The Deadliest Animal on the Planet

The biology of mosquitoes and the diseases they transmit

Mosquitoes are a type of flying insect that people love to hate. They are annoying pests, buzzing in our ears as we try to fall asleep on warm summer nights, and their bites leave us itchy, spotty, and swollen. What’s more, mosquitoes are considered the deadliest animal on the planet due to the infectious diseases they spread, killing hundreds of thousands more people in the world each year than all other deadly animals, such as snakes, sharks, and hippos, combined. At roughly 3,500 species with a near worldwide distribution, mosquitoes are one well-adapted, bloodsucking global predator. The life of a mosquito and its proliferation are dependent on only a few factors: a moist environment;  a food source—nectar for the sustenance, blood for the females to produce eggs; and a source of standing water in or near which the female can lay her eggs. Mosquitoes are also highly mobile: not only can they fly short distances, but they can also be carried hundreds of kilometres on wind currents and thousands of kilometres in cargo holds, shipping containers, and our very own holiday luggage. Since their needs are few, wherever they land, they have a record of success for populating new communities, and this— among other attributes of evolutionary “fitness”—has established mosquitoes as one of the greatest human foes with which we have ever shared our planet.

MOSQUITO EVOLUTION AND BIOLOGY

Based on the fossil record, it is thought that mosquitoes evolved from other flies approximately 80 million years ago in the Cretaceous period. As popularized by the movie Jurassic Park, scientists have indeed discovered mosquitolike insect parts encased in amber from that time period. Although they emerged such a long time ago, modern mosquitoes are quite different from their ancestors; due to a process known as “microevolution,” in which gene expression changes over time within a species rather than above a species, present-day mosquitoes continue to change at a rapid rate compared to, say, mammals, or even other “arthropods” like spiders. Mosquito biology that facilitates such microevolution includes a rapid life cycle, during which mature adults can develop over only a few days from the time of egg hatching, a high degree of variable gene expression, the ability to move around easily, and a large number of offspring produced by each female mosquito. This propensity for rapid change has enabled mosquitoes to evolve into the apex predators that they are today. Their incredible adaptability allows them, depending on the species, to breed in almost any type of standing water—whether cool or warm, brackish or fresh, deep or shallow, polluted or clean—and to occupy all continents except Antarctica. Their adaptability also allows them to “hibernate” over winter (by entering a dormant period called “diapause”), when needed, and to evade our ability to control their ranks chemically via their elaboration of resistance mechanisms against mosquito repellents and insecticides. Mosquitoes succeed best when provided a hot, humid environment, with a readily available food source and water, and due to humanity’s collective global impact—the Anthropocene is so named for a reason, after all—modern-day mosquitoes want for nothing.

After taking a meal of blood from an animal, the female mosquito will lay its eggs in or beside water. Larvae hatch out of eggs after a couple of days and then mature to pupae, from which adult mosquitoes emerge. The entire process may take up to a month, but, on average, this “generation time” is about two weeks, with some species maturing in as few as four days. The generation time of many mosquitoes is dramatically affected by air and water temperature. For example, in Aedes albopictus, the Asian tiger mosquito, a 10° C increase in ambient temperature shortens the adult development time by 40–50 percent (from 15 to 19 days down to 8 to 11 days). Unprecedented human mobility, globalization of trade and transport, and climate warming have all culminated in the ever-increasing competence of mosquitoes to occupy new environmental niches. Originally spread by shipments of used tires, the Asian tiger mosquito is now found throughout the Americas, including parts of southern Canada. What’s more, seasonal spikes in mosquito populations across the globe, which follow typical patterns of precipitation (for example, rainy seasons), are dramatically affected by extreme climatological events such as hurricanes, cyclones, and flooding—the occurrence of which is somewhat unpredictable but undoubtedly increasing in our warming world. Such events are known to lengthen mosquito breeding season and lifespan, and increase mosquito population density. Thus, it is not simply the catastrophic economic, built-environment, and human-capital losses related to climate-associated geophysical disasters that justify climate change as one of the United Nations’ sustainable development goals (https:// sustainabledevelopment.un.org/sdg13) but also the downstream health effects, many of which are directly related to diseases spread by mosquitoes.

MOSQUITO-BORNE DISEASES

Only a small proportion of the approximate 3,500 mosquito species transmit infections to humans, with the most infamous species belonging to the Anopheles and Aedes groups. Collectively, mosquito-borne diseases exact a punishing economic impact on areas of the planet least situated to weather such losses. Malaria—a parasitic infection transmitted by the bite of an infected female Anopheles mosquito—kills close to 500,000 people each year, with more than 90 percent of the 220 million annual cases situated in sub-Saharan Africa, and 60 percent of deaths occurring in children under the age of five. Due to the biting proclivities of Anopheles mosquitoes, most transmission occurs at night, so sleeping under an insecticide-treated bed net is an effective strategy to interrupt transmission. However, like many public health interventions in equatorial low- and middle-income parts of the world, bed-net usage is challenged by logistics (for example, supply chain, distribution, and accessibility of at-risk communities, particularly during rainy season), cost, and socio-political factors. Dengue, one of the “arboviruses”— so named because it is a virus that is “arthropod borne”—is transmitted to people by the day-biting Aedes mosquitoes, and many regions in the tropics are presently in the grip of a large-scale epidemic, which is showing no signs of abating. In some areas of the tropics with entrenched urban transmission of dengue, thousands of cases are being diagnosed every week; recent locally acquired cases have even been detected in more northern areas not known to portend risk, including France, Spain, and Florida. In addition to spreading dengue, Aedes mosquitoes can also transmit both the arthritiscausing Chikungunya virus and Zika virus, which can lead to birth defects and neurological problems in infants born to infected mothers, as well as miscarriage and stillbirth.

While malaria and dengue are major mosquito-transmitted killers in the tropics and subtropics, other serious infections can be spread by mosquitoes in temperate, cooler areas of the planet. In Canada, the most common human infection spread by mosquitoes is West Nile virus, which typically causes a flu-like illness when symptoms occur but can, more rarely, affect the brain and nervous system. West Nile virus is spread to people by Culex mosquitoes, which can bite at any time of the day and acquire the infection themselves from infected birds. Due to our harsh winter climate, West Nile virus is rarely transmitted in Canada outside of the peak mosquito months of May to October. However, like all mosquitotransmitted infections, West Nile-virus frequency and seasonality may be greatly affected by a warming climate and extreme weather events.

Mosquito-borne diseases have shaped the landscape of human history by influencing the outcome of battles and wars, conquests and attempts at colonization, and infrastructure projects of global importance. They—or more precisely, their consequences— are captured in the ancient scrolls and papyri of Egypt, and in the art and literature of Greek writers and philosophers such as Homer and Aristotle. Malaria, specifically, is thought to have caused the fall of the Roman Empire. Yellow fever, another Aedes-transmitted virus, struck down more soldiers in the southern United States during the American Civil War than the war itself and was singlehandedly responsible for decimating legions of workers over the decadeslong construction of the Panama Canal. Closer to home, the construction of our very own Rideau Canal in the early 1800s was beset by mosquito-borne disease, with deaths attributable to malaria vastly outnumbering those related to occupational accidents.

In the late 19th and early 20th centuries, emerging knowledge about the role mosquitoes played in the spread of diseases like malaria and yellow fever, coupled with industrialization and urbanization, led to the deployment of highly effective control measures, including better housing, widespread use of insecticides, destruction of mosquito habitat, and treatment of people with the infections, all of which interrupted transmission. Thus, malaria was wiped out in Canada by the mid-1900s, but since we still have mosquitoes in our country, there is the potential for it to return. The same is true for dengue: the more the climate warms, the more hospitable Canada becomes for the Aedes mosquito; just one bite of one person returning from the tropics with dengue fever could theoretically establish the infection in new Aedes-mosquito populations in Canada. Entomologists and public health scientists have therefore developed screening programs through which they track, trap, and test Canadian mosquitoes for infections that could become either established or re-established after having been previously eradicated. Unfortunately, the insecticides used in the days of the Panama Canal construction are becoming increasingly ineffective due, once again, to the great microevolutionary capacity of mosquitoes. Their rapid adaptability allows them to express mutated genes that confer resistance to insecticide (pesticide) chemicals. Scientists, in turn, have had to up their game in the fight for mosquito control, even turning to laboratory-based genetic modification and then release of, say, sterile mosquitoes in order to reduce breeding populations. Despite these technological advances in the face of rapid mosquito evolution, the major fundamental pillar of mosquito control, which enabled completion of the Panama Canal and eradication of malaria in Canada, continues to apply: destruction of mosquito habitat (that is, “draining the swamp”) to reduce breeding and mosquito population growth. Canadians—and urban dwellers, in particular—must remain vigilant in this regard and clear our local environments of the standing water necessary for propagating generation after generation of mosquito.

PROTECTING YOURSELF AND YOUR FAMILY FROM MOSQUITO-BORNE DISEASES

The approach to protecting oneself  from infections that are spread by mosquitoes should involve multiple strategies aimed at interrupting the mosquito life cycle, avoiding mosquitoes when possible, and—when impossible— repelling mosquitoes to reduce bites. Interrupting the mosquito life cycle rests mostly on removing breeding sites, notably, standing water. Common sources of standing water in urban environments include flowerpots and bird baths, as well as household items such as buckets and cans. Ponds,reservoirs, and flooded drainage ditches may serve as breeding ground in rural areas. Strategies to avoid mosquitoes and their bites include minimizing outdoor exposure from dusk to dawn during peak months, screening on windows, netting around bedding and doorways, and long-sleeved clothing and hats. If exposure is unavoidable, use of a mosquito repellent containing DEET (up to 30 percent, depending on age) or icaridin (20 percent in all persons above six months of age)—both of which have an impeccable record of safety and efficacy—is advisable. While most herbal and other repellent formulations marketed as “natural” have poor efficacy in scientific studies, formulations of lemon eucalyptus oil can offer some limited protection similar to that of 10 percent DEET. Safe and effective repellents are sold in most pharmacies, grocers, and outdoor stores in Canada.

MOSQUITOES AND US

Mosquitoes constitute a group of rapidly evolving, highly adaptable, and tiny but lethal human predators, the ranks of which continue to increase and encroach on new territory due to the intersection of climate, urbanization, and inherent mosquito biology. Having few predators themselves— dragonflies and certain fish being the principal mosquito eaters—and the ability to evade our chemical control tactics through genetic resistance, mosquitoes will continue to not only tax our collective cognitive resources but also spread diseases in catastrophic proportions, often to populations residing in resource-constrained environments with limited potential for effective and sustained mosquito control. And so perpetuates the cycle of this minuscule bloodsucker, shaping the course of our history and our future.

Suggested Further Reading