In our latest veterinary feature from Sussex Equine Hospital, vet Sarah Davidson MRCVS writes about Strangles.
Sarah Davidson MRCVS, Veterinary Associate at Sussex Equine Hospital
Strangles is a disease caused by a bacterium Streptococcus equi subspecies equi and affects the upper respiratory tract of the horse and more specifically the lymph nodes in this area. In 1% of cases, the bacteria can settle out and create abscesses in other organs of the body.
This is termed ‘bastard Strangles’ and can be fatal. It is endemic in the UK horse population, spreads quickly and easily and requires yards to be closed until they are deemed ‘clear’. As with all things, prevention is better than cure and this “article aims to outline how to minimise risk, prevent its spread and if an outbreak does occur, how to manage it and reduce collateral damage.
Yards that frequently see horses moving in and moving out should be more wary of Strangles, as opposed to smaller yards, where the resident horses rarely leave the premises or hack locally around the area.
Protocols that can be employed to reduce the likelihood of Strangles entering a yard are: requesting a blood sample be taken before a horse comes onto the yard; isolating new horses for a minimum of two weeks (10-25m away from resident horses); daily temperature checks; and being conscious of horse interactions when away at shows or competitions.
Strangles is most commonly transmitted by direct contact between an infected horse and a naïve horse, but transmission by equipment, personnel, clothing and boots is not unheard of. The bacteria is discharged from the nose and from any draining lymph node abscesses that may have formed. It can survive up to three days on fences and clothing and even longer in water troughs (up to four weeks). The incubation period, (the time from when a horse is exposed to when the horse shows clinical signs), is three-14 days, sometimes longer, and recovery time is approximately six weeks.
Some 90 per cent of horses recover from Strangles completely, but the remaining 10 per cent can harbour the infection for months or even years in their guttural pouches – a structure unique to equidae.
These horses are called carriers. They are immune to reinfection, show no clinical signs and periodically shed the bacteria creating the potential to infect naïve horses around them. Identifying and treating these horses is vital in disease prevention and stopping the spread.
How to recognise a horse with Strangles:
- Dullness and an increase in temperature will be the first sign
- Loss of appetite
- Nasal discharge
- Swollen lymph nodes that may form abscesses and burst out
While usually this is the extent of the disease, it can become more severe and in some cases be fatal. The swelling of the lymph nodes can obstruct the airway and this where the disease gets its name. Airway obstruction is serious as the horse may require an emergency tracheostomy (a procedure to open the airway in the neck region) to prevent it from asphyxiating.
Another complication is a purpura haemorrhagica syndrome caused by bleeding from the capillaries resulting in small red spots on the skin and mucous membranes and also causing marked swelling of the legs and occasionally the head.
If you have a horse with any of the above symptoms, call your vet immediately for a check. Your vet will carry out a thorough clinical examination and take any appropriate samples to facilitate diagnosis.
This may be a blood sample, a swab from the pharynx, direct visualisation of the guttural pouches and a sample, or a combination of the above. Swabs and samples demonstrate the presence of the bacteria itself and the blood sample shows the immune reaction to the bacteria.
It can take up to two weeks to show a positive response and may remain positive for up to six months following recovery.
If you have a positive diagnosis, your vet will direct you specifically about how to manage the yard, but it is all based on a few basic points:
- The affected horse(s) will be isolated from all other horses. The ideal scenario is a stable or field at least 10metres away with all its own equipment and water source
- Staff members should not move between the isolation area and the ‘clean’ zone and the yard should be completely closed down to horses leaving or entering. This includes hacking out
- Horses will be divided into three groups and managed completely separately, foot dips and over-clothing should be used and changed between groups. If there are enough staff, the groups should be managed by different people, but if not, staff should work from clean to dirty across the day
- RED – horses showing clinical signs even if ‘just’ a raised temperature. Cleanliness between horses is still important
- AMBER – horses that were in contact with the affected horse, but are apparently health, for example, sharing a water trough, sharing equipment or stabled next door
- GREEN – no known contact with horses in either the red or amber groups
- All healthy horses will have their temperature taken daily and any showing a raised temperature should be moved to the red group immediately
- Everyone who owns a horse on the yard should be made aware that Strangles has been diagnosed. There is commonly a lot of fuss and worry surrounding the diagnosis of Strangles, partly because on large-scale yards it can take a long time to bring under control and may end up having quite an economic impact. However, the fact remains that the more open people are about the situation, the better it can be managed by all involved
- Once horses are grouped in the red group, they are to remain there until three consecutive negative nasopharyngeal swabs are taken, or, for gold standard clearance, a one-off guttural pouch lavage with samples submitted for culture and PCR – a process that identifies bacterial DNA. This testing should be carried out a minimum of four weeks after the last horse resolves all clinical signs associated with the disease
- Horses in the green and amber groups should be blood tested
Assuming all tests come back clear, the yard can resume normal activity. Any positive or ‘grey area’ results will be treated accordingly’.
Treatment of a horse with Strangles is largely supportive unless complications arise. The use of antibiotics may be necessary in rare cases, but should be discussed with your vet.
Lymph nodes can be hot-packed to encourage maturation and abscess formation and some literature suggests that better drainage and resolution is achieved from this than by lancing the abscess before it bursts of its own accord. Identified carriers are treated by filling the guttural pouches with a gel form of penicillin followed by retesting two weeks down the line.