Mosquito Disease Transmission
Jonathan F. Day, Professor of Medical Entomology
University of Florida, Florida Medical Entomology Laboratory, Vero Beach


How many bites from an infected mosquito does it take to transmit a disease? None? One? Five? Ten? The answer, you may be surprised to learn, is none.

Mosquitoes transmit a variety of diseases. Among the best known are malaria, West Nile virus, yellow fever virus, and dengue virus. Lesser known mosquito transmitted diseases include dog heartworm and filariasis (a nematode worm that causes human elephantiasis). There are two ways that a blood feeding arthropod (an insect or tick, also known as a disease “vector”) can transmit a pathogen (an organism that causes a disease): mechanical transmission and biological transmission.

In the case of mechanical transmission, the vector (mosquito) acts as a flying contaminated hypodermic needle; picking up a pathogen from an infected animal and carrying it on contaminated mouth parts to an uninfected animal where the pathogen is immediately transferred. Fortunately, this type of disease transmission is very rare with mosquitoes.

For biological transmission, the pathogen undergoes an incubation period, often lasting more than two weeks, in the body of an infected vector. During incubation, the pathogen replicates and infects many of the vectors’ organs, most notably, the salivary glands. This is the reason that an infected mosquito can transmit a disease without actually taking blood.
When a mosquito finds a suitable vertebrate host, the first thing she (male mosquitoes cannot bite) will do is cut and probe the skin looking for a source of blood that is close to the surface. During this process, saliva is introduced under the host’s skin. If the mosquito’ salivary glands contain a pathogen, that pathogen is transferred to the host as the mosquito probes. If the mosquito is disturbed while probing, she will fly off without taking a blood meal, but having infected the host as she probed. This makes infected mosquitoes extremely dangerous. If a WNV-infected mosquito finds a group of five individuals at a 4th of July block party, she could conceivably infect all five with WNV without ever taking a blood meal.

Risk of exposure to any vector-borne disease can be reduced by avoiding mosquitoes and preventing them from probing and biting. The best way to avoid mosquito probes and bites is to avoid infested areas, wear protective clothing and wear insect repellent. For more information, you can visit the UF FMEL mosquito information website at: http://mosquito.ifas.ufl.edu/Index.htm.

DEET: A perspective from a toxicologist and a lifetime DEET user

What has been my experience with DEET?
I write this as someone who has had a long intellectual and personal involvement with DEET as an insect repellent. Growing up in the Upper Midwest in the 1960s my family spent much of the time during our relatively short summers outside enjoying the many wonderful lakes in the region. My memories of summer are those of long sunny days, picnic lunches and fishing. Oddly enough, the smell of DEET repellents also is part of that memory. Those days were years before Lyme disease and West Nile virus were ever heard of. We used DEET simply because it prevented the annoying harassment and bites from what in Minnesota was often referred to as “the Minnesota State Bird,” the mosquito.

Being the child of Great Depression-era parents from immigrant families, we were taught a very important lesson that happened to pertain to insect repellents as well as so many other aspects of life: use only what is necessary and not any more. So, we did, as did my friends. My parents knew nothing of the complex chemical feeding triggers that are picked up by mosquitoes triggering them to seek a host and feed. But my parents had common sense: when the mosquito pressure was intense and we were getting bitten, like on calm days at dawn and dusk, we told to carefully reapply the repellent.

From a professional perspective
I have a doctorate in toxicology (the study of toxicity of chemicals) and am certified in toxicology by the American Board of Toxicology. I have been practicing toxicology for over 25 years in industry and consulting and have spent a significant amount of time studying DEET. Often referred to by experts as “the gold standard” for repellents, DEET has been a most effective repellent, and has a reliable safety record when used according to label directions. As Dr. Mark Fradin says in a landmark medical journal article (http://www.annals.org/cgi/content/full/128/11/931), DEET has a “remarkable safety profile.” I agree.

The proof of DEET’s reliability is in the data, and there’s more scientific data on DEET than just about any other ingredient used by consumers in any kind of product. Much of the data comes from well-designed, properly executed, and appropriately interpreted studies. I played a leadership role in a major study (The DEET Registry of Adverse Events) that included a team of physicians, nurses and scientists that collected reports on “adverse events,” or health incidents, reported over a seven-year period in some way associated with the use of DEET.

We thoroughly reviewed each report to determine if DEET might have caused any of the more serious health issues. Over seven years, we logged 242 cases that warranted further analysis. Our team determined that just one case of major severity was probably related to DEET. It was likely an allergic reaction (non-neurologic) to the product. We found 12 cases of neurological effects of major severity that we categorized as possibly related to DEET. The “Possibly Related” category means that the individuals had used DEET prior to the reaction and we could find no other reasons why they experienced the adverse events. Even if we assume that DEET had played a role in these reports, 13 cases is amazingly few, given the millions of users and billions of applications of DEET-based repellents over the seven years of the Registry.

It also is noteworthy that our data showed no indication that use of DEET in combination with common medicines or other compounds resulted in adverse effects in humans. Recognizing the extensive use of DEET by Americans, and considering the relatively few adverse events described in the Registry, I believe that the risk of serious health effects in humans following use of DEET repellents according to label directions is quite low.

Despite the overwhelmingly positive data on DEET, exceptions exist. These include laboratory studies that some critics of DEET inappropriately cite to make inferences about toxicity to humans. In many cases, it has been my job to carefully review these studies and determine their relevance and applicability to humans. Without exception, I have done so with an abundance of caution, because—as mentioned above—I too am a DEET user. None of these studies have caused me to change my view of DEET. Moreover, you can rely on the recommendations from the CDC, the AAP and the EPA for guidance as their physicians and scientists have combed the data as well.

So how do I approach the use of DEET now?
When I consider the use of DEET either on myself or on my family, I consider the risk versus the benefit. If we lived in an ideal world, no consumer product would ever cause adverse events in anyone, under any conditions. But most do. Consider the valuable pain-reliever acetaminophen. The FDA reports that it causes more than 400 deaths annually in the United States. A 2009 study by a CDC team found that overdoses in children account for more than 7,200 emergency room visits annually. We find acetaminophen valuable, so we all still have it in our homes. We’re just reminded each time we use it to read the label instructions.

Some things have changed since I was growing up. Mosquitoes are more than just annoyances—some carry diseases that can be debilitating and even fatal. Ticks, which were very uncommon when I was a child, are far more prevalent now and Lyme Disease is a serious public health issue. I personally know several people who have had the disease in Virginia over the past year. A good change: new DEET repellent formulations have improved scents and skin feel, and are barely reminiscent of the formulations used when I was a child.

As much as we can lament the emergence of mosquito- and tick-borne disease, happily some things have not changed. The warm summer days and long daylight hours are just as delightful as ever and must be savored to the fullest. I still take pleasure personally in those, but now also get the joy of seeing my children enjoy the outdoors. Mosquitoes are just as annoying as before. And DEET repellents are just as effective as when I was a child. Moreover, what my parents taught me when I was a child is just as true and prudent today as it was then: use only what is necessary and not any more.

Regulatory Toxicology and Pharmacology, Feb 2010

Mosquito Infection Rates

Jonathan F. Day, Professor of Medical Entomology, University of Florida, Florida Medical Entomology Laboratory, Vero Beach

What is your real risk of contracting a mosquito transmitted disease like West Nile virus? The answer depends on three factors: where you live, your daily mosquito exposure rate and, most importantly, local mosquito infection rates.

Where you live is important because of the geographic distribution of WNV and other mosquito-transmitted diseases. There are regions of the United States where the virus is common and other regions where WNV is rare. If you live in California, Texas, Nebraska or Colorado where the virus is seasonally common, your risk of infection is higher than if you live in Maine or New Hampshire where the virus is rare.

Likewise, mosquito exposure rate is an important risk factor. People who live in homes without window screens or air conditioning are at a much higher risk of infection than people who live in mosquito-proof homes. Individuals such as night watchmen and emergency responders who work outdoors at night are at higher risk of infection than individuals who work in air conditioned buildings.

Finally, the local mosquito infection rate is the most important risk factor. During most summers, mosquito infection rates are extremely low. For example, a typical WNV mosquito infection rate in Florida is one in 10,000 (that is, one WNV-positive female mosquito is found in every 10,000 females that are sampled). The likelihood of a human WN infection under these conditions is very low. Contrast this with the mosquito infection rate of 1 in 100 observed in Colorado during the 2003 WN epidemic. If one out of every 100 female mosquitoes can transmit WNV, the risk of human disease, especially in humans with high mosquito exposure rates, increases tremendously.

Risk of WNV exposure can be reduced by avoiding mosquitoes and preventing them from biting, especially when mosquito infection rates are known to be high. The best ways to avoid mosquito bites are to avoid infested areas, wear protective clothing and wear insect repellent.

UPDATE: Due to current environmental conditions, FMEL has released the sixth FMEL Arboviral Epidemic Risk Assessment for Florida of 2010. Details can be found in the latest update (PDF) at my lab's website at http://fmel.ifas.ufl.edu. Go to the Mosquito Information Web Site and look in "MWTD Risk Analysis."

The year my life changed forever
Mel Lacy
Life was good for me in 2006. I was 49, successful, wealthy and healthy, with great endurance. I had the world by the tail—with a lovely wife and family, a great career as a builder and a zest for living.

Suddenly, without warning, toward the last week in July, my health deteriorated rapidly. By early August, I had severe headaches, muscle weakness, and disorientation. In each of four separate doctor visits, I was told bed rest and fluids were the best treatment for whatever it was I had (no one knew.)

I continued to deteriorate and by Aug. 5 I could not stop vomiting. Going to the emergency room is the last thing I remember until Oct. 17, when I regained consciousness. The news was horrific. I was paralyzed from the neck down. Lucky to have survived, I had a tracheotomy, was hooked to a ventilator and had a stomach feeding tube. I had lost 45 lbs. of muscle. I looked like a concentration camp survivor. I was flooded by anxiety—what would happen to me, to my family?

In mid-August, while I was still unconscious, physicians had finally concluded that I had West Nile Virus neuroinvasive encephalitis, which was causing the paralysis. West Nile virus is transmitted to people from infected mosquitoes. Who would have thought that something as tiny as a mosquito weighing less than an ounce could have felled a big healthy guy like me with just one bite? Before this happened, mosquito bites were just an annoyance. They had never been life-threatening. I suspect I was bitten in my own back yard in mid-July. Who would have thought?

After weeks in intensive care, I was transferred to a rehabilitation facility. Rehab for someone in my condition, it turns out, was not a good idea. They sent me home after six weeks and four days later, I had respiratory failure. On the way to the emergency room, my heart stopped. I was revived and spent the next 30 days in intensive care.

During that period, one of the physicians thought I had been misdiagnosed. He sent me to a teaching hospital in Portland, Ore., nearly 500 miles from home. The WNV diagnosis was confirmed. My attending physician said he thought that my head movement was about all I could ever expect. It was a devastating realization. This was, without doubt, one of the lowest of low points.

I was transferred at my own expense to a hospital near my home in Idaho and was finally released on Dec. 26, 2006, still on a ventilator. My wife became my primary caregiver as I lay in a hospital bed in our dining room. A physical therapist visited twice weekly during February. And my wife and I worked at the rehabilitation ourselves. I slowly improved enough so that I could to go to a rehabilitation clinic as an outpatient. The tracheotomy apparatus was finally removed in April.

Progress, when you get WNV neuroinvasive disease like I have, is hard to come by. But in the past three years, I have recovered some movement in my lower arms and hands but not full strength. My shoulders don’t function properly—I cannot raise my arms when they are extended straight out. I have good movement in my right leg and ankle, but not much in my left leg and my left arm is stronger than my right, which is unusual, because I am right-handed. My body core is still weak and I have poor lung function.

Despite the setbacks and the negative prognosis from the outset, there have been some minor victories. These are hard won. I have gained enough overall strength to transfer myself using a slide board which helps me to be a little more independent in my daily routines than in the past. I use a sophisticated, motorized wheelchair, which make a huge difference.

One of the main achievements of the past year is that, thankfully, I am able to drive again. Being given this freedom is a miracle. I transfer to and from the van seat from my wheelchair. I use my right leg for regular pedal controls and have good movement with my arm for steering.

But working harder and longer in rehabilitation, which is effective for most folks with disabilities, is not the right thing to do for anyone stricken by WNV. My doctor informed me that, with this virus, there is a very fine line beyond which I may actually be doing more harm than good. This was tough to hear, having worked so hard and come this far.

So, with that in mind, I no longer go to rehab, but I am able to go to the local YMCA, which has a great pool. There’s a swing chair that lowers down in the water, so I can transfer myself into water and do my therapy. This is wonderful, because once in the water I can walk and am slowly gaining strength.

One of the biggest challenges to overcome these days is muscle fatigue, so I do not have much endurance. As my doctor says, "After you have done your pool workout, that’s the equivalent of a runner having run a marathon."

Will I continue to work to regain mobility and strength? Absolutely. But it will be a long journey and who knows how much more motion I can gain.

My hope is that others will not have to go through something like this. My ability to speak has not been compromised, so I tell everyone I know to take personal precautions to prevent being bitten by infected mosquitoes. Everyone needs to dress appropriately when outside and they need to use an EPA-registered mosquito repellent, like the ones that contain the ingredient DEET, faithfully if you are outside when mosquitoes are biting (typically dawn and dusk).

I never knew this simple information…and often think about the “what ifs” in my life. What if I had used repellent and that mosquito had not bitten me? What if I had not been outside after dark? What if…I had never gotten West Nile virus?

So, I encourage everyone to take precautions. West Nile virus could change your life forever. Don’t let that happen to your family, your friends or yourself. This is a chance you can’t afford to take.

Mel Lacy

Fight the Bite Poster Contest

Greetings!

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While it’s still a bit dreary and cold here in the East, and across much of America, lots of kids are stuck inside due to the winter weather. Here’s a suggestion for fifth and sixth graders seeking an indoor activity during the next two months: create an entry for the Fight the Bite poster contest.

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The CDC and DEET Education Program are once again co-sponsoring the contest, which is open to all fifth and sixth graders in the U.S., including those who are home-schooled. Entries must be postmarked by April 5. Helpful hints, the entry form and rules are at http://www.fightthebitecontest.org/, plus winning posters from prior years, which I’m sure you’ll agree are amazingly good.

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The contest encourages students to illustrate ways to protect themselves and their families from diseases spread by mosquitoes and ticks by using repellent while outdoors. Two winners from each state—one fifth grader and one in sixth—will receive a $50 U.S. Savings Bond and an award certificate. Two Grand Prize winners—one from each grade—will receive a $1,000 U.S. Savings Bond, a plaque and a trip to CDC offices and laboratories in Fort Collins, Colo. (airfare and lodging provided) with their parents for the awards presentation.

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I’ve co-hosted the ceremony the past three years and I can say it’s a fun event that the winners and their families seem to enjoy greatly. There’s even an educational component to the trip, courtesy of our CDC partners.

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“Tips for Teachers” also have been added, based on input from a Missouri teacher who has assisted two national winners. Public health and mosquito control professionals are encouraged to use any of the winning posters for educational outreach, public service materials and similar purposes.

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Keep in mind that tick season is upon us, especially in states that have an early spring. The new ticks, known as nymphs, are really hard to see but they carry diseases like Lyme, Rocky Mountain spotted fever and the like. So, be sure to dress appropriately and use repellent to protect yourselves when you venture into the woods.

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You can post questions, suggestions and comments below, or via info@deetonline.org.

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Best wishes for a wonderful and healthy spring and summer!

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Susan E. Little

Executive Director, DEET Education Program