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/ .

Asian tiger mosquito a focus of last week's training

Keith Haas demonstrates use of a handheld ULV applicator
for treating adult mosquitoes hiding in dense vegetation.

For which important urban insect pest did 70% of pest control companies get more calls last year? For which pest are 88% of pest management professionals (PMPs) confident that control options are better than they were five years ago?  For what pest do nearly 2/3 of companies have callback rates of 4% or less?

According to a 2017 report by MGK^® Co., the repeated answer is "mosquitoes". It appears that pest control customers increasingly want to fight pesky mosquitoes in their backyards, and are willing to pay for it. 

Ultimately, this increased interest in mosquitoes is what brought 15 interested PMPs to the "Practical Mosquito Control" course last week at IPM Experience House. And, as students learned, the driving force for this demand may just be the tiny Asian tiger mosquito (ATM), Aedes albopictus, and its slightly less common cousin the yellow fever mosquito, Aedes aegypti.

A fast and opportunistic biter, tiger mosquitoes are relatively new pests, having arrived from Japan only about 30 years ago. When people complain that, "the mosquitoes are terrible this time of year", chances are they're talking about the day-flying ATM. But ironically, despite it's irritating bite, ATM is not currently a public health threat in most areas.

"The one good thing about ATM", said Scott Sawlis, county entomologist for Dallas County Health and Human Services, "is that it reminds people that they need to wear repellent, and thereby protect themselves from the more dangerous disease-carrying species."  In the Dallas area that would be Culex quinquefasciatus, the stealthier, nighttime-flying, southern house mosquito.  Even though folks don't tend to notice the house mosquito as much, it's the one to carry West Nile virus, our most serious mosquito-borne disease. 

Sawlis and fellow instructors (myself and Dr. Sonja Swiger with Texas A&M AgriLife, and Keith Haas, with Central Life Sciences), spent the day explaining to class attendees about the need for mosquito control, and some of the differences between the target species.  At the end of the day we got to practice what we learned in class by conducting an outdoor inspection and spending some workout time on microscopes looking at these tiniest of pests at a bug eye level. 

During our inspection we discovered mosquitoes breeding (naturally) just a few feet from where class took place.  Afterwards, students got to see fresh-caught mosquito eggs and watch mosquito larvae wriggle through murky breeding media.  Haas demonstrated the ability of a ULV generator to go through and around landscape vegetation, and Sawlis demonstrated proper use of a dipper when trying to determine whether mosquitoes might be breeding in a water source.

Although ATM may be one of the best things to happen to the pest control business in the past few years, it does have a darker side. The ATM is very difficult to control from city spray trucks and even from the air.  And it remains ready to transmit the viruses for Zika and dengue fever, should these diseases arrive in our area like Zika did last summer in Miami, FL and Brownsville, TX.  

One of the most effective tools, our class learned, for fighting tiger mosquitoes is the PMP. While county and city mosquito control staff must patrol streets with sprayers that treat city-blocks at a time (a technique that works well for the house mosquito), only the PMP walks backyards, identifies and treats ground-level breeding sites, and precisely targets sprays to ATM resting sites. This puts the pest control technician in an important role to reduce the most frequent mosquito bites, and to fight Zika and other Aedes-borne illnesses, should they arise here.

If you missed last week's class, and would like to learn more about control of ATM and its biting cousins, several regional training classes will be offered over the next few months.  Stay tuned here for more information.  

West Nile virus risk high in north Texas

Residual insecticides applied via backpack mist blower
sprayer can provide 3-4 weeks of mosquito control  during
times of peak mosquito activity.

July and August are typically the months of highest risk from west Nile virus, and true to form the past few weeks Dallas and Tarrant counties have seen a major increase in not only mosquitoes themselves, but infections within the mosquitoes.

After the major outbreak of WNV in 2012 in north Texas, some health officials made a decision to use something called the Vector Index (VI) as a form of threshold to ramp up mosquito control efforts.  Based on when human cases started to soar in 2012, and on suggestion from the CDC, a VI of 0.5 was determined to be a good threshold to consider going from ground based spray efforts to aerial spraying.

Two weeks ago the VI exceeded that threshold in both Dallas and Tarrant counties.  Both counties publish very interesting reports, available to the public, that include graphs to show  the latest mosquito counts and VI numbers.  To see the trends in Dallas and Fort Worth areas, check out the graphs below.  In the first graph, the Vector Index is the heavy red line.  Last week it exceeded the 0.50 threshold, although there was a drop this week. Note also the numbers of mosquitoes this summer (red bars) compared to average trap catches in 2012 (for the past four weeks, higher than 2012 averages shown by the blue bars). In Tarrant County (Fort Worth and surrounding communities) the VI (green line with triangle points) was likewise up last week, over 0.60 (new data is not yet published).  Note that the most recent 1-2 data points are preliminary estimates and may change as all the data is calculated.

These data are why there is discussion about aerial spraying this week.  In 2012 the number of human cases of WNV in Dallas county reached almost 400, and there were 19 deaths attributed to WNV. Serious business. Last week DCHHS issued a health advisory to the public, and this week the Dallas County commissioners voted to authorize the health department to prepare for possible aerial spray operations should conditions warrant.  

Where does all this leave the PMP who provides residential mosquito control service?  Municipal mosquito spraying actually complements, rather than replaces, mosquito control work on the ground done by professionals.  Aerial spraying generally provides better coverage of the tree canopy where WNV carrying mosquitoes (Culex quinquefasciatus in north Texas) live and mostly feed. Municipal truck mounted ULV sprays provide some control of lower level mosquitoes (Aedes mosquitoes that potentially transmit Zika and dengue fever, among other diseases), but they typically do not provide high level control in backyards or areas protected from spray coverage.  In that sense, the best control of Aedes mosquitoes is accomplished by your boots on the ground, looking for and treating or eliminating mosquito breeding sites, treating doorways, and treating shrubbery and other mosquito resting sites that are difficult to reach from the street.

As you and your technicians visit mosquito control customers this summer, keep in mind that you carry some of the most effective tools in the war against mosquitoes.  This summer, with Zika fears and WNV threats, what you do is more important than ever.

Summer flu often spells West Nile Virus

Dr. Robert Haley, Chief Epidemiologist of the UT Southwestern Medical Center in Dallas, recently wrote a short and easily readable summary of the West Nile virus situation for the Journal of the American Medical Association (JAMA).  Click here to read a free copy of the article.

Based on his medical observations over several years, Dr. Haley concludes that "diagnosis of WNF should be suspected in anyone with unexplained fever from late June through September, the season when other causes of fever are least common. Fever with disorientation, stiff neck, or neurologic deficits suggests WNND."

It's estimated that since its discovery in the U.S. in 1999, over 30,000 Americans have contracted either West Nile fever (WNF) or the most serious form of the disease, West Nile neuroinvasive disease (WNND). While even I may have been tempted a few years ago to dismiss this as an "old person's disease" (older persons with underlying health problems are most likely to die from the virus), at over 50 myself I now fall into the susceptible age group. And I don't feel that old.

It would be a mistake to underestimate this disease. It can even develop in younger people, as a 14 year-old Dallas girl discovered this summer, although this is not common.  Even the milder form of the disease can be debilitating for extended periods of time, with an average recovery time of 60 days in one study.

For PMPs working in residential environments, especially, it's important to take WNV seriously. Higher concentrations of DEET, picaridin and IR-3535 remain the best repellents for outdoor workers due to their longer residual and highly effective repellency for WNV mosquitoes.  For more information and assistance in choosing the right insect repellent for you and your employees, check out the highly useful repellent calculator developed by the National Pesticide Information Center.

The most common mosquito-borne encephalitis

I think that one of the most awesome things our industry does is protect people from disease that can literally destroy a life.  It is also a weighty responsibility when we stand between a customer and the threat of disease.  Numerous pests that are controlled by our industry have the potential for causing disease in humans.  Consider a few examples:

• Rodents are implicated in 55 human diseases.  In Texas, murine typhus is on the increase with nearly 3/4 of all U.S. cases in the southern part of our state.  
• Over 100 pathogens have been associated with house flies, including Campylobacter and the deadly E. coli O157:H7, and possiby community-acquired MRSA.
• Although it's been hard to connect cockroaches with outbreaks of human disease, the German cockroach has been shown to be a carrier of at least 60 species of bacteria, fungi, molds, helminths, protozoans and viruses.  
• Cockroaches have also been implicated in causing asthma, especially in pre-school aged children.  In older people, definitive evidence shows that cockroach allergens exacerbate asthma.
• Mosquitoes may be the worst, with their frequent association with virus transmission, heartworm (in dogs) and other disease. 

The Centers for Disease Control and Prevention recently published the annual report on West Nile virus for 2009.  Since its introduction into the United States in 1999, West Nile virus (WNV) has become the leading cause of arthropod-borne viral encephalitis in the country. The good news is that the number of WNV cases continued to decline this year, with 38 states (and the District of Columbia) reporting 720 cases (down from 1356 cases last year, and a high of 4156 cases in 2002). The bad news is that Texas continues to lead the country (with nearly a quarter of all cases) in reported neuro-invasive cases of WNV.  The term "neuroinvasive" refers to severe cases of the disease that affect a person’s nervous system. These include encephalitis (an inflammation of the brain), meningitis (an inflammation of the membrane around the brain and the spinal cord) and acute flaccid paralysis (an inflammation of the spinal cord that can cause a sudden onset of weakness in the limbs and/or breathing muscles).  None of this is something you want to get. 

I've discussed WNV before in this blog, partly because we have had a number of cases in my Dallas community, but also because WNV is typical of many mosquito borne diseases which pop up periodically in the U.S.  I am convinced that our industry can and should be doing more for the public with regard to mosquito control.  Although most mosquito control is (appropriately) conducted by county and city health departments or mosquito control districts, PMPs can play an important role in the fight against mosquito-borne disease.  Here some of my thoughts on how you can help.

• Train your technicians to be on the lookout for mosquito breeding sites when conducting inspections of residential accounts.  No one thinks their backyard is the one breeding mosquitoes.  It's always the next-door neighbor or the creek down the street.  In one study done by the Dallas County health department a few years ago, 25% of the folks who called the health department to complain about mosquitoes had mosquitoes breeding in their own backyard.  I know from experience that it's easy to forget to turn over the wheelbarrow or put away the kids' toys before rain or irrigation water can sit and breed mosquitoes.  Having an extra set of trained eyes looking over the yard every month can make a significant difference.
• Educate your customers about mosquito control.  How to recognize and report problems, explaining the importance of repellents, and reports on the local mosquito situation in your community(maybe through a newsletter, your website, or monthly bill inserts) can be a much-appreciated service and increase your value to your customer.  A lot of people forget about mosquitoes after the spring tides of floodwater mosquitoes ebb, but now as the summer heats up the risk of mosquito-borne disease actually increases.
• Consider including mosquito control among your summertime pest control services.  In addition to backyard fogging prior to outdoor events, residual sprays applied to mosquito resting sites (via sprayer or backpack ULV blower) can be very effective.  Of course larvaciding in some locations can be helpful also.
• Refer your customers to the Mosquito Safari website for more information about mosquito control and how to find mosquito breeding sites in their own backyards.

New Web Resource for West Nile Virus

In August I posted a story about the human face of West Nile virus. Now there is a website where people suffering from West Nile virus, or others interested in the disease, can exchange information about the topic.

It's called the West Nile Information Exchange, at http://www.westnileexchange.com/, and it provides a forum for folks to talk about the disease and their experiences, or to learn more about research into WNV. The site was developed by Dr. Kristy Murray, an Assistant Professor of Epidemiology for the Center for Infectious Diseases and Associate Director for Research for the Center for Biosecurity and Public Health Preparedness at the University of Texas Health Science Center at Houston, School of Public Health. It has the option for visitors to register and read and post comments about WNV, and subscribe to the comments (like this site).

If you have an interest in WNV, have suffered from this disease or know someone who has, this might be a good place to check out. It's new, so new registrants can help get the discussions going.

The mortgage crisis and West Nile virus

Just when you think the news couldn't look any more bleak, we learn that home foreclosures are sparking a rise in cases of West Nile virus. Huh? Yes, the CDC's online publication Emerging Infectious Diseases will be publishing an article in November that provides convincing documentation that an increase in home foreclosures (and accompanying abandoned swimming pools) in Kern County, California has resulted in an increase in Culex mosquito breeding in suburban neighborhoods.

Kern County has been especially hard hit by the mortgage crisis, with the number of mortgage defaults increasing 300% from 2006 to 2007. During the same time the number of human West Nile virus cases there increased by 276%. Aerial surveys confirm the high number of abandoned or neglected pools, jacuzzis and hot tubs, as evidenced by their green algae-rich color. It's bad enough to live in a neighborhood of foreclosed homes, but it's worse when those backyard pools turn into mosquito factories.

Several species of mosquitoes, including the principle vectors of West Nile virus, thrive in fish-free water rich in algae and bacteria. In California, both Culex quinquefasciatus and Culex tarsalis larvae have been collected from pools. The situation is similar to what was seen after hurricane Katrina in New Orleans, with thousands of scummy pools in abandoned backyards after the storm.

The situation does not appear to be limited to California. According to an online article in Newsday.com even Senator Hillary Clinton has become aware of the issue. She recently asked the CDC to look into the possibility that the foreclosure crisis and vacant homes on Long Island, NY are leaving pools of stagnant water where the virus-carrying mosquitoes breed.

So what's all this got to do with the pest control industry? If your company does residential pest control, you should alert your technicians to the potential health threats posed by abandoned pools. Technicians should report abandoned or neglected pools to customer or the local health department. Health departments should be interested in contacting the owners of such sites as a potential public health nuisance.

If a dirty pool or jacuzzi belongs to a customer, and you have permission, temporary control of mosquitoes is possible by treating pools with Bacillus thuringiensis or methoprene-containing products labeled for mosquito control.

If your company is located in one of the hurricane zones in Texas, noting and reporting potential mosquito breeding sites is even more critical. And if you've been looking for a way to help out in your community, providing chemical or biological control of mosquitoes in backyards might be one of the most important things you could contribute to the re-building effort. If your company is doing something along these lines, let me know. I'd be honored to spread the word about your efforts.

The human face of West Nile virus

Dallas, TX. We hear of rare illnesses and diseases nearly every day in the newspaper or on television. After a while, I think most of us become a little jaded, and tend to forget or ignore threats that have not affected us personally. One of these relatively obscure diseases is a mosquito-borne illness called West Nile virus. Since 2002 Dallas county has averaged 45 clinical cases and two deaths a year.

These numbers may not seem like much...on paper. But these are real people who contract mosquito-borne disease every year. They include our neighbors. Friends. Parents. Teachers. The tiny bite of a Culex mosquito can, if infrequently, lead to a life-changing illness. Many of us reading this post may have already been infected by this virus. Only 20% of infected persons show any signs of the disease, and most of those will have little more than the sniffles. One in 150 infected persons, however, will contract the more serious central nervous system form of the disease.

Despite my awareness of West Nile virus and the clinical implications of the disease, until I met actual victims I don't think I really appreciated the power of this virus to potentially affect my life or the lives of loved ones.

Dr. Don Read and his wife Roberta lead a support group for victims of West Nile virus in the Dallas area. There are only a handful of such groups in the country, but they can be a godsend for people who have suffered from one of the more serious cases of infection. Last week I had the chance to attend one of their support group meetings at Medical City Dallas Hospital. It was a sobering experience to listen as people shared their encounters with the disease.

This month's meeting was covered by Reporter Melissa Maynarich from Oklahoma City's CBS Channel 9 News. She and her photographer posted a video story of some of the folks at this meeting. If you are interested in the human side of the West Nile virus story, check it out at http://www.news9.com/Global/story.asp?s=8764354

It might make you look at mosquitoes a little differently. I know I'm reaching for the repellent more these days. If you know of someone who has suffered from West Nile virus, or just to learn more about the disease and the support group in Dallas, check out the Dallas County Health Department's website on West Nile virus.