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/ .

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.

Bracing for Zika

Will Zika be the next mosquito-borne disease to capture headlines in 2016?  Or will it be the little disease that few (at least in the U.S.) have heard of?  That's the question being debated by public health officials this year.

Current (2015) worldwide range of zika virus. Central and South
American countries were first reported with the disease in
2015 (U.S. Centers for Disease Control).

For many years it seemed like new things happened relatively slowly in public health in Texas. In the mid-1980s entomologists reported the Asian tiger mosquito in Texas for the first time--a daytime-flying mosquito from Japan that is not shy about biting humans. Then in 2002 the first cases of west Nile virus hit our state.  Carried by the southern house mosquito, WNV affected a couple of hundred people or less each year. This was the case until the blazing hot summer of 2012 when over 1800 cases were reported, including 83 deaths. Health departments throughout the state are still reeling, in some ways, from the impact.

Now health officials are bracing for another mosquito-borne disease caused by Zika virus (abbreviated ZIKV by epidemiologists).  A cousin of west Nile virus and dengue fever, ZIKV has been thought of as a less severe form of these flavivirus.  Most people who get ZIKV show no, or very mild symptoms.  Others exhibit a rash, conjunctivitis (inflammation of the eye), and flu-like symptoms.  Most people do not get as sick with ZIKV as with dengue fever or chikungunya, and recover relatively quickly.

For this reason, since its discovery in 1947 until 2007, it was not on the radar of many public health experts. But in 2007 ZIKV cases started to spread throughout Micronesia French Polynesia, and eventually Easter Island. There it was thought to possibly be the cause of a twenty-fold increase in cases of Guillain-Barre syndrome--an autoimmune disease of the central nervous system that can be highly disabling, at least temporarily.

In 2015 the disease made its appearance in Brazil and has since spread to at least nine other member states of the Pan-American Health Organization (PAHO/WHO), and prompting that organization this month to issue an alert to all of member public health agencies.

So here's where things get a little scary.  Since the arrival of ZIKV to Brazil, the virus has been detected in babies born with a condition known as microcephaly.  Microcephaly is a relatively rare condition where the brain fails to develop normally. It may result in miscarriage or in babies being born with under-sized brains.  There is no cure for the condition.  The PAHO/WHO alert noted that the number of diagnosed cases of microcephaly has increased to 2700, a 10-fold increase, in Brazil this year.  Health officials there are worried that there might be a connection between this unprecedented increase in microcephaly and the arrival of ZIKV.  And last month, unusual nervous system birth defects were also reported in Polynesian mothers who tested positive for flavivirus antibodies.

Public health officials guess that these cases may result when a pregnant woman who is bitten by an infected mosquito contracts the virus.  The virus then infects the developing fetus, resulting in this serious condition.

So far there is no hard proof of a connection between ZIKV and microcephaly or Guillain-Barre syndrome, but medical researchers are rushing to learn more about the virus and its possible effects on human health.  According to one expert, quoted in the New York Times, it could be that the risk of microcephaly is increased among people who have previously contracted dengue fever or chikungunya, neither of which diseases are common to Texas or the U.S. If this hypothesis proves correct, the risks to the unborn in this country would likely be negligible.

Currently, ZIKV is thought to be transmitted by the yellow fever mosquito, Aedes aegypti.  This mosquito, along with its close relative, Aedes albopictus, the Asian tiger mosquito, are both very common throughout Texas and the southern U.S.  Unlike west Nile virus, which is primarily a disease of birds, ZIKV is solely a disease of primates.  To be spread among people, it must be picked up from another infected human.

Dengue and chikungunya are similar, human-only, viruses that have not been quick to spread in Texas or other U.S. locations.  This may be the result of lower rates of mosquito biting in the U.S., perhaps due to our more indoor lifestyles, or more common use of repellents.  Some experts argue that for similar reasons ZIKV is likely to be slow to establish in the U.S.  Nevertheless, Brazil shows that given the right conditions, this virus is capable of establishing itself very rapidly, with 85,000 known infections in its first year of spread.

A few U.S. cases of ZIKV have been reported in 2015, but all from travelers who contracted the virus elsewhere.  Mexico has also seen a few cases this year.  There are still no known cases of ZIKV, however, contracted within the U.S.

So be prepared to hear more about the zika virus this year.  It may turn out to be a big event, or it may not.  Even if we didn't need more reasons to dislike biting mosquitoes, now we have one more reminder of the importance of residential mosquito control, and putting on the insect repellent when venturing outdoors.  

Understanding chikungunya

The black and white Asian tiger mosquito is one of the two most common potential vectors of chikungunya nationwide

Fortunately for all of us who live and work in the U.S., insect-borne disease is not rampant in our country. But this isn't something to take for granted either, as we have seen this summer with the rapid spread of the chikungunya (chik-un-GOON-ya) virus throughout the Caribbean.

Over 30 years ago as a graduate student taking a course in medical entomology, I learned about all kinds of diseases spread by insects.  By far, most of these were tropical and exotic-sounding. Chikungunya virus was one of those diseases I memorized way back then, and have since mostly forgotten.

Chikungunya was first described in 1952 during an outbreak in southern Tanzania (east Africa).  The name comes from the Makonde language and means "that which bends up", referring to the contorted, bent-over appearance due to joint pain suffered by those who contract the disease.  Sounds bad, doesn't it?

While not as serious as some mosquito-borne diseases, including malaria or Eastern equine encephalitis, or even the neurological form of West Nile virus, this virus is nothing to sneeze at.  While some people have mild cases, it frequently comes with a very bad headache, joint pain, rash and fever.  There is no treatment for chikungunya, and there is no vaccine to protect you if you go where the disease is active.

When I learned about chikungunya in college, it was found only in eastern Africa and parts of India and Southeast Asia.  That distribution has spread in recent years as outbreaks occurred in parts of western Africa and Europe.  In December 2013 the first epidemic on our side of the world was reported when the disease made the jump into the islands of the Caribbean.  Since the beginning of 2014 the disease has been spreading like wildfire, with more than a quarter million cases, and over 20 fatalities in the Caribbean. 

Chikungunya has been on the radar of U.S. health officials in recent years largely because its vectors are very common in our country.  The principal mosquito vectors of chikungunya are in the genus Aedes. They include the Asian tiger mosquito, Aedes albopictus, and its close relative, the yellow fever mosquito, Aedes aegypti. These are the species that inevitably bite me whenever I chance a trip outside without repellent in my backyard here in the Dallas area.

Unlike West Nile virus, birds are not a part of the chikungunya disease cycle.  As far as we know, the virus is only viable in mosquitoes and primates.  In Africa, the disease lurks both within humans and non-human primates, such as baboons and monkeys.  For the virus to take hold here in the U.S., it would have to be common enough to reach an epidemic tipping point.  This would occur when enough people were infected to start a cycle from human to mosquito to human again.

We may not have yet reached that tipping point, but we could get there easily.  In July the first locally acquired cases were reported in Florida. These were the first cases where the victims had not recently traveled to the Caribbean, or other places where the disease is endemic. According to the CDC, between 2006 and 2013 the U.S. averaged about 28 cases of chikungunya annually from travelers. In the first six months of 2014 alone there were 243 travel-associated cases in the U.S.  And, according to a July 15 release by the Texas Department of State Health Services, five cases have been reported in Texas so far this summer.

So should you or your customers be worried about chikungunya?  If you plan travel to the Caribbean this year, definitely!  Make sure to carry repellent and use it liberally during your travels to any Caribbean island. At home there is little risk yet; but this could change.  As the number of sick travelers returning from Caribbean cruises and high school missions trips increases, the risk of locally acquired chikungunya will increase.

All of this increases the value and importance of all residential pest control services, especially where Aedes mosquitoes are active. Technicians servicing homes should be on the lookout for any sources of standing water.  These may take the form of storm sewer catchment basins, leaking septic systems, containers or plastic items that catch and hold water, house plant pots with dishes that hold water, bird baths, wheel barrows, boat covers and other items. You may not be servicing your account specifically for mosquitoes, but pointing out mosquito risk factors is an add-on service to your customer and can increase your value in their eyes.

If you have a newsletter or email service to your customers, now's a great time to remind them of the four Ds:

• Drain and dump standing water 
• Dusk and dawn are the highest risk times for mosquitoes (though the Aedes mosquitoes are active all day) 
• Dress in long sleeved shirts and long pants when outdoors 
• DEET is an ingredient to look for in your insect repellent (or another effective choice such as lemon oil of eucalyptus, picaridin, p-menthane 3,8-diol or IR3535). 

For more information about where mosquitoes can breed, and how to identify Aedes and other mosquitoes, see http://mosquitosafari.tamu.edu