Guest News Brief by Kip Hansen – 29 July 2020

“What makes mosquitoes so dangerous? Despite their innocuous-sounding name—Spanish for “little fly”—they carry devastating diseases. The worst is malaria, which kills more than 600,000 people every year; another 200 million cases incapacitate people for days at a time. It threatens half of the world’s population and causes billions of dollars in lost productivity annually. Other mosquito-borne diseases include dengue fever, yellow fever, and encephalitis.”   — this quote and this essay’s title come from the blog of Bill Gates:

A recent biology study asks the interesting question:  “Why do  some mosquitoes prefer humans?”  The new study is titled “Climate and Urbanization Drive Mosquito Preference for Humans”.

Let’s hit the new study’s high point right from the start:

Despite the use of the word “climate” in the paper’s title, Climate Change does not drive mosquito preference for humans.  Climate, however,  has  played a role in the evolutionary selection for the human-preferring sub-species of Aedes aegypti – which is the main insect vector that spreads Zika, yellow fever and dengue. (This is not the same mosquito that spreads malaria, that is the Anopheles mosquito.)

The NY Times coverage of study states : “The Current Biology paper focused on evolutionary history, but its findings might have implications for public health. The results, combined with climate and population data from the United Nations, suggest that there will be more human-biting mosquitoes in sub-Saharan Africa by 2050, caused mostly by urbanization.”  When it says “there will be more”, what is really meant here is that the Aedes mosquito population will shift towards the human-preferring sub-species.  The actual number of mosquitoes will depend on the effectiveness of (or lack of) mosquito control efforts of each urbanized area.

The reason they expect the shift is that the study found that the “more human-loving mosquitoes tended to come from areas with a dry climate and dense human population.” [NY Times]. In the areas of sub-Saharan Africa studied, the shift of rural populations to densely urbanized areas over the next few decades is projected to continue.

The hypothesis on how this came to be is based on the fact that mosquitoes need  small pools of still water for reproduction.  In drier climates, in which the rainy season is short, mosquito reproduction depends on human-supplied pools of water for much of the year – like flower pots, old tins cans, abandoned automobile tires and household water barrels.  Urbanized areas thus supply not only the blood-meal needed by female Aedes for reproduction, by providing plenty of humans to feed on,  but the small still  pools of water needed for egg-laying as well.  This two-barreled advantage, they believe, has favored the human-preferring genes (which they consider a sub-species) especially in these densely populated urban areas.

One of the authors of the study, Dr. Carolyn S. McBride, in this quote from the NY Times article, sounds disappointed that they were unable to blame Climate Change:

“I think it’s counterintuitive, because people know the climate is changing rapidly, so that should be the driving force,” Dr. McBride said. “But the features of the climate that we found to be important for this mosquito aren’t predicted to change in strong and clear ways that would affect the mosquito.”

Urbanization, in contrast, is occurring very quickly. “You could easily imagine that having an effect on disease transmission in big cities,” Dr. McBride said.

From the paper’s Summary:  “Our findings suggest that human-biting in this important disease vector originally evolved as a by-product of breeding in human-stored water in areas where doing so provided the only means to survive the long, hot dry season. Our model also predicts that the rapid urbanization currently taking place in Africa will drive further mosquito evolution, causing a shift toward human-biting in many large cities by 2050.”

This study is really about the evolution of the Aedes mosquito and tracking the gene flow of the specific genes they have identified as being associated with the sub-species that seems to prefer biting humans (as opposed to other red blooded animals).  The author team bravely tried to come up with a finding that would blame bad future outcomes on Climate Change (see the methods section of the study’s “Extended PDF”) but it was just no good.  Model predictions of climate variables just didn’t change the finding:  the shift had to be laid at the feet of human urbanization.

As with all modern news about mosquitoes, it is necessary to clear up common misunderstandings.

1.    “….Aedes aegypti mosquitoes, which are responsible for Zika, yellow fever and dengue.”  And, from the Bill Gates’ blog at the beginning of the essay: “they carry devastating diseases.”

Mosquitoes are not responsible for any of those diseases.  Mosquitoes simply spread the disease from one infected host (usually human) to another host – mosquitoes pick up the disease from one infected human source and carry it around a while, then drop it off in another human.    The diseases do not originate in the mosquitoes.

“…the infected mosquito carries the disease from one human to another (acting as a “vector”), while infected humans transmit the parasite to the mosquito, In contrast to the human host, the mosquito vector does not suffer from the presence of the parasites.”  CDC

and

“people serve as the primary vertebrate hosts for Ae. aegypti and Ae. albopictus mosquitoes spreading chikungunya, dengue, yellow fever, or Zika virus.”  CDC  [my bold – kh]

The presence of mosquitoes does not mean the presence of mosquito-vectored diseases.  For instance, Aedes aegypti [left panel] are found through much of the southern parts of the United States, yet we are almost entirely free of Zika, yellow fever, and dengue.

Today, dengue is only reported to be locally acquired in the very southern tip of Florida, where many residents come and go from the Caribbean Islands. So far this year, there has been one (1) locally transmitted case of dengue. [ source ]

The same is true for the malaria-vector mosquito, Anopheles:

As can be seen, malaria was ubiquitous throughout the Eastern United States in 1882, except for in the eastern mountain ranges. By 1932, it has been beaten back to a few hold-out areas, but broke out in 1934-1935.

After a long campaign against malaria, the CDC currently reports:  “Now approximately 1,500 malaria cases and five deaths are reported in the United States annually, mostly in returned travelers.”

2.    To nearly eliminate human cases of mosquito-vectored diseases is fairly simple in a country like the United States, and almost impossible in less-developed countries.

In the United States, humans sick with such diseases are taken to hospitals, where they cannot be bitten by mosquitoes and therefore cannot further transmit the disease. Neighborhoods where the disease showed up are heavily treated to knock out the existing generation of mosquitoes that might have been responsible for the transmission of known cases and the neighborhood is searched diligently for further cases.   Then, with no (or very few) sick humans and few mosquitoes, there is no further transmission.

In the developing world, where health care systems have fewer resources and the people have less access to that system, sick people only end up in the hospital when they are already very sick (if then) and have, in all probability, already infected many local mosquitoes, who are busy infecting other humans.  In this case, the procedure is for local officials to stage a wide area campaign of mosquito control by spraying, distributing treated mosquito nets, running a Anti-Mosquito Breeding Sites campaign, and bringing in nurses and doctors to find,  quarantine and treat the sick.

A typical campaign poster (this one in the Caribbean) to eliminate breeding sites:

These campaigns are not limited to the Third World – see this  Fight The Bite  game from Miami/Dade County, Florida.

In my personal experience in humanitarian work in the Caribbean, the local officials almost never have the equipment or chemicals necessary for wide-spread mosquito control and have only limited, already-stretched-to-the-limit medical resources.  It is heart-breaking.

The long fight against mosquito-vectored diseases has involved DDT – which itself is a very controversial issue – but is not the primary focal point of the fight.  Many local mosquito populations have developed varying degrees of resistance to DDT.  It is, however, still effective when used to treat indoor walls and bed-nets.

[And NO – “bringing DDT back” into wide use in Africa will not be an (or the)  instant silver bullet solution to mosquito-vectored diseases. That is a myth.  DDT is already widely used in Africa. ]

Then there are:

Permethrins  Treat clothing and gear

• • Use permethrin to treat clothing and gear (such as boots, pants, socks, and tents) or buy permethrin-treated clothing and gear.
    • Permethrin is an insecticide that kills or repels mosquitoes.
    • Permethrin-treated clothing provides protection after multiple washings.

For years, while living on our sailboat in the Caribbean, we regularly treated our hatch screens and cabin surfaces with permethrin – and had great success with it.

In the United States, synthetic pyrethroids are used in aerial spraying to control adult mosquitoes along with malathion and naled.

Mosquito-vectored diseases are a major, world-wide health problem and the use of insecticides in their control remains a hugely controversial topic at all levels of government and a matter of much concern from health and environmental groups.   The controversies swirling around the issue are highly politicized.

One thing that is certain:  The propaganda meme that Climate Change will spread mosquito-vectored diseases is categorically false and based on gross, seemingly intentional,  misunderstanding of the mechanisms involved in disease spread.

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Author’s Comment:

I was reluctant to bring up DDT – in the past it has overwhelmed the comment section.  I suppose it will again today, so I had my say on the topic.  I will not be responding to the controversy in the comments here.  After all, there have been whole books written on the topic, on both sides, and yet the controversy remains – it will not be resolved here.

On a personal note, I am allergic to mosquito bites.  When I get them they swell up, they itch sometimes for weeks and if I get very many, I get body-wide allergy symptoms and have to rely on medication. Living in the Caribbean for so long was challenging.  I nearly drove my wife crazy with my repeated rants and mania about mosquitoes.  Suffice it to say: “I don’t like mosquitoes.”

All that said, neither my wife or I ever contracted any of the many nasty mosquito-vectored diseases endemic to the Northern Caribbean.

The featured study presents an interesting finding on why some mosquitoes like humans better than birds or dogs or goats.

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