utensils, grooming tools, tack and other equipment. In addition, feed, water and even human hands are capable of transmitting the bacteria to a naïve horse. The organism does not persist long-term in the environment; however stalls or trailers that were recently contaminated with infected nasal discharge may be source of infection to other horses. In many cases, the introduction of strangles to a farm can be traced to travel of one or more horses, or arrival of asymptomatic carrier horses.
A diagnosis of strangles is usually made when the horse exhibits the classic clinical signs of fever, lymph node swelling or drainage and purulent nasal discharge. To confirm the diagnosis, a sample of the nasal discharge from either a swab or a nasal washing, or pus from a draining lymph node can be sent to laboratory for bacterial culture. However, early in the disease process, before Streptococcus equi has emerged into the nasal passages, the culture may be negative. Equally challenging is the diagnosis of the carrier state, which may require three sequential samples using specialized equipment (long swabs or endoscope) to sample the nasopharynx or guttural pouches. Polymerase chain reaction (PCR) is a newer test that has recently been developed to detect very small quantities of Streptococcus equi DNA. Unfortunately, this test detects both live and dead bacteria, so its usefulness may be limited to detecting asymptomatic carrier horses and determining when the infection has been eliminated from a recovering horse.
Treatment for strangles depends on the stage and severity of the diseased individuals. Many veterinarians argue whether or not to use antibiotics in the treatment of strangles. All agree however, that any affected horse should be quickly isolated and maintained separate from the remaining herd.
Horses with early clinical signs of fever and depression, may be placed on antibiotics as an initial means of controlling an outbreak by arresting the bacteria and preventing abscess formation. However, most veterinarians do not advocate antibiotics in mild clinical cases, since it may inhibit the development of protective immunity to the bacteria. Once a horse develops lymph node abscessation, hot packs and poultices are applied to aid in maturation and drainage. In some cases the abscess will require lancing if spontaneous drainage does not occur. Once open, the abscesses are often flushed with antiseptic solutions to promote healing. Anti-inflammatory medications, such as phenylbutazone or flunixin meglumine, may be helpful to control the pain and swelling which promotes eating and drinking.
Antibiotics should be avoided unless the infection has been longstanding or the horse has advanced signs of the disease. These include obstruction of the upper airway by enlarged lymph nodes, pneumonia or abscesses in other body organs (such as liver, kidney, brain, spleen, lung or intestinal tract). If antibiotic therapy is administered, it should be continued until complete resolution of the disease, since clinical signs may return if the antibiotics are stopped prematurely. When the infection extends into the guttural pouches it must be treated with flushing through a specialized catheter, an endoscope or through a surgical incision.
To reduce the introduction of strangles to a farm, various strategies are recommended. New arrivals should be held in quarantine for 2-3 weeks prior to mingling with other horses on the farm. While in quarantine, a rectal temperature should be taken daily and the horse observed for cough or nasal discharge. Personnel handling these horses should wash their hands and use alcohol-based hand cleansers when they are finished dealing with the animals.
When a strangles outbreak occurs, all the horses with signs of strangles and the horses who have been in contact with them, should be immediately quarantined. Symptomatic horses should be managed appropriately and exposed horses should be monitored as mentioned previously. Individuals handling these horses should use latex gloves and manage these animals after handling non-exposed horses on the farm. Buckets, feeder and other equipment should be disinfected daily with dilute bleach or other disinfectant if available (quaternary ammonium or phenols). Bedding should be composted separately or removed from the premises and the stalls walls and flooring disinfected daily. Effective fly control should be implemented to help spread the disease to unaffected horses.
Vaccination for the control of strangles is a source of controversy since a solid immunity to the disease is usually not attainable, and there are potential complications with vaccine use. There are two types of products available: extract vaccines and a live vaccine. Extract vaccines contain the components of the S. equi organism and are given intramuscularly. The most common side effect of this vaccine is pain, swelling or abscess at the sight of injection. The live vaccine is composed of a non-infectious strain of S. equi and is given intranasally. Side effects of this vaccine are infrequent, but can include lymph node swelling or abscessation especially if concurrent injections are being administered. The development of immune response with vaccination takes 7 to 10 days, so during an outbreak only those horses without contact of affected horses should be vaccinated. Horses with current signs of strangles or that have recovered from a previous strangles infection should not be vaccinated. An uncommon but serious adverse reaction that can occur with vaccination (or more commonly with natural infection) is a vasculitis called purpura hemorrhagica. However, as researchers continue to study S. equi, newer and safer vaccines are on the horizon.
In conclusion, strangles is a common and sometimes very serious contagious disease of horses. However, with careful vigilance, preventative measures and early recognition, it is a disease that can be managed and controlled.
