Strangles is a highly contagious bacterial infection of the upper respiratory tract in horses, caused by the bacterium Streptococcus equi. It gets its name from the swollen lymph nodes that can become so enlarged they compress the airway, making it difficult for the horse to breathe. In a group of horses that has never been exposed, infection rates can reach 100%, though the disease is rarely fatal in otherwise healthy animals.
What Causes Strangles
The bacterium targets the lymph nodes of the head and throat, particularly those under the jaw and around the throatlatch area. Once the bacteria enter through the nose or mouth, they colonize the tissue lining the upper airway and migrate to nearby lymph nodes, where they multiply and trigger abscess formation. These abscesses fill with thick pus and swell dramatically, sometimes to the size of a grapefruit.
Strangles spreads through direct contact between horses or through contaminated objects: water troughs, feed buckets, grooming tools, fencing, and human hands or clothing. The bacteria survive one to two days on dry surfaces, but in cold, wet conditions that window extends to 34 days. A contaminated water source in winter can remain infectious for over a month.
How Symptoms Progress
The incubation period ranges from 3 to 14 days after exposure. A sudden fever above 103°F is usually the first sign, appearing two to three days after infection. Over the next several days, the horse develops a watery nasal discharge that quickly turns thick and yellow or white.
About one week after infection, the lymph nodes under the jaw and behind the throatlatch begin to swell. These nodes become hot, firm, and painful to the touch. As they enlarge, the horse may have difficulty swallowing and stop eating. In severe cases the swelling partially obstructs the airway, causing labored, noisy breathing. Most abscesses eventually rupture on their own, draining large amounts of pus, and the horse begins to improve within a few days of drainage.
Other common signs include lethargy, depression, and a cough. Young horses and those with no prior exposure tend to develop the most severe symptoms.
Complications to Watch For
Most horses recover from strangles without lasting problems, but several serious complications can develop.
Bastard Strangles
Sometimes the infection spreads beyond the head and throat to lymph nodes deeper in the body, particularly in the abdomen and, less often, the chest. This is called metastatic strangles or “bastard strangles.” Abscesses can form in internal organs, and in rare cases the bacteria reach the brain. Streptococcus equi is actually the most common cause of brain abscesses in horses, though this remains uncommon. Bastard strangles is far more dangerous than the typical form and can be difficult to diagnose because the abscesses are hidden internally.
Purpura Hemorrhagica
In some horses, the immune system overreacts to proteins in the bacterial cell wall. This triggers inflammation of blood vessel walls throughout the body, leading to widespread swelling in the legs, head, and belly, along with small hemorrhages visible on the gums and inside the nostrils. Affected horses may develop painful, firm swellings in their muscles and under the skin, and in severe cases the condition damages the intestines and lungs. Purpura hemorrhagica can occur after natural infection or, rarely, after vaccination.
Diagnosis and Testing
Veterinarians can often suspect strangles based on the classic combination of fever, nasal discharge, and swollen lymph nodes, but laboratory testing confirms the diagnosis and is essential for identifying carrier horses that look healthy but still harbor the bacteria.
Two main testing methods are used. Bacterial culture grows live organisms from a nasal swab, a wash from the back of the throat, or pus from a draining abscess. The limitation is that samples containing large amounts of thick discharge sometimes come back negative on culture even when the bacteria are present. PCR testing, which detects the bacterium’s DNA, is more sensitive and can return results the same day. However, PCR picks up DNA from dead bacteria and from the live intranasal vaccine, so a positive PCR result can persist for up to 30 days after vaccination.
For horses recovering from strangles, guttural pouch washes are the gold standard for confirming a horse is no longer carrying the bacteria. The guttural pouches are air-filled sacs connected to the ear canal where the bacteria can hide for months. Clearing a horse requires three consecutive negative washes, each performed seven days apart over a 21-day period.
How Strangles Is Treated
Treatment depends on the stage and severity of the infection. In straightforward cases, the primary goal is to let the abscesses mature and drain. Warm compresses applied to the swollen lymph nodes can speed this process. Once an abscess ruptures or is lanced by a veterinarian, the site is flushed to keep it clean, and most horses improve quickly.
The role of antibiotics in strangles is a point of ongoing debate among veterinarians. Antibiotics given very early, before abscesses form, can sometimes halt the disease. But if abscesses have already started developing, antibiotics may slow their maturation without clearing the infection, prolonging the illness and increasing the risk of complications like bastard strangles. For this reason, many veterinarians reserve antibiotics for horses that are severely ill, struggling to breathe, or showing signs of the infection spreading internally.
Supportive care matters. Horses with strangles often have sore throats and may eat more readily if offered softened feed or a mash. Ensuring access to fresh water and a quiet, comfortable space helps recovery.
How Long Horses Remain Contagious
Most horses continue shedding the bacteria for about one month after they recover. Some become long-term carriers, harboring the organism in their guttural pouches without showing any symptoms. These silent carriers are a major reason strangles keeps circulating through horse populations: a seemingly healthy new arrival at a barn can spark an outbreak months after its own infection resolved. Guttural pouch washes with PCR testing are the only reliable way to identify carriers.
Vaccination
Two types of strangles vaccines are available, and neither provides complete protection.
- Killed (injectable) vaccine: Reduces the clinical attack rate by roughly 50% after the full booster series. It carries a higher rate of injection site reactions compared to other equine vaccines, including soreness, local abscesses, and occasional cases of purpura hemorrhagica.
- Modified live (intranasal) vaccine: Delivered into the nostril rather than by injection. It stimulates stronger immunity against experimental challenge than the killed vaccine. A small number of horses, especially those under two years old, develop temporary nasal discharge or mild lymph node swelling afterward, though these signs are not considered contagious.
Because purpura hemorrhagica can be triggered by vaccination in horses that already have high antibody levels from recent exposure, vaccinating during an active outbreak is generally avoided. Vaccination works best as a preventive measure in horses at higher risk, such as those that travel frequently or live at facilities with a lot of horse traffic.
Biosecurity and Prevention
Preventing strangles comes down to quarantine discipline and environmental management. New horses arriving at a facility should be isolated for two to three weeks and ideally tested before mixing with the resident herd. During an outbreak, infected horses need strict separation, with dedicated equipment, water sources, and handlers who change clothing and wash hands before moving between groups.
Cleaning contaminated areas requires attention to the conditions. In warm, sunny, dry weather, the bacteria break down within about two weeks on exposed surfaces. In cold or damp conditions, pastures and paddocks may need six weeks of rest before they are safe. Stalls, buckets, and tools should be disinfected thoroughly. The bacteria die quickly once surfaces dry out, so good drainage and ventilation help.
Because carriers can silently reintroduce the bacteria, any horse with a history of strangles in the previous few months should be tested with guttural pouch washes before being allowed contact with other horses. Three consecutive negative results, spaced a week apart, provide reasonable confidence that the horse is no longer shedding.