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Friday, July 12, 2013

The Year of The Mosquito

The Year of The Mosquito
This year is shaping up as a bad one for people who are particularly annoyed by mosquitoes.
Michigan has about 60 species of mosquitoes.  Not all species of mosquitoes bite humans.  Not all human-biting mosquitoes carry disease? Some mosquitoes breed in wetlands.  Others breed in your sump pump, a bird bath, street catchment basins or any number of other locations. 
Although mosquitoes breed in wetlands,in healthy wetlands with stable water levels year after year, fish and other predators keep mosquito numbers low.
 Wetlands are definitely a glass-half-full/half-empty test of perception.  When you look at a wetland, you may see habitat for turtles, frogs, ducks, birds, spawning fish, deer and any number of other interesting species.  When a public health official looks at a wetland, he or she may see an incubator for life-threatening diseases.
It was only in the 1800’s that the link between malaria and mosquitoes was established. Previously, people believed that a miasma, or poisonous atmosphere rising off swamps, made people sick.  Real estate prices came to reflect that.  Wealthier people lived on higher ground; poorer people lived by the swamps.  Even the terminology has come to carry connotations:  wetland is more likely to be used by people who see the virtues of wet places; swamp is more likely to be used by someone who wants to drain them.  The first Europeans to arrive in Michigan found a land filled with wetlands and mosquitoes.  They didn’t find malaria, however.  That, they brought with them.  Later settlers found malarial swamps.  When the federal government made gifts of land for our state capitol and for the nation’s first land grant college, the Michigan Agricultural School (now Michigan State University), in the mid-1800’s, there were some strings attached.  Before the land could be comfortably used, it would first have to be drained.  Nonetheless, the school had to be closed several times because so many faculty and students were suffering from malaria.
Prior to the mid-1800’s, drainage was accomplished primarily by ditches.  Underground drainage tiles were first used in Michigan in a major way by Zachariah Chandler.  He owned a lot of land around Lansing and became a wealthy man by draining and selling it.  Portions of the march which bears his name still exist today, mere shadows of the great wetland which stretched from Bath, where the Bengel Wildlife Center stands today, to the Michigan State University Campus about 4 miles away.
To stand today on the eastern edge of one of the sod farms located in what was once the Chandler Marsh, and look into the sunset over miles of flat green grass, inspires the viewer to imagine the lush diversity of plant, animal and birdlife that must have once lived there.  To stand in the same place 150 years ago, and wonder if that mosquito at the back of your neck was carrying malaria must have been a very different experience—enough to inspire men to drain swamps.  And drain they did—Michigan today has only 50 percent of the wetlands it had when the Europeans arrived. 
It wasn’t the drainage that actually got rid of the malaria, however.  Malaria is transmitted to humans by Aedes aegypti mosquitoes.  To eliminate malaria from an area, you can, in theory, drain the swamps to eliminate mosquito habitat, or kill mosquitoes, or you can eliminate the malaria from the humans.  That’s ultimately how we got rid of malaria.  Window screens aren’t just a convenience they’re a public health measure.  Chloroquine and other drugs, along with window screens, eliminated malaria from the human population.  The mosquitoes keep breeding and biting, but they have no malaria to pass along.  If a traveler brought back malaria to Michigan today, the disease would not become established, because we have drugs to eliminate the parasite from humans.  That’s why malaria remains a disease of poverty and lack of education in some places in the world.  People cannot afford the drugs, or if they can, they may take them until they feel better and then share them with a sick family member.  Not completing the full course of antibiotics is what has led to the emergence of drug-resistant strains of malaria.
Malaria is in some ways simpler than other diseases, however.  West Nile virus, for example, survives in avian hosts.  Even if it were possible to treat or isolate every person who got the West Nile virus, it would be impossible to eliminate future cases of the disease.  That’s because some mosquitoes seek their blood meal from both birds and humans.  Killing all the birds seems an unlikely and unpopular solution.
As many as 43 other species of mosquitoes around the world have been found to harbor the West Nile virus and it is not known how many Michigan species carry the virus and bite humans.  Culex pipiens, a primary vector, represents a major problem however.  Standing water, whether in a cow’s hoof print, a clogged gutter, or under a potted plant, provides breeding habitat for Culex pipiens.  That makes control measures difficult to implement.
In the hierarchy of preferred ways to eliminate mosquitoes, eliminating habitat is near the top, in the view of some.  Depending upon the species of mosquito, this may or may not have an impact on other forms of wildlife.  Since culex pipiens breeds in many human artifacts, eliminating those habitats should be high priority.  You can reduce mosquito populations in your vicinity by eliminating standing water. 
 Most mosquitoes lay their eggs either singly or in a tiny “raft” on the surface of the water.  Larvae emerge from the eggs and go through further states of development in the water, eating organic matter and microorganisms.  BTI (Bacillus thuringiensis israelensis) is a bacterium that disrupts the digestion of the larva and kills it.  The bacteria don’t harm humans or pets, so among the control options, it has a smaller impact.  However, finding all the possible breeding locations is nearly impossible.  Chemical larvicide is also sometimes used, with more significant implications for other forms of wildlife.  Some species of mosquitoes lay their eggs in cattail roots, in which the larvae develop.  These survive larvicides. 
Insecticides are usually the option of last resort for mosquitoes.  Populations can be reduced, but not eliminated.  The pesticides are indiscriminate and land on mosquitoes, people and desirable forms of wildlife.  In Mosquito—Man’s Deadliest Foe, the authors report that truck spraying will…”kill at least some mosquitoes, and it does assure local residents that the outbreak is being taken seriously.”  In the period of public concern surrounding outbreak of a mosquito-borne disease, the public often initially supports spraying of pesticides.  Then the balance shifts and vociferous objections are heard.  It is not possible to make any ironclad scientific case for or against spraying, and decision-making is complicated by the fact that values must be accounted for as well. 

Because mosquito control decisions must be based on value judgments as well as scientific data, public participation is essential.  Find out what your community has planned to do to deal with mosquito outbreaks.

In the 60’s and 70’s, the occurrence of heartworm, a mosquito-born disease of dogs, skyrocketed in Michigan.  What had been a disease of the American south became endemic here.  Dogs are not routinely treated to kill the tiny worms.  Similar microfilaria causes the mosquito-borne elephantiasis in parts of Africa.  Ivermectin, one of the drugs used to treat heartworm, is also used to treat people suffering from elephantiasis.  Andrew Spielman, coauthor of Mosquito:  Man’s Deadliest Foe, reports that U.S. sales of the drug for canine use may subsidize donations of Ivermectin by a U.S. Drug company to the World Health Organization for human use in Africa.

The Michigan Mosquito Control Association provides good general background information at  On the site, you can download a 109-page booklet to help communities plan for mosquito control, prepared in cooperation with the Michigan Department of Agriculture.
Official State of Michigan in on West Nile virus is at

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