Q. What is the “acute flaccid paralysis” that sometimes occurs with WNV infection?
A. In addition to West Nile fever, meningitis, or encephalitis, some people who become infected with WNV can develop “acute flaccid paralysis”—a sudden onset of weakness in the limbs and/or breathing muscles. In most persons, acute flaccid paralysis is due to the development of West Nile poliomyelitis—an inflammation of the spinal cord that causes a syndrome similar to that caused by the poliovirus. West Nile poliomyelitis was first widely recognized in the United States in 2002. Persons with West Nile poliomyelitis may develop sudden or rapidly progressing weakness. The weakness tends to affect one side of the body more than the other, and may involve only one limb. The weakness is generally not associated with any numbness or loss of sensation, but may be associated with severe pain. In very severe cases, the nerves going to the muscles that control breathing may be affected, resulting in rapid onset of respiratory failure. It is important to recognize that this weakness may occur in the absence of meningitis, encephalitis, or even fever or headache—there may be few other clues that the weakness is due to WNV infection.

Q. How often does West Nile poliomyelitis occur?
A. We don’t know for sure how often West Nile poliomyelitis occurs, but it does occur less frequently than meningitis or encephalitis. Scientists are continuing to monitor persons with West Nile poliomyelitis to get a better understanding of how often, and in whom, it occurs.

Q. Are there other types of weakness or “acute flaccid paralysis” caused by WNV infection?
A. The vast majority of persons with WNV “acute flaccid paralysis” suffer from West Nile poliomyelitis (an inflammation of the spinal cord). Some persons with WNV infection may instead develop an illness similar to Guillain-Barré syndrome, which is a disease of the peripheral nerves and not the spinal cord. Weakness of the facial muscles may also develop in persons with WNV infection. While many persons with WNV infection experience fatigue and feel weak all over, this is not the same as “acute flaccid paralysis”.

Q. Who tends to be affected by West Nile poliomyelitis?
A. People of any age can be affected by West Nile poliomyelitis. While persons over the age of 65 are at highest risk for all forms of WNV neuroinvasive disease, including poliomyelitis, persons of younger age groups (e.g., in their 30’s and 40’s) can also develop West Nile poliomyelitis. West Nile poliomyelitis may affect people who are otherwise healthy and without prior medical conditions.