Originally Published: September 1, 2004 7 a.m.
PRESCOTT – Arizona's latest total of 316 reported cases of West Nile virus is a common number among Arizona newspapers, but health officials believe that thousands of other people are already infected.
Arizona Department of Health Services spokesman Michael Murphy said the number does not reflect the thousands of unreported infections with mild symptoms.
According to state statistics, 80 percent of the infected population will experience no symptoms at all. Twenty percent will experience flu-like symptoms, and 1 percent will suffer the most serious symptoms: encephalitis and meningitis. Murphy said 117 West Nile victims endured meningitis and 71 suffered the brain-swelling effects of encephalitis.
Dr. Peter Kelly, an infectious disease specialist for Arizona's Office of Public Health Emergency Preparedness and Response, said the human immune system marks the difference between the seriously ill and the mildly affected. The most threatening symptoms are common only among the older population, he said.
"There are very few people that get this prior to the age of 30," Kelly said. The latest state update on the virus shows that 75 percent of the state's reported infections have occurred in people 31 or older. Arizonans between 41 and 70 years of age account for 192 infections.
"This is a disease that affects adults, and then disproportionately goes after the older adults," he said. "These people have a lot of concurrent chronic illnesses," he noted, which undoubtedly enhances their vulnerability.
Concurrent chronic illnesses weaken the body's initial response to the virus, he said. The 80 percent of people unaffected by the infection simply produce antibodies that effectively destroy the disease.
Kelly said doctors are able to recognize who has this disease, because the antibodies will remain in the blood for long after the virus particles are eradicated.
Viruses such as the human immunodeficiency virus (HIV) are notorious for quickly evolving new genetic code to defeat the antibodies. Fortunately, Kelly said there have been no major changes to the genetic structure of the West Nile virus since becoming prevalent in 1999.
"As far as we know, it is the same virus we were isolating then (in 1999)," he said. "We may not know for quite a while as to whether there was a minor change in the genetics of the virus."
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