Strangles is a contagious disease of horses caused by the bacterium Streptococcus equi. Typical signs include fever, loss of appetite, soft cough, purulent nasal discharge and swollen lymph nodes of the face, which may often abscessate and burst.
The swollen glands can restrict the airways – hence the name “Strangles”. In some cases, however, the disease may be very mild, causing only slight nasal discharge without a raised temperature or swollen glands. A carrier state without any obvious clinical signs is also possible.
Horses of any age can be affected. Younger animals (up to 5 years of age) are more often affected as they have had less opportunity to develop resistance. The disease occurs throughout the world and causes heavy economic loss in terms of the cost of treatment, quarantine measures and occasionally the death of affected animals. It is one of the most frequently diagnosed infectious diseases of horses.
What causes Strangles?
Streptococcus equi is a gram-postive bacterium, classified as Lancefield group C. It is not a normal inhabitant of the upper airway of the horse, and does not require prior infection with viruses to allow it to become established and cause disease. There are two features of the organism that contribute to its ability to cause disease:
- It is surrounded by a capsule of hyaluronate, which has a strong negative charge that appears to repell phagocytic cells (such as neutrophils – one of the white blood cells). Hyaluronate is also widepread throughout the body and its presence in the bacterium is likely to help “disguise” it from the body`s defence mechanisms.
- M-protein- a component of the cell wall – enables the organism to adhere to the epithelial cells of the nose and throat. It also protects the bacterium from digestion by phagocytes.
The severity of cases of strangles can vary. This may be because of differences in immunity in the animal or different strains of the bacteria. In one group of outbreaks, nearly a quarter of the S equi isolated lacked 20% of the surface M-protein but were still able to cause strangles.
What are the signs of strangles?
The incubation period may be as short as 3 days or as long as 14 days. The first sign may be fever and a clear nasal discharge, which soon becomes purulent. Swelling develops in lymph nodes of the head . The swelling may be so severe that the horse has difficulty breathing. The affected lymph nodes usually take 7 – 10 days to rupture. In uncomplicated cases, the disease usually runs its course in about 3 weeks. Infection is usually restricted to the head and neck and most horses recover uneventfully.
In up to 8% of cases, it may spread to other organs, when it is known as “bastard strangles”, and can be fatal. Abscesses can occur anywhere in the body, but are most often found in the lungs, abdominal lymph nodes, liver, kidney, spleen and brain. Another complication which is usually fatal, but less common, is “purpura haemorrhagica”. This is an immunecomplex reaction to the streptococcal antigen.
Affected horses are depressed, and have marked swelling of the legs and belly. They may bleed into the gums and other organs such as the lungs.
Other complications that have been reported include laryngeal paralysis, infection of the heart (endocarditis, myocarditis) and brochopneumonia.
How does it spread?
The most important way strangles spreads is by direct contact between infected and susceptible animals. This requires fairly close contact between the horses. Indirect transmission is also common. S. equi will live for long periods in shared water sources. As a result, the disease can spread quickly in grazing animals sharing the water supply. The infection can also spread on tack or handlers and their clothes. Particular care should be taken with sleeves and other areas likely to come into contact with the horse’s face. The ease with which the disease spreads through groups of animals depends largely on management practices. The incubation period of strangles is usually less than 14 days. However, the interval between new cases in an outbreak can be far longer – up to 3 weeks or more – because infected horses can excrete S. equi for long periods after clinical signs have disappeared.
Strangles can appear without the introduction of obviously sick horses. There is increasing evidence that carrier horses play an important role in the spread of infection to susceptible animals. A recent study found that 15 / 22 outbreaks (68%) produced at least one horse from which S equi could be isolated more than 4 weeks after the disappearance of clinical signs.
The guttural pouches are the main site of S. equi colonisation. Carrier animals may remain a potential source of infection for a considerable time. An earlier investigation found that some horses continued to harbour S. equi for many months after signs had gone.
One horse remained infected for over three years. Carriers are often released into a susceptible population in the belief that they no longer present a risk. So, many outbreaks of strangles occur after new animals, which are unknown carriers, are introduced into groups. According to Richard Newton of the Animal Health Trust in Newmarket, England, “People can’t rely on the fact that horses just get over strangles and no longer pose an infectious threat after a particular period of time. This is a concept that veterinarians are only just coming to terms with, and so it may be quite new to most horse owners.”
How is it diagnosed?
The clinical signs are characteristic. A horse with a swollen throat and a thick yellow purulent discharge almost certainly has strangles. The diagnosis should be confirmed by culturing pus from abscessed lymph nodes, nasal discharge or throat swabs. “Nasopharyngeal” swabs, taken from the back of the nose and throat, are most likely to detect the organism .
All confirmed cases of S. equi infection, in Thoroughbreds or animals in contact with Thoroughbred horses should be notified to the Thoroughbred Breeders’ Association. Cases in non-Thoroughbreds should be reported to the Australian Horse Industry Council.
Identification of carrier animals
Animals carrying S. equi can be difficult to detect and a negative result from bacteriological culture of a single nasopharyngeal swab does not prove absence of infection. Three consecutive swabs over a 2-week period greatly increase the chance of detecting a carrier, particularly if the horse has only recently recovered from the disease. Established carriers, however, can go undetected by culture of repeated nasopharyngeal swabs for 2-3 months.
Researchers at the UK Animal Health Trust have developed a PCR test to detect the DNA of S. equi.4 Polymerase chain reaction (PCR ) is a very sensitive technique. It can detect an individual molecule of target DNA, and so is able to identify a single organism. PCR in conjunction with culture is much more sensitive than culture of swabs alone.
The PCR test on nasopharyngeal swabs is most useful as a screening test to indicate which horses warrant further investigation. The next step in detecting carriers is to examine the guttural pouches. Most carrier animals show signs of disease in the guttural pouches – either purulent discharge or “chondroids”. “Chondroids” are hard dried aggregates of purulent material.) S. equi can be cultured from washings taken from the pouches.
How is strangles treated?
1. Treatment of individual animals.
Strangles is difficult to treat effectively because antibiotics do not penetrate the centre of an abscess where there is no blood supply. However, resistance to antibiotics commonly used against S equi is very rare, and so early treatment with antibiotics may be helpful if lymph nodes have not become enlarged. Each case should be assessed individually.
- In the early stages of the disease, before abscesses have started forming, penicillin is very effective at killing the bacteria responsible.
- Once an abscess has formed it is best to allow the absecces to burst. Antibiotics at this stage tend to “damp down” the infection, but not eradicate it completely, so it may flare up again once treatment stops. Fomenting the abscessed lymph nodes with hot cloths or compresses may encourage them to burst and drain. Most cases recover completely and are soon free from infection.
2. Prevention of further spread.
The spread of infection can be controlled by detecting infected horses early and isolating them until they are free from infection. Shedding of S. equi usually ends rapidly after recovery but may be intermittent. To detect carrier horses, it is recommended to take three nasopharyngeal swabs at 5-7 day intervals over a two week period and culture the swabs for S. equi. Three negative swabs indicate freedom from infection in the great majority of cases – but not all. Recovered cases may still be carriers despite undergoing three negative swab tests.
Young animals are most susceptible to infection and should be monitored closely. One approach to controlling an outbreak is based on that used successfully by workers at the Animal Health Trust (UK):
- Isolate infected horses and those that had been incontact with them. Strict hygiene measures should be employed: eg dedicated equipment for each group, disinfection for stable staff, thorough disinfection of stable and equipment. Stop all movement of horses on and off the premises.
- Monitor horses that have been in contact with sick animals or have shared the same pasture. Check their temperatures twice daily. An increase in temperature might indicate that the horse is about to develop the disease. Antibiotic treatment is likely to be effective at this stage.
- Take nasopharyngeal swabs from recovered cases and in-contact animals on three occasions at weekly intervals. These samples should be cultured for Streptococcus equi. They can also be tested by PCR, for evidence of S equi M-protein, which is more sensitive.
Place the horses in two groups according to the results of the swabs:
1. non-infectious :(S equi is not cultured, and at least the last PCR test is negative.)
2. potentially infectious: (S equi is cultured or detected on PCR.)
• Monitor the infected group by examining the guttural pouches. Carry out bacterial culture and PCR on samples from the guttural pouches.
• Treat any carriers:
1. flush the guttural pouches to remove any infectious discharges, and remove any “chondroids”.
2. give antibiotics (usually penicillin or potentiated sulphonamides.)
Is there a vaccine?
Over the years there have been many attempts to produce a vaccine to protect against strangles. No vaccine is currently available in the UK. Most of the commercially available strangles vaccines available worldwide contain the M-protein from the S equi cell surface- either in an extracted form or as part of the inactivated whole cell. These vaccines, however, frequently produce adverse effects and induce poor immunity against experimental infection. Recent research carried out in the Netherlands and Ireland in the search for a safe and effective vaccine against strangles has produced encouraging results.
Dr Tom Jacobs and his colleagues tested three different vaccines and three different vaccination routes in Shetland ponies. Two weeks after the last vaccination the ponies were challenged by intra-nasal application of a virulent strain of S. equi. A live vaccine, produced from genetically modified S equi and administered by injection on the inside of the upper lip appeared to be a safe and effective method of vaccination.
Researchers at the Animal Health Trust are currently working to identify different sub-types of S. equi using a variety of techniques including pulsed field gel electrophoresis and DNA analysis. They hope that by correlating their findings with information on the severity of the disease caused by each subtype they may be able to locate the genes which are responsible for severe disease. Hopefully this information will allow them to develop an effective vaccine.
NOTE FROM HORSES AND PEOPLE EDITORS: In Australia there is a vaccine that is injected intramuscularly. This vaccine, like all strangles vaccines worldwide, does not claim to completely prevent infection but is expected to reduce the likelihood and severity of infection if used properly. Like other vaccines, it can also cause reactions in some horses.
Horses affected with strangles or known to have strangles within the past year should not be vaccinated, due to the possibility of developing an immune reaction. During an outbreak, only horses unlikely to have been in direct or indirect contact with infected animals should be vaccinated.
Strangles is a highly-contagious disease with potentially devastating consequences. Horses likely to have contact with a variety of other horses should be considered for vaccination. Decisions regarding vaccination should take into account risk of exposure, any possible reactions and the consequences of infection, and are best taken in consultation with your veterinarian.