West Nile Virus Signs and Symptoms

A list of clinical signs the animal may or may not display.

Signs and Symptoms in Horses

1. Horses are affected by West Nile virus much more often than any other domestic animal.
2. The percent of infected horses that go on to develop symptoms and illness is not known.
3. Of those horses that do present clinical signs, about one-third die or need to be euthanized. The clinical signs for horses include weakness or paralysis of hind limbs, muzzle twitching, impaired vision, incoordination, head pressing, aimless wandering, convulsions, inability to swallow, circling, hyperexcitability, or coma.
4. The incubation period for horses is 5 to 15 days.

Signs and Symptoms in Humans

1. About 80 percent of the infections are mild without any symptoms.
2. About 20 percent of the infected people develop a mild flu-line illness. This is referred to as West Nile Fever. West Nile Fever is a mild human illness caused by the virus and includes flu-like symptoms such as fever, severe headache, sore throat, backache, mylagia, fatigue, stiffness of the neck, nausea, abdominal pain, diarrhea, rash. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects.
3. Less than 1 percent of West Nile virus infections lead to more serious illnesses - encephalitis, meningitis, or meningoencephalitis. These are more severe human illnesses caused by the virus. People with these illnesses often present with symptoms of fever, headache, stiff neck, mental confusion, disorientation, tremors, convulsions, muscle weakness, paralysis, and - in about 15 percent of these cases - coma.
4. The incubation period for humans is 3 to 14 days.