By Gemma Sherlock BVetMed MRCVS. Ashbrook Equine Hospital.
The disease ‘Strangles’ is so called because of the clinical signs that a horse may present with; enlarged lymph nodes which can result in respiratory obstruction and difficulty swallowing. Strangles is caused by the bacteria Streptococcus equi equi and is a highly infectious disease that is host-adapted to affect equines. There is currently no regularly used, single way to prevent the spread of strangles and, as there is always a low level of the disease present in the horse population, the key aspect is to implement strict hygiene measures both preventatively and also in the presence of an infection.
The first sign is a fever and this is usually followed within 24-48 hours with depression (and inappetance) and nasal discharge (starting as a clear discharge and becoming thicker and more pus like). The lymph nodes of the head then become enlarged and most horses develop a cough. Horses with a residual immunity, vaccinated horses and foals with maternal antibodies may have milder clinical signs but can still spread the infection. Horses less than 5 years old usually display the most severe signs.
Lymph node abscesses typically rupture between 7 days and 4 weeks following infection (either internally causing a nasal discharge or externally)
Transmission of the bacteria can be by direct contact but also through fomites (e.g. grooming kits, water buckets or owners). It can survive for 4 weeks in the environment in the ideal conditions. The incubation period (the time from exposure to the bacteria and the development of clinical signs) is 3-14 days. Horses are then infectious from a few days to approximately one month. To reduce the number of cases, it is very important to isolate horses as early as possible.
Diagnosis is by bacterial culture of the discharge from abscesses or nasal swab samples alongside clinical signs. Endoscopy and guttural pouch washes may also be performed if the cultures are not diagnostic.
Treatment & Management
Affected horses should be isolated and the ‘traffic light system’ can be implemented. The environment should be dust free and the horses should be cared for by individual people wearing protective clothing and using separate equipment (this should be cleaned and disinfected following use). Warm, soft palatable foods are more likely to be eaten. Temperatures of all horses possibly exposed to the bacteria should be taken twice daily and any horses developing an increased temperature (>38.5°C) should also be isolated. Horses are often not infectious until a couple of days after the development of a fever – this is why early diagnosis and isolation can significantly lower the risk of spread.
Warm compresses may be applied to the enlarged lymph nodes to encourage rupture of the abscesses – drainage of the abscesses will speed recovery. Ruptured abscesses may then be flushed with iodine until they are no longer discharging.
Anti-inflammatory drugs (such as ‘bute’) will often be provided to reduce the fever and pain associated with the enlarged lymph nodes and to improve appetite.
Antibiotics may be provided on a case by case basis. They are not always indicated as they may reduce the level of immunity that the horse develops and lengthen the time that the horses are affected, as they delay the rupture of the abscesses. However, in horses who are extremely unwell, they may be required.
Temporary tracheostomy tubes may be placed in severe cases.
Horses should remain in isolation for approximately one month. Removal from isolation depends on three negative nasopharangeal swabs at 4-7 day intervals or endoscopy and a negative guttural pouch wash.
Traffic Light System
This involves 3 groups of horses – red, amber and green.
- – Red group – horses with any clinical signs
- – Amber group – horses in direct or indirect contact with any horse from the red group
- – Green group – horses with no contact with the affected horses
Following infection with the strangles bacteria, some horses may become carriers – these are horses who carry and may shed the bacteria but are not clinically infected. They may have ‘chondroids’ in the guttural pouch which are balls of pus. Carriers may be recognised by blood samples and/or endoscopy and guttural pouch washes at least 3 weeks following infection.
The following complications are rare although can happen following infection with the strangles bacteria.
This is spread of the bacteria causing abscesses in other lymph nodes in the body, particularly in the abdomen and sometimes in the thorax. It is the most common cause of brain abscesses in horses (although rare). It is usually fatal.
This is a vasculitis that can vary in severity from a mild reaction to a fatal disease. Clinical signs include oedema (usually of the limbs), oozing and sloughing of the skin. The oedema can sometimes also affect internal structures resulting in colic, muscle pains and respiratory abnormalities. Treatment of this is with steroids although it is not always successful.
There is a lot of stigma associated with strangles and, for this reason, outbreaks are often not declared to the local community. However, it would be easier to reduce the spread if fellow horse and yard owners were aware. Strict biosecurity is the best way to limit the spread of the disease. New horses onto the horses should be quarantined and monitored for 2-3 weeks. Blood samples can be taken to check for exposure to the bacteria, however two samples are required 10-14 days apart to fully eliminate the risk of exposure. Care should also be taken with people coming on and off the yard, this includes visitors, vets, farriers, instructors etc.
There is currently no widely used vaccination for strangles. A vaccine was designed but then removed from the market. Most horses that have been affected by strangles will have some immunity although this will reduce over time and so a horse could get strangles more than once.
If you have any concerns that your horse may be affected by strangles, it is important to notify your yard owner and get the vet to visit to ensure quick diagnosis.
It is important to remember that although strangles has a high morbidity (high numbers of in-contact horses become affected), the mortality (death rate) is low. Spread of strangles can be dramatically reduced by notifications and biosecurity.