Parkside Veterinary Group

Strangles is characterised by a raised temperature, depression, Head Infection

nasal pus and enlarged glands of the head and neck which frequently become abscesses. The swollen glands can restrict the airways hence the name "Strangles". It is caused by the bacterium Streptococcus equi.

In some cases the disease may be very mild, causing only slight nasal discharge without a raised temperature or swollen glands.

Although most animals recover quickly and uneventfully, some cases become very ill for several days and a few may even die. Severe cases may take 3 to 4 weeks to make full recoveries.

Horses, ponies or donkeys can be affected by strangles which is the most common disease reported to the International Breeders Meeting/Animal Health Trust International Collating Centre.

Infection is usually restricted to the head and neck but, in up to 8% of cases, it may spread to other organs and cause abscesses. This is usually fatal and is known as "bastard strangles". Another complication which is usually fatal but less common, is known as "purpura haemorraghica". Affected horses bleed into the gums and other organs such as the lungs.

Diagnosis should be confirmed by culturing pus from abscessated lymph nodes, nasal discharge or throat swabs.

Can strangles be treated with antibiotics?
Antibiotics are not always useful as they cannot penetrate the centre of an abscess where there is no blood supply. However, early treatment with antibiotics are helpful if lymph nodes have not become too enlarged. Each case should be assessed individually.

Is it possible to vaccinate against strangles?
There is now a vaccine from Intervet, though it only lasts for 6 months.Immunity following infection is incomplete and short-lived. The Animal Health Trust is exploring the way that S. equi avoids the horse's immune system. This information is important in the development of a longer-lasting vaccine.

Spread of infection between animals
Transmission of S. equi infections requires fairly close contact between infected and susceptible animals and mechanical transmission is also common. Spread occurs frequently through shared water sources where the bacterium will live for long periods. Therefore outbreaks in grazing animals with shared water supplies can be fairly explosive. Spread of infection can also occur via tack or handlers and their clothes.

Particular care must 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 is largely dependent on management practices.

The incubation period of strangles varies from 7 to 14 days. However, because infected horses can shed the bacterium for long periods, the interval between new cases in an outbreak can be far longer, up to 3 weeks or more.

Shedding after clinical signs have disappeared
Once infected, most animals will eliminate the organism fairly rapidly. However, a significant proportion will excrete S. equi, which usually resides in the guttural pouch, for long periods after clinical signs have disappeared. In more than 50% of 22 outbreaks studies recently, at least one animal was found to carry the organism for more than a month after clinical signs had disappeared.

All 4 outbreaks where convalesced cases were examined by endoscopy by the Animal Health Trust had long term guttural pouch carries of S. Equi. Amongst all of these convalesced cases 10% were carriers. One of these horses has now carried S. equi for almost 5 years.Carriers of this type are often released into a susceptible population in the belief that they no longer present a risk. Therefore, many outbreaks of strangles occur after new animals, which are unknown carriers, are introduced into groups.

Detection and treatment of carriers
Animals carrying S. equi can be difficult to detect and negative results from bacteriological culture of a single nasopharyngeal swab do not prove absence of infection. Three consecutive swabs over a 2-week period greatly increase the chance of detecting a carrier, particularly within a few weeks of the end of convalescence. Established carriers, however, can go undetected by culture of repeated nasopharyngeal swabs for 2-3 months.

The Animal Health Trust has developed a PCR test to detect the DNA of S. equi which greatly enhances the ability to presumptively detect carriers. The Trust has also developed strategies for the treatment of carriers to eliminate infection from the guttural pouches using endoscopy*.

Control
Due to the chronic nature and common occurrence of carrier animals, it is impossible to tell when it is safe to mix convalesced horses with others. It is safer, at least in the medium term, to show that animals are no longer likely to be infected through culture and PCR of three swabs taken a week apart and by the continued isolation, investigation and treatment of presumptively identified carriers.

Segregation of infected from uninfected animals on premises can be an effective and important means of control. If extreme care is taken, it may be possible to resume normal operations on uninfected parts of premises. In these circumstances, personnel and equipment used in infected areas should not be allowed into clean areas.*This work is generously supported by the Home of Rest for Horses.

The Horserace Betting Levy Board has, for the first time, included Guidelines on Strangles in their 1997 Code of Practice.These guidelines suggest:

Monitor all new arrivals closely for 2 weeks.
Segregate and swab all horses that develop a nasal discharge.
Attempt to identify shedders amongst newly affected horses and their in-contacts by taking 3 nasopharyngeal swabs over a 2 week period and culturing for S. equi.
Place all infected horses and their in-contacts in strict isolation under veterinary supervision.
Do not allow new animals to enter the premises unless they can be kept in strict isolation.
Do not release from isolation any animal that has not had 3 consecutive negative swabs over a 2 week period.
Investigate in depth any animal that has carried S. equi for more than 2 months and treat the site of infection (eg guttural pouch). Particular care must be taken at all times to ensure that infection is not spread through handlers, shared water troughs, direct contact, bedding etc.

All outbreaks of strangles should be notified to the Thoroughbred Breeders' Association or, for non-Thoroughbreds, the Welfare Department of the British Horse Society.

The number of strangles outbreaks in the UK has increased in recent years and particular care must be taken to ensure that animals do not become infected and pass the infection on to other horses. Strangles is a very unpleasant and potentially fatal disease which causes a great deal of suffering. However, with care, outbreaks can be controlled and the worst effects avoided.