After infection, the virus invades the central nervous system, including the spinal cord and brain. The virus cannot be transmitted for horse to humans. Neither can it be transmitted from horses and humans to mosquitos. The only way the virus is transmitted it if a mosquito bites an infected bird and then bite a human. Only about one out of three people bitten by an infected mosquito will get the disease.
Infection can cause a range of illnesses.
For people with infection of the central nervous system, a sudden fever and severe headache can be followed quickly by seizures and coma. About half of these patients die from the disease. Of those who survive, many suffer permanent brain damage and require lifetime institutional care. Symptoms usually appear 4 to 10 days after the bite of an infected mosquito. Diagnosis is based on tests of blood or spinal fluid.