Environment

Flirting With the Ultimate Swat: Should We Kill All Mosquitoes?

Given that mosquito-borne diseases are amongst the biggest killers on Earth today, going for a complete eradication is never out of health officials’ minds.

A mosquito. Credit: slgc/Flickr, CC BY 2.0

A mosquito. Credit: slgc/Flickr, CC BY 2.0

If not for a Cuban scientist’s audacious claim and an American physician’s outrageous experiment, the Panama Canal may never have been built by the Americans when it was. The completion of the historic canal had seemed almost impossible for a long time – not because the technological prowess required to create an artificial waterway connecting two oceans was too much, but because of the diminutive mosquito.

Unfortunately, it was not known at the time that it was mosquitoes, abundant in the tropics, which were causing the deathly yellow fever. The prevailing miasma theory postulated that diseases like yellow fever spread through bad air and filthy environmental conditions.

It was Carlos Finlay from Cuba who first made the suggestion to the medical community in his 1882 letter to the Annals of Royal Academy of Sciences of Havana:

… on account of various reasons which it would be idle to relate I asked myself if it was not the mosquito that transmitted the yellow fever poison.

Finlay’s naming of the stegomyia mosquito (a group that includes Aedes aegypti) as the culprit was met with much resistance. First, he had only associative and observational, not experimental, evidence for this. More importantly, he was attempting to overturn the centuries-old miasma theory.

Around the same time as Finlay’s announcement, the French had started work on constructing the Panama Canal. Led by Ferdinand de Lesseps – of the Suez Canal fame – the French spent 11 long years excavating at the Isthmus of Panama. However, they were forced to abandon their efforts in 1892 after one-third of their workforce was wiped out by yellow fever.

Treatment and prevention programmes of the time were still based on the miasma theory. They usually involved city-wide clean-ups and sanitation drives to get rid of filth and waste that was presumed to spread the virus. While cleaner cities is always a good thing, some practices adopted by hospitals were not as harmless. For example, windows were frequently kept open and potted plants dotted several treatment rooms. Although the rooms were airier and the corridors prettier, the mosquitoes that streamed in and bred in the pots were making more patients sick. In all this time, Finlay’s theory lay ignored.

Take two

After the French retired, the US decided to take over the construction in the 1900s. They may have met with the same fate as their predecessors if not for one of their military physicians, Walter Reed. When Reed heard about Carlos Finlay’s mosquito theory, he decided it was worth studying.

In October 1900, his first results were published. Reed got proof in the most straightforward way possible: human experimentation. A stegomyia mosquito was first made to bite a yellow fever patient and, subsequently, a bunch of healthy volunteers. When the latter contracted the disease, Reed knew the breakthrough had been made.

American workers began arriving at the tropical Panama in June 1904 and construction began. Despite Reed’s breakthrough, and a repurposed sanitation campaign by Chief Sanitation Officer William Gorgas, old beliefs persisted. It took a bad yellow fever outbreak over the next year for the Theodore-Roosevelt-led government to give the campaign a boost. Gorgas finally had the required resources and found himself heading what was, according to a report in Historia Medicinae, “the most expensive, concentrated public health campaign in history”.

One-hundred-and-twenty tonnes of pyrethrum powder, 300 tonnes of sulphur, 2.2 million litres of oil, 3,000 garbage cans, 4,000 buckets, 1,000 brooms, and 1,200 fumigation pots later, it seemed yellow fever was finally under control. By late 1906, no more people were dying of it. By 1914, the Panama Canal opened for business.

A brush with success

A couple of decades later, Brazil undertook the major effort to eradicate Aedes aegypti, which was killing large numbers by spreading yellow fever. The government flagged off an eradication programme in 1942. Correctly recognising that the cooperation of neighbouring nations was necessary to achieve any lasting results – otherwise the virus would waltz right back in – the whole region was persuaded to sign up. In a massive effort led by the Pan American Sanitary Organisation (known today as PAHO), 18 countries achieved eradication of Aedes aegypti by 1962.

However, things slowed down soon after. People were travelling more and political will was weakening. And sure enough, mosquitoes started leaking into the “safe” countries from regions where they still persisted, like the USA, Venezuela and Cuba. By the 1980s, the region was back to square one.

Thanks to the development of a vaccine and mass vaccination programmes in endemic nations, yellow fever is under control today. But this doesn’t mean the Aedes aegypti mosquito is out of the picture. It is ubiquitous on the planet and is a carrier of dengue fever, chikungunya, and more pertinent today, the Zika virus.

To spread, vector-borne diseases require not just the presence of the carrier but also an infected person available for the mosquito to bite. So once the number of cases hits zero in a region, it doesn’t matter if there are still mosquitoes because there is no virus to transmit. And as long as a virus doesn’t reenter from the ‘unsafe outside’, the disease is unlikely to re-erupt. The problem in this globalised world is getting the virus to stay put.

Going for the kill

Given that mosquito-borne diseases are amongst the biggest killers on Earth today, going for a complete eradication is never out of health officials’ minds. An “ultimate swat” will not be easy, especially considering the agricultural use of the quick-fix chemical DDT (dichlorodiphenyltrichloroethane) has been banned almost everywhere. Even if we figure out a way to programme this, could the eradication of a species ever be a good idea? After all, some of these mosquitoes have had a place in Earth’s ecological system for 50 million years. Moreover, it’s only females that bite; the males feed on nectar from flowers. And mosquitoes are one of the thousands of species that transmit diseases.

One school of thought among experts says we’ll do just fine without mosquitoes. An article in the journal Nature, from 2010, concluded thus:

“They don’t occupy an unassailable niche in the environment,” says entomologist Joe Conlon, of the American Mosquito Control Association in Jacksonville, Florida. “If we eradicated them tomorrow, the ecosystems where they are active will hiccup and then get on with life. Something better or worse would take over.” The romantic notion of every creature having a vital place in nature may not be enough to plead the mosquito’s case.

The cons of eradication

There are still those who feel that obliterating a species may not be the most practical method to make these diseases disappear. In fact, it may not even work, as illustrated by the many failed programmes around the world. “Eradication of Aedes aegypti alone is unfeasible,” says University of Florida ecologist Phil Lounibos. “This species has a highly resistant egg stage that protects it from drought and detection and facilitates its transport from one place to another.”

Another frightening possibility, warns Lounibos, is the emergence of an alternative carrier. “Eradication would leave an open ecological niche that would be filled by an alternative species, such as the Asian Tiger mosquito, which is also a good vector of dengue, chikungunya, and zika viruses,” he explains.

According to him, a more realistic strategy than eradication would be to either suppress or reduce local populations of the Aedes aegypti species to a level that, in turn, reduces the risk of pathogen transmission. Lounibos is also optimistic about population replacement with a modified vector that can no longer transmit these dangerous viruses.

For example, the ‘Eliminate Dengue‘ research programme is introducing Aedes aegypti mosquitoes containing a common bacteria called wolbachia into the wild. The wolbachia mosquitoes have reduced capabilities to transmit dengue. If they sufficiently breed with wild populations, then newer generations will be much less of a dengue-threat. So far, according to the researchers, trials conducted in cities in northern Australia and Brazil have shown encouraging results.

Acclaimed science writer David Quammen is more vehement about his anti-speciecide views. He points out that mosquitoes may be the last barrier between humans and tropical forests. In his brilliant 1981 essay titled Sympathy for the Devil, he goes as far as to call them “the great ecological heroes of planet Earth” when it comes to rainforest preservation. This memorable extract from the essay sums up his views:

So as Europe was being stripped of its virgin woods, in India and China and the North American heartland, the rainforests escaped, lasting into the late twentieth century – with some chance at least that they may endure a bit longer. Thanks to what? Thanks to ten million generations of jungle-loving, disease-bearing, blood-sucking insects: the Culicidae, nature’s Viet Cong.