West Nile Virus—Is Michigan having an “epidemic?”

West Nile Virus—Is Michigan having an “epidemic?”

There are currently over 90 cases of human West Nile virus disease reported in Michigan; included among these are 5 deaths. It is early in the West Nile virus disease activity season (usually runs from mid-August through early October), so the number of cases are going to rise, in my opinion substantially, before the 2012 case count has concluded.

I publicly stated that Michigan is having an epidemic of West Nile virus disease (see link). This has apparently bothered some members of the public who accuse me of using inflammatory words to scare people. Let’s explore the terms.

We refer to diseases that are occurring in a steady state as being endemic. According to Last’s Dictionary of Epidemiology, an endemic disease “may refer to the usual prevalence of a given disease within a given geographic area.” Over the past 9 years, the usual annual prevalence of West Nile virus disease in Michigan has been 28 cases and 2 deaths (means) and 19 cases and 2 deaths (medians).

An epidemic, according to Last, is disease activity “clearly in excess of normal expectancy.” In my opinion, 95 cases and 5 deaths in the first two weeks of the typical West Nile virus season in Michigan meets the definition of an epidemic. These are not inflammatory words meant to scare people but are an apt description for the West Nile virus season we are experiencing.

The causes for this higher than expected case count likely have their origin in the hot and dry weather conditions that occurred in Michigan this summer. These are the conditions most conducive to population increases among the Culex species of mosquitoes—the only ones known to transmit West Nile virus to humans in our state.

Michigan is having an epidemic of West Nile virus disease. We should inform the public of this occurrence so that they can adopt appropriate preventive precautions.

4th person dies in West Nile virus epidemic, Michigan officials say