What’s Buggin’ You?

Mosquitoes, Temperature, and the West Nile Virus

By Chrystine Kern

Pop quiz! Get your pencils out! Question one: what mosquito-borne virus has infected humans in 45 US states, resulting in fever, brain inflammation, and human fatality?

You guessed it (and if not, don’t worry: this won’t be graded)—the West Nile Virus.

This virus first came to the US in New York during the summer of 1999. According to the US Centers for Disease Control and Prevention (CDC), the West Nile Virus has resulted in 959 deaths in the US since its first arrival in 1999 up through last year. Although the CDC confirms 2003 as the worst epidemic year so far in the US (with a total of 9,862 people infected and 264 killed), the numbers for last year remain high with 4,268 infected and 177 killed.

So why the virus’ sudden appearance in the US?

Two recent studies from the Journal of Medical Entomology explain a link between warmer environmental temperatures and the ability of mosquitoes to transmit the West Nile Virus.

The first of these studies was published in 2002. For this study, a group of scientists—including David J. Dohm, Monica L. O’Guinn, and Michael J. Turell—noted that although previous studies had shown a link between environmental temperatures and the ability of mosquitoes to transmit arboviruses, there was a lack of information showing whether or not environmental temperatures played a role in the transmission of the West Nile Virus specifically. And if so, what role exactly did they play?

So, to investigate these questions, Dohm and his group of scientists studied a group of Culex Pipiens—which is just the fancy Latin name for the common house mosquito, the best-known carrier of the West Nile Virus as well as the most largely distributed mosquito in the world. They allowed these mosquitoes to feed on three-day-old leghorn chickens infected with the West Nile Virus, and the results were astonishing.

The scientists found that mosquitoes incubated in higher temperatures were more likely to get infected with West Nile Virus, while mosquitoes incubated in lower temperatures were less likely to become infected. Not only that, but transmission rates were also consistently lower with mosquitoes held at lower temperatures (18^o C/50 ^o F) and consistently higher with mosquitoes held at higher temperatures (30 ^o C/62 ^o F), thus showing there is a positive correlation between temperature and transmission rates. In other words, as temperature increases, so does the ability of mosquitoes to transmit the West Nile Virus!

These findings are consistent with several recent outbreaks of the West Nile Virus, including the 1999 outbreak in New York. As 1999 was a year of record-high temperatures in New York, this may explain how the West Nile Virus was transmitted so quickly. In addition, fairly recent West Nile outbreaks in Russia (1999) and Romania (1996) were also associated with above-average temperatures, confirming the correlation between environmental temperatures and West Nile transmission.

Similarly, a second study was conducted by scientists William K. Reisen, Ying Fang, and Vincent M. Martinez. It was published in 2006 as a follow-up to Dohm’s 2002 study. The scientists in this particular study, however, examined a different kind of mosquito, the Culex Tarsalis—another nifty Latin-derived name for a widely distributed North American mosquito that has become recognized as a key transmitter of the West Nile Virus from Canada to Mexico (with the US in between).

The study procured results similar to those from the 2002 study—namely, that mosquitoes held at higher temperatures were proportionally more likely to contract and transmit the West Nile Virus than those held at lower temperatures. So, the study concludes, the rapid transmission of the West Nile Virus during the epidemic summers of 2002-2004 were directly linked to the above-average summer temperatures. The study also notes, however, that West Nile outbreaks in the southern latitudes are not linked to above-average temperatures, probably because temperatures in these areas are already high enough for the disease to spread.

So, if temperatures are on the rise, we can expect West Nile transmission to be on the rise as well! And according to the Intergovernmental Panel on Climate Change (IPCC), current temperature trends indeed seem to be increasing, but let’s not forget that mosquitoes thrive in humid environments—so both temperature and rainfall factor into the spread of the West Nile Virus, as Dohm and his colleagues mention in their study. And since the IPCC also predicts increased precipitation in most areas of North America (excluding only the South-West US), we can expect to see more West Nile cases in the upcoming years.

Since there is currently no human vaccination for this virus, I’d like to leave you all with some helpful hints from the Maryland Department of Agriculture to avoid exposure to it (tips first cited in The Capital):

• Avoid outdoor activities during dawn and dusk when mosquitoes are most active
• Wear insect repellent on skin and clothing
• Wear long sleeves and pants
• Install or repair window/door screens
• Remove standing water from household containers, including flower pots, trashcans, and pet food/water bowls

So put on some bug spray and stay on the lookout, as recent studies and current climate trends caution us against this growing threat.

Bibliography

CDC: Centers for Disease Control and Prevention. 2007. 28 November 2007

            < http://www.cdc.gov/>.

Dohm, David J. et al. “Effect of Environmental Temperature on the Ability of Culex        Pipiens to Transmit West Nile Virus.” Journal of Medical Entomology 39 (2002):        221-225.

Furguson, E.B. “West Nile Mosquitoes found in Cape St. Claire.” The Capital 21         September 2007: A1.

Manning, Anita. “West Nile Virus has depleted bird species from coast-to-coast.” USA            Today 17 May 2007: 9.