“Strangles” in horses is a highly contagious disease caused by the bacteria Streptococcus equi. It is spread by inhalation or oral intake of the organism during direct contact with the respiratory secretions of an infected horse or by fomites – contaminated equipment, tack, stalls, feeding buckets, and water troughs. Insects, and even handlers can serve as vectors, though the causative bacteria are only known to infect equids (horses, donkeys, mules).
Strangles gets it name from the clinical appearance and symptoms of infected horses. Lymph nodes of the face and neck often become quite enlarged, causing the horse to experience trouble breathing and painful swallowing. Most horses recover from Strangles within a few weeks; however some cases progress to life-threatening infections or persistent complications. The disease is often fatal in foals that did not develop adequate protection through colostral antibodies. Prevention measures are vital to keeping this disease from spreading throughout the herd.
The horse’s symptoms and the identification of S. equi through nasopharyngeal and lymph node cultures are used to diagnose Strangles. Symptoms develop in 3 days to 3 weeks after exposure and begin with a moderate fever, reduced appetite, and mucopurulent or snotty nasal discharge. The horse will begin to experience throat pain and stand with the neck extended as regional lymph nodes become infected and engorged with pus. Respiratory distress occurs when swelling becomes severe. At that point, the lymph nodes may abscess, or rupture and drain through the skin beneath the mandible, or lower jaw.
Streptococcus equi can occasionally invade the thorax and abdominal cavities through lymph circulation. This condition is referred to as “Bastard Strangles” and is certainly life-threatening. It can occur weeks or months after the outward symptoms of infection have abated.
Another possible complication of S. equi infections is an immune reaction called Purpura haemorrhagica. This is an inflammatory disease of the peripheral blood vessels resulting in severe edema (tissue swelling) in the limbs and head. Tiny bruises may be visible in the mucous membranes as this “allergic reaction” to the bacteria develops.
All suspected cases of Strangles should be immediately segregated from other horses and examined by a veterinarian. Infected horses can be contagious for as long as 2 months after symptoms develop, so quarantine measures are vital to prevent the spread of the bacteria. Thorough disinfection of all equipment, stalls, troughs, grooming supplies, etc., is crucial. Handlers should change clothing and wash with antibacterial soap to reduce the chance of infecting healthy animals.
Veterinarians sometimes avoid the use of antibiotics in the treatment of Strangles, especially after symptoms have developed, reserving the drugs for complicated cases. Some evidence indicates that symptomatic horses may be more likely to develop internal lymph node abscesses if they are started on antibiotics, but this is still open to debate. Prophylactic antibiotic dosing in exposed but asymptomatic horses may reduce the chance of contracting the disease, but this practice carries a risk of inducing resistant bacterial strains and inhibiting natural immunity in the animal. Nonetheless, Streptococcus equi is usually susceptible to Penicillin, so the doctor may choose to treat very early cases in order to head off advanced symptoms and avoid an outbreak in the herd.
Warm compresses can be applied to swollen areas to reduce pain. If swelling impinges upon breathing and eating, the veterinarian may lance the lymph nodes to provide relief.
Whether the veterinarian prescribes antibiotics or not, a culture and susceptibility swab will usually be collected to confirm the presence of S. equi in the laboratory. This can help to predict the risk to other horses that may benefit from vaccination or prophylactic treatment.
Strangles vaccination may be recommended to reduce the severity of the disease in exposed horses. Currently available vaccine is not able to entirely prevent the disease, but it may reduce the symptoms and make lymph node abscess less likely. The vaccines also have a risk. If the horse has a high M-protein titer (blood test), the horse has a higher risk of getting Purpura haemorrhagica from the vaccine. In addition, the modified live intranasal vaccine should be handled with caution (abscess formation in muscle if administered intramuscularly).
About 10% of horses will become chronic carriers of Streptococcus equi and continue to act as reservoirs of the disease. A DNA test can help to identify these horses.
New horses should be quarantined before introduction to the herd. If no symptoms develop within three weeks, it is usually safe to allow them into the group. A veterinarian should always examine new horses to reduce the risk of infecting other animals with contagious diseases like Strangles.