By Nora Grenager, VMD
Published in Bay Area Equestrian Network September 2006
Strangles is a highly contagious respiratory disease of horses caused by the bacteria Streptococcus equi subspecies equi. The disease was initially given this name because it can cause abscesses of the lymph nodes in the throatlatch region, which if severely enlarged can compress the airway and suffocate the horse.
If strangles is suspected on a farm, all horses at that location should be divided into three groups: infected horses with clinical signs, horses that have no clinical signs but may have been exposed to the bacteria or to a sick horse, and horses without clinical signs who have not been exposed. Rectal temperature should be taken daily of all horses on the property (or at least those that have been exposed). Transmission can be direct (horse-to-horse nose contact) or indirect (shared housing, shared water or feed containers, shared equipment, and people).
Infected horses typically get a fever 3- 14 days after exposure to the bacteria. This is followed by a large amount of mucopurulent (yellow-white snot) nasal discharge and painful swelling of the submandibular and retropharyngeal lymph nodes. Affected horses may be depressed, lethargic, have a sore throat, and be reluctant to eat. The lymph nodes will enlarge and eventually rupture, draining creamy yellow-white pus. Horses typically feel better once the lymph nodes have ruptured.
Culture of nasal swabs or abscess material is the best way to diagnose strangles. Cultures are diagnostic approximately 70% of the time. Another test called the polymerase chain reaction (PCR) is rapid and very sensitive at detecting bacteria, but cannot distinguish between live and dead bacteria. Serology (blood samples to detect the horse’s antibody levels to the bacteria) can be helpful but often necessitates two samples taken two weeks apart to confirm current infection. Multiple horses on a property with the typical clinical signs is also Abscessed submandibular lymph nodes Nasal discharge in horse with strangles suggestive of a strangles outbreak and should be treated as such.
If Strangles is suspected on a farm, all horses at that location should be divided into three groups: infected horses with clinical signs, horses that have no clinical signs but may have been exposed to the bacteria or to a sick horse, and horses without clinical signs who have not been exposed. Rectal temperature should be taken daily of all horses on the property (or at least those that have been exposed).
Horses that have clinical signs of enlarged lymph nodes should generally be treated with supportive care aimed at enhancing lymph node abscess maturation and drainage. Antibiotics are not typically recommended because they just delay, not prevent, the abscess maturation. It is best to use a hot pack or topical drawing agent to promote maturation of the abscess until it opens and drains on its own. Sometimes if the abscess is mature (has a soft center), drainage can be surgically performed by a veterinarian. Once the abscess is open, it should be flushed once or twice daily with a dilute antiseptic solution until there is no more drainage. Horses may be given anti-inflammtory medications such as Banamine or bute to help reduce fever and any pain.
Horses without clinical signs that develop a fever (if their temperature has been monitored daily) may be given antibiotics for 5-10 days (veterinarian-dependent) to prevent lymph node abscessation. They can also be treated with anti-inflammatories like the horses in the previous group. These horses will remain susceptible to reinfection after the antibiotic therapy is discontinued.
If strangles is suspected or diagnosed on a farm, a plan should immediately be implemented to prevent spread of infection. Every situation is unique and requires the veterinarian and barn manager/owners to develop the best quarantine and treatment plan for that location. Movement of all horses on and off the property should be stopped. Horses should be segregated according to the three previously mentioned categories (sick, exposed but not sick, not exposed and not sick) with no mixing of equipment (especially water troughs) or people between the three groups. All horses should have their rectal temperatures taken once daily to identify any new infections as early as possible. Ideally recovering horses should have 3 negative cultures or PCR samples prior to being considered noninfectious but this is often not economically practical. It is not entirely clear how long the bacteria can live in the environment, but most veterinarians recommend quarantining infected pastures or stalls for 4 weeks. Equipment can be cleaned with a dilute bleach (1:10) solution.
Up to 10% of horses can become carriers of strangles and shed infectious organisms, even without clinical signs. Most often these horses are harboring the Strep. equi bacteria in their guttural pouches. This infection can be a result of a retropharyngeal lymph node abscessation into the guttural pouch. It is easiest to diagnose a guttural pouch infection by endoscopically visualizing the guttural pouches. Your veterinarian may be able to perform endoscopy at the farm, but often it will need to be done at a referral facility. It is also possible to lavage the guttural pouch and collect the fluid for culture or PCR. This is a useful way to monitor a horse with a diagnosed guttural pouch infection.
Preventing strangles is obviously preferable to dealing with an outbreak but it can be difficult. When possible, a horse being brought to a new location should be isolated Culturette for 3 weeks to evaluate for any clinical signs. This is sometimes not feasible given that horses often mingle with other horses at events.
Horses that have had the disease usually develop a good natural immunity for up to 5 years. Vaccination and good biosecurity are ways to prevent outbreaks. Two basic types of vaccine exist; both require a booster at 2-4 weeks, and both should be given once a year. There are an intramuscular vaccine and an intranasal vaccine. The intranasal vaccine is generally associated with better immunity and less adverse side effects. It is best to talk with your veterinarian about whether your horse should be vaccinated, and with which vaccine. As with most diseases, vaccination decreases the severity and duration of clinical signs, but does not completely prevent the disease. Vaccine reactions are rare but include purpura hemorrhagica and guttural pouch empyema (discussed later). Some horses may get a mild strain of the disease with fever and lethargy following vaccination. The vaccine manufacturers recommend not vaccinating horses in the face of an outbreak unless there has been no possible exposure. This, however, should be tailored to each specific situation.
Most horses that get strangles need rest and supportive therapy but recover from the disease without complication. However up to 20% of horses can have complications such as guttural pouch empyema (infection), “bastard strangles,” or purpura hemorrhagica.
Guttural pouch empyema can be treated with an indwelling catheter lavage system, guttural pouch antibiotics, and systemic antibiotics. Occasionally the pus can become dried out and form little concretions called “chondroids,” which may need to be removed surgically. Horses should be quarantined while being treated for guttural pouch empyema. The only way to definitively prove that the infection is cleared is three negative guttural pouch cultures at weekly intervals.
Bastard strangles is when an abscess forms in a lymph node at a distant site such as the lung, abdomen, liver, spleen, kidneys, or brain. These infections have varying clinical signs depending on the location, and can be difficult to diagnose. There is a blood test (for “SeM-specific antibody titer”) that can be useful. These infections are hard to treat and require long-term antibiotics and sometimes even surgery. Normal guttural pouch Guttural pouch with chondroids and indwelling catheter.
Purpura hemorrhagica is an immune-mediated vasculitis, which means the horse’s blood vessels are compromised by immune complexes secondary to the bacteria. The compromised blood vessels lead to edema (swelling of the limbs and belly), petechiation of the mucous membranes (spotty gums), and fever all of variable severity. This is uncommon but can be a very serious complication requiring intensive treatment with a guarded prognosis.
Strangles is considered one of the three most significant respiratory diseases of horses. Because it is so highly contagious, and horses are a very mobile population, achieving prevention and control can be difficult. It is important to be aware of the typical clinical signs and discuss vaccination of horses at risk with your veterinarian.