Posted by In a post yesterday we talked about West Nile Virus. It causes a mosquito borne disease and most people will have mild or even asymptomatic infections. But you don’t want to the be the exception for this one. So what to do?
Here’s the typical response in many urban environments:
Yuba-Sutter’s annual battle against mosquitoes and West Nile virus will hit the streets tonight as spraying begins in residential areas of Robbins and Meridian.
The weekly spraying will begin tonight in Olivehurst, Live Oak, Linda, Plumas Lake, Marysville, Sutter, Tierra Buena and Wheatland, said Ron McBride, manager of the Sutter-Yuba Mosquito & Vector Control District.
Yuba City is not on the list because its mosquito count is not high enough, said McBride.
Spraying will begin at dusk – about 8:45 p.m. – and last about two hours. Residents are advised to avoid or minimize their exposure to the pesticide by staying indoors and closing windows. Residents who use swamp coolers instead of air conditioners should turn them off to avoid drawing spray into the house, said McBride.
As usual, the insecticide will be the chrysanthemum-based Pyrethrin.
Using truck-mounted foggers, workers will spray along every other street, letting the wind spread the insecticide to the other streets, said McBride.
California has had just one known human victim so far this year, a Kern County man who had to be hospitalized, he said.
Yuba-Sutter’s first West Nile victim of the year – a crow – was found early this month at Yuba College, said McBride. (Appeal-Democrat)
This spraying technicaue is called adulticiding. Biologically active chemicals are put in high volume into the human environment to kill adult mosquitoes. You are exposed along with the mosquitoes. With any luck, only the mosquitoes will be harmed. Like the virus itself the chemicals may be safe for most people. Risk – benefit trade-off, right? If we apply the same criteria for spraying as we would use for a vaccine against WNV, we would require it to be both safe and effective. But spraying has not been shown to be either, although it is assumed to be both. A vaccine could be voluntarily administered. Exposure to spray is not voluntary. It doesn’t matter what your desires or your risk factors are.
First, safety. Notice care is taken by public officials in the article to identify the pesticide as “the chrysanthemum-based Pyrethrin.” Pyrethrin does come from chrysanthemums, but most pyrethrins used as adulticides are synthetic versions like Resmethrin or Permethrin. They are chemicals, with no relationship to any botanical. Not that it matters. Botulism toxin is “natural” but is one of the deadliest toxins we know. Moreover, when permethrins are used for adulticides they are invariably used in combination with a knock-down agent, piperonyl butoxide (PBO). PBO inhibits an enzyme in the mosquito’s P450 system that detoxifies permethrin. This additive boosts the permethrin toxicity to the insect. But humans also have P450 systems. It is not known if exposure might, say, alter the drug metabolism of older subjects in ways that would have an adverse effect. Other possible side effects, like effects on the immune system, of children or asthmatics have yet to be thoroughly investigated in humans. Remember, these are biologically active compounds. If they weren’t they wouldn’t be useful. Probably these chemicals are relatively safe and non-toxic to most people. But we cause promiscuous exposure without a thorough check for safety.
For the sake of argument, though, let’s say the spray is perfectly safe or nearly so. It then might be justified if we knew adulticiding were effective in interrupting the transmission of disease from birds to humans. Unfortunately this is the biggest gap. There is no evidence that adulticiding has any discernible effect on interrupting the transmission of disease. This may seem counterintuitive. because if mosquitoes are the vector, killing mosquitoes would seem the obvious intervention. Here are some reasons why adulticiding in an urban area is problematic.
- bridge vector mosquitoes (those that bite both birds and humans) prefer birds at rest. These are uncommon in the fronts of buildings and on the street, where truck-based spray is applied.
- birds often roost above the level of spray.
- the lateral spread of spray is restricted by buildings on the street, thus blocking backyard roosting areas.
- most products used in sprays have relatively short duration of effectiveness.
The buildings that block the backyards also increase the likelihood of human and pet exposure. Mosquitoes breed best on the same warm summer nights that lead to open windows and ventilating fans. Keeping windows closed for the evening doesn’t seem reasonable and would depend on residents knowing the spray schedule as well. A CDC report, “Epidemic/Epizootic West Nile Virus in the United States: Revised Guidelines for Surveillance, Prevention, and Control (April, 2001)” essentially made the same points: “ground applications are prone to skips and patchy coverage in areas where road coverage is not adequate or in which the habitat contains significant barriers to spray dispersal and penetration.” Effectiveness of urban spraying will certainly be less, perhaps substantially so, than laboratory based benchmarks.
Spraying doesn’t kill all mosquitoes, either, but we don’t know the proportion that have to be affected before disease transmission is interrupted. Whatever that proportion is is quickly returned to the environment’s carrying capacity because mosquitoes reproduce very rapidly. Some of their predators, however, don’t breed as quickly and to the extent they are also affected by the spray, adulticiding may actually worsen the situation. Nor do we know that infected mosquitoes are equally susceptible. It is possible that they will reproduce more successfully than sensitive ones or be less sensitive to the spray. There are many unknowns and experience in areas where adulticiding is used intensively and continuously don’t seem to show reduced incidence of WNV infections. Whatever benefits there may be are certainly very short term, requiring repeated and frequent reapplication.
Where does that leave us? The current best advice for avoiding WNV is don’t get bitten by mosquitoes: wear long pants or long sleeves between dusk and dawn when you are out and about and use a mosquito repellent like DEET. Mosquito control in the spring with larvaciding is thought to be a good investment. It involves local treatments in speciified breeding grounds. Deny breeding areas by emptying standing water from cans, flowerpots, garbage cans and clogged gutters. Not very reassuring you say? Not as comforting as spraying mosquitoes?
Too bad. Get used to one more risk of modern life. As global warming induces climate changes and weather extremes, we will have more mosquito-borne diseases to face and in more places now unaccustomed to them.
Have a nice summer.
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