Tetanus - This severe disease progresses very quickly. Affected horses experience stiffness, rigidness, overreaction to noise and stimuli, inability to open the mouth, difficulty breathing, and recumbency. Fatal if untreated and sometimes despite early, aggressive treatment. Usually occurs consequent to a wound in a non-vaccinated horse.
Encephalomyelitis (EEE, WEE, WNV, VEE) - Spread by mosquitoes, these viruses affect the brain and spinal cord. Infected horses have severe depression, weakness, incoordination, ataxia, stiffness, fever, difficulty eating, and abnormal behavior.
WEE is fatal in about 25% of cases. EEE is nearly always fatal. WNV fatalities are relatively rare (about 30% of horses with neurological signs of WNV infection will die) although recovered horses might or might not retain neurologic deficits. VEE is often fatal.
Rhinopneumonitis (Equine Herpesvirus I and IV) - This virus is extremely well dispersed. Type 1 is commonly associated with respiratory disease, weak foals, and abortion (and rarely neurologic disease); type 4 is primarily associated with respiratory disease (and rarely weak foals and abortion). The neurologic form of the disease (EHV-1) appears to be on the rise recently; it causes horses (sometimes in groups) to lose control of their hind legs and bladder (and other things). There have been several epidemics of the neurologic form in the past couple of years; vaccination is likely not protective against the neurologic form.
Influenza - Influenza is usually not life-threatening, but it increases vulnerability to other diseases, including pneumonia. Clinical signs include fever, lethargy, cough, nasal discharge, muscle aches, and inappetence.
Rabies - Clinical signs include weakness in the limbs, loss of neurologic control of limbs, loss of ability to swallow, profound depression, or furious states where the animal aggressively attacks objects or people. Always fatal.
Potomac Horse Fever - This disease can cause severe diarrhea, severe laminitis or founder, and abortion.
Equine Protozoal Myeloencephalitis (EPM) - Clinical signs can include weakness, lameness, incoordination, inability to move correctly (especially in the hindquarters) or to stand up, seizures, weight loss, blindness, loss of balance, disuse of a single limb, and/or inappropriate sweating. Lack of treatment can lead to permanent nerve damage and death.
Strangles - Easily transmitted by other horses and by intermediaries such as people, buckets, and tack. Early clinical signs include nasal discharge, cough, inappetence, and fever. Later, the horse often developes swellings in the throat, between the jaws, and/or under the ears. Occasionally, abscesses affect other parts of the body, causing colic or signs of neurological disease.