West Nile Virus
So far in 2012 in there have been over 1000 reported cases of West Nile Virus in the U.S. which have resulted in over 40 deaths. The graphic at the top of the page from the CDC shows the locations of the confirmed 2012 cases through August 28. The news reports I have read thus far on West Nile Virus have been uniformly insubstantial. They discuss the statistics of the outbreak, whether there have been any local cases or deaths while leaving the question of what it means for a new endemic disease to have entered the U.S. entirely unexamined. Fortunately, Common Science is here to fill this gap.
West Nile Virus (WNV) circulates among birds, mosquitoes and mammals. Birds – robins and crows in particular – serve as what are called “amplifying hosts” since their bodies build up high levels of WNV when infected. Mosquitoes that bite infected birds can then transfer the virus from bird to bird and bird to mammal. Infected mammals do not build up sufficient concentrations of virus for mammal-to-mammal transfer via mosquito to be possible. These factors make the summer, when birds, mosquitoes, and people are likely to be together, the high season for WNV. (Just for emphasis, you can’t get WNV from another person, either directly or through mosquito bites, so you need not be concerned about being around an infected person.)
Eighty percent of people who are infected with WNV do not experience any symptoms. Most of the other 20% experience relatively mild symptoms including headache, fever, stiffness and fatigue which are often misdiagnosed as the flu. Only 1 in 150 (0.67%) of people who are infected develop “serious disease” which may include the invasion of the nervous system. An episode of serious disease can result in permanent nerve damage and require a lengthy and difficult period of rehabilitation. Approximately 1 in 2000 (0.05%) people infected with West Nile Virus die. Those who are infected, symptomatic or not, who survive are immune to future infections.
West Nile Virus was first identified in humans in Uganda in 1937. The first documented human case of WNV in the U.S. occurred in New York City in 1999. Since then, WNV has been found in people, birds, and mosquitoes in all 48 contiguous states. At present there is no vaccine for WNV and we are 5-10 years away from one being available.
How is the disease likely to progress in the United States? We can look to the Nile region for some insights. A study in Egypt in 1950 found that 90% of adults over the age of 40 were immune to WNV based on having been infected at some prior point in their life. This data indicates that transfer of disease from bird to human by mosquitoes is quite efficient. So is the U.S. on track to become like Egypt, where nearly everyone is infected with WNV before they are 40 years old?
While hearing of 40 deaths is concerning, the media reports of 1000 cases do not give me the impression that WNV is something to be particularly concerned about. But let’s remember that most infections are asymptomatic or misdiagnosed. Therefore, we can be confident that there have been many more than 1000 cases in the U.S. so far this year. Consider what the math tells us. Approximately 1 in 2000 infections results in a death. We’ve had 40 deaths in the U.S. so far in 2012. This translates to 80,000 West Nile Virus infections! Further, since we are only half way through the height of mosquito season, we can anticipate that a total of 100,000-200,000 Americans will be infected with West Nile Virus this year. So it would appear that, yes, we are on track to have a large percentage of the population how become infected (and then immune) at a relatively young age.
Health departments across the country are telling everyone to wear long sleeves and bug spray to limit mosquito exposure. In my experience, these measures are, at best, partially effective. Therefore, although there will be ebbs and flows, cases of WNV in the US will continue to grow until such time as a vaccine is developed. It is reasonable to assume that prior to the time a vaccine is available it will be common for the U.S. population to experience millions of WNV infections per year. Millions of infections per year will mean increased patient load on doctors and hospitals and economic impacts from lost work days. Further, people whose systems are weakened by fending off a viral infection are more susceptible to other diseases. I’d like to see the development of a vaccine for WNV to be a national priority. The payback on investment would be fantastic, but so far our leaders are not paying attention.
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