Brazil has been in the spotlight this year not only because of the Olympic Games, but also due to the Zika virus outbreak. Zika virus is a mosquito-borne disease, which has infected thousands of pregnant women and disabled their babies with neurological defects. In just two years, mosquitos have delivered Zika virus to almost every country in the Americas . But this is not the first time that mosquitos have profoundly affected global health. In fact, by carrying disease-causing pathogens, they have shaped certain facets of human history.
Bill Gates called mosquitos “the deadliest” creatures in the world . In addition to Zika virus, mosquitos are also carriers of many deadly diseases including malaria, yellow fever, dengue, chikungunya, Western equine encephalitis, and West Nile virus . Every year, these mosquito-borne diseases afflict approximately 700 million people and kill more than one million, exceeding the number of people lost in war annually in recent decades .
Mosquitos are low-key culprits. They appear small, slender, delicate and humble, but they carry what some might consider the most destructive weapon in the world, a six-needle-formed mouthpart called a proboscis. By inserting their proboscis into skin, they can suck blood and transmit life-threatening diseases. Only female mosquitos bite because they need blood to nourish their eggs. In addition to blood, their reproduction also requires ambient temperature and abundant water; they like to lay eggs in lakes, puddles, swamps, or any other still-water reservoirs. Thus, tropical regions with seasonal storms provide an ideal natural habitat for mosquitos. And these mosquitos, by spreading devastating diseases, became the most unforeseen of enemies to Westerners who tried to colonize the Americas.
Mosquitos Fostered the First Lawful Union of Scotland and England
When England began to prosper from foreign trade in 17th century, Scotland was still floundering in severe famine, political turmoil and recurrent battles. To save the country from despair, a Scottish banker William Paterson proposed an ambitious plan: the colonization of America, which would result in more trade, an improved economy, and gains in political power. This wild idea received passionate support from desperate Scottish bankers, who invested 25%-50% of Scottish money in circulation to Paterson’s colonization plan. In 1698, Paterson and 1200 Scots embarked on their historic journey to Darien, the isthmus of Panama.
The Scots were satisfied with Darien’s dense forest, palatable fruit and wild creatures, and hospitable Native Americans to trade with. They established a fort in an isolated bay: a low ground surrounded by steep cliffs, backed by the bay, guarded by big rocks and supplied with plenty of fresh water. It was a perfect fort for military purposes, but unfortunately, it was also a perfect hotbed for mosquitos. When the wet season came, the mosquitos also had everything they needed to build their new kingdom: hot weather, water accumulated in mire and reservoirs, and human blood. Swarms of mosquitos quietly laid eggs in the fort and gradually infected the Scots with malaria and yellow fever. At first only five Scots died each week, but eventually the rate of death climbed. Half a year later, they had lost a quarter of their soldiers, with most surviving soldiers weakened by disease. The survivors fled back to Scotland, ending their first failed colonization attempt. But soon they recruited another 1400 healthy Scots and launched a second expedition to Darien. Unfortunately, they chose to live in the same fort with mosquitos carrying yellow fever and malaria, so the unimmunized and undernourished Scots again underwent a dramatic loss. To make matters worse, Spanish soldiers attacked their fort, so the decimated Scots had to surrender. And when the Spanish stepped into the fort, there were only 300 wretched Scots left—the mosquito-borne diseases had saved the Spanish’s bullets. Eventually, only 100 Scots returned home to Scotland.
This patriotic colonization attempt turned out to be the last brutal strike to Scotland. Thousands of people lost their lives and Scotland lost almost half the currency in circulation. Many Scottish aristocrats lost so much of their fortunes that they had to ask England for financial help. Ironically, William Paterson, who promoted his colonization plan as a way to protect the independence of Scotland, now had to advocate for the union of Scotland and England to save Scotland from bankruptcy. Eventually Scotland and England signed the Act of Union in 1707 - due in part to the havoc wrecked by mosquitos .
Besides hindering Scottish soldiers in Central America, mosquito-borne diseases also disrupted the colonization plans of other Western countries. For example, malaria diminished many British soldiers and assisted George Washington’s winning in the Battle of Yorktown . Mosquito-borne diseases tortured French soldiers in America so ruthlessly that Napoleon had to give up America and sold an area of 15 states to the United States . Two possible reasons can explain the good luck of the founders of America. First, the American soldiers might have had a slightly better resistance to malaria and yellow fever than their European enemies who had never experienced these diseases. Second, some American troops might fortunately have chosen to live in highlands, avoiding natural water reservoirs, while the European armies settled near landing ports in lower ground, where mosquitos easily found home in the mires and puddles after the rain.
Prevention of Mosquito-Borne Disease—An Ongoing Battle
Although mosquitos disturbed Western colonists in America since the 16th century, they were not discovered to be disease-carriers until 1897 . For hundreds of years, mosquitos traveled with explorers from Africa to almost every country except Iceland (which has extremely low temperatures), acting as potential menaces to human health. We’ve tried to defend ourselves with multiple chemical and biological strategies, but have yet to find a way to eradicate mosquitos without huge economic and environmental costs. Insecticides such as DDT are effective, but can be harmful to many other organisms too. We have developed vaccines against some known mosquito-borne diseases like yellow fever, but still lack vaccines for malaria, dengue, and Zika virus. And pathogens make prevention and treatment even more challenging by their ability to rapidly evolve. For example, Zika virus can mutate more quickly than humans and has gone through significant genetic changes in the past seventy years .
One interesting and promising strategy is to modify the genetics of mosquito, to generate a sterile strain called OX513A. They can mate with wild type mosquitos, but produce offspring that die prematurely. When these OX513A are released in the wild, they can reduce the mosquito population by ten-fold [10,11]. Hopefully, the genetically modified mosquitos will be safe enough to release globally, weakening and eventually eliminating all mosquitos. And this may be the most-deserved punishment for mosquitos: They have been meddling with human health and history for ages, and now it’s our turn to change theirs.
Xin Liu (firstname.lastname@example.org)
Guest Contributor, Signal to Noise Magazine
PhD Candidate, Molecular Biology Interdepartmental Doctoral Program (MBIDP), UCLA
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