Dear Doctor: West Nile virus is active again in southern California. What happens when you are infected? Is there a vaccine?
Dear Reader: There’s been a surge of West Nile virus in people this summer, with southern California, Nevada and central Arizona recording the highest number of cases of the mosquito-borne illness. According to statistics released by the Centers for Disease Control and Prevention at the beginning of September, 326 confirmed cases of West Nile virus disease in humans have been reported in 35 states. Of those, close to two-thirds resulted in neuro-invasive diseases such as meningitis or encephalitis.
West Nile virus has been identified as the leading cause of mosquito-borne disease in the continental United States. It’s caused by the bite of an infected mosquito and is seen in birds, humans, horses and other mammals. Mosquitoes acquire the virus when they feed on an infected bird. Since many species of birds migrate great distances, and since mosquitoes inhabit virtually every corner of the world, West Nile virus has spread rapidly. The virus isn’t transmitted between humans or other mammals. Although it can be acquired via a blood transfusion or an organ transplant from an infected donor, the U.S. blood supply is tested for the virus and is thus considered safe.
West Nile cases typically spike during late spring to late autumn. That’s when warm temperatures and wet weather create ideal conditions for mosquitoes to breed and spread. Scientists say that our rapidly warming world is also playing a role in the global spread of the disease. First identified in Africa in 1937, the disease made its first appearance in the U.S. in New York City in 1999. The virus now is seen across a majority of the continental U.S. As rising temperatures extend the breeding season of mosquitoes, scientists warn that cases of West Nile will continue to rise.
The good news is that about 80% of people who become infected with the virus will have no symptoms at all. About 20% will have symptoms that are mild and not life-threatening. These include flu-like symptoms such as fever accompanied by headache, body aches and nausea. Additional symptoms of mild illness may include rash, joint pain and stiffness, as well as fatigue, which can persist for weeks. A fraction of the population -- about one in every 150 people infected with the virus -- will develop potentially serious illnesses, including neurological conditions such as encephalitis, which is inflammation of the brain, or meningitis, in which the membranes surrounding the brain and spinal cord become inflamed.
Symptoms of severe illness include high fever, headache, muscle weakness, stiffness in the neck, changes to cognition, tremors, numbness and paralysis. If infection with West Nile virus is suspected, seek medical help. Although vaccines are available for horses, there is not yet a vaccine for humans.
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