Climate change and infections


It was shirt sleeve weather once again this week – 68 ° F at Christmas – here in western Maryland at the foot of the Appalachians. Where we spend the summer in Maine, it is now increasingly hot and humid. My concern about climate change led me to the Identification Week session on Climate Change and Infections.

Stanford biologist Erin Mordecai has shown how sensitive mosquitoes are to small changes in temperature. This affects their ability to reproduce, and optimal temperatures vary for different species.

For example, the transmission of malaria by Anopheles gambiae peaks at 25 ° C; for dengue, transmitted by Aedes aegypti, it is 29 ° C. Mordecai showed how mosquitoes exhibit “non-linear or bump-like responses to temperature, in which the rate of development or the rate of survival or the rate of biting or other aspects of the vector and the parasite increases. from a minimum temperature to an optimum temperature, then often decrease very sharply.

For this reason, in Africa malaria will move east and to higher altitudes. Now more localized in West Africa, dengue will spread in a wide band across sub-Saharan Africa by 2050. The same will be true for other arboviruses transmitted by A. aegypti, like chikungunya.

Research has already shown such changes in the disease in Kenya, where malaria peaked in children when the temperature was 25 ° C. Non-malaria fever from chikungunya and dengue increased with temperature. A similar change from malaria to the increase in dengue, Zika and chikungunya, has been shown in South America.


I recently read a lot about Brazilian Jair Bolsonaro and how his anti-science policies fueled Covid-19. They have also led to charges of committing crimes against humanity (which I have spoken about here). Mordecai’s talk about how Bolsonaro’s policies also fueled malaria also intrigued me.

Mordecai explained that deforestation increases forest edge habitat, preferred by Anopheles Darlingi. She and her colleagues discovered a two-way pattern of malaria. Initially, there were 6.4 more cases of malaria per square kilometer of forest cleared. Malaria subsequently lowers the rate of deforestation because people become too sick to work. Under Bolsonaro, she said, there were 62,000 more cases of malaria associated with a 30% increase in deforestation.

Small-scale gold mining also results in more areas on the edge of the forest. For every kilogram of gold mined, Mordecai’s group found 59 P. vivax malaria case. They estimate that 24% of P. vivax malaria is attributable to mining. Oddly enough, the additional cases of malaria occurred up to 250 km away. This was apparently because the miners spread the parasite as they returned home in squalid conditions conducive to the spread of mosquitoes and malaria.

It’s more than mosquitoes we need to worry about, of course. Rachel Harold reviewed some of the more commonplace infections, such as gastroenteritis, one of the leading killers in the world, with 1.7 million deaths per year. For every degree of temperature rise, there was an 8% increase in diarrhea. We are also seeing a more prolonged infectious season for some organisms, such as Vibrio, which infects seafood.

Harold described climate change as a “threat multiplier”. For example, people are displaced by climatic disasters. Then, epidemics of cholera, Shigella and other sanitation-related diseases hit the refugee camps. His speech introduced me to the Medical Society’s Climate and Health Consortium, which has a variety of resources.

One of the other dominant messages from various discussions was that we are playing beating a mole, struggling to find vaccines and new treatments for a variety of emerging pathogens. Instead, as Mordecai said in a previous interview, “If you have an ecological change that results in 96 million cases of dengue fever per year, you should target that causal relationship even if you are trying to find a vaccine against it. dengue fever and get it to as many people as possible. possible.”

Many disastrous epidemics or pandemics are due to our disruption of the environment and our encroachment on wildlife – Ebola, Nipah, Zika, HIV, and possibly COVID-19. Ecohealth Alliance attributes around a third of emerging infectious diseases to land use change. Katarina Zimmer also provides an excellent overview of the topic in National Geographic.


One thing that concerns me now is the fungal infections that we are likely to see from the tornado surge in the South and Midwest and the dust storms that moved to Colorado and then to the Midwest.

Coccidiomycosis, the fungus that causes “valley fever” is fairly well known in California and Arizona. It causes around 15,000 cases per year. It is likely that many people, including doctors, are unaware that the range of Cocci extends further to the Midwest. A variety of other fungal infections have followed natural disasters.

There have been four cases of mucormycosis (fungal infections caused by Mucor) in Arkansas this year, which is quite unusual. There has also been a cluster of cases in Honduras. By far the greatest number of cases of mucositis is found in India. While most doctors dismiss this as due to diabetes, steroid use and Covid, India has had a disproportionate number of cases before Covid. Dr Jessy Skaria had an unconventional hypothesis as to why mucor is so important in India, linked to the reverence for cows by many. Mucor is common in herbivorous droppings, which are often burned in India, both as fuel and in purification rituals. In her latest article, Skaria asks scientists to study her hypothesis by “measuring the environmental loads of Mucorales spores before, during and after a festive cow dung fire”. She also suggests that researchers compare samples from different parts of India, including those that do not use such lights. It seems like an easy study to do. I also hope that the Indian Medical Association (IMA), Indian Council of Medical Research (ICMR) and mycologists undertake such a study. If the hypothesis is proven, they will have a powerful new tool to fight the Mucor epidemic. If they can’t find aerosolized Mucor spores, that’s strong evidence against that hypothesis. You won’t know without study.

In the meantime, I hope the CDC and state health departments do similar studies here, looking for fungal spores after storms. They also need to better educate doctors and communities about the risk of fungal infections. Most people are not used to this.

And we all need to step up our work on climate change, or we’ll have more to worry about than Covid.

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