West Nile Virus Diagnosis

Information about how the disease is diagnosed.

Horses

Clinical Signs in Horses:
Horses show signs of encephalitis. A fever was not often observed. Clinical signs of encephalitis in horses may include a general loss of appetite and depression, in addition to any combination of the following signs:

It is important to note that not all horses with clinical signs of encephalitis have West Nile encephalitis. Certain other diseases can cause a horse to have symptoms similar to those resulting from infection with West Nile Virus. WNV can be confused with Western Equine Encephalitis (WEE) or sleeping sickness. Rabies cases may also present with similar neurological signs. If you are concerned that your horse may be exhibiting signs of encephalitis, please contact your veterinarian. Laboratory tests are necessary to confirm a diagnosis.

Diagnosis of Horses:
This disease is considered a Foreign Animal Disease by the USDA. Due to the zoonotic potential of West Nile Virus, horses who develop neurologic signs from May to October should be considered WNV suspects. To test for WNV, serum samples should be submitted to the Montana Veterinary Diagnostic Laboratory. Serum is tested for IgM levels, which indicates exposure to WNV. (An IgG response is present following West Nile vaccination.) Complete histories should be submitted with the serum samples. If the horse dies or is euthanized, samples from the spleen, liver, kidneys, and lungs should be submitted. If the horse is demonstrating neurologic signs prior to death, the brain and cervical spinal cord should also be submitted for WNV testing. If a post-mortem is performed in the field, precautions should be taken to minimize exposure to potential pathogens. Other diagnostics include PCR of CNS tissues and immunohistochemistry.

Humans

Diagnosing West Nile virus infections is generally done through a combination of observing signs and symptoms along with specialized molecular biologic testing for the virus itself.

Only people with severe symptoms will require further testing. Confirmatory diagnosis of West Nile virus infection is generally done by blood and spinal fluid samples, obtained by lumbar puncture (also called a spinal tap). If you have West Nile virus, an analysis of your blood sample may show:

• A rising level of an antibody to the West Nile virus. Antibodies are protein products from your immune system that attack foreign substances, such as viruses, in your body.
• A positive ribonucleic acid (RNA) test for the West Nile virus.
• A positive culture of the West Nile virus.