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.