Pesticides and Mosquito Control for West Nile Virus: Potential Toxic Effects on Aquatic Systems

By Roberta Attanasio, IEAM Blog Editor

The influence of climate change on the spread of infectious diseases is a topic that generates intense debate, mostly because these effects depend on a variety of intertwined, variable factors – wealth of nations, healthcare infrastructure, availability of vaccines and drugs as well as ability to control vectors such as mosquitoes, ticks, snails, and others. Vector control is, indeed, one of the most important measures included in the existing global strategy to fight infectious diseases.

Aedes aegypti

Aedes aegypti, taking a bloodmeal. Courtesy James Gathany/Centers for Disease Control, Wikimedia Commons.

Mosquitoes transmit – among other infectious pathogens – the West Nile virus, which was first detected in New York City in 1999. It spread rapidly throughout the continental United States. Outbreaks of West Nile virus fever and encephalitis now occur across the nation each summer, when mosquitoes are most numerous and active. The virus moves from birds to mosquitoes and from mosquitoes to birds. Infected female mosquitoes spread the virus to a variety of hosts – including humans – when taking a blood meal. Thus, the best way to prevent West Nile virus infection is to control mosquito populations through integrated pest management programs that incorporate vigilant surveillance, habitat reduction, pesticides, and public education.

The use of pesticides to prevent disease outbreaks in humans is often necessary to reduce the number of biting female mosquitoes, especially during the summer months. These pesticides are sprayed from hand-held application devices, trucks, or aircraft to surface water where mosquitoes breed, which raises concerns about their potential toxicity to aquatic organisms immediately after spraying events, and brings up the need to balance risks to environmental and human health.

How can we determine whether or not these concerns are justified? Results from a new study published in Integrated Environmental Assessment and Management (March 21, 2014) help to answer this question. During the summers of 2011 and 2012, Bryn Phillips, at the University of California, Davis, and his collaborators monitored water column and sediment samples in California to determine the environmental effects of pesticide applications.

Pesticide field spraying

Photo courtesy of tjmartins, CC BY-NC-SA 2.0.

The goal of the study, “Monitoring the aquatic toxicity of mosquito vector control spray pesticides to freshwater receiving waters,” was to determine whether or not toxicity testing could contribute important information for evaluating the impact of pesticides on aquatic systems, as compared to the analysis of the active ingredient alone.

Phillips and collaborators performed a combination of aquatic toxicity tests and chemical analyses on pre- and post-application samples from agricultural, urban, and wetland habitats. They monitored a variety of pesticides used for mosquito control: the organophosphate pesticides malathion and naled; the pyrethroid pesticides etofenprox, permethrin and sumithrin; pyrethrins, and piperonyl butoxide. The study results show that about 15% of the water samples collected following application of pesticides were significantly toxic, in many cases because of dichlorvos, a breakdown product of naled. In other cases, toxicity was likely caused by synergism between piperonyl butoxide and pyrethroid pesticides.

These results emphasize the importance of toxicity testing for the detection of effects due to breakdown products and synergism. In their article, the investigators conclude “Toxicity testing can provide useful risk information about unidentified, unmeasured toxicants, or mixtures of toxicants.” Overall, however, the results indicate that many of the spray pesticides examined do not pose a significant toxicity risk to invertebrate animals in aquatic habitats.

Interestingly, the study was prompted by the California State Water Resources Control Board in cooperation with the Mosquito Vector Control Association of California and in the context of the Clean Water Act, following the adoption of a National Pollutant Discharge Elimination System General Permit. However, the final permit decision (summarized on page 2, number 8, of the State Water Resources Control Board Amended Monitoring and Reporting Program) states that: “In lieu of water quality monitoring for these active ingredients, the amended MRP requires reporting of corresponding application rates and incidents of noncompliance.”

In a telephone interview, Phillips said, “While the study was designed to determine the potential environmental effects of spray pesticide applications, it was clear, based on the final permit decision, that the policy makers were trying to strike a balance between human and environmental health.”

We need pesticides to protect human populations from West Nile virus. At the same time, we want to protect human populations and aquatic systems from the damaging effects of pesticides. The balance between these factors is likely to be influenced by climate change, where temperature shifts and altered patterns of precipitation directly affect mosquito populations, even in drought conditions. Warmer temperatures and prolonged periods of drought can actually boost mosquito populations. Indeed, warmer temperatures increase the rate of mosquito development from egg to larva to adult, as well as the rate of viral replication in infected insects. In addition, mosquitoes feed more often in warmer temperatures, thus increasing the spread of the virus. Pools of standing water, so often found in periods of drought, provide breeding grounds, and the lack of rainfall ensures that developing mosquitoes are not washed away.

It has been said that the chance of a West Nile virus outbreak hinges on a fine line between drought and rainfall. We can expect an increased use of pesticides when the right conditions for mosquito spread occur. Hopefully, additional toxicological studies will be carried out to identify the still undetected, environmentally harmful breakdown products and synergistic activities of pesticides – prompting a reassessment of water quality monitoring. This information is necessary to properly guide policy in a constantly changing environmental landscape subjected to temperature increases and varying precipitation patterns.

13 thoughts on “Pesticides and Mosquito Control for West Nile Virus: Potential Toxic Effects on Aquatic Systems

  1. Ray Kinney

    ,While the goal of the study was ” was to determine whether or not toxicity testing could contribute important information for evaluating the impact of pesticides on aquatic systems, as compared to the analysis of the active ingredient alone”, it would appear that, yes, toxicity testing does contribute important information. However, acute toxicity testing is not a very complete toxicologic assessment upon which to base a very important public safety and ecotoxicologic decision. We really know so little about the chronic low dose, low dose, and epigenetic effects of these chemicals and all of their degradates, adjuvants, “inert ingredients”, manufacturing contaminants, chemical degradation components, mixtures, effects on already weakened organisms (including human embryos, young developing, and older health compromised individuals. Epigenetic effects are not well understood at all yet. To widely disperse mixtures such as these across the landscape certainly places such glarring datagaps prominently in front of intellectual integrity, where red flags of insufficiency can be seen to be waving on all of the surounding hilltops. Far greater adverse toxics effects potentials, than West Nile, could be hidden within these datagaps.
    Widespread applications across whole geographic regions could adversely expose whole populations of humans and wildlife to increased risks we have no adequate ability to monitor effectively.
    “In lieu of water quality monitoring for these active ingredients, the amended MRP requires reporting of corresponding application rates and incidents of noncompliance.”
    This language scares the #$%^% out of me. It appears that chemical trespass is going to be the mode of public action, onto millions of people that will constitute a massive ecotoxicologic experiment. I don’t know anywhere near enough about the risks involved, yet I have spent a very large amount of effort for many years reading the sci lit. I’m not the sharpest dude on the block, and my training and “qualifications” are sorely lacking, but to base such a decision on such little actual scientific integrity appears to be highly dangerous, and not even based on the ‘best available science’. This study was a good start toward determining ‘best available science’ but that is scientifically JUST A START.
    In a public health emergency, such as may possibly be the case here, the politic takes over from adequate science-driven knowledge, but scientific integrity calls out for strenuously cautioning the politic that the science is inadequate to support any assurance of safety in view of the glaring datagaps. This decision should be on the head of the politic, not the science. IMHO

  2. Mark Jankowski

    This paper addresses the potential for impacts on avian immunity in birds infected with West Nile virus.
    Jankowski, M. D., J. C. Franson, E. Möstl, W. P. Porter, and E. K. Hofmeister. 2010. Testing independent and interactive effects of corticosterone and synergized resmethrin on the immune response to West Nile virus in chickens. Toxicology 269:81-88.

  3. Ray Kinney

    Environmental monitoring SAVES us money, by allowing us to discover water quality mistakes we are making. This gives us the chance to avoid continuing to make these same mistakes over and over into the future, and having to needlessly pay over and over into the future. It does not make good sense to avoid funding water quality toxic contaminant assessment. The legislators need better education.

  4. Johanna Humphrey

    The idea of the use of pesticides causing great harm to our aquatic systems is a frightening topic. With warming temperatures, West Nile Virus is becoming a more prevalent virus in America. I believe that we should explore other options to control the mosquito population before increasing the use of pesticides. If the overuse of pesticides cause more harm than good, officials should come up with alternative methods to resolve the problem at hand. I do agree that toxicity tests should continue to be performed to avoid any possible damage. Educating officials of the pros and cons of pesticides will help them to make better decisions when it comes to this issue.

  5. rallen27

    According to the National Pesticide Information Center, pesticides have different effects and some inhibit certain pests while others can kill pests instantly. But even if the pesticide’s intended effect is to kill pests it could still harm other creatures that come into contact with it and be termed toxic. It is understood that toxic means harmful to living creatures, but I was curious to know what the active ingredients in pesticides (like the one used to repel mosquitoes) could be doing to impair the living creatures. It could be that the immune system of the organisms (which could include the mosquito population) are being weakened or it could just be that the reproductive systems are being disrupted but these are just hypotheses. A real-life example of how living organisms are being harmed by ingredients in pesticides can be found in, “Testing independent and interactive effects of corticosterone and synergized resmethrin on the immune response to West Nile virus,” and the researchers help to highlight the fact that even though there are strict guidelines that the National Research Council use there should be more research done on not just the human risks but also on how these pesticides are interacting with the immune system in all animals. Until there is more elaborate research done to aid in the fight against the West Nile virus, the pesticides will be effective at both promoting more infections in other animals and decreasing the transmission of the West Nile virus.

  6. Melanie Washington

    It is clear from this article that climate change is a contributing factor to the spread of infectious diseases. The warming temperatures in the United States and other parts of the world allow mosquitoes to grow and reproduce more quickly and thereby spread the West Nile Virus. Pesticides seems to be the easiest solution to controlling the spread of the virus through mosquitoes but this has an impact on human and aquatic health. Another possible idea to controlling the spread of the virus is the possibly look at the bird pollution. If there is a way to treat infected birds, then possibly there could less use for pesticides. A mosquito that takes a blood meal from a bird that has been treated from West Nile would not be able to spread the virus to humans. Even though it is difficult to control and treat infected bird populations because of migratory patterns, it would keep the aquatic life in the waters thriving and also improve human health since pesticides are not being used to control the spread of West Nile via mosquitoes.

  7. Nabila

    This was an informative article. Mosquitoes are the carrier of many infectious diseases such as the West Nile Virus. There are conditions that are required to support the mosquito population so that mosquitoes can transfer the West Nile Virus to humans. The temperature needs to be warm, and the air needs to be dry in order for a mosquito population to thrive. These mosquitoes grow and reproduce in or near water sources. When conditions are right, mosquito populations thrive and carry infectious diseases to humans.
    There are also preventive measures that can be taken in order to slow down the spread of the West Nile Virus. One of these measures are pesticides. Pesticides help keep mosquito populations under control, even when the climate conditions are ideal for mosquitoes. However, there are setbacks to this preventive measure. Just as pesticides can harm mosquitoes, they can also harm other plants and animals nearby. For instance, pesticides are sprayed upon water sources in which mosquitoes thrive. The toxic chemicals from the pesticides not only harm the mosquitoes, but also harm the aquatic plants and animals within that water source. It is quiet difficult to find the amount of pesticides needed to kill the mosquito populations, and at the same time would not be harmful to any aquatic plants or animals.

  8. Elaheh

    while reading this article, a question that comes to my mind is what is more costly to the human health between the risk of mosquito bites which may transmit the West Nile Virus or the toxicity of aquatic environment? There are so many other ways that our aquatic environments are being polluted and toxic so does this source of toxicity add exponentially? If not, I believe that using pesticides to lower the risk of transmission of the West Nile Virus is a lot more important than being worried about the toxicity of aquatic environments. The people in charge can come up with a way of lowering all these toxicities if they are willing to invest in this issue especially because this is not the only source that is the cause of aquatic toxicity. Also, I believe having people be aware of facts about West Nile Virus can help to prevent people from being at risk; for example, if during summer time because of warmer temperatures, the chance of being bitten by a mosquito that could perhaps give us the virus is higher, why not try to be extra careful about the places we go to or getting the habit of carrying a spray that could prevent the mosquitoes from being close to us?

  9. S McKellar

    The intense spread of this infectious disease known as West Nile can potentially be fatal to one’s immune system by weaken it tremendously. Considering the progression of mosquitos throughout the population that are affected with this disease, pesticides are a source of relief to diminish these infected mosquitoes. However, with the use of these pesticides there are always good and bad aspects that have to be considered; that being high aquatic toxicity levels as well as affects on aquatic wildlife. Due to the threat of toxic waters with pesticide treatments, I agree that water toxicity testing is crucial in figuring out what balance is needed for safe human use all while minimizing mosquito levels.

    1. Ray Kinney

      If, “The intense spread of this infectious disease known as West Nile can potentially be fatal to one’s immune system by weaken it tremendously.”, are we sure that we are not weakening our immune systems even more by contaminating vast amounts of water with these formulations? Pesticides and many of their known degradates are showing up where they should not be, contaminating waters all across the nation and the world. We really do not know enough about public health effects, and about subtle aquatic health effects.

  10. cnesbitt1

    Controlling the vector is said to be the biggest impact on the spread of infectious diseases, such as the West Nile Virus. What’s interesting is that the spread of the virus is not only determined by the vector, but also by the climate that the vector is in. This is a little disturbing because we as humans have no control over whether the summer season will be filled with rain or drought, but we can suffer the consequences. I think evaluating the toxicity of pesticides is a great idea considering it could harm our aquatic systems, but I think we should look at a bigger spectrum, such as evaluating how humans can become “immune” to the West Nile Virus. Could we possibly wear some type of repellant such as a bracelet, like MosRepel , so mosquitoes are no longer a threat to us? Or could some vaccine be created similar to the flu vaccine that we can receive each year? With these two methods it seems as though we could possibly reduce the levels of toxicity in the different environments and also protect ourselves.

  11. Ray Kinney

    A quote from the article: Overall, however, the results indicate that many of the spray pesticides examined do not pose a significant toxicity risk to invertebrate animals in aquatic habitats.”

    Was subtle behavioral effects testing done on all aquatic species? Could chronic low dose effects limit behaviors that support these populations? Were any reproductive, food acqusition, predator avoidance abilities altered? Were olfactory abilities altered? Is any human drinking water taken from downstream sites? Were any known formulation degradates found in water twenty miles downstream? Were any unrecognized degradates
    present in any subsequent drinking water sources many miles downstream? How hard did the detection process work to find potential contamination? Data gaps hide risk. A lack of data does not mean there is a lack of significance. If you stop looking too early into the process the potential risks are greater, and the actual risks remain hidden and continue to alter the wellbeing of populations. It is far too easy to see the grant money running out and just say “were done looking, everything seems great, let’s write it up to say ‘it’s cool’. Significance hides. It takes existential responsibility to drive the decision that enough has been investigated and cross your fingers that more ‘significant’ overall effects don’t still exist. If the ‘results’ in the quote above are still inadequate, their value in determining a finding of ‘no significant toxicity’ is very questionable, and does not imply that adequacy has been reached. The toxicity testing completed was a good start, but that does not necessarily imply adequacy.


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