Where is West Nile virus this year?

If it seems you're hearing less about West Nile virus (WNV) this summer, you may not be imagining it.  Although mosquitoes have been abundant in north Texas this year, for some reason the virus has remained relatively quiet.

Where has WNV gone?

A paper written by epidemiologist Dr. Wendy Chung and colleagues in 2013 may offer some insights on the absence of the virus this summer. Those of us who lived in Dallas in 2012 may remember that summer as the worst human outbreak of WNV ever.  Nearly 400 cases were reported in Dallas County alone, and 19 people died of the disease. The epidemic was so bad that Dallas county resorted to spraying the entire county for mosquitoes by plane--something not seen in north Texas since an encephalitis outbreak in 1966.

Chung and colleagues charted the course of the disease during 2012 and saw high infection rates of mosquitoes early in the summer, followed by a rapid increase in human cases. Looking back over previous years and case numbers, the researchers concluded that an unusually mild winter followed by rainfall patterns ideal for mosquito breeding in the spring (and a very hot summer--West Nile virus multiplies quickly in mosquitoes at higher temperatures) created ideal conditions for an outbreak.

So what's different about 2019? We had a relatively mild winter, with only three days at or below 28° F, and a wet spring--both conditions mosquitoes love. But the summer, at least by Dallas standards, has so far been cool.  Until this week, the DFW Airport weather station saw only two days over 100° F. By the end of July the area usually has experienced more than seven days over 100° F.

These graphs show 2019 mosquito abundance and Vector Index (V.I.) estimates compared to previous years. Although mosquito numbers are high this year, the V.I. has remained low for both Tarrant (=Fort Worth-top) and Dallas counties (bottom). In 2012 the V.I. exceeded the danger level of 0.5 for multiple weeks (blue dotted line). Source: Tarrant County Public Health and Dallas County Health and Human Services.

Predicting WNV

One of the tools used by health departments to predict disease risk for WNV is a statistic called the vector index (V.I.).  The V.I. is calculated weekly from mosquito trap data, and combines information on both average abundance of Culex quinquefasciatus (the main carrier of WNV) and disease incidence in the trapped mosquitoes.  A V.I. of 0.5 or higher for two or more weeks is considered a crisis indicator by some health officials.

The graphs shown here are provided by epidemiologists in Dallas and Fort Worth, and show both mosquito abundance and V.I. estimates for both counties. Despite higher mosquito numbers, the V.I. hasn't ventured above 0.1 for either Dallas or Tarrant counties this summer. Most of the season the V.I. has been closer to zero, hence less need for mosquito spraying and fewer people getting sick. In Dallas county this year there have been no human cases of WNV. Tarrant County (Fort Worth) reports only one case this year with a very low V.I., near zero most weeks (top graph).

According to statistics from the Texas Department of State Health Services, low WNV incidence seems to be true for the whole state this year with no reported human cases as of the end of July.  Harris County (Houston) also reports a light year for WNV, according to the acting director of Mosquito and Vector Control, Chris Fredregill.

Looking Ahead

With this week's string of 100° days in many areas will risk go up?  Certainly West Nile virus remains a threat to all of us through the end of the summer and into the fall; but this late in the season the chance of a major outbreak is probably low. On the other hand, hot weather favors the virus. It's no time to forget about mosquitoes. I expect Aedes mosquitoes (yellow fever mosquito and Asian tiger mosquito) to become more abundant after last weekend's rains.  This week is a good time to get out and dump standing water.  Although Aedes mosquitoes are not major disease risks, they cause most of the itchy mosquito bites we get during the day--and we don't want that.

Why Surveillance Reports?

Integrated pest management is just as relevant for mosquito control as it is for all other forms of pest control.  One of the principles of IPM is to base treatments on pest numbers.  Because mosquito monitoring is expensive and requires special expertise beyond what most PMPs possess, few companies monitor mosquito numbers or disease. However, high quality data may be available from your local health authorities, depending where you live. A pest management company can use this data to alert customers to times of higher disease risk and changes in mosquito abundance.

Every community's mosquito situation will be different.  If you are doing business in a larger metropolitan area, or a mosquito control district, you may have access to the kind of data shown here. To find out, contact your local or regional health department and ask if they provide reports of mosquito abundance and disease prevalence.

In Dallas, weekly reports may be obtained by emailing Epidemiology@dallascounty.org and requesting to subscribe to the weekly Arbovirus Surveillance Report. For Tarrant County, email RWHill2@tarrantcounty.com and request to receive the Arbovirus Surveillance Report Weekly.  Unfortunately, not all counties have equivalent reporting systems. Harris County provides mapping of areas with virus detection.  And the Texas Department of State Health Services provides weekly reports throughout the summer for the whole state.

An additional source of information for both PMPs and your customers is the Mosquito Safari website. At the Safari you can take a virtual tour of a field and a backyard and learn important facts about mosquitoes. 

If you need more intensive training, our Extension medical entomologist, Dr. Sonja Swiger, is offering classes this year for pesticide applicators wanting to prepare for their Public Health (Category 12) license.  In the fall she also offers several 3-day Master Vector Borne Disease Management Courses around the state.  To learn more, or to register, go to https://livestockvetento.tamu.edu/workshop-registration/ .

Pesticides and ADHD

In case you haven't seen or heard the story this week about scientific research showing a link between pesticides and attention-deficit/hyperactivity disorder (ADHD) in children, I think it's important enough to pass on.  According to the Time magazine story, researchers at the University of Montreal and Harvard University this week reported in the journal Pediatrics a statistical correlation between ADHD and exposure to organophosphate (OP) pesticides.

Before discussing the results, we should note that there are different ways to study environmental causes of disease.  The most straightforward approach is to take a randomly selected group of subjects from a population you are interested in (say males of a certain age), and subject them to a level of the environmental chemical or condition, say pesticides, or microwaves from cell phones, etc.  You then compare the health of this exposure group over time to a randomly selected group of subjects without the exposure.  This type of controlled experiment is one of the more rigorous ways to test an hypothesis and show a cause-effect relationship.  However, this sort of approach is rarely possible for practical and ethical reasons.  Who wants to volunteer themselves for a study of the possible effects of a toxic chemical on your health?  Fifty dollar stipend anyone?

Another approach would be to look for people who have been naturally exposed (via their lifestyle, or home environment, etc.) to an environmental chemical and compare their health parameters to a group who has not been naturally exposed.  This is a valid research method and has been used many times to study the effects of environmental chemicals on human health.  It's important to realize, however, that this approach measures correlation, which is not the same thing as cause and effect.  If they looked, for example, researchers might find a correlation between heart attacks and watching at least two football games on TV each week.  This would not, however, prove that watching football causes heart attacks.  The actual cause could be something entirely different.  Something co-related to football watching, like lack of exercise and big bowls of greasy nachos!!?  The point is that it is very easy to have a correlation without a cause and effect relationship.

Of course this was the same argument cigarette companies made for many years.  Correlations, if done properly, can provide valuable suggestive evidence, but must always be looked at critically to make sure that all relevant variables have been properly controlled. Typically correlative studies are done as preliminary to a more rigorous study, or as a second choice when more rigorous studies are not possible to conduct.  We cannot afford to dismiss correlations as uninformative.

In the case of this study from the University of Montreal, Maryse Bouchard and a team of colleagues analyzed the levels of chemicals called dialkyl phosphates in the urine of more than 1,100 children ages 8 to 15.  Dialkyl phosphates (DAPs) are metabolic "breakdown" products of OPs. Like nacho crumbs left in the sofa cushion, DAPs are left behind in the blood and the urine after someone has been exposed to OP pesticides. Measuring DAPs in blood and urine is much more sensitive than acetylcholinesterase tests (used by many PMPs in the past to measure OP overexposure), and an accepted way to document previous exposure to OP pesticides in epidemiological studies.  Bouchard's study found that children in the study group with the highest levels of which are breakdown products of OPs had the highest incidence of ADHD.  In other words, there was a correlation between DAPs and ADHD.  To see a free abstract of the study, click here.

The researchers, as all good researchers should, made it clear that this is a correlation which falls short of proof that organophosphate insecticides are a cause, or even a contributing factor to, ADHD.  I am no expert on ADHD, nor on the validity of the chemical analyses or the epidemiological methods used by the authors of this study, but studies like this serve as a cautionary tale for the use of pesticides in urban environments or on food.  The possibility of subtle health effects on humans is always present with the widespread use of synthetic chemicals. But the potential risks always must be weighed against the undoubted benefits of pest control.

Here are a few things to keep in mind should you find yourself in the position of justifying to local media or your customers the use of pesticides as pest control tools.

• The pesticide class which was the focus of this study is, for the most part, no longer widely used in the pest control industry.  Organophosphates (such as diazinon, malathion, chlorpyrifos, acephate, and safrotin) have been largely supplanted by pyrethroid insecticides and even newer classes of chemistry.  The results of this study apply only to OPs and not to all pesticides.
• The study did not attempt to prove a cause-effect relationship between OP pesticides and ADHD.  The study, for example, as the authors admit, could not fully rule out that some behavior factor common to ADHD children might cause them to have higher frequency of contact with OP pesticides. 
• The source(s) of OP exposure was not identified by the study.  Much of the exposures of children to these older pesticides likely come from dietary sources, especially since OP use by the pest control industry was already in decline during the years when the data was collected. 
• Any pest control company practicing IPM and safe application methods should at best be a minor and occasional source of pesticide exposure to customers.  Risk, or toxicity, of a pesticides is a product of two factors: (1) the toxicity of the pesticide and (2) how much exposure one has to that pesticide.  If labels are being followed, care is being taken to place pesticides only in places where people are unlikely to contact them, and lowest risk pesticides are being used, then the risk of harm to human health should be very small.

The results of this study do not negate the regulatory process that evaluates the safety of pesticides in use by the industry today.  Testing required by the U.S. Environmental Protection Agency for pesticides remains some of the most stringent consumer product testing in the world.  That said, no regulatory system can guarantee safety of any product or practice.  It's often said you can't prove absolute safety, because that's the same thing as trying to prove a negative--it's beyond the scope of experimental science.  We can only hope to guarantee a certain level of safety.  That is why a 100-fold safety factor is built into our current pesticide regulatory system, and why continual review of pesticides goes on between agency and manufacturers.  If a causal relationship is determined to exist between OP insecticides and ADHD, this new data will be taken into account by the EPA, and regulatory changes will undoubtedly be made, as they have for other pesticides.

If I were convinced that we could do IPM effectively without pesticides, I would certainly be a supporter of "no more pesticides".  But not all pesticides are equally risky, and the reality is that pesticides can be useful and even essential tools in our competition with pests. 

Until we find even safer ways to control pests, I will continue to study the use of pesticides, teach and preach pesticide safety, and recommend their use.  And in the meantime, if you really want to protect your health... cut back on the nachos. : )