Strangles - What You Need To Know!
Posted on 27th September 2022 at 11:35
Strangles is a respiratory infection of horses, ponies and donkeys (and other equines) which is caused by the bacteria Streptococcus equi var equi. It is endemic to the UK, and many other countries around the world – this means that it is constantly present at a low level in the equine population. Outbreaks are seen commonly, therefore knowing how to recognise the signs and symptoms of strangles, and how to respond in an outbreak situation is important for any horse owner, or anyone running an equine establishment.
Signs & Symptoms
These can vary between individuals depending on their age and health status, and whether they have any immunity to strangles (either from previous exposure or vaccination).
Some of the most common symptoms are:
High temperature (>38.5°C) – this is often the first sign seen
Snotty nose (both nostrils)
Swelling or abscessation of lymph nodes in the jaw area (submandibular lymph nodes) or throat region (parotid lymph nodes)
In rare cases, strangles can cause abscesses elsewhere in the body (e.g. internal abscesses in the horse’s thorax, abdomen, brain) which can be fatal in a small number of horses.
These symptoms can take 3-14 days to develop from the point at which a horse or pony has been exposed to the infection.
How Is Strangles Spread?
The bacterium which causes strangles is highly contagious and therefore spreads very easily between
horses – either by direct cont act or fomite transmission – which means the transfer of the infectious agent on objects such as clothing or yard equipment. The bacteria can survive outside the horse in the environment for four to five days, or under exceptional circumstances for up to four weeks e.g. water troughs. Without preventative measures strangles will spread around most horses and ponies on a yard over a period of days to weeks. Waves of infection can close access to yards for a significant length of time.
As horses recover from strangles, the bacteria will survive within some horses (around 10%) from a few weeks up to a number of years. These “carriers” show no symptoms but can infect other horses with strangles – another reason why strangles can unexpectedly suddenly appear on yards.
Treatment is largely supportive in the form of non-steroidal anti-inflammatories e.g. phenylbutazone “bute” and encouraging horses to eat/drink to maintain their hydration. Sometimes surgical draining of abscessed lymph nodes is required, if they reach a size which is causing problems swallowing or breathing. Antibiotics are generally not recommended or required, as they can delay abscesses rupturing and therefore delay them starting to resolve.
Managing an Outbreak
Once infection is present on a yard the aim must be to prevent it from spreading to other horses.
This should include:
Stopping all horses and ponies moving on or off the yard to prevent spread to other yards
Minimising visitors to the yard where possible
Monitoring horses and ponies’ temperatures twice daily as this is often the first clinical sign seen
Separating horses and ponies into groups using a traffic light system:
“red” high-risk group – any horses with symptoms
“amber” medium risk group –potential contacts of those “red” horses
“green” low - risk group – horses with no symptoms nor contact with “red” horses
Isolation stables or fields should be as far as is practically possible away from the rest of the yard (although ideally in sight of other horses or ponies) – however, with strangles, direct contact or fomite transmission is required for infection, so isolation does not need to be as far away as with viral infections where airborne transmission is also a factor.
Infected groups should ideally have different carers and equipment i.e. feed/water buckets, hay nets, grooming kit, mucking out tools etc.
Disinfectant foot baths for dipping feet, hand sanitiser or hand washing facilities, protective clothing and provision for cleaning equipment should be available when entering and leaving each group of horses.
After the infection stops spreading between horses on the yard, they should be screened to make sure they have not become carriers using guttural pouch endoscopy – this testing can be costly and time - consuming but does ensure that affected individuals are not going to continue to spread the disease.
Preventing strangles from coming onto a yard in the first place should be a priority for horse owners and yard owners – firstly to prevent infection, but also to prevent a long period of lockdown where horses and ponies cannot leave the yard, and lastly to stop the high cost of treating affected cases then testing for freedom from strangles. It should also be noted that although horses that have been infected with strangles previously are usually protected from re-infection, the length of time varies from a few months to years, after which all will be able to catch strangles again – therefore they are not excluded from the prevention strategies outlined below:
Two different types of vaccination against strangles are currently available – a mucosal vaccine which is injected into the horse’s inner lip surface, and more recently an intramuscular vaccine has become available. The primary course of both vaccinations is similar, with a first vaccination and then the second vaccination 4 - 6 weeks later. Re-vaccination/booster vaccination varies between the two vaccines currently available, and may also depend upon the risk status of your horse or pony. It is worthwhile discussing strangles vaccinations with your veterinary surgeon, especially if you feel your yard is a high risk, or you are regularly leaving your yard to go to competitions and/or mix with other horses.
It is important to have biosecurity measures in place for new arrivals at your yard, ideally in the form of a written policy, to reduce the risk of a strangles outbreak being caused by a new arrival. Horses and ponies new to the yard should be isolated for a minimum of two to three weeks (ideally three weeks), ensuring no direct contact with existing horses on the yard (in the stables or when turned out) and no potential for fomite transmission of disease through the sharing of yard/grooming equipment, tack and rugs etc. They should be monitored carefully for the development of any respiratory signs (or any evidence of skin disease or other health problems) and their temperatures monitored once or ideally twice daily (normal range 36.5-38.5°C) whilst in isolation.
Isolation stables should have multi-purpose disinfectant footbaths and hand sanitiser points at their entry and exit points and be thoroughly cleaned and disinfected using multipurpose disinfectant or a sanitising fogger between horses.
Some yards require screening for strangles before horses are allowed onto yards – blood testing is available, but a one-off blood sample does have its limitations as it is not guaranteed to rule out the risk of a horse bringing strangles onto the yard. It is therefore important to remember that these tests do not replace the need to isolate new arrivals for two to three weeks prior to mixing with other horses. The blood test indicates whether or not horses are likely to have had recent exposure to strangles (within the last six months or so) – but it is not perfect – it can be negative in horses or ponies who are only just starting with an infection if there hasn’t been enough time for their immune system to respond to the infection, and can also be negative in “carriers”. A follow - up blood sample 10 - 14 days after the first sample can check for an increasing level of antibody response, and may increase or decrease the index of suspicion for recent exposure.
If positive, the blood test does not necessarily indicate that live bacteria are still present – any horse having a positive blood test will need to be further tested to see if the bacterium is still present using a wash of the guttural pouches, which is a more expensive procedure, but gives a much more accurate test result i.e would confirm or disprove current/active infection. Guttural pouch endoscopy is also the test of choice when confirming whether an infected case has cleared their infection – this is typically done four to six weeks after the resolution of clinical signs in all cases on an affected yard.
Article written by Dr Jessica Putnam BVMedSci(Hons) BVM BVS(Hons) MRCVS – October 2022
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