The Mosquito Read online

  To understand the sprawling and stealthy influence of the mosquito on history and humanity, it is first necessary to appreciate the animal itself and the diseases it transmits. I am not an entomologist, a malariologist, or a physician of tropical medicine. Nor am I one of the countless unsung heroes fighting in the trenches of the ongoing medical and scientific war against mosquitoes. I am a historian. I leave the complex scientific explanations of the mosquito and her pathogens in the hands of these experts. Entomologist Dr. Andrew Spielman advises us, “To meet the health threats that are growing worse in many corners of the world, we must know the mosquito and see clearly her place in nature. More importantly, we should understand many aspects of our relationship to this tiny, ubiquitous insect, and appreciate our long, historical struggle to share this planet.” In order to best appreciate the rest of our story, however, we first must know what we are up against. To encapsulate Chinese general Sun Tzu’s timeless fifth-century BCE treatise, The Art of War: “Know your enemy.”

  According to an orthodox quotation erroneously attributed to Charles Darwin, “It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is most adaptable to change.”* Regardless of its origin, the mosquito and its diseases, most notably malaria parasites, are the quintessential example of this passage. They are masters of evolutionary adaptation. Mosquitoes can evolve and adapt quickly to their changing environments within a few generations. During the Blitz of 1940–1941, for example, as German bombs rained down on London, isolated populations of Culex mosquitoes were confined to the air-raid tunnel shelters of the Underground Tube along with the city’s resilient citizens. These trapped mosquitoes quickly adapted to feed on mice, rats, and humans instead of birds and are now a species of mosquito distinct from their aboveground parental counterparts.* What should have taken thousands of years of evolution was accomplished by these mining sapper mosquitoes in less than one hundred years. “In another 100 years time,” jokes Richard Jones, former president of the British Entomological and Natural History Society, “there may be separate Circle Line, Metropolitan Line and Jubilee Line mosquito species in the tunnels below London.”

  While the mosquito is miraculously adaptable, it is also a purely narcissistic creature. Unlike other insects, it does not pollinate plants in any meaningful way or aerate the soil, nor does it ingest waste. Contrary to popular belief, the mosquito does not even serve as an indispensable food source for any other animal. She has no purpose other than to propagate her species and perhaps to kill humans. As the apex predator throughout our odyssey, it appears that her role in our relationship is to act as a countermeasure against uncontrolled human population growth.

  In 1798, English cleric and scholar Thomas Malthus published his groundbreaking An Essay on the Principle of Population, outlining his ideas on political economy and demography. He argued that once an animal population has outpaced its resources, natural catastrophes or checks such as drought, famine, war, and disease will force a return to sustainable population levels and restore a healthy equilibrium. Malthus bleakly reasons, “The vices of mankind are active and able ministers of depopulation. They are the precursors in the great army of destruction, and often finish the dreadful work themselves. But should they fail in this war of extermination, sickly seasons, epidemics, pestilence, and plague advance in terrific array, and sweep off their thousands and tens of thousands. Should success be still incomplete, gigantic inevitable famine stalks in the rear.” Enter the mosquito as the main human Malthusian check in this grim apocalyptic vision. This unrivaled dealing in death is primarily inflicted by only two perpetrators with no harm to themselves—Anopheles and Aedes mosquitoes. The leading ladies of these two species circulate the entire catalogue of more than fifteen mosquito-borne diseases.

  Our Aedes Enemy: A female Aedes mosquito in the process of acquiring a blood meal from her human host. Aedes mosquito species transmit a catalogue of mosquito-borne disease including the viruses that cause yellow fever, dengue, chikungunya, West Nile, Zika, and various encephalitides. (James Gathany/Public Health Image Library-CDC)

  Our Anopheles Enemy: A female Anopheles mosquito obtaining a blood meal from a human host through her pointed proboscis. Note the secretion droplet being expelled to condense the protein content of the blood in her abdomen. Anopheles mosquito species are the sole vectors of the five human types of malaria plasmodium. (James Gathany/Public Health Image Library-CDC)

  Throughout our existence, the mosquito’s toxic twins of malaria and yellow fever have been the prevailing agents of death and historical change and will largely play the role of antagonists in the protracted chronological war between man and mosquito. “It is not always easy to remember to give yellow fever and malaria their due. Mosquitoes and pathogens left no memoirs or manifestos. Before 1900, prevalent understanding of disease and health did not recognize their roles, and no one grasped their full significance,” upholds J. R. McNeill. “Subsequently historians, living in the golden age of health, normally failed to see their significance either . . . But the mosquitoes and pathogens were there . . . and they had effects on human affairs that we can see reflected in archives and memoirs.”

  Yet malaria and yellow fever are only two of over fifteen diseases that the mosquito bestows upon humans. The others will provide the supporting ensemble cast in our story. Mosquito-borne pathogens can be separated into three groupings: viruses, worms, and protozoans (parasites).

  The most abundant are the viruses: yellow fever, dengue, chikungunya, Mayaro, West Nile, Zika, and various encephalitides, including St. Louis, Equine, and Japanese. While debilitating, these diseases, aside from yellow fever, are generally not prolific killers. West Nile, Mayaro, and Zika are relatively new entries to the index of mosquito-borne disease. There are currently no vaccines, save that for yellow fever, but for the most part, survivors are blessed with lifetime immunity. Since they are closely related, common symptoms include fever, headaches, vomiting, rashes, and muscle and joint pain. These symptoms usually begin three to ten days after contagion from a mosquito bite. The vast majority of those infected recover within a week. Although exceptionally rare, severe cases can result in death caused by viral hemorrhagic fevers and a swelling of the brain (encephalitis). The old and young, pregnant women, and those with escorting medical issues make up the disproportionate bulk of casualties from these viral infections, which are all spread predominantly by the Aedes mosquito. Although globally present, the highest infection rates occur in Africa.

  Occupying the top tier of the virus class is yellow fever, which often amplified and accompanied endemic malaria. It is an accomplished killer, first stalking humans in Africa about 3,000 years ago. Until recently, it was a global historical game-changer. This adversary targets healthy, young adults in the prime of life. Although a successful vaccine was discovered in 1937, between 30,000 and 50,000 people still die annually of yellow fever, with 95% of fatalities occurring in Africa. For about 75% of those infected with yellow fever, symptoms mirror those of its viral cousins mentioned above, and usually last three to four days. For the unlucky 25% or so, after a day of respite, they enter a second toxic phase of the disease complete with fever-induced delirium, jaundice due to liver damage, severe abdominal pain, diarrhea, and bleeding from the mouth, nose, and ears. The internal corrosion of the gastrointestinal tract and kidneys induces vomit of bile and blood, the consistency and color of coffee grounds—giving rise to the Spanish name for yellow fever, vómito negro (black vomit)—which is followed by coma and death. The latter, usually occurring two weeks after initial symptoms, might well have been the last pleading wish of many victims.

  While this portrayal paints a grisly picture, it also embodies the gnawing terror that yellow fever implanted in pacing and brooding populations across the world, especially in the European colonial outposts of the New World. The first definitive outbreak in the Americas occurred in 1647, disem
barking with African slaves and fugitive mosquitoes.* It must have been agonizing to wonder when and where “Yellow Jack,” as the British christened it, would strike next. While fatality rates from yellow fever averaged around 25%, depending on the strain and conditions of an epidemic, it was not uncommon for death rates to reach 50%. A handful of outbreaks reached 85% in the Caribbean. The salty sea stories of ghost ships like the Flying Dutchman are based on true accounts; whole crews might succumb to yellow fever, months passing before the aimlessly drifting ships were corralled. Boarding parties were greeted with nothing but the stench of death and the rattle of skeletons with no revealing clues as to the cause. Luckily for survivors, who are left incapacitated for weeks, yellow fever is a one-shot deal. Lifetime immunity is imparted to those who defang the dogged virus. Although dengue, thought to have its 2,000-year-old ancestral origins in monkeys of Africa or Asia (or both), is far more benign than its close cousin, yellow fever, the two viruses can provide limited and partial cross-immunization.

  Stigmata: This engraving from a 1614 British medical textbook depicts a woman showing the unmistakable symptoms of filariasis or “elephantiasis.” (Diomedia/Wellcome Library)

  Spread by the Aedes, Anopheles, and Culex mosquito breeds, the sole member of the worm category is filariasis, commonly referred to as elephantiasis. The worms invade and obstruct the lymphatic system, causing an accumulation of fluids resulting in extreme, if not spectacular, swelling of the lower extremities and genitals, while also frequently causing blindness. Engorged scrotums, easily surpassing the size of large beach balls, are not unusual. For women, the labia can become almost as grotesque. Although this stigmatizing disease is treatable with inexpensive modern medicine, unfortunately, 120 million people annually still suffer from filariasis, predominantly in the tropics of Africa and Southeast Asia.

  Malaria stands alone in the protozoan, or parasite, classification. In 1883, Scottish biologist Henry Drummond called parasites “a breach in the laws of Evolution and the greatest crime against humanity.” Malaria is the unsurpassed scourge of humankind. Currently, almost 300 million unlucky people annually contract malaria from the bite of an Anopheles mosquito, the very same one that bit you and stole your blood during your camping vacation. Without your having the slightest clue, the malaria parasite has entered your bloodstream and is making a mad dash for your liver, where it can rest and recuperate while it plans its procreative assault on your body. You, however, are back home from your camping trip, madly scratching your mosquito bites with the malaria parasite furtively hibernating in your liver. How sick you become, and your likelihood of death, is dependent upon which strain of malaria you have contracted.

  It is possible to be infected with more than one species at a time, although usually within this battle the deadliest strain outperforms the others. They are all perpetrated by 70 of the 480 species of your Anopheles offender. There are over 450 different types of malaria parasites vexing animals across the world, with five of them afflicting humans. Three types, knowlesi, ovale, and malariae, are not only extremely rare but have a comparatively low mortality rate. Knowlesi recently made the zoonotic jump from the macaque monkey in Southeast Asia, while the uncommon ovale and malariae now exist almost exclusively in West Africa. We can rule out that you have contracted any of these three, which leaves us with the two most dangerous and widespread contenders battling for hegemony of your health and life—vivax and falciparum.

  The malaria parasite roosting in your liver will traverse through an impressive seven-stage life cycle. It must have multiple hosts to survive and procreate—the mosquito and an army of secondary vectors: humans, apes, rats, bats, rabbits, porcupines, squirrels, a volery of birds, a congress of amphibians and reptiles, and a swarm of others. Unfortunately, you are that host.

  Following that fateful mosquito bite, this miscreant will mutate and reproduce inside your liver for one to two weeks, during which time you will show no symptoms. A toxic army of this new form of the parasite will then explode out of your liver and invade your bloodstream. The parasites attach to your red blood cells, quickly penetrate the outer defenses, and feast on the inner hemoglobin. Inside the blood cell, they undergo another metamorphosis and reproductive cycle. Engorged blood cells eventually burst, spewing both a duplicate form, which marches forward to attack fresh red blood cells, and also a new “asexual” form that relaxedly floats in your bloodstream, waiting for mosquito transportation. The parasite is a shape-shifter, and it is precisely this genetic flexibility that makes it so difficult to eradicate or suppress with drugs or vaccines.

  You are now gravely ill with an orderly, clockwork progression of chills followed by a mercury-driving fever touching 106 degrees. This full-blown cyclical malarial episode has you in its firm grip and you are at the mercy of the parasite. Lying prostrate and agonizingly helpless on sweat-soaked sheets, you twitch and fumble, curse and moan. You look down and notice that your spleen and liver are visibly enlarged, your skin has the yellowing patina of jaundice, and you vomit sporadically. Your mind-melting fever will relapse at precise intervals with each new burst and invasion of the parasite from your blood cells. The fever then subsides while the parasite eats and reproduces inside new blood cells.

  The parasite uses sophisticated signaling to synchronize its sequencing, and this entire cycle adheres to a very strict schedule. This new smart hub asexual form transmits a chemical “bite me” signal in our blood, significantly boosting the chances of being picked up by a mosquito from an infected human to complete the reproductive cycle. Inside the stomach of the mosquito, these cells mutate once more, into both male and female varieties. They quickly mate, producing threadlike offspring versions of the parasite, which make their way out of the gut and into the salivary glands of the mosquito. Within the saliva glands, the malaria parasite shrewdly manipulates the mosquito to bite more frequently by suppressing the production of her anticoagulant to minimize her blood intake during a single feeding. This forces her to bite more frequently to get her required fill. In doing so, the malaria parasite ensures that it maximizes its rate and range of transfer, its procreation, and its survival. Malaria is a remarkable example of evolutionary adaptation.

  It was this salivary configuration of the parasite that was transferred to you by that damned mosquito on the camping trip you took over two weeks ago. But the question remains: What type of malaria has left you incapacitated with enervating recurrent symptoms? If it is the dreaded falciparum, you may recover, or you may enter a second phase of the disease called cerebral or severe malaria. Within a day or two, you will experience seizures, coma, and death. The fatality rate from falciparum is dependent upon strain, location, and numerous other factors, but nevertheless straddles 25% to 50% of those infected. Of those who survive cerebral malaria, roughly 25% will have permanent neurological damage, including blindness, loss of speech, severe learning disabilities, or paralysis of the limbs. Malaria takes a life every thirty seconds. Sadly, 75% of the deceased are children under five years of age. Falciparum is the vampiric serial killer, accounting for 90% of malarial deaths, and Africa currently houses 85% of all global malarial deaths. Unlike yellow fever, malaria hunts the young and immunologically weak. Pregnant women also suffer disproportionately.

  In this unfortunate scenario, if you are lucky enough to have contracted vivax, you probably will not die. Vivax is the most common form of malaria, especially outside of Africa, and is responsible for 80% of all malaria cases, but it is not generally a killer. Its mortality rate hovers around 5% in Africa, with an even lower 1–2% in the rest of the world.

  It is almost impossible to describe the scale of the devastation the malarial Anopheles mosquito can wreak. Even today, the horror of malaria is hard to comprehend. So it is near unfathomable to imagine malaria in the historical context when causes were unknown and treatments did not exist. J. A. Sinton, an early-twentieth-century malariologist, conceded that the disease “constitutes one of
the most important causes of economic misfortune, engendering poverty, diminishing the quantity and quality of the food supply, lowering the physical and intellectual standard of the nation, and hampering increased prosperity and economic progression in every way.” Add to this description the physical, emotional, and psychological effects of such an enormous death toll. Currently, it is estimated that endemic malaria costs Africa roughly $30–$40 billion a year in lost commercial output. Economic growth in malarious countries is 1.3% to 2.5% lower than the adjusted global average. Cumulatively, spanning the modern era after the Second World War, this equates to a 35% lower gross domestic product (GDP) than it would have been in the absence of malaria. Malaria sickens and cripples economies.

  Thankfully for you, the odds were in your favor and you shook off your stint of vivax malaria within a month. I am sorry to tell you, however, that your suffering is likely not over. Neither falciparum nor knowlesi causes malarial relapse. A second communicable bite from a malarious mosquito is required for reinfection. Regiments of parasites from the other three types of malaria, including vivax, however, lie in ambush in the liver, and can generate repeated relapses for up to twenty years. A British veteran of the Second World War had a malarial encore forty-five years after his infection in 1942 during the Burma campaign. In your case, the time frame for vivax is generally one to three years. Nevertheless, you can always be reinjected by another mosquito bite.