STRANGLES is a disease which strikes fear in the hearts of horse owners; even its very name is emotive. Strangles is the common name given to the bacterial infection, affecting horses and donkeys, which is characterised by abscessation of the lymph nodes in the head and neck. It is one of the most frequently diagnosed respiratory infections in horses worldwide and it has been described for over 800 years. This infection has the potential to cause major economic losses to the equine industry both within Ireland and worldwide due to its prolonged course, extended recovery period and associated serious complications, including possible fatalities.

Recent research has highlighted the extent of the problem within Ireland, estimating that over 40% of horses in Ireland have been exposed to this disease and may be chronic carriers of infection. Disease prevalence is generally considered to be much higher than official reports. The Irish Equine Centre diagnostic laboratory cultures the causative organism from many new outbreaks annually. Established outbreaks may last for months or even years particularly in large horse populations with frequent new arrivals that provide a supply of susceptible animals. Due to the nature of the equine industry, with widespread movement of horses for sales, breeding and competition, the potential for spread of this contagious disease is immense.

Transmission

Strangles is caused by the bacterium Streptococcus equi subspecies equi. Infection can spread rapidly through a population of horses particularly where horses are housed in close proximity to each other, for instance a stable yard. Up to 100% of in-contacts can become infected in a naïve herd. Disease spreads quickly and can persist in a yard for many months or even years. Disease can be spread by direct contact with secretions from infected horses or indirect transmission may occur via contaminated housing, water sources, feed or feeding utensils, equipment and tack. The clothing of handlers, caretakers, farriers and veterinarians may also play a role in the transmission of disease. Shared drinking bowls have been identified as a common reservoir of the infection and it is thought that the bacteria could survive in water for several weeks.

Clinical signs

The clinical signs of strangles are highly variable which highlights the importance of submitting swabs for testing from any horse with a purulent (pus containing) nasal discharge. Typically a high temperature (103-106°F) will be the initial presenting sign. Anorexia, lethargy, depression and a nasal discharge develop 24-48 hours after the rise in temperature. Initially the nasal discharge is clear but quickly becomes purulent. With progression of disease, the lymph nodes in the head and neck regions become hard and painful. Difficulty swallowing and breathing may occur due to the enlargement of these lymph nodes and the horse may stand with an outstretched neck to assist breathing and relieve lymph node pain. Often these lymph nodes rupture releasing a thick purulent heavily contaminated discharge. In uncomplicated cases, horses recover one to two weeks later. Some lower virulence strains of S. equi may cause milder clinical signs particularly in older horses.

These strains result in milder clinical disease with or without rupture of the lymph nodes. The majority of horses will recover fully from the infection. However, the disease may be prolonged and can cause considerable suffering to the affected horse. A developing abscess is extremely painful and undoubtedly affected horses really suffer. It is also estimated that up to 10% of horses develop serious complications known as “Bastard Strangles”. Furthermore, up to 10% of these latter cases eventually die, despite treatment.

Diagnosis

Diagnosis is initially based on recognition of common clinical signs. However, owners should call a vet to take a swab from the back of the nose from any suspect cases. Detection rates are high due to advances in the testing methods available. Once disease has gone through a unit, swabbing is a useful method to determine if animals remain infectious. Some experts recommend three nasal swabs at weekly intervals from recovered cases in order to evaluate when a yard can reopen after an outbreak.

Control

In the event of an outbreak, isolation is key to preventing the spread of infection. All exposed and clinically ill horses should be isolated for a period of four weeks if possible. Avoid indirect transmission by using dedicated equipment and work wear for isolated animals. Advise personnel about disinfection techniques and minimising traffic to and from the isolation area. Maintain a closed yard until the infection is controlled. Ideally no horses should enter or exit the farm until outbreak is over. Disinfect areas and equipment that have been used by infected and suspect horses, including stalls feeders, and remove as much organic waste materials as possible. Perform regular rectal temperature checks (at least every 12 hours) as feverish animals can be detected and removed from cohort animals before they begin shedding bacteria. Regular clinical checks are also useful to identify cases early. Identify the carriers of infection and treat or remove from the herd.

Prevention

Until recently prevention of strangles depended solely on management practices such as strict biosecurity for newly introduced horses and the maintenance of appropriate disinfection regimes. Stress should also be minimised to reduce the risk of infection.

A vaccine against strangles has recently returned to the market following a period of unavailability. Vaccination in conjunction with strict hygiene and biosecurity management can form a vital part of a comprehensive control programme. Vaccination is recommended if there is a risk of introduction of disease due to contact with horses from areas where this pathogen is known to be present. This live vaccine is available to reduce clinical signs and the occurrence of lymph node abscesses in exposed horses.

It is recommended that all horses stabled together are vaccinated. It is recommended that you would consult with your veterinary practitioner before using the vaccine, to discuss the risk profile for your horse/horses and a possible vaccination programme.

Treatment

International scientific opinions differ with regard to the treatment regimes for strangles. It is agreed, however, that the treatment plan for strangles infection should vary depending on the stage of the infection and clinical signs. For this reason treatment plans are best designed in conjunction with your veterinary surgeon. In early cases of disease, the veterinary surgeon may recommend antibiotic therapy or supportive care. Occasionally the veterinary surgeon may decide to lance affected lymph nodes to ease the pain experienced by the horse.

In more complicated cases of strangles, painkillers or steroids may be required. Endoscopic guided procedures may also be performed particularly for cases of guttural pouch empyema. There is no single treatment protocol for strangles; it is therefore wise to consult your veterinary surgeon for advice on the most suitable treatment options.

Joanne Cregg graduated from University College Dublin in 2007 with a degree in veterinary medicine. She initially worked in mixed practice in the midlands for seven years before joining MSD Animal Health in 2013. She is currently providing technical support on products and disease management to animal owners and veterinary practitioners throughout Ireland.