Many of the deadliest diseases in horses are infectious diseases (diseases that can be spread from horse to horse). In the UK we typically see diseases such as strangles, equine herpes virus and equine influenza, says veterinary surgeon Emma Trott from Paragon Veterinary Group.
However, in other parts of the world diseases such as rabies, West Nile virus, African horse sickness, glanders and equine Infectious anaemia are important deadly diseases that may one day become important in the UK.
Not all deadly diseases are contagious: diseases such as tetanus and equine grass sickness are some of the most dangerous diseases in the UK horse population.
World's deadliest horse diseases
Tetanus
When asked for the deadliest horse disease, tetanus, also known as lockjaw, is likely to come up highly on most equine veterinarians’ lists.
Tetanus is caused by the bacterium, Clostridium Tetani which is present throughout the UK in the soil which infects horses by entering open wounds. The bacteria multiply in the wound and produce a toxin (one of the deadliest natural poisons in the world) which spreads throughout the body. Horses with tetanus have difficulty moving and eating, dilated nostrils, prolapsed third eyelids, alert ears and are very sensitive to noise and light.
As the disease progresses, the muscle spasms increase, leading to breathing difficulties, respiratory failure and eventually death. Sadly, even with early detection and intense medical management, most horses with tetanus sadly die from the disease.
Luckily, vaccination for tetanus is very effective and we rarely see the disease in vaccinated horses. The tetanus vaccination is typically administered with the equine influenza vaccination but can be given separately.
Equine grass sickness
Equine grass sickness is a poorly understood and often fatal disease that causes damage to the nervous system. Equine grass sickness was first identified in the early 1900s following an outbreak in army horses near Dundee.
Equine grass sickness is seen throughout Europe but has the highest incidence in the United Kingdom. We typically see equine grass sickness cases in young horses with access to grass, with a peak in May each year. Cases present with three different forms of the disease – those with acute signs often present with severe paralysis of their gastrointestinal tract causing severe signs of abdominal pain (colic) which is often fatal.
Horses presenting with chronic grass sickness, tend to show mild, intermittent signs of abdominal pain, weight loss and difficulty eating. With intensive management some of these horses can recover. With no known cause, sadly there is no easy way to manage, prevent or cure the disease at present but it is an area of significant research.
Rabies
In the UK we are lucky that we have eradicated rabies and our strict rules on the import of animals reduces our risk of the disease entering the UK. Early signs of rabies can include behavioural changes, noise and light sensitivity followed by aggression, increased saliva production and eventually general paralysis and death. Rabies is spread by the saliva of infected animals, therefore in areas where the disease is present, any animal bite should be treated by a medical professional quickly.
West Nile Virus (WNV)
West Nile Virus is a non-contagious, infectious insect vector borne disease spread via mosquitoes, probably the world's deadliest animal.
Birds are natural carrier of West Nile Virus, and the disease is spread through mosquitos. These mosquitos pass the disease on to other mammals such as horses or even humans.
Infected horses typically show signs relating to swelling of the brain and spinal cord 3-15 days after being bitten. These signs can include depression, incoordination, head pressing, seizures and death.
Luckily, WNV is not currently circulating in the UK and the main risk to UK horses is from travelling to areas where the virus is present.
Strangles
One of the most common infectious diseases is strangles or streptococcus Equi Equi. This is a bacterial infection that is spread through direct contact with infected material that can cause a nasty respiratory disease. It generally causes fever, depression, thick nasal discharge and abscesses around the head and neck.
Did you know that strangles is one of the oldest known infectious diseases? It was first reported in Roman times and was given the name strangles due to the possibility of severe disease causing restriction of the airways and possible strangulation.
Luckily, with modern diagnostic tests and medicines we can help manage this disease, but it can still be fatal in many cases. There is an intramuscular vaccination for strangles which can help reduce the spread of the disease and can reduce clinical signs.
Equine herpes virus (EHV)
There are four main types of equine herpes virus: EHV-1; EHV-3; EHV-4; and EHV-5. EHV1 and EHV-4 are typically seen in the UK. EHV-1 typically causes mild respiratory disease in young horses, however, it can also cause abortion storms in late gestation mares, and neurological disease in horses of any age.
EHV is spread through direct contact with nasal sections and infected aborted material and can be spread on people and equipment.
The neurological form of EHV can range from mild incoordination of the hindlimbs to severe paralysis. As there is no specific treatment for these cases, we rely on intensive supportive care and sadly the neurological form of the disease can often be fatal. Vaccination for EHV 1 and 4 is often recommended in pregnant mares and competition horses.
Glanders (Farcy)
Glanders is a contagious and often fatal bacterial disease in horses, causing ulcerative growth in the respiratory system and skin. Glanders has been eradicated in many countries worldwide and was last reported in the UK in 1928. Strict import rules reduce the risk of disease spread into the UK.
Equine infectious anaemia (EIA)
Equine infectious anaemia, otherwise known as swamp fever, is transmitted between horses via infected blood. This can be through insect vectors, contaminated equipment or infected blood products. EIA is a notifiable disease in the UK and clinical signs can include fever, haemorrhage, anaemia, weight loss, ataxia and jaundice.
African horse sickness (AHS)
African horse sickness is an infectious non-contagious viral disease spread by the Cullicoides midge. AHS can cause fever, respiratory and cardiac disease, and has a high death rate. AHS is a notifiable disease in the UK.
Equine influenza (EI)
Equine influenza is a highly infectious viral respiratory disease that spreads easily among susceptible animals. The virus can spread through the air, through nose-to-nose contact or through infected material on equipment. Infected horses will show signs of a fever, a dry, harsh hacking cough, lethargy and nasal discharge.
Although rarely fatal, the disease can significantly damage the horse’s respiratory tract, and we advise that horses have a week off exercise for every day they have a fever! To reduce the risk of diseases in our horses, vaccination is advised at intervals determined by your horse's age, risk status and whether they are pregnant or competing.