Strangles is a highly contagious respiratory and abscess causing disease of horses caused by the bacteria Streptococcus equi subspecies equi (Strep equi). This bacteria can only cause clinical disease in horses, donkeys, and mules.
Infection is classically characterized by fever, pharyngitis (inflamed/sore throat), and then subsequent abscess formation in the lymph nodes of the head and neck region. The disease can occur in any age although young horses seem to develop more severe clinical signs. Fever and lethargy are often the first signs seen within 3-14 days of exposure. The pharyngitis will cause horses to be reluctant to eat or drink, hold their head in an abnormal position, have nasal
discharge, and sometimes a cough. The lymph nodes (most commonly under the jaw and just behind the jaw) become swollen and form abscesses that typically rupture at 1-4 weeks post infection. The abscesses have thick walls and when rupture drain a thick white odorless discharge. Abscesses usually rupture externally but can rupture into the guttural pouches which can result in a chronic infectious state. Occasionally swelling from the abscesses/pharyngitis will cause obstruction of the upper respiratory tract and require placement of a temporary tracheostomy. Horses may also have difficulty swallowing feed or water due to inflammation of the surrounding nerves. Occasionally the abscesses are seen in other sites of the body (known commonly as bastard strangles) such as the brain, abdomen, and mammary gland.
Strangles is spread by the purulent discharges of infectious animals through normal equine behavior such as nose/head contact and attaches to the cells of the tonsils through the nose/mouth. It is most commonly from direct contact with an infectious horse but can also be from indirect transmission with a fomite, such as contaminated water sources, feed, bedding, tack, and clothing and equipment of handler . It has also been shown that the higher dose of
bacteria the horse is exposed to may result in more serious disease.
Nasal shedding of S. equi begins 2-3 days after onset of fever, and persists for 2-3 weeks in most animals, but can be up to 6 weeks or longer in horses with persistent guttural pouch infection. A common source of infection is a horse that is clinically healthy but still carries the organism in their nasal secretions. An average of 10% of horses become long-term carriers of Strep. equi due to persistent infection in their guttural pouches. The photo below depicts the location of the guttural pouch in horses, an expansion of the eustachian tube (leading from pharynx to the middle ear) unique to horses and a few other species. Abscesses behind the jaw in the retropharyngeal lymph nodes can rupture into the guttural pouches instead of externally.
Although clinical signs and bloodwork supportive of a bacterial infection can be highly suspicious for strangles the confirmation is by culture or PCR (polymerase chain reaction) of Strep. equi. Sampling can be from an abscessed lymph node or a deep swab of the nasal discharge. If the horse does not currently have large amounts of discharge, or is clinically healthy, a nasopharyngeal wash can yield better results. There is a blood antibody test available that is used for the purpose of diagnosing internal strangles infection, to support a diagnosis of purpura hemorrhagica, or to evaluate horses post-exposure if safe to vaccinate.
The majority of strangles cases require supportive care only. This consists of isolation, rest, good housing, soft palatable food, and easy access to water. Bute or Banamine can be given to help control fever and inflammation. Antibiotics are unnecessary in most cases and may cause a delay in maturation of abscesses or recurrence of disease when antibiotics discontinued. Approximately 75% of horses develop natural immunity to strangles secondary to exposure/infection but may not if antibiotics are used. Antibiotics (usually Penicillin) will be used in horses that are in respiratory distress, have internal abscessation, guttural pouch infections, and horses with very high fevers and lethargy. Lymph node abscesses will usually rupture on their own, but surgical drainage is sometimes necessary, although is not performed until abscess is mature as evidenced by a thinned wall. Daily flushing with dilute betadine is performed after the abscesses have opened until they are healed.
Complicated cases with guttural pouch infections or internal organs infected require more involved treatment, long term antibiotics (Penicillin), flushing of the pouches or abscesses, and supportive care as needed. Occasionally a temporary tracheostomy is needed due to swelling of the pharynx causing respiratory difficulty/distress.
There are two different types of immune mediated complications secondary to Strep. equiexposure. Purpura hemorrhagica is a vasculitis (inflammation of blood vessels) with symptoms of edema/swelling and hemorrhage under the skin. It can be associated with infection with Strep. equi or vaccination. It can vary from mild signs to a severe, fatal disease and is treated with steroids, antibiotics, and supportive care (fluids/bandaging/hydrotherapy). It is not recommended to vaccinate a horse with a high antibody titer as it is much more likely to develop purpura. Myositis (muscle inflammation) is another type of complication after exposure to Strep. equi. Clinical signs are stiff gaits with swelling along back and gluteal muscles, and can result in dramatic muscle loss after recovery. Treatment is similar to purpurahemorrhagica.
Vaccination. There are two types of vaccination available, an intramuscular protein extract vaccination, and an intra-nasal modified live vaccination. The intramuscular vaccination has shown to be only 50% effective and is therefore not used in our practice. The modified live vaccine is given to healthy horses greater than 9 months of age with an initial 2 does series (3 weeks apart) and then once yearly. This vaccine should not be used during an outbreak unless
the horse has had no contact with exposed or infected horses. Horses that have had strangles within the last year should not be vaccinated, due to higher risk of developing the immunemediated disease purpura hemorrhagica. Quarantine: All new arrivals to a property should be quarantined for 3-4 weeks. If suspect, can perform screening with culture or PCR before introduction to the herd.
In suspected outbreak, stop all movement on/off affected premises immediately. If Strangles is confirmed, quarantine should last a minimum of 3 weeks post resolution of all clinical signs. Isolate all sick horses. Take temperatures of all horses daily – fever precedes nasal shedding so can help stop spread if febrile horse if isolated immediately. Biosecurity: Dedicated personnel and equipment must be used, or always treat/feed/clean sick horses last and disinfect after. Always disinfect all potentially contaminated equipment. Strep equi is susceptible to most common cleaners, so thorough cleaning including initial removal of organic debris and then use of a disinfectant should be effective. Resting of pastures for a couple weeks is recommended if possible but Strep. equi has not been shown to survive on fencing or soil longer than a couple days. However, it can remain viable in water for up to 4-6 weeks, so cleaning/disinfecting of all troughs is necessary.
Biosecurity is also important when traveling with your horse and exposing them to horses with unknown vaccination/infection status not just for Strangles but for many other contagious diseases. This includes cleaning/disinfecting of stalls and feeders, bringing your own equipment (grooming/buckets/manure forks), and preventing nose to nose contact with unknown horses.