Strangles infection in horses and ponies
This disease is more common in the winter. It is characterised by a raised temperature, depression, purulent discharge from the nose and swollen head and neck glands, which often become abscesses. It is caused by the bacterium Streptococcus equi.
Some horses recover quickly when the disease is mild, causing only a slight nasal discharge, with no rise in temperature or swollen glands however some become very ill for several days even taking 3 to 4 weeks to make a full recovery, a few may even die.
Usually the infection is restricted to the head and neck however in up to 8% of cases the bacterium spreads to other organs, causing abscesses. This is known as 'bastard strangles'. Another complication that may occur is 'purpura haemorrhagica'. Affected horses and ponies bleed into the gums and other organs.
Strangles can be confirmed by culturing the bacteria from abscesses, nasal discharges or throat swabs.
Antibiotics are not always useful, except in the early stages, as they cannot reach the centre of abscesses. There is no vaccine currently available that prevents infection however animals that have had the disease usually become partially immune. The disease is spread between affected and non-affected horses by direct contact, shared drinking water, handlers or their clothes etc.
The incubation period is 7 - 14 days although infected horses can shed the bacterium for long periods and around 10% become carriers of the disease for at least a month after they appear to have recovered.
Horses that are carrying S. equi can be difficult to detect; negative results from a single throat swab do not prove absence of infection. Three consecutive throat swabs over a 2-week period improve the chances of detection, as does the PCR test developed by the Animal Health Trust. Due to the common occurrence of carrier animals and the chronic nature of the disease it is impossible to tell when animals are completely free from infection. Segregation of infected from non-infected horses on premises assists control.
From 1st March 2008 the Animal Health Trust is offering a new blood test which will distinguish those horses that have been exposed to S. equi during a strangles outbreak and may of become carriers. It can be used to screen horses prior to movement, competition or sales etc. Further details can be found at www.aht.org.uk