Infections, Vectors and the Microbiome
Insect Vectors
There is ample evidence that climate change can alter the distribution of insects that carry arthropod (insect) borne infections through alterations in temperature, rainfall and habitat.
A 2022 paper estimated that almost three of five pathogen-driven diseases are likely to be altered by climate factors. Most will extend their range, but they also found distributions of some pathogen-driven diseases were improved.
Why might climate change sometimes improve the situation? The weather may be too hot or dry for some pathogens. It may be the pathogen itself that can’t survive, or the vector, the organism that spreads the pathogen, that is impacted. Mosquitoes need standing water; droughts could be good for preventing malaria, if you are careful to look out for small remaining collections of standing water, since droughts decrease the amount of fast running water as well, so there can be areas where slow moving or standing water increases!
Sometimes the range of the vector and pathogen simply shifts, endangering some people while helping others.
When the range shifts, even if it decreases the incidence of infection in one area, there may be a net worsening burden of disease: the newly exposed populations may be larger, live in higher density, may not have acquired biological immunity, or they may have less resources and experience with the disease, so may not be as prepared to deal with the infection as populations that have adapted to it.
An Aedes mosquito having lunch (of blood) on you.
Viral infections
The most common infectious disease vectors globally are mosquitoes, in particular mosquitos of the genus Aedes. These small mosquitos don’t need much water at all, so they are relatively drought resistant.
The pathogens these mosquitos spread are viruses, called arboviruses, that is, viruses spread by arthropods (primarily insects). These are infections such as dengue, Zika, chikungungya, and yellow fever. They may sound exotic to us, but in many parts of the world they wreak havoc.
The Intergovernmental Panel on Climate Change (IPCC) AR6 (2022) is particularly concerned about dengue spreading due to climate change. Billions of more people in sub-Saharan Africa, Europe, Central and South America and Asia will be at increased risk.
The Aedes mosquito is in the United States. Zika was found in the United States in 2016-2017. Twelve cases of chikungungya were thought to be locally acquired in Florida and Texas in 2014. Similarly some small locally acquired outbreaks of dengue were reported in Hawaii, Florida and Texas in 2013 and 2020.
Aedes species invaded southern California from China in 2001 and are moving north. So far none carrying dengue, Zika, chikungungya, and yellow fever have been reported… stay tuned.
Another mosquito-borne viral infection is West Nile virus. Infection often results in central nervous system involvement, which can be fatal. West Nile virus is found throughout the contiguous 48 states of the United States, most commonly mid-summer into the fall. There are concerns it is spreading due to climate change, though in the United States the annual incidence has varied enough that patterns aren’t clear.
Parasitic infections
Another mosquito vector is the Anopheles mosquito, responsible for spreading malaria. Many of us in the United States fail to appreciate that more than 200 million people suffer from malaria, at great economic and personal cost in areas and people who can ill afford it, resulting in 435,000 deaths/year (in 2017). Malaria used to be found in the United States, and controlling it (like trachoma mentioned below) was a great victory. There is concern it may return to the United States and other areas where it has not been seen in centuries.
Bacterial infections
It isn’t just viral infections and mosquito vectors that can be altered by climate change. Lyme disease, caused by the bacterium Borrelia burgdorferi, is transmitted to humans by a tick that is changing its distribution west and north in the United States. As with many of these infections, it is hard to tease out how much to attribute to climate change directly, but it likely plays a large role.
It is thought that the distribution of an insect vector of trachoma, an infection that affects the eyelids and that can lead to corneal scarring and blindness, in some areas of Africa particularly (in the past it was not rare in some areas of the United States), may be altered by climate change.
Infections that are not associated with insect vectors.
Fungal infections
Valley fever (coccidioidomycosis), caused by the soil fungus Coccidioides, is found in the southwestern United States, particularly the central California San Joaquin Valley, but also in Arizona, New Mexico, Nevada, Texas and Utah, as well as parts of Mexico and South America. It causes a potentially fatal systemic disease and pneumonia. Being associated with hot, dry climates, it is expected to spread with climate change and there is evidence that is happening. In 2013 it was first reported in Washington State. A 2019 article by epidemiologists predicted the further spread of Valley fever.
Farm workers are at particular risk, since anything that disturbs the soil releases the fungus. However, it is said you can be exposed just driving through the central valley of California!
Coccidioidomycosis can cause a devastating infection in the eye. They are very hard to treat, even with very aggressive medical and surgical approaches. Blindness, or even loss of the eye, is a common outcome.
Bacterial infections not associated with insect vectors
A documented example is Vibrio vulnificus, that can result in a “flesh eating” infection and sepsis resulting in death; it thrives in warm salt water and infections have been more common after hurricanes, for example in the Fall of 2022 in Florida with Hurricane Ian. Hot sea waters in the summer of 2023 have also led to new concerns about infections caused by Vibrio vulnificus in warmer coastal waters, especially in the United States on the East Coast and Gulf of Mexico. V. vulnificus lives in salt or brackish water. The range of this organism has been expanding its range northward at a rate of 48 km (29 miles)/year, as the waters warm up and have been found as far north as Connecticut and New York. Infections increased 18-fold from 1988-2018. V. vulnificus primarily causes wound infections that can cause necrosis (the tissues die and break down). It is far from the most common Vibrio infection, with only about 200 infections reported per year, but besides horrible skin infections about one in five infected people die from the infection. Infections are more dangerous in people with liver disease, diabetes, and poor immune system function.
The CDC has suggested therapies for clinicians to use and also gives recommendations for the public for 2023.
Amoebic infections
The amoeba Naegleria fowleri, dubbed “brain eating” because it infects the brain, which is usually fatal. The organism lives in warm fresh water and soil. People are exposed by swimming in the water where the organism lives. The amoeba enters through the nose. Infections have been spreading northward in recent years from southern to the midwestern United States. It may be shifting its territory not only because it thrives in warmer water, but because disrupted soil leeches iron, a mineral that the organism needs.
Global Climate Change and Human Health, from science to practice. Jay Lemery, Kim Knowlton, Cecilia Sorensen. Jossey-Bass (Wiley), 2021. Chapter 8 has a good summary.
See also: Climate Change, Fires, Floods, and Infectious Disease. The Lancet Microbe. Editorial. | VOLUME 2, ISSUE 9, E415, September 01, 2021.
Antibiotics
Antibiotic resistance, a major health issue, is not just due to the medical overuse of antibiotics in humans. The overuse of antibiotics in industrial meat production also generates resistant bacteria. Antibiotics are also a form of water and land pollution. Antibiotics get into the water supply and the ground because they are found in urine (the livestock’s urine, and ours!). Sewer water is full of our medications, including antibiotics, and full of bacteria as well, a recipe for inducing antibiotic resistance.
The Microbiome
The microbiome is our body’s natural ecology with microbes. We have more bacterial cells on our skin and mucosa (especially the gut) than we have human cells in our entire body. There is vast evidence that the microbiome influences human health, from the propensity to obesity to the immune response. There are complex ways climate change may affect our microbiome. How this will alter human health is not clear, but it is something to watch out for.
Global climate change, diet, and the complex relationship between human host and microbiome. Towards an integrated picture. Catania F et al. Bioessays 2021 Jun;43(6):e2100049.doi: 10.1002/bies.202100049. Epub 2021 Apr 8.)
There is also the “atmospheric microbiome” which may be changing.
Climate change impact of fungi in the atmospheric microbiome. Hanson MC et alTotal Environ 2022 Jul 15;830:154491.doi: 10.1016/j.scitotenv.2022.154491.Epub 2022 Mar 10.)
The microbiome of ocean organisms also may be affected. A couple of examples:
Future climate change is predicted to affect the microbiome and condition of habitat-forming kelp. Z. Qiu, M. A. Coleman, E. Provost, A. H. Campbell, B. P. Kelaher, S. J. Dalton, et al. Proc Biol Sci 2019 Vol. 286 Issue 1896 Pages 20181887
Cross-generational effects of climate change on the microbiome of a photosynthetic sponge. H. M. Luter, M. Andersen, E. Versteegen, P. Laffy, S. Uthicke, J. J. Bell, et al.Environ Microbiol 2020 Vol. 22 Issue 11 Pages 4732-4744 Accession Number: 32869905 DOI: 10.1111/1462-2920.15222
We don’t understand yet what this might do to the health and vitality of these and myriads of other organisms, and through them, ecosystems.
We are dealing with complex systems made up of complex systems. Along with our overuse of antibiotics and other practices, anti-vaccine sentiment in some quarters, as we learned from the Covid pandemic, infectious diseases are here to stay and are a looming threat. Environmental burdens will continue to add to the difficulties.
Other ways soil degradation and climate change can make matters worse
In addition, extreme weather events, such as heavy rains and floods, can be a set-up for infections. Think of sewers failing and backing up, rats clambering around, dead animals, and people huddled together in shelters with insufficient bathing and bathroom facilities, a set-up for infections whether or not insect vectors are involved.
Similarly, if it is hot and clean water is not plentiful, especially if power is not consistent or refrigeration is not always available, it is a set-up for the spread of food-borne and other diarrheal diseases, a major killer in the developing world. This is common after climate disasters.
Droughts and loss of soil lead to displacement of populations, leading to refugees and similar conditions that can breed the spread of infections.