Infectious diseases are one of the leading causes of death in horses. Furthermore, some of these diseases, such as rabies, are also particularly dangerous for humans as they can be transmitted from horses to humans. Many of these diseases can be effectively prevented. Infections occur when one or more microorganisms such as bacteria or viruses enter the animal's body. Some bacteria or viruses can harm the animal, for example, by creating toxins or destroying tissue. If the damage is extensive enough to be expressed as a symptom of an illness, we speak of an infectious disease.


Tetanus (lockjaw)

Tetanus is caused by a bacterial toxin normally found in the soil and in the feces of horses. The bacteria that produce the tetanus toxin need a decreased oxygen supply to multiply, so any area where there is a deep puncture wound or where a wound has healed over (such as the navel stump of a newborn foal) is an area where tetanus can thrive. Symptoms of tetanus include an elevated third eyelid and stiff neck, progressing to overall muscle stiffness causing a 'sawhorse' stance. Tetanus is often fatal, but a yearly vaccine can prevent it, and the vaccine is a good idea because small cuts can go unnoticed and become infected.


Equine Encephalomyelitis (sleeping sickness)

This is a disease that affects the nervous system, and can be caused by equine encephalomyelitis viruses (Eastern, Western, and Venezuelan), which are carried by mosquitoes. Signs include depression and a high fever, followed by a period when the horse appears blind, nervous and uncoordinated, which progresses to muscle tremors, yawning, and eventually, complete paralysis. Proper vaccination and good mosquito control are important to help prevent this disease.


Equine Influenza

This viral disease is spread by inhalation of drops of infective material. Signs include a dry, hacking cough, sudden onset of fever, watery nasal discharge, weakness, stiffness, loss of appetite and depression. Infection with equine influenza is rarely fatal but can cause problems such as emphysema, pneumonia or bronchitis.

Equine Herpesvirus (rhinopneumonitis, rhino, viral abortion)

There are 2 types of equine herpesvirus: EHV-1, which causes respiratory disease (fever, cough, nasal discharge), reproductive problems (abortion, stillbirth), and neurological problems (hindlimb weakness, difficulty walking, sometimes paralysis); and EHV-4, which is limited to respiratory problems and is usually only a problem in younger horses. Once a horse has been infected with EHV-1, he will always be a carrier, and the virus may re-activate within the horse during times of stress. A horse that has been infected with EHV-4 will always test positive for it also, but usually will not show clinical signs of it again after the initial infection.


West Nile Virus

Horses get WNV by being bitten by an infected mosquito; most horses do not show any signs and recover on their own, but in some horses the infection affects the central nervous system and causes signs including fever, weakness or paralysis of the hind limbs, impaired vision, lack of coordination, head pressing, convulsions, inability to swallow, and coma.



This is a viral infection of the central nervous system, and although it is not common in horses, rabies can be transmitted to horses by the bite of an infected animal such as a skunk, raccoon, fox, dog or bat. Rabies can be transmitted to people. We recommend that you check with your veterinarian regarding recommendations for rabies vaccination for your horse.


Strangles (shipping fever)

This contagious respiratory disease is caused by a bacterial infection. Signs include a fever, thick, yellow, nasal discharge and swollen, abscessed lymph nodes under the jaws. The infection is spread by infected material from nasal discharge or abscesses contaminating stalls, feed troughs, pastures, etc. Young horses are the most susceptible to strangles and many horses seem to have a lifetime immunity after recovering from this disease.


Potomac Horse Fever

This disease is a bacterial infection of the blood and tissues and is thought to be transmitted to horses by arthropod vectors such as ticks, lice, mites, and fleas. It is much more common in some areas of the country than others. Signs include a high fever, depression, decreased gut sounds, and a profuse, watery diarrhea that can lead to laminitis, colic, dehydration, shock, and death.



The best and most cost-effective weapon for preventing infectious diseases is appropriate vaccination. Basic immunization of foals should start at 2-4 months. The basic immunization is followed by certain revaccination intervals. Contact a veterinarian about which vaccinations are recommended for horses in general and if there are any special vaccinations necessary due to age, exposure risk, and geographic location.


Basic immunization should include: 








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