Strangles, also known as equine distemper, is a highly contagious upper respiratory disease of horses, mules, and donkeys. Strangles infection, or the fear of strangles infection, tends to unsettle many horse owners despite that fact that strangles itself is rarely fatal. The stigma attached to this disease is fuelled by its highly contagious nature and the striking, often “messy” clinical signs that accompany it.
Any age of horse can become infected with strangles, although generally it is a disease of the younger horse, less than five years of age. Cool and damp conditions tend to favour its occurrence.
Most horses contract the disease through contact with infected nasal discharges from a horse that has an active case of strangles, or from a horse that has recently recovered from strangles and is shedding the bacteria. Symptoms of infection begin to appear two to 14 days following exposure to the bacterium, Streptococcus equi. This bacteria initially invades the tonsillar tissue of the horse, then spreads to the lymph nodes of the head and back of the throat.
Within days the lymph nodes in the neck and throat-latch become enlarged, swollen and painful. Fever, poor appetite, depression and heavy nasal discharge commonly accompany infection. Afflicted horses may show difficulty swallowing and develop a soft, moist, guarded cough, often standing with their necks outstretched in an attempt to find relief.
When the infected and enlarged lymph nodes become increasingly enlarged, the horse’s breathing is likely to become impeded/obstructed. The resulting distressed/strangulating breathing sounds are what give this disease its name.
With time, abscesses in the lymph nodes mature, rupture and drain a creamy pus. The drainage is highly contagious. The lymph nodes in the space between the jawbones are commonly affected and drain externally. Other lymph nodes in the head and throat may also be involved, but are not as apparent because many of them will drain into the horses’s throat and nasal cavity.
Most horses recover
Typically the disease runs its clinical course in three to four weeks. Some horses exhibit a milder form of the disease with fever, cough, and nasal discharge, but no draining abscesses. These horses often have a level of immunity from prior exposure.
Most horses that contract strangles recover without complications, acquiring a lasting immunity. Symptomatic care while the disease runs its course is usually sufficient to aid recovery. This may include hot packing abscesses to hasten their maturation, followed by flushing of the ruptured abscess with dilute iodine solution. Strict hygiene is highly advised when attending infected horses.
Antibiotic therapy remains controversial and is often avoided in strangles cases. Veterinary involvement is advisable with animals in respiratory distress or with unrelenting malaise. Severe and complicated cases generally do require antibiotics and other supportive therapies.
Complications are most likely to arise when horses are immunocompromised or stressed and thus struggle with clearing the bacteria. This includes animals that are very young, old, heavily parasitized, in crowded or inadequate and unsanitary housing conditions or animals receiving inadequate nutrition.
When the immune system fails to contain the bacteria to the lymph nodes of the head and neck, the lymph nodes of the thorax, abdomen or brain can become infected. This form of the disease is known as “bastard strangles.” Another complication of strangles is an immune-mediated disease called purpura hemorrhagica. Both scenarios often have a poor outcome.
The most frustrating complication of this disease is the development of the chronic asymptomatic carrier. It is estimated that up to 20 per cent of all recovered horses shed S. equi from their nasal secretions for up to six weeks following resolution of clinical signs. It is these horses that tend to spread the disease as owners often feel that the horse is “recovered” and of no harm to other horses. Contaminated water buckets, feed tubs, housing or handlers can also source the infection. A very small percentage of horses continues to shed the bacteria for months to years beyond the original illness, serving as an ongoing source of recurrent infections.
The carrier horse can be very difficult to identify. Currently consecutive negative nasal swabs are cultured as a means to identify carriers. Maintaining good biosecurity measures, isolation and meticulous hygienic procedures are necessary to prevent introduction and spread of the disease.
Field experience suggests that vaccinating horses reduces the incidence and severity of strangles. Vaccination is not an absolute prevention nor is it deemed useful in all situations. It can be considered a tool in strangles management, not a replacement for good stable management and biosecurity. Thus far, intranasal vaccination results in the best local immunity.