West Nile Enters Difficult Teenage Years

West nile virus expected to cause problems for decades to come

/ Author:  / Reviewed by: Robert Carlson, M.D

(RxWiki News) West Nile virus is thirteen years old. It made its first appearance in 1999, and grabbed the nation's attention this summer with an outbreak that was unmistakably teenage behavior.

“Disturbingly unpredictable, disagreeable, and difficult to control” - that's how West Nile virus experts describe the disease's first season as an adolescent, in an article for the New England Journal of Medicine.

West Nile isn't likely to fade away, they say, and will continue to cause unpredictable and sporadic outbreaks for decades to come.

"When outside, take precautions against mosquitos."

The article was co-authored by Dr. Lyle Petersen, Director of the Division of Vector-borne Infectious Diseases at the Centers for Disease Control and Prevention, and Dr. Marc Fischer, medical epidemiologist with CDC’s Arboviral Diseases Branch.

The final count for 2012 – one of West Nile's most infectious years to date – was 3142 cases, which includes 1630 cases of severe West Nile and 134 deaths. Most cases were reported in Texas, but the virus hit every state in the continental US except for Maine.

“To judge from past reporting trends, these figures suggest that this year's West Nile virus outbreak will be among the largest ever recorded,” the authors wrote.

West Nile's other big years were 2002 and 2003, when there were almost 3000 cases of severe West Nile, which turns the mosquito-borne illness into a serious disease that affects the brain. Those years, more than 260 people died.

But West Nile wasn't much cause for concern for the next several years, dipping down to its lowest levels of neuroinvasive disease in 2009 and 2011. It swayed some public health experts to wonder if West Nile would ever make a resurgence.

This year's outbreak proved a point, and cemented the virus' personality as unpredictable and difficult to control. In fact, the authors say that creating a long-term forecast for West Nile outbreaks might be impossible.

Why? There are so many factors at play in an infectious disease like West Nile including environmental conditions, weather, and the distribution and population of mosquitos and birds, who transmit the virus to mosquitos.

These factors have proved difficult to sort out in infectious diseases. The authors write that even if the exact conditions were replicated between them, there might be an element of chance that could spin the wheel a different direction.

This year, the city of Dallas was heavily hit by West Nile and retaliated by spraying insecticide from small airplanes. No short-term health consequences have been found as a result of the spraying, the authors write, and these measures do reduce the numbers of insects.

There is still no treatment for the neuroinvasive form of West Nile that can cause death and brain damage. It's difficult to test treatments on an illness that is so seasonal and affects people only sporadically from year to year. The authors don't have a good outlook for a treatment coming soon.

“Unfortunately, we cannot predict the distribution or incidence of West Nile virus for the next year, let alone the next decade,” Drs. Petersen and Fischer conclude. “Clearly, a long-term perspective is necessary; West Nile virus is likely to be the adolescent that never grows up.”

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Review Date: 
October 1, 2012
Last Updated:
October 5, 2012