Controlling Strangles
This information is provided by the Animal Health Trust (AHT).
WHAT IS STRANGLES?
Strangles is a worldwide disease of horses, ponies and donkeys, but it is less common in thoroughbreds. It is reported as commonly as any other infection of horses (Animal Health Trust’s International Collating Centre). Caused by the bacterium Streptococcus equi, strangles cases typically develop a yellow nasal discharge; coughing and swollen lymph nodes of the head and neck which form abscesses that restrict the airway and oesophagus (giving the disease its name). In severe cases, the abscesses break through the skin, but milder cases with just a short-lived temperature do occur. Infection in 1-5% of cases may spread to any other organ in the body: this is usually fatal and is called bastard strangles. The incubation period is approximately one week and signs of the disease may last three to four weeks before a typical uneventful recovery.
WHY IS STRANGLES STILL A TROUBLESOME AND IMPORTANT DISEASE?
Many types of bacteria live on the skin and mucous membranes of their host animal largely without causing disease. However, the strangles organism is, with an important exception (see below), usually found only in association with disease. This should mean that there is considerable potential for controlling strangles by isolating cases and their contacts until all evidence of infection has gone, after which, they can be released into the general population.
In practice, strangles is difficult to control with flare-ups occurring as outbreaks seem to be coming under control or with apparently healthy horses spreading infection when they are moved to new homes. Unfortunately, we have found that approximately half of outbreaks produced a healthy long-term carrier, some of which were difficult to detect. This type of carrier could be unwittingly released into a susceptible population in the belief that it no longer presents an infectious risk.
Horses frequently make nasal contact, which probably means that the carrier animal is the most important vehicle of spread, even though the strangles organism can survive in the stable environment for several weeks, albeit in a state of uncertain infectivity.
CAN STRANGLES BE VACCINATED AGAINST?
Intervet produce a submucosal vaccine, ‘Equilius StrepE’. This consists of two primary doses, four weeks apart, with three monthly boosters for high-risk horses and six monthly boosters for medium risk horses.
CAN STRANGLES BE TREATED WITH ANTIBIOTICS?
Antibiotics may not always be useful, as the drugs cannot penetrate into the centre of an abscess where there is no blood supply. Nonetheless, early treatment with antibiotics may be helpful (i.e. at the first sign of a temperature) before lymph nodes have become enlarged. However, every case is different and each will need to be seen by a veterinary surgeon to make the judgement.
WHAT CAN BE DONE NOW TO CONTROL STRANGLES?
The main method of control is to strictly isolate affected animals, and all of their contacts, until there is no longer laboratory evidence of infection. Bacteriology culture of swabs that sample the back of the throat as well as the nose is used to detect the organism. The organism is often found only intermittently so three swabs taken at weekly intervals, which are negative is an indicator of freedom from infection for most animals. However, a few carriers harbour the organism in their guttural pouches (an enlargement of the Eustachian tube which runs between the middle ear and throat) and, so far, the only way to reliably detect these is to culture a sample taken with the aid of an endoscope.
It is not practicable to use widespread endoscopy so there is a need for a more reliable test utilising nasopharyngeal swabs. Molecular techniques, which offer this prospect, are now being investigated. However, until a more reliable test becomes available the ‘three negative swab’ regime can still help to reduce the release of infected animals and is extremely useful for managing strangles outbreaks.
Should you wish to discuss any of these issues in more detail, please consult your veterinary surgeon.