Strangles: Streptococcus equi subspecies equi infection in horses
Streptococcus equi subspecies equi (S. equi var. equi) is the bacterium which causes a contagious disease in horses known as strangles. Strangles is primarily an upper respiratory infection characterized by abscessation of the lymph nodes in the head and upper neck. Strangles most often affects young horses but horses of any age can be infected and develop disease. Following natural infection, a carrier state of variable duration may develop and intermittent shedding may occur.
The organism is transmitted by direct contact with infected horses (showing clinical signs) or sub-clinical shedders (not showing clinical signs but harbor the bacteria), or indirectly by contact with contaminated objects including tack, grooming equipment, feed and water sources and contaminated handlers.
Additionally, insects can transmit the bacteria mechanically. S. equi var. equi typically survives in the environment for 2-4 days under natural circumstances but may survive up to 4 weeks in the environment under ideal conditions.
The time frame from infection to the appearance of clinical symptoms (incubation period) of strangles is 3–14 days, and the first sign of infection is fever. Fever is typically followed by nasal discharge, depression, and lymph node swelling. The submandibular and /or retropharyngeal lymph nodes may be enlarged and cause difficulty swallowing, respiratory noise, cough and extended head and neck posturing. Older animals with residual immunity and animals with strong immunity from vaccination may develop milder clinical disease. On occasion, the bacteria may spread to, and cause abscessation of, lymph nodes in other parts of the body, particularly the abdomen. This is a clinical syndrome known as metastatic abscessation or ‘bastard strangles’. Additionally, some horses may develop purpura hemorrhagica resulting from overproduction of antibodies. These antibodies become attached to blood vessel walls and activate a strong immune response resulting in hemorrhage and edema. The primary clinical symptoms of purpura hemorrhagica in horses is swelling of the lower limbs, chest and abdomen. The skin may die in severe cases. Purpura hemorrhagica may occur after infection or vaccination. S. equi var. equi is highly host adapted but infection in debilitated humans has been reported, although rare. Caretakers should take particular caution to avoid respiratory and oral contamination with exudates.
Definitive diagnosis is made by PCR (DNA amplification) or bacterial culture of exudate from abscesses, nasal swabs or pharyngeal lavage samples. PCR is more sensitive (less chance of a false negative result) than culture and the results are reported faster.
Controlling spread of the disease is facilitated by segregation, isolation and quarantine. Affected horses should be separated from unaffected animals and cared for by separate caretakers wearing protective apparel. The temperature of all exposed horses should be obtained twice daily, and febrile horses should be isolated and tested. Contaminated equipment should be cleaned with detergent to remove organic debris then soaked with an appropriate liquid disinfectant.
Treatment of clinically ill horses typically consist of supportive therapy. Warm compresses may be applied to swollen lymph nodes to facilitate maturation and drainage and speed recovery. Ruptured abscesses are flushed with an appropriate antiseptic for several days until drainage ceases. Anti-inflammatory medications can be administered to reduce pain and fever and to encourage eating and drinking. Some abscesses may need to be surgically incised to facilitate decompression and drainage. Occasionally, in complicated cases with retropharyngeal abscessation and pharyngeal compression respiration may be impaired. In extreme casestracheotomy may be required.
Intranasal and intramuscular vaccines are available to assist with infection and disease prevention. Most horses develop prolonged immunity after exposure, infection or disease. Some horses develop very high titers (a measurement of how much antibody) and may be at increased risk of pupura hemorrhagica if exposed to infected horses or vaccinated against strangles. It is important to work closely with an experienced equine veterinarian to develop a prevention protocol for individual horses, herds and equine facilities.
For more information contact Dr. Shannon Moreaux, Montana Extension Equine Specialist: (406) 994-7689, firstname.lastname@example.org or visit the American Association of Equine Practitioners website: https://aaep.org/horsehealth/understanding-equine-strangles.