Effect Measure

West Nile policy: nothing changes

It’s mid summer, so it’s time to drag out what we say each summer about spraying for West Nile Virus. First, why we have to say it:

Sacramento County authorities plan to launch a mass aerial-spraying campaign to combat the West Nile virus on Monday.

Mosquito control officials will spray insecticide over 55,000 acres of urban neighborhoods north of the American River. About 375,000 people live in the area.

Last week, county health officials announced that two people have contracted the virus, and West Nile had reached an epidemic rate in the region’s mosquitoes.

“We are seeing infected mosquitoes at a rate that we know can transmit the virus to humans, so we are doing what we can to reduce those numbers,” said David Brown, manager of the Sacramento-Yolo Mosquito and Vector Control District.

Aerial spraying — last conducted in Sacramento in 2005 — is considered a last resort to halt the spread of infected mosquitoes. This summer’s spraying is scheduled for three nights. (San Francisco Chronicle)

Sigh. From Effect Measure, June 3, 2005:

West Nile and other mosquito arboviruses like Eastern Equine Encephalitis are rare but serious diseases. The reservoir for these viruses is usually birds. Humans get sick when a mosquito bites both an infected bird and a human. Mosquitoes that bite both birds and humans are called bridge vectors. What we need to know is whether mosquito adulticiding (spraying to kill adult mosquitoes) actually interrupts human transmission of these diseases. There is surprisingly little evidence that it does (in fact I know of none). Adulticiding only reduces the mosquito population temporarily, and the amount of the reduction, particularly in an urban setting, is difficult to determine. Repeated applications would be necessary if this control mechanism is chosen, while the impact on the ecosystem of all the effects of pesticides, both acute and lingering, is unknown.

A number of factors contribute to reduced effectiveness of reaching target mosquitoes with typical truck-based application, particularly in urban environments like New York City:

  • the most prevalent bridge-vector mosquitoes prefer birds, particularly birds at rest, of which there are few in the street and building-front areas when most sprays are applied;
  • roosting areas may be higher than the reach of the spray;
  • buildings close to the street restrict the lateral spread of the spray;
  • backyard roosting areas are not effectively reached because close spacing of buildings limits penetration beyond the buildings;
  • the period the spray is effective and airborne is of relatively short duration.

In any environment, ground and aerial applications produce skips and patchy coverage, especially where habitat produces barriers to spray dispersal and penetration.

Because of these and other factors, the effectiveness of spraying will likely be significantly less than the laboratory measured effectiveness of the pesticides. Nor is it known what proportion of mosquitoes must be affected to have a meaningful and positive effect on transmission of the disease to humans. Arguments can and have been made that adulticiding may even have the paradoxical effect of increasing risk by its effects on mosquito predators (like dragon flies) or the selection of healthier mosquitoes with longer lives. These uncertainties are a reflection of our substantial ignorance about important facts, an ignorance which should provoke further caution. (Effect Measure, June 3, 2005; minor edits)

Obviously it’s easier for me if I don’t have to write new posts every summer. But I’m not looking for “easy.” I’m looking for rational and science-based.

Help me out, please.


  1. #1 PalMD
    July 31, 2007

    Interesting question. We do know that mosquito control is effective for malaria (obviously not the same kind of organism, but still, a mosquito biting a human is necessary for both). Programs that go after “hot spots” like back yard reservoirs, etc, are probably more effective and safe than mass ariel or truck spraying.
    I saw a ton of West Nile the first season it hit my area, and have seen almost none since. Jays and crows are coming back, too.

  2. #2 phytosleuth
    July 31, 2007

    Entomopathogenic fungi have been sprayed on trap sheets hung out to lure mosquito vectors of malaria. Still, this focus is on the bridge vector, but may be safer to children, critters, and our environment.

    Anyone looking at helping birds resist pathogens by assisting THEIR health; nutrition? immune system?

  3. #3 rnnp
    July 31, 2007

    When I worked (left in February of this year) at a wildlife conservation park in NYC (where the health of birds certainly outweighed the health of humans), the emphasis was on larviciding along with elimination of standing water. Even if this was in the interest of protecting the collection, it seems more sensible than random spraying to try to limit adult mosquitoes.

  4. #4 bc
    August 1, 2007

    We’d have much better scientific data if we could just persuade the residents of a community with a homogeneous risk for West Nile throughout to agree to spray one half of the city with actual adulticide, and one half with “sham” adulticide, and see what happens.

    Somehow, I believe it is unlikely that this would be possible.

    In our community, there isn’t even uniform risk of WNV from one part of the city compared to another, either in numbers of vector mosquitoes or in infection rates of those mosquitoes. The time when adulticiding might have the greatest impact is when few or no human cases have been reported. There are different vectors in different parts of the country, with very different habitats and habits, so studies done in one area may not be applicable to another area. There are numerous options for adulticides, and truck-based and aerial spraying as means of applying them, further complicating studies of effectiveness. Our community has implemented aggressive larviciding in the past few years, but I am concerned that it may actually contribute to greater WNV transmission, as it seems to have more effect on reducing the non-vector mosquito species than the vector mosquitoes, giving people less incentive to use repellent or stay indoors and increasing the likelihood that any given mosquito bite will be from a vector mosquito.

    Draining water is a fine idea, but doesn’t work well when the major vector breeding area is in irrigated crops and pastures on the edges of your community [not true everywhere but in much of the plains states] and you can’t shut down agriculture to prevent a possible WNV outbreak.

    When all the interventions of draining the water that you can, larviciding what you can’t, and educating the community in personal protection still lead to large numbers of infected adult vector mosquitoes and large percentages of the population who take no protective measures, what should we do? If we wait till all the infected mosquitoes die from natural causes, I think thousands of residents in our community will become ill, dozens will develop neuroinvasive disease, and 6-12 will die [which is what happened here a few years ago.] It is unlikely that adulticide spraying will cause anything like that level of harm to the community. In the absence of other choices, in the absence of a vaccine, spraying now–before large numbers of human cases are reported–seems a reasonable decision. [If you wait till you have hundreds of human cases, adulticiding is likely much less useful, as most of those who ultimately become ill will have already been infected.]

    I’m not enthusiastic about adulticiding, and have many other public health priorities I’d like to spend the money on, but in the absence of significant evidence of risk from spraying, my precautionary principle leans towards spraying.

    What suggestions do you have for scientifically establishing the best course of action?

  5. #5 William Armbruster
    August 1, 2007

    So, revere writes that, because truck spraying in Manhattan is ineffective, arial spraying in Sacramento should not be done == and offers no alternative. Then sits back and tut tuts that this hasn’t changed in three years.

  6. #6 revere
    August 1, 2007

    William (and bc): A fair characterization, I think. Here’s how I would defend it. The problems with truck spraying in NY and aerial spraying in Sacramento aren’t different. Aerial spraing still has the skipas and apthicness and gets only bridge vectors at the tops of trees compared to the ones lower down (there is evidence there are differences in infection and perhaps species or subspecies). Plus all the other problems of knowing whther adulticiding interrupts transmission to any meaningful extent, the possible side effects and the effects on non-target species remain. So in my view the burden to show my objections are non-transferable to the Sacramento situation rests on the proponents of spraying.

    And that is the nub of the problem, isn’t it, for you and for bc. Which side here has the burden.

  7. #7 local epi
    August 1, 2007

    They also do the hard work of larviciding. If you look at the Sacramento Bee there is also an article on 7/26 describing a vector specialist searching out the green pools and standing water. I’m thinking that those efforts aren’t enough this year so they are also adulticiding. We do what we can.

  8. #8 bc
    August 1, 2007

    So do you have any suggestions for going about scientifically assessing the effectiveness [or lack of effectiveness] of adulticiding for West Nile Virus disease? What kind of studies would you do to answer the question?

  9. #9 revere
    August 1, 2007

    bc: I’m not a medical entomologist but I would think that one measure would be the pesence of WNV carrying mosquitoes in representative areas where people are found before and after spraying in multiple localities and over several weeks. There is probably a lot of temporal and spatial variation which would require a significant effort, so it is isn’t surprising we don’t know this. OTOH there are reasons to think it doesn’t work very well as mosquitoes breed exponentially and would come back very quickly, although their predators wouldn’t. Also the patchy application of the pesticide.

    WNV can be a nasty disease, so if adulticiding is effective I wouldn’t oppose it and I support larvaciding in the spring. But boradcast sprahying has its own hazards and without good evidence you are doing some good I would not do it at this point.

  10. #10 M. Randolph Kruger
    August 1, 2007

    Revere-I personally launch 50,000 dragonflies in the early spring here in the bottom lands of the Miss. River Valley and it seems to work one helluva lot better than spraying any day. Nothing funnier than watching a dragon fly dodging a finch on patrol. Daily battle.

    Anyways… here is a question. WNV is pretty dangerous if you get it and it goes to encephalitis. But, with the costs involved with spraying and in relation to human lives, I put it at nearly 2.8 million dollars for no human deaths in West Tennessee last year and only one case. The spraying is done on an as needed basis and most counties dont spray at all, you are on your own.

    Even with the spraying the city/county we reported only 3 cases of confirmed WNV… finally to the question. With the effects of WNV being mosly flu like symptoms and occasionally an encephalitis, and even more rarely a death does the cost to disease ratio work out in your mind?

    Also, what in Hell does that stuff do to all the good bugs out there?

  11. #11 tymbuktu
    August 2, 2007

    I lived on the island of Saba in the Caribbean for two years. Unlike most other Caribbean islands they have NO mosquito problem.

    Three factors – one is highly porous volcanic soil that prevents the formation of ponds and natural standing water.

    The second is a high profile public awareness campaign regarding standing water and the fact that mosquitos hang out in your closet during the day. So chase them out so the anole lizards -there must be hundreds of thousands – can eat the adults.

    But the most interesting is the “Guppie Man”. Since there is no public water system on the island, all drinking and grey water is collected in cisterns by way of one’s roof. Every three months or so the Guppy Man comes by and adds some more guppies, including at least one pregnant female, to your cistern (yes the water is filtered before it exits your faucets). No mosquitos.

    On nearby St. Maarten, you may be eaten alive if you are in the wrong area. Different soil but no one cares, no state run program.

  12. #12 marquer
    August 2, 2007

    Also, what in Hell does that stuff do to all the good bugs out there?

    Randy, during the 1982 Mediterranean fruit fly outbreak in California, my family home and the surrounding area were aerially sprayed with malathion.

    The Medflies died. So did every single beneficial pollinator. It took five years for the garden to return to a semblance of normality.

    I note that Singapore is suffering anew from mosquito-borne dengue. Their response is to go after even the smallest reservoirs of standing water. If you are a Singaporean apartment dweller with a balcony, you had better make damned sure you have no vessels there in which water might collect. The hand of the state is heavy in SG, but moves with a speed which belies its mass.

  13. #13 revere
    August 2, 2007

    Randy: You, tymbuktu and marquer note other control measures. Eliminating standing water is one of the most important, IMO. Regarding your quesiton, “Also, what in Hell does that stuff do to all the good bugs out there?”, spraying also knocks down insect predators, noe of which bounce back as fast as mosquitoes after spraying. This means the new crop of mosquitoes will not have any predators for a period and can multiply faster and attai higher population levels, at least in theory. One of the things we don’t know is whether this is an imporant effect offsetting mosquito killing, which in any event is temporary. We also don’t know the effects of the pesticides on the birds (one possibility is that infected birds are differentially affected), or whether infected mosquitoes or more or less affected by the pesticide.

    Regarding the cost benefit connection, that isn’t a scientific question. If you were to make the same calculation for the War on Terror, you’d get the idea.

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