Over the past two weeks as temperatures finally rebounded into the seventies in the Washington region, we began to see an uptick in insect activity outdoors. Last week we invited pest-slaying parasitic wasps in for an evening snack of honey. This week we turn our attention to mosquitoes. On a warm day last weekend while cycling on the spectacular C&O canal, I stopped for a short water break and was instantly mobbed by a band of robust mosquitoes. These dark bodied vampires were likely Anopheles quadrimaculatus, a fresh water breeder commonly found throughout the eastern United States. Adults overwinter in protected locations such as human-made structures and tree holes and actively seek blood meals with the return of warm weather in spring.
Catching the scent of fresh human prey, hungry Anopheles mosquitoes swarm the hand of this foolish person.
About this time last spring we met two other mosquitoes found in the Washington region, the pale marsh mosquito, Ochlorotatus dorsalis, and the yellow fever mosquito, Aedes aegypti. Aedes aegypti gained notoriety last year for its role in transmitting a nasty arbovirus called Zika. For most people Zika virus is not a serious health risk. Only about 20% of those infected will show symptoms and for those that do fall ill symptoms are usually mild and include fever, rash, joint pain, red eyes, muscle pain, and headache. These symptoms usually pass in a few days to a week. However, for pregnant women, Zika poses serious health risks to unborn children. Infection with Zika during pregnancy has been linked to severe developmental disorders of the fetus including a condition known as microcephaly (micro = small, cephaly = head). These babies with abnormally small heads sometimes suffer potentially severe problems including seizures, developmental delays, impaired balance, hearing and vision, and reduced intellectual capacity. A recent study of pregnant women infected with Zika in Rio de Janeiro found adverse outcomes in 46% of offspring, several of which were born with microcephaly. These abnormalities were found irrespective of which trimester the Zika infection occurred. Microcephaly appears not to be the only disease linked to Zika virus. Evidence from French Polynesia and other locations link the autoimmune disease of the nervous system called Guillain–Barré syndrome to Zika.
Where did Zika come from and where is it now? Zika virus was first discovered in the Zika forest of Uganda in 1947. It spread across Africa to Asia and the first human outbreak occurred in Micronesia in 2007. It hopscotched across several islands in Oceana in 2014 and then jumped to several Pacific islands before arriving in South and Central America in 2015. As of March 2017 the Centers for Disease Control (CDC) listed travel advisories and Zika risks for more than 100 nations worldwide.
Here in the United States 2016 was a breakout year for Zika as the virus made landfall in several states and was reacquainted with its old friend the yellow fever mosquito. Some recent accounts placed the yellow fever mosquito primarily in southern states lining the Gulf of Mexico and the Caribbean. However, historical chronicles as early as the 1700s report many northern cities in the US plagued by yellow fever mosquitoes and diseases they carried. In 1793 Philadelphia was struck with an outbreak of yellow fever that killed some 5,000 people. Baltimore experienced yellow fever in 1794 and in nearby Norfolk and Portsmouth more than 3,000 died in the yellow fever epidemic of 1855. A recent study found a population of yellow fever mosquitoes that successfully overwinters in the Capital Hill neighborhood of Washington, DC. We met a similar population of Culex mosquitoes living underground in New York City a few weeks ago. These remarkable scalawags have made a hibernal home in the relative warmth of yet unknown subterranean redoubts somewhere beneath the city. During summertime and early autumn when scientists conducted their studies, larvae of yellow fever mosquitoes were collected from water-containing vessels including a birdbath, fountain, trash can, plant saucer and from a storm water basin. These clever human-loving species spend winter snug underground and bubble up in the summer to breed in the warmth of the Washington summer and dine on humans in the heart of DC.
According to the CDC, between January 2105 and March of 2017 some 5,158 cases of Zika have been reported in the US. The great majority of these involve travelers to foreign lands who acquired the virus elsewhere and returned with it to the US. Unfortunately, but not surprisingly, by the end of July 2016 indigenous populations of yellow fever mosquitoes near Miami completed local transmission of the virus from mosquito to person. By the end of the year, more than 216 cases of local mosquito-borne transmission of Zika occurred in the Sunshine state. In November of 2016, Texas became the second US state where Zika was presumed to be transmitted locally. Here in the District-Maryland-Virginia region, according to the CDC between January of 2015 and March of 2017, 34 cases of Zika were found in DC, 114 occurred in Virginia, and 134 were reported in Maryland. To date none of these cases are believed to have involved local transmission. According to the Maryland Department of Health and Mental Hygiene in 2017, 6 cases of Zika have already been discovered.
Female mosquitoes like this yellow fever mosquito biting my hand use human blood to produce eggs. While feeding, viruses like Zika are transmitted from the mosquito to the human in saliva injected into the wound by the mosquito.
What will 2017 bring with respect to mosquitoes and Zika virus? Insect development is strongly related to ambient temperatures. A simple truth is that within certain limits of hot and cold, the warmer it is, the faster insects, including mosquitoes, develop. This means hot regions enable mosquitoes, including the yellow fever mosquito, to complete more generations each year thereby increasing the likelihood of Zika transmission. Could local transmission of Zika occur in places like Washington DC? Well, some of the ingredients are present – humans infected with the virus and yellow fever mosquitoes present. Working in our favor is the fact that our summers simply may not be warm enough or long enough for mosquitoes to reach population densities high enough to transfer the virus from one person to another. Effective management of mosquito populations in our cities and rarity of Zika in the human population may also mitigate the chances of person to person transmission.
What can you do to avoid contracting this potentially dangerous illness? Let’s start with vacationers and those heading away for a spring break. My first suggestion is to consult a physician or reputable website such as the CDC website for travel advisories regarding illnesses before you travel abroad. For pregnant women or women who plan to become pregnant who are living in or traveling to countries with Zika, beware of significant health risks to your unborn child. Also be aware that Zika can be transmitted sexually and take precautions if your partner has travelled to countries or regions in the US with Zika. Excellent advice for women concerning Zika and unborn children can be found at these two links:
If your destinations include locations where Zika is endemic, be sure to use personal protection when you are outdoors. Travelers returning to the US from Zika regions should continue to use mosquito repellents for at least three weeks according to the World Health Organization. This way, travelers who may unknowingly return home with the Zika virus in their blood will help prevent the local transmission of Zika here in the US by keeping the virus out of native mosquito populations. Although mosquito bites are most common at dawn and dusk, Aedes aegypti seek blood during daylight hours. Protect yourself from aggressive biters by wearing light-weight, long-sleeved shirts and pants when working or playing outdoors. Certain brands of clothing are pretreated with mosquito repellents such as permethrin. I have worn these in tropical rainforests where mosquitoes were fierce and the clothing really did help. Permethrin aerosols can be purchased and applied to clothing. Permethrin is not to be applied to skin.
Many other insect repellents can be applied to exposed skin before you go outdoors. Some will provide many hours of protection, while others provide virtually none. The “gold standard” of mosquito repellents is the compound DEET. Higher percentages of DEET in a product generally result in greater levels and duration of protection. However, surveys indicate that some people avoid using DEET for a variety of reasons. In recent years many botanically-based products have come to the marketplace. Scientists discovered that wild tomato produces a compound, 2-undecanone, and created a product which prevents mosquitoes from landing on humans. Picaridin is another active ingredient that provides long lasting protection against mosquitoes. Other products containing oils extracted from lemon eucalyptus, Corymbia citriodora, and products combining oils of soybean, geranium, and caster bean protected people from mosquito bites as effectively as products containing DEET in field trials, but often for shorter time intervals. One new repellent with a cryptic moniker called IR3535 has also made the CDC preferred active ingredient list. Products based on citronella and other essential oils derived from plants vary greatly in repellency with average protection times ranging from 5 minutes to 2 hours. So, you may have to apply these products more frequently to be protected.
Questions always arise regarding the use of repellents on children. Repellents carry precautionary statements on their labels. Always read the label carefully and follow directions and precautions exactly. You should help children apply repellents and consult a pediatrician before applying any product to the very young. Some products state that children should not handle the product and even some botanically-based products warn against use on kids under the age of 3. If your travel destinations include places that will be clothing light, be sure to use sunscreen to protect your beautiful skin from harmful UV and use insect repellent. Repellant is applied after, not before, sunscreen and should be replenished after swimming.
Watch this video to learn tricks for reducing problems with mosquitoes around the home.
The excellent CDC web sites on Zika virus, the information rich “Mosquito Information Web Site” of the University of Florida, and the fascinating articles, “Evidence for an Overwintering Population of Aedes aegypti in Capitol Hill Neighborhood, Washington, DC” by Andrew Lima, Diane D. Lovin, Paul V. Hickner, and David W. Severson; “American Disasters: 201 Calamities That Shook the Nation” edited by Ballard C. Campbell ; and “Zika Virus Infection in Pregnant Women in Rio de Janeiro” by Patrícia Brasil et al. were used in the creation of this episode. To learn more about Zika and yellow fever mosquitoes please visit the following web sites: