Strangles in Horses
What is “Strangles” infection?
“Strangles” is the lay term that was given to horses suffering from pharyngeal lymph node swelling that causes difficulty eating, swallowing, and in rare instances, even breathing (hence the name “strangles”). Strangles is caused by Streptococcus equi bacteria, the most important of which is the highly contagious Strep. equi spp. equi. Another subspecies, Strep. equi spp zooepidemicus, is a commensal bacterium of horses that can also create Strangles-like symptoms, but because it is a commensal bacterium, it does not have the same contagion risk.
What are the clinical signs of Strangles?
The first sign is typically fever and lethargy occurring 3-14 days after exposure. Fevers can be very severe, up to 42C (107.6F)! Fevers typically persist until the lymph node abscess ruptures.
The next sign that appears is pharyngitis, making horses reluctant to eat or drink, holding their head in abnormal, stretched-out positions. Occasional soft cough may be heard while eating. Squeezing the larynx can cause significant pain or gagging, followed by a cough.
Lymph nodes swell in the submandibular and retropharyngeal regions, and occasionally the parotid and cranial cervical regions as well. Abscesses then rupture around 7 days to 4 weeks after infection. The abscesses mature, oozing serum from the skin before rupturing and draining thick purulent material. If the abscess ruptures into the airway or the guttural pouch, horses can present with thick nasal discharge. If large amounts of discharge are observed when coughing, eating or the head is lowered, a guttural pouch empyema or infection, is likely.
Chronic infection of the guttural pouches occurs in some instances and can lead to pus drying and forming hard balls, called “chondroids”. These chondroids harbour the bacterium and the horse becomes a chronic, intermittent shedder of Strep equi without clinical signs.
When infection with Strep. equi is not confined to the upper airway, “bastard strangles” may occur, forming abscesses in
How is Strangles diagnosed?
Diagnosis of Strangles begins with a physical examination. Many of the clinical signs are suggestive of a Streptococcal infection including sudden fever, submandibular lymph node swelling, difficulty swallowing, and thick nasal discharge.
Confirmation of Strep. equi infection requires pathogen identification using qPCR or bacterial culture. Samples can be collected from the nasal discharge, pharyngeal lavage, or the guttural pouch. As the bacterium is harboured within the guttural pouches, this is the preferred sampling location to ensure accurate diagnosis. Sampling from the nose or pharynx may yield “false negative” results, meaning the horse is actually infected but the samples did not reveal the bacterium.
Endoscopy of the guttural pouches is essential to confirming retropharyngeal abscessation, and helpful for collecting samples. Endoscopy also provides visualization of the severity of infection within the guttural pouches and identification of chondroids, if present.
In some instances, routine blood analysis is also recommended to help define the severity of disease and guide treatment when symptoms beyond the lymph nodes are observed.
How do you treat Strangles?
In mild cases involving only general malaise and external lymph node swelling and abscessation, the disease is allowed to run its course without further intervention. This has been demonstrated as effective, generating a good immune response, and limiting secondary disease or prolonged shedding. Hot packs can be applied on the lymph node to promote the abscess softening and rupturing more quickly. Occasionally, anti-inflammatories are provided to improve comfort and reduce fevers.
In more severe cases, where swallowing is affected or vasculitis is present, systemic antibiotics may be prescribed in addition to anti-inflammatories. Treatment with antibiotics and anti-inflammatories can slow the progression of disease, and may reduce the amount of immunity generated. Both lead to prolonged shedding of the bacterium potentially, therefore a “risk vs. benefit” discussion is required for each case presented.
When the guttural pouches demonstrate signs of infection or pus accumulation, local therapy may also be recommended. Rinsing the guttural pouches to remove the purulent debris and then infusion of the pouch with penicillin is highly effective in treating more severe or chronic Strep. equi infections. Treatments may be performed once, in mild cases, or repeated daily for one week if significant pus is present.
When chronic infection of the guttural pouches has led to the formation of chondroids, then removal of the chondroids must be achieved in order to prevent future bacterial shedding. Small chondroids can be removed using endoscopy-guided basket retrieval. When larger and more numerous chondroids are present, surgical debridement of the guttural pouch may be required.
How do you prevent Strangles?
Strangles prevention requires a 2 pronged approach: unexposed, clinically normal horses can be vaccinated, and exposed or clinically diseased horses must be quarantined.
An attenuated live, intranasal vaccine, Pinnacle IN, should be administered only to healthy non-febrile animals free of nasal discharge. The vaccine is given in a schedule of 2 doses at 2–3 week intervals, and an annual booster. As the vaccine is live, there is a risk of residual virulence with formation of slowly developing mandibular abscesses in a proportion of vaccinates, nasal discharge, and occasional cases of immune-mediated vasculitis (purpura).
Vaccination should not be administered to any horse that may be in contact or have been in contact with a horse shedding Strangles, or horses who have recently (<1 year) recovered from Strangles. Severe reactions can occur and can be fatal in some instances.
Serologic values are not a measure of protection; the SeM-specific titer cannot be used to determine carrier status and a single value is not a measure of active infection. Titers wane over time and horses that received antibiotic treatment during an outbreak seem to mount a reduced immune response and remain susceptible to reinfection. If you are unsure if your horse was exposed to Strangles, and you wish to vaccinate, a serologic test is recommended to ensure that the titers are low (SeM ELISA < 1:3200), indicating that a reaction to the vaccine is unlikely.
How do you quarantine horses?
Quarantine is the most important method of preventing the spread of Strangles. New horses to a property should be isolated from others for at least 3 weeks, and ideally should be tested for Strangles before being introduced to the herd. New horses, or suspect cases should not have nose-to-nose contact with other horses, should be handled with dedicated clothing, gloves, grooming supplies and mucking tools. All bedding, feed, buckets and blankets should also be kept separate from other horses.
Surfaces should be cleaned with a foaming soap agent to remove organic material, rinsed and then thoroughly soaked in an appropriate liquid disinfectant used according to the manufacturer’s guidelines and allowed to dry. Some products commonly used include household bleach, Lysol, Dettol, Virkon, and Peroxigard. Use of high pressure systems create risk of aerosolization of bacteria, spreading contagion. Although there is no evidence for prolonged survival of S. equi on pastures, those used to hold infectious animals should be rested for several weeks after animals are removed. The bacteria is best inactivated by dry weather and direct sunlight.
During an outbreak, S. equi is normally not present on the mucosa until 24–48 hours after the onset of fever, and so horses monitored by daily measuring of rectal temperatures during an outbreak may be recognized early and isolated to limit transmission of S. equi. Furthermore, approximately 10% of exposed and infected horses never develop guttural pouch drainage and chondroids. These horses with chronic infection have been found to shed bacteria for over 60 days after the onset of the outbreak, and some become life-long carriers. Unfortunately, those horses infected with Strep. equi who develop clinical signs and lymph node drainage can still shed the bacterium for up to 3 weeks after resolution of clinical signs, making screening tests vital to controlling outbreaks.
When do you lift the quarantine?
At the end of the outbreak, horses and facilities need to be released from quarantine. This is a difficult process as subclinical shedding can spread and start a new outbreak.
Infected horses should be tested again either 3 weeks after resolution of clinical signs, or at least 3 weeks after antibiotic therapy – or whichever is longer. Exposed horses that did not develop clinical signs should similarly be tested 3 weeks after potential exposure.
Animals are considered safe to move out of quarantine when they are free of clinical signs AND have a negative status. Negative status is determined either by 1 negative guttural pouch lavage and qPCR, or 3 negative nasopharyngeal lavages performed once weekly for 3 weeks.
For more information:
ACVIM consensus statement 2018: Streptococcus equi Infection in Horses: Guidelines for Treatment, Control, and Prevention of Strangles – Revised Consensus Statement, J Vet Intern Med. 32: 633-647.