What is a Vector Index?

As of 2016, Chester County is using the Vector Index method of using mosquito data to predict the West Nile Virus threat to humans. Here is a summary from “2013 West Nile Virus in the United States: Guidelines for Surveillance, Prevention, and Control” at the CDC site, p. 18 (boldface added):

The Vector Index (VI) is an estimate of the abundance of infected mosquitoes in an area and incorporates information describing the vector species that are present in the area, relative abundance of those species, and the WNV infection rate in each species into a single index (Gujaral et al. 2007, Bolling et al. 2009, Jones et al. 2011). The VI is calculated by multiplying the average number of mosquitoes collected per trap night by the proportion infected with WNV, and is expressed as the average number of infected mosquitoes collected per trap night in the area during the sampling period. In areas where more than one WNV vector mosquito species is present, a VI is calculated for each of the important vector species and the individual VIs are summed to represent a combined estimate of the infected vector abundance. By summing the VI for the key vector species, the combined VI accommodates the fact that WNV transmission may involve one or more vectors in an area. Increases in VI reflect increases in risk of human disease (Bolling et al. 2009, Jones et al. 2011, Kwan et al. 2012, Colborn et al. 2013 in press) and have demonstrated significantly better predictive ability than estimates of vector abundance or infection rate alone, clearly demonstrating the value of combining information for vector abundance and WNV infection rates to generate a more meaningful risk index (Bolling et al. 2009). As with other surveillance indicators, the accuracy of the Vector Index is dependent upon the number of trap nights used to estimate abundance and the number of specimens tested for virus to estimate infection rate. Instructions for calculating the Vector Index in a system with multiple vector species present are in Appendix 2.

For the actual VI formula and a detailed example, see pp. 64-66 of the above Guidelines.

Risk analysis needed before spraying permethrin

Contributed by one of Dontsprayme’s consulting scientists, in response to spraying activity this summer

I am concerned about the recent decision to spray in an area of Chester County for West Nile carrying mosquitoes, considering what is currently known about permethrin, the availability of less toxic alternatives and methods for mosquito control, and the demonstrated resistance of mosquito populations to this pesticide. Even if there are some West Nile positive mosquitoes in the vicinity, has a risk analysis been done to see that the perceived benefits of spraying outweigh the long term risk to human health?

While permethrin was studied at length in 1994 by the US Army and found to be relatively safe, this early study should be taken in context: more American soldiers have died from insect-borne illness than of enemy fire. For troops deploying to tropical areas, and who have already willingly put their lives on the line for our country, permethrin is the lesser of two evils. Since the 1994 study, there has been a great deal of research into the toxicity of permethrin, and the picture grows more and more grim with the passing years. Work that supports the use of permethrin, such as the EPA’s cumulative risk assessment (2011)[1], is very thorough at the surface, but consider limited endpoints: specifically, those derived from the a priori known ways in which pyrethrins and pyrethroids disrupt neural function.

As complete as the EPA study seems to be, its flaw is in its failure to consider other endpoints besides neural function. A recent review article[2] identified 29 studies in which permethrin-induced toxicity was identified in various species (and cited a number of other studies where human toxicity was shown). It also goes into far more detail than the Army study about the mechanisms of toxicity in the various bodily systems.

From the article:

Although it was believed that PER showed low mammalian toxicity, an increasing number of studies have shown that PER can also cause a variety of toxicities in animals and humans, such as neurotoxicity (Carloni et al., 2012, 2013; Falcioni et al., 2010; Gabbianelli et al., 2009b; Nasuti et al., 2014, 2008, 2007b), immunotoxicity (Gabbianelli et al., 2009a; Jin et al., 2010; Olgun and Misra, 2006), cardiotoxicity (Vadhana et al., 2010, 2011a, 2011b, 2013), hepatotoxicity (Gabbianelli et al., 2004, 2013), reproductive (Issam et al., 2011), genotoxic (Turkez and Aydin, 2012, 2013; Turkez and Togar, 2011; Turkez et al., 2012), and haematotoxic (Nasuti et al., 2003) effects, digestive system toxicity (Mahmoud et al., 2012; Sellami et al., 2014b, 2015), anti-androgenic activity (Christen et al., 2014; Xu et al., 2008), fetotoxicity (Erkmen, 2015), and cytotoxicity (Hu et al., 2010) in vertebrates and invertebrates.

Additionally (Vadhana et al., 2013):

Early life environmental exposure to PER could play a critical role in the onset of age-related diseases (Carloni et al., 2012, 2013; Fedeli et al., 2013; Gabbianelli et al., 2013; Vadhana et al., 2011b). Previous findings demonstrate that early life pesticide exposure to low doses of the PER insecticide has long-term consequences leading to toxic effects such as cardiac hypotrophy, increased Ca2 ©≠ level and increased Nrf2 gene expression….

In fact, there is evidence that effects of this nature are transgenerational and that there are epigenetic changes that ensue due to exposure. What’s clear is that the pesticide research community has NOT signed off on the harmlessness of such pesticides to humans despite the EPA guidelines or material safety data sheets. 

In addition its toxicity, it’s also fairly clear that mosquitoes evolve resistance to permethrin and other pesticides relatively rapidly. From Ramkumar et al (2015), after exposure to permethrin, within 10 generations, the 50% lethal dose concentration (LC50) of permethrin increased 17-fold. 

Ramkumar, G., & Shivakumar, M. S. (2015). Laboratory development of permethrin resistance and cross-resistance pattern of Culex quinquefasciatus to other insecticides. Parasitology Research, 114(7), 2553–2560.

Research on West Nile carrying mosquitoes indicates that when field collected mosquitos were tested for pesticide resistance, in one case there was a 299-fold increase in dosage to reach the LC50.

Kasai, S., Shono, T., Komagata, O., Tsuda, Y., Kobayashi, M., Motoki, M., … Tomita, T. (2007). Insecticide resistance in potential vector mosquitoes for West Nile virus in Japan. Journal of Medical Entomology, 44(5), 822–829.

An alternative to using such pesticides is a larvicide, BT, which has been studied extensively. This appears to be safe at the moment (except for mega-doses, or deviant genetic strains), and is a champ at killing mosquito larvae. 

Ibrahim, M. A., Griko, N., Junker, M., & Bulla, L. A. (2010). Bacillus thuringiensis. Bioengineered Bugs, 1(1), 31–50.

So the question is: if permethrin has already been shown to be dangerous to animals and humans AND it’s been shown to have diminishing effects on mosquitoes, and there are alternative measures that work, why is there such a strong push to spray? One must remember that where spraying of this nature is used by the WHO, it is used as the lesser of two evils in regions where the risk of mosquito-borne illness and subsequent death or disability is high enough to justify its use. Are there enough cases of West Nile in our area that spraying is justified? Has there been enough sampling of mosquito populations? What is the correlation between the ratio of mosquitoes with West Nile and the number of diagnosed cases? Are larvicide or other control measures being optimally used?

As a scientist who teaches the physical sciences and who does health-related research, I’m struggling to understand how the data can possibly support a decision to spray.

[1] US Environmental Protection Agency; Office of Pesticide Programs. (2011). “Pyrethrins/Pyrethroid Cumulative Risk Assessment.” Retrieved from US Environmental Protection Agency.

[2] Xu Wang et al., “Permethrin-induced oxidative stress and toxicity and metabolism. A review,” Environmental Research, Volume 149, August 2016, Pages 86-104.

West Nile Virus identified in mosquito sample in Chester County

July 19, 2016
Chester County Health Department, News Release #6
For more information call 610-344-6752

West Nile Virus identified in mosquito sample in Chester County

West Chester, PA – The Chester County Health Department is informing residents that a mosquito sample collected in Tredyffrin Township on July 7, 2016 has tested positive for West Nile virus (WNV).

This is the first mosquito sample that has tested positive in Chester County in 2016. The Chester County Health Department sets mosquito traps to collect and test adult mosquitoes for WNV as part of routine surveillance. Traps are placed in highly populated areas, known mosquito breeding areas, and in areas where a resident has previously been identified as having a confirmed case of WNV infection. Traps are also placed in response to complaints from residents regarding high levels of mosquito activity. The Chester County Health Department will continue to monitor these areas as well as surrounding areas and will consider mosquito control activities when appropriate.

The chances of contracting WNV from an infected mosquito are small and chances of becoming seriously ill are even smaller. However, the Chester County Health Department encourages county residents to “Make you and your home a bite-free zone”, reducing the risk of West Nile virus and other mosquito-borne diseases. Because mosquito-borne diseases are spread through the bite of an infected mosquito, residents can reduce their risk by using insect repellent and other personal protection and getting rid of standing water on their property.

The Health Department advises:

Limit outdoor activities at dawn and dusk during warmer months since most types of mosquitoes bite most frequently during these times. Be aware though that some types of mosquitoes bite most frequently during the daytime.

Wear long-sleeved shirts and long pants, and socks. Choose clothing that is light colored and made of tightly woven material.

Stay in places with air conditioning and window and door screens to keep mosquitoes outside.

Sleep under a mosquito bed net if you are overseas or outside. Continue reading

What can I do to reduce the mosquito population?

Here is the enemy: larvae

But those are mosquito larvae, which don’t bite.

Right, but once larvae hatch, it is really too late for that batch. One female mosquito, with an occasional protein infusion from blood, lays 100+ eggs. The eggs hatch into larvae (as pictured), which fly away as adults in a few days.

Can’t we just spray and kill all the mosquitoes?

Air-borne insecticides, a temporary fix, may kill 80% of adult mosquitoes in the vicinity, but do not affect eggs, larvae, or pupae (the larvae who have already moved to a cocoon-like phase). Within a few days, the population is restored. Besides, many species can fly a mile or more. And even worse, the more we spray, the more likely mosquitoes are to become resistant to the insecticides we use.

So what do we do?

We must eliminate stagnant water! It sounds easy but requires imagination. Empty out birdbaths, pails of water, saucers under plants, and whatever else can hold water. And look overhead: be sure your house eaves drain. Spill out ALL standing water every 3-5 days to be safe.

How about ponds and streams?

A biological agent called Bti, widely available, prevents larvae from growing into adults. If the water is running, no problem. Swimming pools usually have enough chlorine to kill larvae.

Where can I get more information?

West Chester Borough has a new program and web page for residents to help their neighbors eliminate mosquito breeding. See here.

The County has two relevant pages, one on avoiding mosquito-borne diseases generally and one more specifically West Nile Virus. Both stress how to keep mosquitoes from breeding on our property. In fact, the County health regulations require us to avoid breeding mosquitoes.

See also excellent advice from the West Baton Rouge Parish Council Mosquito Abatement program, including video showing where mosquito larvae flourish and what they look like.

And talk to your neighbors, especially any who may be circulating information on mosquito control.

Chester County Health Department offers advice on prevention of mosquito-borne diseases

Press release from CHESTER COUNTY HEALTH DEPARTMENT, May 19, 2016

Make you and your home a bite-free zone

West Chester, PA – Now that the weather is warming up, the Chester County Health Department encourages county residents to “Make you and your home a bite-free zone”, reducing the risk of mosquito-borne diseases like West Nile virus and Zika virus.

Because mosquito-borne diseases are spread through the bite of an infected mosquito, residents can reduce their risk by using insect repellent and other personal protection and getting rid of standing water on their property…

download the full release under the heading “Make your home a Mosquito-FREE zone” at CHESTER COUNTY HEALTH DEPARTMENT