Climate change will spread malaria and dengue fever
Mosquitoes that transmit the diseases will spread as regions warm and grow more humid.
In my day job, working two days a week as a horticulturist for a very small zoo, there are few surprises. I mostly plant plants in the ground, trim them up from time to time, and get rid of as many weeds as I can.
Yet one day in my greenhouse not long ago, it was about midday when I saw a blur above one of the plants. Then there was another. I bent down to take a closer look. They were small and hard to see, but after a second or two, I figured out what they were. Their tall, spindly legs and backs, arched against the sky, gave them away. Hovering above and alighting on the flowers were hundreds of mosquitoes.
My first impulse was to swat and smash as many as I could. Then I thought that I should leave them alone. After all, these are the males, the ones that couldn’t bite you no matter how hard they tried. Females can’t be too far away, but I remained unbitten, thanks to long sleeves, long-legged pants, a hat, DEET, and a little bit of luck.
As a child growing up in West Virginia, mosquitoes were a summertime nuisance. For weeks I scratched their itchy bites on my arms and legs. Poison ivy was avoidable, but mosquito bites were a constant. When I first moved to southern California, one of the pleasures of living here was finding places, more arid areas, where mosquitoes were relatively absent.
We are, however, getting more mosquitoes in southern California. In recent years three species of mosquitoes have invaded, including mosquitoes of the Aedes genus, which can transmit Zika, dengue fever, yellow fever, and other viruses.
We should have seen this coming
A recent review of scientific studies published in the journal Globalization and Health looked into climate change and how our warming world will spread malaria and dengue fever. Studies have taken several methods. In some, researchers took laboratory measurements of how disease vector species—namely, mosquitoes—respond to temperature and environmental changes. Other scientists have analyzed climate change projections. All are trying to to predict where different species of mosquitos will migrate to in the future.
Not all mosquito species can transmit malaria and dengue. For the species that do, the diseases are transmitted when female mosquitos bite infected individuals and then go on to bite uninfected individuals.
Malaria is caused by any one of five parasitic microorganism species that are transmitted by female Anopheles mosquitos. Symptoms of the disease resemble the flu, with chills and fever. Left untreated, folks can die of malaria. In 2020 there were an estimated 241 million cases of malaria worldwide, with an estimated 627,000 people dying. Most of the victims were children in sub-Saharan Africa. Every year, about 2000 people in the U.S. are diagnosed with malaria. Most of these folks are people returning from overseas where malaria is common.
Dengue can be severe or mild and is often confused with other diseases. Symptoms can be nausea and vomiting, aches and pains (notably pain behind the eyes), fever, rash, and pains in joints, muscles, and bones. Severe dengue can cause bleeding from the nose and gums or vomiting blood or passing bloody stools. Symptoms usually last from two days to a week. There is no treatment for dengue, only treatment for symptoms, and most people recover. Dengue is caused by a virus, and a dengue vaccine has been developed.
Climate change will spread the diseases
The thing to remember about climate change is that it’s complicated. Some places may even wind up cooler than they are now. And for mosquitoes, whose larvae live in stagnant pools, they need warm weather and water. Places that get warmer but become drier may not be areas that disease bearing mosquitoes move into. Also, the authors caution that higher temperatures and more rain do not necessarily translate into more disease. Mosquito larvae might get washed out of standing water in places that get a lot of rain. Yet the IPCC has already noted that climate change has geographically altered the risk of vector-borne diseases.1
Many regions are predicted to get warmer and more humid, perfect mosquito climate, with shorter winters and longer summers, two things that are also loved by mosquitoes. Most of the studies predict the incidence of malaria and dengue will spread northward in Europe, North America, and Asia. Within 30 years, the Anopheles mosquitoes are expected to bring malaria to Europe for six months of the year. Southern Great Britain may be climactically suitable for malaria transmission for two months out of the year by 2030, only eight years from now. By 2050 malaria is expected to reach Scotland. Dengue is also expected in England.
Malaria will also extend into the mountains of Latin America, sub-Saharan Africa, the Eastern Mediterranean, and Western Pacific. Mosquitoes grow more active in warmer weather; so as the temperatures increase, the rate of transmission will increase as well.
It’s important to note that malaria was once common in the United States. It’s eradication was part of FDR’s New Deal. Except for travelers returning from overseas, the disease was eliminated by 1951. The predicted spread of the mosquito-borne diseases will also depend on governments’ efforts to combat the diseases.
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IPCC. Climate change 2014: Synthesis Report, Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland; 2014. 151 pp
Well said. And, mosquitoes also carry pathogens that are harmful to other wildlife. Avian malaria in Hawai`i, for example, is disastrous for the ecosystems of that archipelago.