Strangles is caused by bacterial infection with Streptococcus equi subspecies equi (referred to as S. equi). The bacteria typically infect the upper airway and lymph nodes of the head and neck. The disease has been in the equine population for centuries, and was first reported in 1251. The infection is highly contagious in horse populations, particularly affecting young horses, and can recur on farms with previous outbreaks of the disease. It is one of the most commonly diagnosed contagious diseases of the horse, worldwide. The persistence of this infection on farms is multi-factorial. The bacteria can survive in water sources for over a month, but the primary source of recurrent infections is most likely asymptomatic carrier horses that can shed the bacteria to other horses for months to years.
Clinical Signs of Disease
Historically, strangles got its name as affected horses were sometimes suffocated from large, infected lymph nodes that obstructed their upper airway or trachea. The hallmark clinical signs of infection are fever (temperature >101.5ºF), nasal discharge, and enlarged submandibular lymph nodes (in the space between the lower jaw bones), which ultimately abscess. Purulent nasal discharge is typically present, although it may initially be clear. The retropharyngeal lymph nodes (located deep behind the throatlatch) may also become enlarged and abscess. These will sometimes drain into the guttural pouches, which are air-filled spaces within the head that are an expansion of the Eustachian tubes. Guttural pouch infection and pus accumulation (empyema) are often the result of retropharyngeal lymph nodes that abscess and rupture into the guttural pouches. Guttural pouch infection may also occur from bacterial entrance through the pharynx (throat). Anorexia, depression and difficulty swallowing may also accompany signs of infection.
Though strangles is highly contagious and can affect many horses on a farm, most horses with infection recover without complication. Complications from the infection include spread of the infection to lymph nodes other than the head and neck (also known as metastatic infection or bastard strangles), immune mediated disease (such as purpura hemorrhagica), muscle disease and pain and lack of milk production. Horses that develop complicated infection typically require antibiotic and additional therapies based on veterinary examination.
Nasal discharge from the infected horse is the largest source of contamination. Sources of infection can be nose to nose contact with an infected horse, sharing contaminated water buckets, feed tubs, twitches, tack and clothing and equipment of handlers who work with infected horses.
Once the horse comes into contact with a potential source of infection, it may take three to 14 days after exposure before the horse will show the first clinical sign of strangles (fever). Based on this information, a minimum isolation period for introduction of new horses to the farm should be 14 days.
Clinical signs of strangles are highly suggestive of the diagnosis. There are three methods to confirm the diagnosis of strangles:
- Culture of the bacteria from the nasal discharge or abscess
- Polymerase chain reaction (PCR) that detects DNA of the S. equi bacteria
- Blood test (serology) that measures a titer to a specific protein (SeM) of S. equi.
Polymerase chair reaction (PCR) is a very sensitive test that detects bacterial DNA. PCR cannot tell the difference between live and dead bacteria, so is typically used in conjunction with culture. However, if consecutive PCRs are negative, the horse is unlikely to have strangles. Anywhere from four to 50 percent of the horses on farms with recurring strangles are asymptomatic carriers of the infection. Most horses will begin shedding (bacteria can be transmitted from nasal secretions to other horses) the bacteria from their nasal passages a couple of days after the onset of fever. Bacterial shedding occurs intermittently for several weeks. Some horses may continue to shed the bacteria for months to even years, serving a continual source of new infections on the farm. All diagnostic tests and treatment of affected cases should be done under veterinary supervision.
Antibiotic therapy remains controversial for the treatment of strangles. Complicated cases and those requiring tracheostomy for management of respiratory distress generally do require antibiotic and other supportive therapies. There is some evidence that treatment with antibiotics (such as penicillin) at the first sign of fever and in horses with no lymph node enlargement may prevent infection. However, early antibiotic treatment will also prevent these cases from developing immunity to the infection, and subsequently makes them susceptible to reinfection sooner.
Management of an Outbreak
The first and most important thing to remember in a suspected outbreak of strangles is to involve your veterinarian right away to determine the diagnosis and the best control practices for your particular farm. Strangles is a reportable disease in some states, and the state veterinarian may need to be notified as well. Movement of any horses on or off the farm should be stopped, and new horses should not be introduced. Take the temperature of all horses on the farm twice daily. Normal rectal temperature is 99 to 101.5°F. Monitoring the rectal temperature and isolating horses at the first sign of fever is one of the most effective ways to stop the spread of infection. Infected horses can transmit the bacteria to healthy horses one to two days after they develop a fever.
An isolated area should be set up for horses with fever and any other signs of illness (nasal discharge, etc). Extreme care should be taken not to mix horses with infection and horses exposed to horses with strangles to unexposed horses.
Ideally, three groups of horses should be created:
1) Infected horses
2) Horses that have been exposed to or contacted infected horses and
3) Clean horses with no exposure
Unexposed horses should be kept in a “clean” area, and ideally should have separate caretakers, cleaning equipment, grooming equipment, water troughs and pasture. People and equipment can transfer the infection from horse to horse. Extreme care, handwashing and disinfection of supplies must be observed by everyone involved. If different individuals cannot care for infected and healthy horses, then healthy horses should always be dealt with first.
- Dedicated protective clothing such as boots, gowns or coveralls and gloves should be utilized when dealing with infected horses.
- Work with infected horses last if the same personnel cares for the entire farm.
- Use dedicated equipment for infected horses (pitch forks, rakes, tack, etc.).
- Thoroughly disinfect equipment between horses (Read the label instructions on disinfectants to be sure they are used at the correct dilution and are active against S. equi).
- To disinfect stables and vans, remove all manure and debris first. Soak all surfaces with liquid disinfectant or steam treat the area and allow to dry.
- Disinfect water troughs daily that are used by infected horses.
- There is no evidence that S equi can survive for long periods on pasture.
- Pastures used to house infected horses should be rested for 4 weeks.
- Manure and waste feed from infected horses should be composted in an isolated location, and not spread on the pastures.
A serious challenge when dealing with an outbreak of strangles is identifying the horses that are carriers of the bacteria but are not showing any signs of illness. Ideally, all horses on the farm should be tested for strangles. Use of the bacterial culture and PCR, combined identifies carriers with a 90 percent success rate. Nasal pharyngeal swabs or washes can be done to sample the horses for infection. The washes improve the chance of identifying carrier horses. Additionally, all sick horses should be tested three consecutive times and be negative all three times before being put back with healthy horses. Previously infected horses can shed the bacteria for weeks to months, or even years in rare cases, which is why three negative test samples are recommended prior to reintroduction to the healthy herd. For the most accurate diagnosis of carriers and horses without obvious clinical signs, upper airway and guttural pouch endoscopy can be performed. This procedure allows for identification of infections that can develop in the guttural pouch, and culture of that area. Although disinfection, isolation procedures, and diagnosis can be costly; they are certainly cheaper than additional outbreaks on your farm.
Vaccination is one method for prevention and control of infection with S. equi. However, vaccination cannot guarantee disease prevention. With strangles, vaccination will likely reduce the severity of disease in the majority of horses that are infected. Available vaccines can be administered by intramuscular and intranasal routes. Improper administration of the vaccination can result in poor protection against infection and/or complications at the site of injection; therefore, administration by your veterinarian is recommended. The intranasal vaccination results in the best local immunity.
Vaccination is generally not recommended during an outbreak of strangles. If there are horses on the farm with no clinical signs of infection (fever, nasal discharge) and no known contact with sick horses, vaccination may be considered. Horses that have had the disease within the previous year also do not need to be vaccinated. Once recovered from an active infection, 75 percent of horses have immunity for one to two years. Vaccination of horses recently exposed to strangles (that have high antibody levels) may result in purpura hemorrhagica. Purpura hemorrhagica is caused by an over-active immune response within the horse, which can result in limb swelling, swelling of the head, and small hemorrhages on the gums. Vaccination is only recommended in healthy horses with no fever or nasal discharge.
Should I vaccinate my horse? The answer to that question depends on your horse’s chance of exposure to infection, and your personal comfort with the level of risk and in conjunction with your veterinarian. Generally, if your horse travels routinely and is exposed to varied or new populations of horses regularly, vaccination should be considered. Broodmares on farms with a history of strangles should be vaccinated prior to foaling. The initial vaccination requires a booster dose before being effective against infection. It takes about one month from vaccination for immunity to develop. Therefore, be certain to vaccinate your horse in advance of transport or potential exposure to new horses.
Additional Preventative Measures
Here are some additional suggestions for reducing your horse and your farm’s risk for infection:
- Require a current health certificate for new horse arrivals on the farm.
- Ask owners of new horses about a history of strangles and consider testing new horses to see if they are shedding the bacteria.
- If feasible, quarantine new arrivals for 2 to 3 weeks and monitor their temperature.
- All horses should have individual water buckets that are routinely disinfected.
- If shared water troughs are utilized, they should be routinely disinfected.
- When traveling to shows, minimize your horse’s exposure by bringing your own feed, buckets and equipment. Minimize use of shared stalls or pastures at show grounds.
- If horses are pastured together, group them according to their age and risk level (for example, all weanlings together, all broodmares together).
Reducing your horse’s exposure to unknown horses and utilizing routine disinfection measures will decrease the chance of infection with strangles. If your farm does have an outbreak, isolation and containment of sick horses will help reduce the spread of infection. The American College of Veterinary Internal Medicine (ACVIM) has developed strangles control guidelines, and can be downloaded atwww.acvim.org/uploadedFiles/Consensus_Statements/Strangles.pdf with detailed recommendations for diagnosis, treatment and control of infection. You may also visit the AAEP Web site at www.aaep.org/vaccination_guidelines.htm, for the complete AAEP Vaccination Guidelines.
- Sweeney CR, Timoney JF, Newton JR, and Hines MT. Streptococcus equi Infections in Horses: Guidelines for Treatment, Control, and Prevention of Strangles. J Vet Intern Med2005; 19: 123-134.
- Loving, NS. Strangles. The Horse; Nov 1 2007, Article #10688.
Special thanks to Amanda House, DVM, DACVIM, Dana Zimmel, DVM, DABVP, DACVIM and the AAEP Owner Education Committee.
Reviewed by Amanda House, DVM, DACVIM in 2016.