Common Infectious Diseases

There are a large number of common infectious diseases in horses. Some of these include Western Equine Encephalitis, the West Nile Virus, the Equine Herpes Virus, Strangles, and Tetanus. A brief summary outlining the important points of each these diseases can be found below.

Western Equine Encephalitis

Western Equine Encephalitis is caused by an arbo-virus that is extremely infectious. Horses can be affected by Western Equine Encephalitis as well as birds and humans. Other animals may be affected including mules and donkeys. This viral disease can, unfortunately, lead to death in animals suffering from the disease. The disease is known to be transmitted via mosquitoes which inoculate the virus into the horse, or even human. The mosquitoes may acquire the virus from affected birds.

The part of the animal the most affected by the virus is the Central Nervous System (CNS).

Some animals suffering from Western Equine Encephalitis may show no signs or symptoms at all. Those that do, however, may experience listlessness, loss of appetite and a fever. As the disease progresses the horse may become irritable and even paralysed. This may follow to produce fatal results.

Blood may be taken and tested in order to diagnose the disease, following an observation of the signs and symptoms presented. There is no set out treatment, only supportive treatment, for the disease which is mainly found in North, Central and South America. Preventative measures used against the spread of this disease include vaccinations and attempts to reduce the mosquito population in high risk areas.

West Nile Virus

This infectious virus is transmitted to hosts via mostly mosquitoes. As well as affecting horses, this disease can infect many other animals but mainly birds. Other animals which may be infected by the disease include dogs, cats, and rabbits. Humans can suffer from the West Nile Virus (WNV). The mortality rate for affected animals can differ.

The Central Nervous System (CNS) is mainly affected by the harmful virus. It can cause inflammation of the brain and interferes with the functioning of the CNS. Transmission occurs when a mosquito feeds on an infected bird and then inoculates the virus into the blood stream of the horse. It must be noted that horses cannot actually pass the virus directly to humans.

All types of horses can be susceptible to the West Nile Virus, independent of their breed, age and health. The signs and symptoms following infection include lethargy, depression, and loss of appetite. Additionally, the horse may shiver or tremble, show signs of uncoordinated movement, become partially paralysed or recumbent, or convulse. The horse may lack the ability to swallow or have impaired vision. Fatalities can occur in horses suffering from the viral infection.

Horses can recover following infection and any treatment given should be supportive as there is no outright cure. West Nile Virus vaccines do exist and are recommended in areas where the disease is present. Additional prevention can include methods to reduce the amount of mosquitoes in the horse’s living area.

Equine Herpes Virus

This is a highly contagious equine virus that affects horses of all ages. Better known as the Equine Rhinopneumonitis the disease is caused by the equine herpes virus type-1 (EHV-1). The Equine Herpes Virus causes problems with the respiratory and neurological system and may even lead to abortions in pregnant mares. Currently, it is believed that only the equine family can become infected with the virus.

The disease is transmitted via the inhalation of contaminated aerosol droplets expelled by an affected horse. The signs and symptoms are those which come with an infection of the respiratory system and due to the fact that the Central Nervous System can become adversely affected, paralysis may occur. The more susceptible horses are the youngest and the foals, due to their compromised immunity at such a young age.

This is not known as a zoonotic disease as it cannot be transmitted to humans. There is the possibility of vaccinating horses as a preventative measure although they are not always guaranteed to be entirely successful. If a horse does become infected then only supportive treatment can be given in order to aid recovery. If the horse becomes paralysed, however, there is almost always a poor prognosis.


A highly contagious disease, Strangles is one of the most common respiratory problems found in horses. Horses, ponies, and donkeys can all become affected with strangles. Unfortunately, horses affected with this condition can actually die following infection.  Strangles is caused by the bacteria known as Streptococus equi.

Some horses can carry the bacteria without showing any signs or symptoms at all. The majority of cases, however, do experience signs such as depression, loss of appetite, and a fever. The lymph nodes around the throat will then become inflamed and abscesses can form.  Pussy discharge may be visible and the affected animal will have difficulty swallowing, or even breathing. When other parts of the body are affected, this is known as “bastard” strangles and is almost always fatal.

Through direct contact, the strangles bacteria enter the horse’s body orally. The majority of contaminated surface, such as tack or wooden fences, can harbour and thus transfer the disease. Unvaccinated horses in high risk areas are the most susceptible to becoming infected with strangles. With humans who have a compromised immunity, the disease can become zoonotic. Strangles is a preventable condition, and this is done by means of a vaccine.


Tetanus is an infectious disease which is known to affect the horse, suffering from the malady, neurologically. The majority of all animals can become affected by this bacterial infection and this can include dogs and cats. Unfortunately, fatalities do generally occur in the equine family following infections from this disease, especially those which have not been exposed to preventative measures. 

Also known as “lockjaw”, tetanus is caused by the bacteria known as Clostridium tetani which travels around the body via the blood stream.  Once the bacterium comes into contact with an open wound, such as an injury or puncture from a rusted nail, it replicates itself very rapidly. The central nervous system is affected as it comes under attack from a toxin released by the bacteria known as tetanospasmin. Horses which reside in areas where the bacteria are found in the soil are more at risk of contracting the disease. In addition to this, horses can become more susceptible following an operation.

Much like its alternative name, horses suffering from an infection of tetanus have a condition where their jaw “locks” in a position. Additionally, the horse may experience signs and symptoms of stiffness, spasms and an increased sensitivity to its surroundings. The bacteria have also been known to affect humans. Horses can be vaccinated against tetanus and the prognosis for horses affected with the condition, which have not been vaccinated, have a very poor prognosis.


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