Strangles is a disease of the lymph nodes of the equine upper respiratory tract. It is caused by the bacterium Streptococcus equi (s. equi) and is endemic within the horse population of the United Kingdom.
Affected horses typically have a high temperature (> 38.0 degrees), cough, poor appetite, nasal discharge and swollen or abscessed lymph nodes. Some infected horses may become very ill and the disease may become fatal if the bacterium spreads to other parts of the body (bastard strangles) or if the respiratory tract is occluded by swollen lymph nodes (hence the name “Strangles”).
Transmission of Disease
Direct contact between infected horses is the most obvious means of transmitting the infection but the hands and equipment of staff, farriers or veterinary surgeons can spread it indirectly. The bacterium is discharged from the nose and draining lymph nodes, and it may survive for a prolonged period of time in the environment, particularly in water troughs. The incubation period is usually one week, but can be longer in certain cases.
A small but important number of horses that have recovered from Strangles become persistently infected, and can harbour the bacterium in their guttural pouch for months or even years. These carrier animals are immune to re-infection, have no obvious clinical signs and can intermittently shed the bacterium to other horses. The identification and control of those carrier animals is one of the most important factors in disease prevention.
Ideally, all horses entering any stable premises should be quarantined for a period of 3–4 weeks and monitored closely, particularly in the period immediately after arrival. Any horse that develops nasal discharge, dullness, poor appetite or fever should be isolated and tested for Strangles before coming into contact with the resident horses.
Strangles can be diagnosed either by bacterial culture of nasal discharge, a pharyngeal swab, a guttural pouch lavage, or by a blood test. Please note that the blood test detects antibodies (acquired immunity after infection) rather than the organism itself. Newly exposed horses take 2 weeks to develop sufficient antibodies to give a positive result and may remain positive for up to 6 months after recovery.
The chronic carrier state is diagnosed or excluded by 3 sequential nasopharyngeal swabs taken one week apart, or by a one time lavage of the guttural pouch.
Control of Infection
The spread of Strangles may be limited by the early detection of shedders among newly affected horses and their in-contacts by appropriate testing. Any suspected cases should be isolated immediately.
Young and elderly horses are most susceptible to infection and should be monitored closely.
Regular disinfection of water troughs should be performed in order to minimize the infectious dose that in-contact horses receive and so reduce the severity of disease.
Horses should not enter affected premises unless they can be kept in strict isolation from all possible sources of infection. No infected or in-contact animal should be released from isolation until they have been tested negative for active shedding and the chronic carrier state.
Treatment of acutely affected animals is mainly supportive. Sometimes antibiotics are added to the treatment plan, but this should be carefully discussed with your veterinary surgeon.
Treatment of chronic carrier animals includes topical antimicrobial treatment of the guttural pouch followed by retesting procedure.
Testing of horses at the end of a Strangles outbreak
Following an outbreak of Strangles, the best time to detect a carrier horse is a minimum of 30 days after the last clinical signs are seen. Shedding usually ends rapidly after recovery although it may continue intermittently in chronic carriers.