Malaria and yellow fever have plagued humanity for generations.  I grew up in Cleveland, and local history there indicated that after Moses Cleaveland planned the city, early settlers focused their attention on the “heights” to avoid the malaria carried by mosquitoes in the Cuyahoga River valley.   Through the 1800s, malaria also plagued Tucson, and yellow fever outbreaks frequented New Orleans.  Draining swamps, spraying pesticides, and installing window-screens virtually eliminated malaria, yellow fever and dengue from the U.S., although the latter shows new inroads of establishment in Florida and Puerto Rico.

Malaria and yellow fever are diseases that are transmitted by vectors—here mosquitoes of the genera Anopheles and Aedes, respectively.  These mosquitoes don’t guarantee the presence of the disease, but they are a necessary condition for the occurrence of malaria and yellow fever.  About 1 million people each year die from malaria, largely in tropical climates.

In the past couple of years we’ve heard about new mosquito-borne diseases, including West Nile Virus, which arrived in the U.S. in 1999 and Zika, which arrived in Brazil in 2013.  Carried by the Culex mosquito, West Nile virus has now been recorded in 45 states, and Zika has been brought to the U.S. by travelers contracting it from Aedes mosquito bites in South and Central America.  Zika has the potential to cause devastating birth defects, especially microcephaly.  West Nile and Zika virus are examples of Emerging Infectious Diseases.    Malaria and Yellow Fever were bad enough; why are we now plagued by these new diseases?

Two aspects seem germane to understanding the spread of mosquito-borne disease.   First, with the rapid, interconnection of modern air travel and transport, a disease from anywhere is likely to be transported everywhere.   If it arrives in a spot where the local mosquito populations are appropriate for further transmission of the virus, an outbreak is possible.  West Nile virus was a tropical disease, now thought to have arrived by air in New York City, and by 2003, it had spread across the country.

Secondly, with the ongoing changes in global climate, particularly warming, which brings us shorter and less severe winters, the tropical mosquitoes that carry these diseases are expanding northward in the United States and elsewhere.  The mosquitoes are following similar changes in the distribution and abundance of migratory birds, which show earlier spring arrivals and more northerly nesting populations in recent years.  Indeed, some birds, like tree swallows, are likely following the insects.  Warming climate is also implicated in the northward spread of Lyme disease, which seems destined to afflict eastern Canada by mid-century.

Long, cold winters do not favor reproduction in mosquitoes, whereas hot, wet summers allow rapid development of larvae and additional generations.  Shannon LaDeau and her colleagues at the Cary Institute of Ecosystem Studies find that some of these mosquitoes can overwinter successfully in the buried sewer systems and stream culverts of urban areas.

Unfortunately, most of the emerging infectious diseases have no vaccine and no immediate cure.  Even malaria, which has been around for centuries, has no vaccine and shows increasing resistance to some of the better drugs known to prevent it.  Many communities engage in broadcast spraying of pesticides to kill mosquitoes, with incomplete knowledge of the side effects of those pesticides on humans.   Although some promising programs using sterile genetically-modified mosquitoes are being tested in certain regions, window screens and liberal use of DEET-based insecticides are recommended this summer. (See http://blogs.nicholas.duke.edu/citizenscientist/cant-beat-the-deet/ ).

You may doubt the reality of climate change, rising sea level, crop failures and drought.  Perhaps a diminutive mosquito will change your mind—perhaps even before the fall election.

References

Faria, N.R., R. Azevdeo, M.U.G. Kraemer et al. 2016.  Zika virus in the Americas: Early epidemiological and genetic findings.  Science 352: 345-349.

Hales, S., N. de Wet, J. Maindonald, and A. Woodward.  2002.  Potential effect of population and climate changes on global distribution of dengue fever: An empirical model.  Lancet 360: 830-834.

Harrigan, R.J., H.A. Thomassen, W. Buermann and T.B. Smith. 2014.  A continental risk assessment of West Nile virus under climate change.  Global Change Biology 20: 2417-2425.

Jones, K.E., N.G. Patel, M.A. Levy, A. Storeygard, D. Balk, J.L. Gittleman and P. Daszak. 2008.  Global trends in emerging infectious diseases.  Nature 451: 990-994.

LaDeau, S.L., B.F. Allan, P.T. Leisnham, and M.Z. Levy. 2015.  The ecological foundations of transmission potential and vector-borne disease in urban landscapes.  Functional Ecology 29: 889-901.

Leighton, P.A., J.K. Koffi, Y. Palcat, L.R. Lindsay, N.H. Ogden.  2012.  Predicting the spread of tick invasion: an empirical model of range expansion for the Lyme disease vector, Ixodes sapularis in Canada.  Journal of Applied Ecology doi: 10.111/j.1365-2664.2012.02112.x.

Martens, P., R.S. Kovats, S. Nijhof, P. de Vries, M.T.J. Livermore, D.J. Bradley, J. Cox, and A.J. McMichael. 1999.  Climate change and future populations at risk of malaria.  Global Environmental Change—Human and Policy Dimensions 9: S89-S107.

Vitale, J. and W.H. Schlesinger. 2011.  Historical analysis of the spring arrival of migratory birds to Dutchess County, New York: A 123-year record.  Northeastern Naturalist 18: 335-346.