OREGON STATE UNIVERSITY

Does mosquito control prevent disease? Maybe not

09/27/2005

CORVALLIS, Ore. - A new study suggests that environmental or other impacts on a species can change not only population numbers, but also the age and life expectancy of individuals - findings that may affect issues ranging from fishery management to pest control and disease prevention.

The research has outlined a mathematical formula that confirms this phenomenon and can be used in specific ecological situations to predict the probable changes. And the system may have some important applications - such as explaining why some mosquito control programs do not seem to reduce the incidence of human disease the way they were intended to and might actually make problems worse.

The findings were published in a professional journal, Mathematical Biosciences, by scientists from Oregon State University, the School of Public Health at Harvard University, and CSIRO Marine and Atmospheric Research in Australia.

This study looked at the ecological effect of what the scientists call "press perturbations"- a long-term change that has an impact on one or more species, which may be caused by environmental shifts, pollution, habitat loss, land use alterations, human impacts or many other forces.

"It's quite clear that significant ecological impacts can change the population levels of species, causing them to go up or down," said Phil Rossignol, a professor of public health entomology at OSU. "But what we now understand is that these impacts can also cause major changes in the age or lifespan of individuals within a species, sometimes when the overall population is not even affected."

These less apparent age impacts on a species, the researchers say, can in turn have ecological effects that are profound in their own way.

"This is very relevant to marine fisheries, for instance," Rossignol said. "In many cases we need older fish for reproductive success, and anything that interferes with that is a serious concern."

A lake that is suffering eutrophication from too much nitrate pollution may have populations of fish that are as numerous as ever, but younger and smaller in size. An older animal may be able to resist predation, whereas a younger one will not. But one of the most compelling, and possibly important implications of this phenomenon may occur with vector-borne diseases, such as mosquitoes that can transmit malaria or West Nile Virus, the scientists said.

"With mosquito-transmitted disease, the life expectancy of the mosquito is often more important than the overall population numbers," Rossignol said. "For instance, it takes two weeks for a mosquito to become infective with malaria, which in natural systems is often longer than the average lifespan of an individual. Controlled studies have confirmed that a small increase in life expectancy can have a large increase in ability to transmit disease."

Some approaches to mosquito control, especially biological control, can knock down the overall population numbers of mosquitoes. That's fine if the only concern is avoiding nuisance bites from larger numbers of the pest. But if the concern is disease transmission, this approach can actually result in some of the surviving members living longer than they would have otherwise, Rossignol said.

"Mosquitoes ordinarily live in a very difficult environment and are eaten by lots of predators," he said. "If you nurture some mosquito species with plenty of food in protected laboratory conditions, you can keep them alive for a year and they grow to huge sizes. But that same mosquito in the wild is small, defenseless, not very fast, and might be competing with thousands of others for the same very limited food supply. It could die in a week." Pest control programs which reduce just the overall number of mosquitoes may result in less competition for the survivors and allow them to reach older ages, becoming much more effective carriers of disease.

"We have observed in working situations that sporadic mosquito control efforts are highly ineffective at reducing the levels of some disease, such as malaria or West Nile Virus," Rossignol said. "If the nature of the control program allows some of the insects to reach an older age, we may be making the problem worse instead of better. Our new findings can help explain why this phenomenon occurs, and why some of the most common approaches to reducing mosquito-borne disease don't seem to work."

In this case, Rossignol said, systems that effectively target only older mosquitoes may be much more successful in preventing transmission of human disease. Unfortunately, some of those approaches involve the most invasive use of toxic pesticides and are also the most publicly unpopular.

"As we use these new mathematical algorithms to learn more about how various perturbations affect not only the population of a species but also its age and lifespan, we will have a better idea how to deal with some of these issues," Rossignol said.