Stream pollution makes mosquitoes happy

This week the Ecological Society of America is having their annual meeting, and several papers of entomological interest have been presented. One found that increased pollution in urban streams leads to increased mosquito populations:

“Luis Fernando Chaves, a post-doctoral researcher at Emory University, and his team discovered mosquitoes in abundance in a sewage-contaminated stream in Atlanta, but rarely in a nearby clean stream. They also found that mosquitoes were largest in streams with high levels of organic minerals – in this case, nitrogen and phosphorous – that originated from the sewage treatment plants.”

The issue of sewage runoff is a major one here in Michiganclosed beach sign

Many of our cities use the same pipes for sewer water and storm runoff.  This means that in a heavy storm, excess water–with all that sewage too–goes right into the watershed.  This usually means the Great Lakes or other bodies of water.

Signs posted warning against swimming because of high fecal bacterial counts are a pretty common sight here.  Sadly, because of Michigan’s state budget woes, the annual required report of wastewater release has been suspended, effective April 2009.  Right now, there is simply not enough staff to monitor our water safety.

Chaves’ work suggests a double whammy–all those bacteria are wonderful food supplies for the bacteria that feed mosquitoes. And the mosquitoes are bigger, which means they can live longer–and have a better shot at transmitting a mosquito-borne disease, of which there are many.

You can read some research about storm runoff closer to home in the Journal of Great Lakes Research–here’s a couple of representative papers:

McLellan, S., Hollis, E., Depas, M., Van Dyke, M., Harris, J., & Scopel, C. (2007). Distribution and Fate of Escherichia coli in Lake Michigan Following Contamination with Urban Stormwater and Combined Sewer Overflows Journal of Great Lakes Research, 33 (3) DOI: 10.3394/0380-1330(2007)33[566:DAFOEC]2.0.CO;2

Tracie M. Jenkins, Troy M. Scott, Mechelle R. Morgan and Joan B. Rose (2005). Occurrence of Alternative Fecal Indicators and Enteric Viruses in Michigan Rivers Journal of Great Lakes Research, 31 (1), 22-31 : doi:10.1016/S0380-1330(05)70235-5

Link to Chaves’ home page

Great Lakes Assessment of Urban development and Water Quality

Not all mosquitoes are the same
Welcome to the second day of World Malaria Day [week] at the Bug Blog! I’ve talked several times about the way in which different mosquitoes respond differently to pesticides for malarial control, but here’s a new twist.

In a recent study to compare different species of mosquito in their ability to be a malarial vector, there were very large differences!

As a review– a disease vector is an organism that does not cause disease itself, but transmits an infection by transporting pathogens from one host to another. The malarial parasite is alive inside the mosquito, although they don’t get malaria–but they can give it to us, the host. In this research, different mosquitoes were compared in their abilities to serve as a vector for malaria.

One of the claims frequently made by the “DDT will solve everything” crowd is that just spraying enough DDT will kill all the malarial mosquitoes.  This ignores that mosquitoes vary widely from population to population, and species to species, in their ability to resist DDT.  They aren’t all the same, and there is no one size-fits-all control method.

The research I discussed yesterday is another good example of the variability problem–predicting malaria using weather and other environmental data in different areas of Africa required different solutions.

In this experiment, you can get a sense of another layer of difficulty in controlling malaria. The experiments looked at two different strains of malarial parasite (Thai and Korean), and 3 different mosquito species.  That’s a lot of potential variation!

What did they find out?malaria_lifecycle

I won’t go into the specifics–you can read the paper if you want to see technical words like “sporogony”–but basically, not all of the mosquito species were able to support the malarial parasites’ life cycle.  (BTW, this diagram is probably the single greatest cause of drinking in invertebrate zoology students. The names! The stages! Ugh!)

Infecting a mosquito with the malarial parasite isn’t enough–the parasite has to change, migrate out of the gut of the mosquito, and into its salivary glands.  Not all mosquito bodies are equally friendly to this process–there were large differences between the 3 mosquitoes tested in this experiment.

Hopefully this gives you a sense of the complex layers of difficulty surrounding malarial control–in addition to the environmental variability  from location to location discussed yesterday, and variation in pesticide resistance which I’ve discussed before, there are also large genetic differences within mosquito species, mosquito populations, and malarial parasites. It’s a spaghetti tangle of variables, many of which we have no control over.

This is why there is no easy solution to malaria, and why after centuries, we are only now beginning to make progress.

But we are making progress! Just not as fast as we’d like.

Joshi, D., Choochote, W., Park, M., Kim, J., Kim, T., Suwonkerd, W., & Min, G. (2009). The susceptibility of Anopheles lesteri to infection with Korean strain of Plasmodium vivax Malaria Journal, 8 (1) DOI: 10.1186/1475-2875-8-42