Competition horses commonly travel long distances and mix with large numbers of other horses at shows and competitions, providing the ideal conditions for infectious diseases to spread.
Thus, a single horse infected with the influenza virus, for example, may spread the infection to numerous susceptible horses at a show or event. These horses then carry the infection home, and spread it to more susceptible horses, which in turn spread it to other horses.
The ease and speed by which such an epidemic can spread depends on many factors, including the nature of the infectious agent, the way the infection is spread, the level of immunity among horses, environmental conditions, and more.
Currently in the UK there are a limited number of contagious equine diseases that we need to be aware of and try to control. However, with the increasing international travel of horses, as well as possible climatic changes, there is an ever-present threat of the emergence of new or “exotic” diseases, such as West Nile virus.
Respiratory infections are currently the most important types of contagious diseases that can spread among the horse population in this country. These include the influenza virus, equine herpes viruses-1 and -4, and strangles.
Equine flu is a major and economically important cause of acute respiratory disease. The incubation period is short — two to three days — and the virus can therefore spread rapidly. Infected horses can shed the virus for up to 10 days.
Horses of all ages are susceptible, but infection is most common in young horses (two- to three-year-olds) and unvaccinated horses. Low-level infection may occur in vaccinated horses, but they are unlikely to develop serious disease and are less likely to spread the virus to others.
Equine herpes viruses
There are a number of different types of equine herpes viruses, but types 1 and 4 are the most important. Infection by equine herpes virus-4 usually results in respiratory disease, whereas type 1 can result in respiratory disease, abortion, the birth of sick foals and neurological disease.
Infection occurs by inhalation — as for influenza. The incubation period is two to 10 days. Respiratory disease is most common in young horses. A vaccine against equine herpes virus types 1 and 4 is available, which provide some immunity against the respiratory and abortion forms. It is currently unknown whether the vaccine protects against the neurological form.
Strangles is a highly contagious infection that classically results in upper respiratory catarrh and abscess formation in the lymph glands around the head. Infection is most common in young horses, but any age can be affected.
Although highly contagious, spread is slow compared with the respiratory viruses, and requires direct contact. The copious discharges result in the rapid contamination of the environment. Most recovered cases develop a good immunity. Currently, no strangles vaccine is available in the UK.
Reducing the risk
There are many measures that can be taken to reduce the risk of infection. The nature of these vary with the individual diseases, and depend on such factors as the natural history of the disease, its means of spread, the immune response and status of the horse.
For diseases such as the influenza virus, widespread vaccination is important in preventing epizootics. Although vaccination may not eliminate the risk of infection in an individual, it will significantly help to prevent its spread, and is a major component in the overall preventive strategy.
- Be familiar with the clinical signs associated with the main infectious diseases
- Seek veterinary attention immediately if you suspect a contagious disease in your horse. Confirmation is vital so that appropriate control measures can be implemented
- If there is a known outbreak of a contagious disease in the area, seek veterinary advice about the risks and the measures that can be taken to minimise the chance of infection
- Maintain an adequate vaccination programme. Influenza vaccination is recommended in most horses, and is essential if your horse attends shows. Consider increased frequency of vaccination for “high risk” horses. Equine herpes virus vaccination is recommended in pregnant mares and may be advisable for other horses
- Establish a quarantine procedure for new arrivals on to a yard. Isolation for three weeks is generally adequate. Obtain veterinary advice concerning testing new arrivals
- Maintain adequate ventilation rates for all stabled horses, especially in barns
- Maintain high levels of hygiene. Seek advice concerning routine use of disinfectants and “cleaning-up” following an outbreak
- Age segregation of horses, and segregation of pregnant mares, may be advisable in certain situations, eg on studs
This article was originally published in Horse & Hound (26 February, 2004).