MCWD dispells West Nile concerns

With concerns about infectious diseases pervading the news, the Minnehaha Creek Watershed District (MCWD) has just released a new white paper on the relationship between urban wetland environments and the spread of the West Nile Virus. The virus is transmitted exclusively by mosquitoes, making it a particularly great hazard during the summer. “As a water resource agency that manages water quality and quantity through BMPs [environmental water management systems] and storm water facilities, we face potential inquiries from the public,” the paper notes. “The MCWD can expect questions or concerns on whether a BMP or storm water facility created to a single family home, school or park acts as a breeding area for mosquitoes potentially carrying West Nile.” The MCWD is hoping that its report will help Minnesotans understand the ecological circumstances that support the spread of the disease.

The West Nile Virus found a good home in Minnesota last year with the giant, bat-like mosquitoes that are widely regarded as the unofficial “state bird.” Mosquitoes are a perennial nuisance in the state in part because they rely on aquatic environments to reproduce. Mosquito larvae are born in the water and live off of aquatic bacteria until they mature. The vast wetland environments in Minnesota naturally support a large mosquito population.

However, the MCWD report asserts that only a small fraction of the state’s mosquitoes are effective transmitters of the West Nile Virus. “Out of some 2,500 or more different mosquito species worldwide, approximately 50 can be found locally and only a couple are targeted as potential [carriers] of WNV,” it says. Furthermore, “it is believed that the two most common Minnesota mosquitoes, the vexans mosquito and common cattail mosquito, are not effective transmitters of WNV.”

That news has left most researchers concentrating on the culex mosquito as the primary source of the disease in the state. The culex variety feeds almost exclusively on birds, which fits with the widespread hypothesis that West Nile is originally an avian disease. Indeed, research has shown that mosquito larvae are not born with the virus if their mother is a host; rather, any mosquito carrying West Nile must get it from a bird carrier before passing it on to a human or other animal.

As for the aquatic environments that play host to mosquito populations, the MCWD asserts that the wetland management strategies that it applies to municipal bodies of water do not, in general, promote the growth of mosquito larvae. “According to experts we spoke with, ideal water conditions for mosquito development are found at the edges of semi-permanent wetlands or in wet meadows. These ideal environments consist of very shallow water conditions (1”-12”), thick vegetative cover and fluctuating water levels, all of which contribute to bacterial growth through rotting vegetation.”

It concludes, “storm water facilities tend not to support the diverse conditions and characteristics mentioned above.” Water features called extended storage ponds or dry ponds come closest to matching the ideal breading conditions; but “most are designed to hold large amounts of water,” making them too deep to support much larvae growth. Also, most storm water storage areas are designed to induce continuous water flow and support populations of fish, frogs, bats and birds, the likes of which naturally control and limit mosquito populations.

“Based on assumptions of these local experts,” it concludes, “MCWD facilities are no more, if not less, likely to act as breeding grounds for [infectious] mosquitoes than a puddle, birdbath or rain gutter.” However, it notes that research on West Nile is still in its early stages, and results should not be taken as conclusive. The paper also points out that residents can help prevent the growth of mosquito populations by avoiding the unnecessary or excessive application of pesticides which, “while trying to control mosquito populations, could have potentially toxic effects on aquatic life and untargeted biological species.”