Dead birds found infected with WNV by county. More maps available at USGS. West Nile Virus is an arthropod-borne virus (aka arbovirus) of the genus flavivirus. Originally and commonly found in Africa, West Asia, and the Middle East, it has been detected and tracked in the U.S. since 1999, beginning in the New York area. West Nile is closely related to yellow fever and St. Louis encephalitis virus which is also found in the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals. Its typical human vector is transmitted by mosquitoes from birds to humans and other mammals. Because birds are an early vector for this disease, the Centers for Disease Control (CDC) and local officials have asked citizens to report unusual sightings of dead birds since the earliest cases were seen in 1999; once the virus is verified in an area, citizens are asked to cease reporting of these sightings.

Less than 1% of those individuals infected with West Nile virus will develop severe illness and 80% of those infected will be asymptomatic while the remaining 19% will exhibit only mild flu-like symptoms according to the CDC; of the 1% who develop severe illness, typically immunocompromised individuals and those over 50, there is a 3%-15% fatality rate [August 2005]. West Nile symptoms range from fever, nausea, paralysis and meningitis. The most severe type of human disease resulting from infection with the West Nile virus is a “neuroinvasive disease” that affects the nervous system. Specific types of neuroinvasive disease include: West Nile encephalitis, West Nile meningitis or West Nile meningoencephalitis — symptoms include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. West Nile Fever is a mild flu-like illness that occurs in about 19% of those who become infected — it is characterized by fever, headache, tiredness, aches and sometimes rash. West Nile Fever can be as brief as a few days, though even healthy people may become sick for several weeks. Should you develop a high fever with severe headache, the CDC recommends you consult your health care provider.

In summer 2005, infection rates in the Sacramento area peaked at 10% of the mosquitoes along Sacramento's American River found carrying the disease. On 2005-08-18, officials pronounced 60-year old Joan Randall, a Davis resident, to be the first human West Nile case contracted in Yolo County. As of August 2006, the confirmed human infection rate is twelve persons in Yolo County, seven of whom are from Davis and five of whom are from Woodland. Nine are male, three are female.

For the year 2006 (through mid-August) there have been 50 cases statewide, and one recent death in Butte County. Nationwide there have been 388 cases, and 13 deaths.

To prevent a potential epidemic, Sacramento-Yolo Mosquito and Vector Control District instituted a pesticide-spraying program using pyrethrins in Sacramento-area counties, including Yolo County. Sacramento was blanketed by aerial spraying while Davis was slated to be sprayed by truck. Davis residents came out in storm to protest the spraying, saying it might hurt "humans, and also cats, bees, earthworms, and natural predators of mosquitoes." Residents wishing to add their address to a "no-spray" list can do so on the Sacramento-Yolo Mosquito Vector Control District website.


Local Controversy

On August 23rd, 2005, a open panel on the issue of ground spraying for mosquitoes dissolved in chaos when the attendees refused to follow the planned format. The format had been prearranged; a panel of experts would answer questions from the public. The questions were to be submitted in writing and Mayor Ruth Asmundson was to read them to the panel. After two questions, the gathered public began shouting at the panel that they should "learn to practice Democracy" and moved toward the front of the room. In response, the panel and Mayor closed the meeting and left out the back. This caught the attention of the local news, and even ABC news in San Francisco. The City Council fiasco was particularly unfortunate because the meeting was called to explain the spray-response plan to the public, however, the citizens who disrupted the meeting thought it was called because the city was already going to spray. The scientists invited to speak at the meeting went home and did not have the opportunity to answer questions. Ironically, the disruption may have harmed the democratic process by preventing the dissemination of information.

Since then, a group called Stop West Nile Spraying Now has organized (site) to lobby against using pesticides to control mosquitoes. They also educate the public on how to reduce the incidence of mosquitoes through the elimination of habitat, and changes in outdoor behavior. The group can be seen tabling at the Farmers Market and the Davis Food Coop.

Mosquito Spraying August 2006

Map of area to be sprayed in Davis. On 2006-07-27, two additional human cases of WNV were confirmed, raising the Vector Control District's alertness level to 4 out of a possible 5, triggering the spraying of pesticides. On 2006-07-29, the VCD announced its decision to spray adult mosquitoes in Davis and other areas, on the 3rd and 4th of August. The Davis City Council met on Monday July 31 to discuss the issue of spraying adult mosquitoes to control WNV, and Councilmember Lamar Heystek suggested that neighborhoods ought to be allowed to opt out of the treatment, but it was voted down 4-1. The Davis Enterprise reported on Tuesday 2006-08-01 that the spraying would go forward, and that the people on both sides of the issue called for putting aside politics and claimed that science was on their side.

The August 3 spraying was canceled due to wind, so the spray dates were at first pushed to Friday 2006-08-04 and Saturday 2006-08-05, however, the Davis Enterprise reported that spraying was canceled for that weekend. This gave the scientists scrambling to set up experiments to study the effects of the spray on non-target insects more time to prepare. On the 3rd and 7th of August, the third and fourth cases of West Nile in Yolo County were confirmed, both in Davis.

On Monday (2006-08-07), the situation was re-evaluated and the Vector Control District scheduled the sprays again for Tuesday (2006-08-08) and Wednesday (2006-08-09).

Spray Times & Logistics 2006

Fighting the Bite over South Davis, 2006-08-08 8:47pm

The Spray treatments were scheduled for Tuesday and Wednesday the 8th and 9th (August 2006), weather permitting.

The pesticide mixture will be sprayed from a plane that will fly over Davis East to West about 15 or 16 times, depending on the weather. The plane will fly around 150mph and release at around 300ft above ground. The spraying will begin at about 8:20 pm, and it will take 60 to 90 minutes to complete the spraying for Davis. Read more about it in the Sacramento Bee. For spraying to be effective, winds must be below 10 mph. Wind causes fewer mosquitoes to be airborne, for example.

Pesticide information

The pesticide mixture being used is an adulticide (kills adult mosquitoes) containing synthetic pyrethroids and Piperonyl Butoxide, or PBO. Pyrethrins are found in the African flower Chrysanthemum cineriaefolium. They operate by blocking chemical signals in the nerve junctions of insects. PBO is a synergist that inhibits cytochrome P450, a class of enzymes that break down pesticides, thus increasing the effectiveness of low amounts of pyrethrins. It allows the pyrethrins to act longer inside the insects, but it is not itself a pesticide. For more information, see The VCD FAQ. The name the district uses for the pesticide is "Evergreen 60-6."

There is no such thing as a safe pesticide, however, synthetic pyrethroids are some of the safest insecticides. The pesticides being used will also degrade in the sunlight. Yet, if the pesticide reaches our waterways, it may have negative effects on the environment.

While they are considered safe for organisms living in the water column the toxicity of pyrethroids to sediment dwelling organisms is relatively unknown, with this precursory study showing negative results on an indicator species for sediment toxicity. Here is a quote:

"About one-fifth of our Central Valley sediment samples are toxic to a standard testing species due to a class of pesticides no one has tested for before, for which there are little data on their toxicology when sediment-bound, and which are being promoted as an alternative to the increasingly restricted organophosphate insecticides...I don't want to give the impression that pyrethroids are destroying the streams, since that has not yet been shown, but if we are serious about maintaining stream health, we have to consider the sediments and not limit our sampling just to the water above."

Also, this study done after the spraying in Sacramento in 2005 shows that lingering PBO increases the toxicity of existing pesticides in the environment, even while no detectable amount of pyrethrins from the mosquito spray could be found. Here is a quote:

"The research team found that the level of PBO from the mosquito spray was high enough to double the toxicity of pyrethroids already present - primarily bifenthrin, one of the more toxic chemicals of the group. PBO, when in a pesticide product, can increase pyrethroid toxicity tenfold or more, but to see even a doubling of toxicity due to PBO in creek waters was unexpected."

What to do during treatments

Although the danger to humans from the pesticide is relatively low, Davisites should probably stay indoors after 8 pm as much as possible. Pets should also be taken indoors, and beekeepers will want to protect their bees. (If you see feral colonies of bees marked with 3x5 cards, that is because a local bee fanatic is surveying the effects of the spraying on their colonies.) This specific pesticide is destroyed by sunlight, so the day after the spraying it will degrade. (except in places that do not get much sun.)

Although many have thought it best to hose off outdoor toys and surfaces after the spraying, it is recommended that the sun be allowed to destroy the pesticide. This study suggests people leave outdoor furniture unused in the sun for a day or two to let the pyrethrin and PBO residue degrade rather than send the runoff into urban creeks. So if you do any hosing, do it on the grass, and not into the storm drains.

Vegetables grown outside should also be washed before being consumed. But you should be doing that already anyway, whether conventional or organic. (Why?) - Organic produce has fewer pesticide residues on them than conventional, but it is not zero. Also, there are "organic" pesticides, fungicides, etc, which aren't safe to eat either, and finally, manure used in organic agriculture raises the count of E. coli on the surface of the veggies. Plus, there are natural bacterial pathogens and spoilage agents on anything grown outside, so washing your produce extends its life in the fridge.

Community Forums

The Moon illuminates the pyrethrin spray Wednesday night. (8/9) If it wasn't poison, it would be pretty

You could voice your opinion at the next Davis City Council meetings, write letters to local papers, or stand on the street and shout, but there are currently no community forums as yet planned for this specific issue. The science show on KDRT 95.7 Thursday evening would have been an ideal time to discuss the issue right before the first spray treatment, but the station was under reconstruction on that day and no live shows aired.

However, there WILL be a show on K-DiRT specifically set up for the community to voice their opinions on Friday (2006-08-11), at 8:00 pm. At least one guest will be featured, entomologist Sharon Lawler, and there may also be representatives from the Sacramento/Yolo Vector Control District and Stop West Nile Spraying Now. You can book a specific time for talking by calling 530-757-2419 or emailing, but you may call in even if you don't. The call-in number is 530-792-1648.

On Thursday (2006-08-10), Karl Mogel interviewed several people involved in the mosquito spraying issue during his show, The Inoculated Mind. Guests include Rick Roush, the head of the Statewide Integrated Pest Management program, Jim Northup from Stop West Nile Spraying Now, and Sharon Lawler, an entomologist who is part of the research effort put together to study the effects of the mosquito spraying. There may also be a representative from the California Department of Health Services. Some of the issues that will be discussed include the scientific certainties/uncertainties surrounding the spraying issue, where do we put our threshold for action, discussion of the ethical issues raised by spraying versus not spraying, and discussion of some of the results from the studies conducted by UC Davis researchers during the spray treatments. Finally, Karl will reveal the data he collected from his own study of the effect of the spray on the bees. The show begins at 6:00pm, and the interviews begin at 6:30pm after the news, with the show ending at 7:30pm. The number to call in is 530-792-1648, and the frequency is 101.5 FM. The show was replayed on Sunday 2006-08-13) from 9-10:30pm.

Mosquito Spraying August 2014

The Sacramento-Yolo Mosquito and Vector Control District conducted aerial spraying on August 6th and 7th from 8pm-12am. This is in response to the 132 Mosquito samples, 26 dead birds, and 1 chicken that tested positive for the West Nile Virus. It is advised to stay indoors during these times and to thoroughly wash any fruits or vegetables grown from local gardens.

Sources & More info


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2006-08-08 22:25:16   I have the perfect location for photographing the plane, if anyone wants to join me tomorrow. Clear, dark, and during an organic-farm evasion maneuver. —KarlMogel

2006-08-09 08:42:55   I spent my 2006-08-08 evening trying to carefully note the pre-spray fauna, smells, and such. I left a white piece of paper, face up to the sky out last night. This morning, 2006-08-09 (post-spray), it is covered with light-gray spots. The air has the smell of burnt plastic, and I do not hear any insects buzzing at all. Haven't seen any dead ones yet, though. —SteveDavison

2006-08-09 16:56:24   Preliminary results of my bee study and the UC Davis studies on mosquitoes, spiders, and dragonflies came out pretty good for the bees, flawless for the spiders and dragonflies, and devastating for the mosquitoes. I'll know more about it all tomorrow, and I'll post news about my show on this and other pages. —KarlMogel

2006-08-10 07:17:14   You know, I happily spray my mice fairly often with pyrethrins, and have been handling it for years now. It's also in pretty high amounts in horse shampoo. Just to add a bit of perspective. —JabberWokky

2006-08-10 07:19:05   Incidently, if you want to get some, walk into PetCo, as they have shelves of the stuff... with applicators for just about any animal. They are especially good for animals that are highly sensitive to chemicals, like birds. —JabberWokky

  • You could also get a head-lice treatment that will give you a much higher dosage of the same stuff. So the risks are pretty low compared to what else we expose ourselves to (You're personal exposure from a can of Raid might even be higher), however, I find the argument interesting that one does not have the right to spray an unwilling person. Then again, at the same time I find compelling the argument that one can't force the non-action against WNV-infected mosquitoes on people who are susceptible to the virus either. In any case, what I think most people can agree on is that more intensive preventative action might avert future sprays in the years to come. I think the goal of Stop West Nile Spraying Now should go in this direction. -KarlMogel

2006-08-29 14:44:22   This summer, I've been working for a company in Livermore called Aerial Services. We've surveyed a ton of counties in the bay area, looking for clouded or green pools. We then photograph them and send the information to the respective vector control districts. We do it for relatively cheap compared to some counties' methods of hiring a helicopter and ground crew. Stagnant water is a great place for disease-carrying mosquitoes (or any mosquitoes for that matter) to breed. We're trying to get sac/yolo to hear us out but we're waiting for a response or explanation. What would you rather have? Blanket spraying for places that may or may not be affected, or localized testing and treatment for specific problem areas identified by aerial photos? Send me an email if you have any questions.

Additionally, we were featured by ABC news for our survey of the most affected part of Santa Clara county. I was in the plane if you watch the video. :)


2007-3-29 15:14:37   I found a dead crow on the perimeter of the University Mall parking lot, a few days ago. I notified WNV authorities through the proper site, but they rely on civilian description to determine whether or not specimens are eligible for testing. Not the most reliable way of monitoring an infectious disease, if you ask me. —LeonardMarque

2007-03-30 13:09:29   Looks like the WNV season has begun, as I also found a dead crow today, on Villanova near Sycamore. It's creepy to see a whole, untouched animal just lying on the ground. It clearly had not been hit by a car, but had just fallen out of a tree. I, too, contacted the DHS site. Time to start putting on bug spray. —DanLawyer