West Nile virus (WNV) is a worldwide pathogen. Mosquitos get the virus from infected birds and transmit it to horses and humans. WNV creates the greatest threat to the health of horses in the US, as the leading cause of arbovirus encephalitis. Summer is the most dangerous time for the virus and it's just around the corner. Therefore, it is important for horse owners to pay attention to clinical signs of the disease which can progress from non-specific signs, such as fever, loss of appetite, and depression to severe neurological signs that include weakness of the legs, lameness, muscle twitching, inability to stand, ataxia (including stumbling, staggering or wobbly gait), partial paralysis, recumbency, blindness, or grinding of teeth. If your horse manifests any of these signs you must contact a veterinarian for an accurate diagnosis of the potential infection. A diagnosis of WNV is determined by detection of the IgM/IgG antibody against the virus in horse blood serum. Thus, a blood test is necessary to confirm a diagnosis of the infection. Sometimes retesting horses with clinical signs of WNV is recommended, if test results are negative the first time. Although some horses may not show any signs of disease, WNV usually causes encephalomyelitis (inflammation of the brain and spinal cord). Therefore, it is very important to obtain a definitive diagnosis in your horse and to document the number of cases in a region. Fortunately, horses can be protected against WNV via annual vaccination. Remember, approximately 30% of horses die from WNV encephalitis or complications associated with encephalitis. Please, be prepared for the upcoming mosquito season and vaccinate your horse in springtime.