Global Climate Change: Driver of Infectious Diseases?

By Roberta Attanasio, IEAM Blog Editor

Sec Sebelius - WHO assembly 2012

US Health and Human Services Secretary Kathleen Sebelius addresses the 65th World Health Assembly (2012). Credit: United States Mission Geneva, CC BY-ND 2.0

The impacts of global climate change on human health and well-being are undeniably alarming. Safe drinking water, sufficient food, and secure shelter are threatened by rising sea levels and severe weather events. Heat waves dramatically increase death rates not only from heat strokes, but also from complications arising from cardiovascular, respiratory, and cerebrovascular diseases. According to the WHO, climate change is expected to cause approximately 250,000 additional deaths per year between 2030 and 2050, mostly from malnutrition, malaria, diarrhea, and heat stress. Unfortunately, these estimates take into account only a subset of the possible health impacts and assume continued economic growth and health progress. The global situation is likely to be much worse.

In addition to malaria and diarrhea, climate change is expected to have a substantial effect on the burden of many other infectious diseases. However, such expectations have generated considerable debate in the past few years—in many instances, factors such as socioeconomic drivers and disease control measures can confound the detection of effects mediated by climate change. Sonia Altizer, Professor of Ecology at the University of Georgia, said: “For a lot of human diseases, responses to climate change depend on the wealth of nations, healthcare infrastructure and the ability to take mitigating measures against disease. The climate signal, in many cases, is hard to tease apart from other factors like vector control and vaccine and drug availability.”

Indeed, while the effects of climate change on some infectious diseases, such as malaria, seem to be established, causal links are harder to assess for others. The report Under the Weather: Climate, Ecosystems, and Infectious Disease points out: “Since the dawn of medical science, people have recognized connections between a change in the weather and the appearance of epidemic disease. Roman aristocrats retreated to hill resorts each summer to avoid malaria.”

Mosquitos, particularly the Anopheles species, which transmits malaria, require temperatures above 16°C to complete their life cycles. Process-based (mathematical) models show that temperature increases of 2-3ºC would increase the number of people at risk of contracting malaria by 3-5%—a major effect, considering that in 2013 an estimated 198 million cases of malaria occurred worldwide, and 500,000 people died. In addition, according to the models, the seasonal duration of malaria would increase in many endemic areas. Thus, rising temperatures could elevate malaria as a great public health concern.

However, a few years ago, Kevin Lafferty, a USGS research ecologist, wrote: “Although the globe is significantly warmer than it was a century ago, there is little evidence that climate change has already favored infectious diseases. While initial projections suggested dramatic future increases in the geographic range of infectious diseases, recent models predict range shifts in disease distributions, with little net increase in area. Many factors can affect infectious disease, and some may overshadow the effects of climate.”

Due to the current uncertainty of the climate change-infectious disease relationship, it is not surprising that a new paper published in the journal Philosophical Transactions of the Royal Society B (Evolution in action: climate change, biodiversity dynamics and emerging infectious disease”) is drawing extensive attention. Daniel Brooks, one of the two authors, said that the appearance of infectious diseases in new places and new hosts, such as West Nile virus and Ebola, is a predictable result of climate change. He warns that humans can expect more such illnesses to emerge in the future, as climate change shifts habitats and brings wildlife, crops, livestock, and humans into contact with pathogens to which they are susceptible but have never been exposed to before.

Indeed, the paper brings up a novel paradigm. According to Brooks and his co-author Eric Hoberg, scientists have long assumed that infectious pathogens don’t quickly jump from one species to another—as they adapt to a specific host species, they become less effective at infecting other species. Brooks calls this phenomenon the “parasite paradox.” Over time, hosts and pathogens become more tightly adapted to one another.

However, Brooks and Hoberg found that such jumps happen more quickly than anticipated. Even pathogens that are highly adapted to one host are able to shift to new ones under the right circumstances. Climate change represents one of these circumstances. For example, as temperatures increase in some regions, many host species move into new habitats along with their pathogens, thus increasing the risk that their pathogens will encounter and infect native hosts. Brooks said that the new hosts are more susceptible to infection and get sicker from it, because they haven’t yet developed resistance.

“It’s not that there’s going to be one ‘Andromeda Strain’ that will wipe everybody out on the planet,” Brooks said, “There are going to be a lot of localized outbreaks putting pressure on medical and veterinary health systems. It will be the death of a thousand cuts.”

Malaria family

Anti-malaria infrastructure (bed netting and window screens), Ifakara, Tanzania, 2007. Credit: Gates Foundation, CC BY-NC-ND 2.0

It is not difficult to imagine who will be mostly affected by these localized outbreaks: children, the elderly, the sick, and the poor, especially those in regions with weak health infrastructures and in developing countries. What are possible solutions? Maria Neira, WHO Director, Department of Public Health, Environmental and Social Determinants of Health, recommends general actions and others specific for the control of infectious diseases: “Surveillance systems for climate-sensitive infectious diseases like malaria, dengue, and cholera should be fortified. Countries should make better use of early-warning information to predict the onset, intensity and duration of epidemics. Such predictions allow health officials to pre-position medicines and vaccines, which can reduce the death toll.”

However, forecasting appropriate responses poses a significant challenge. It will require the combined efforts of, among others, evolutionary biologists, biomedical scientists, epidemiologists, geoscientists, and social scientists—such combined efforts could eventually lead to the design of multi-faceted, effective approaches to mitigate the effects of climate change on the spread of infectious diseases.

16 thoughts on “Global Climate Change: Driver of Infectious Diseases?

  1. WeaverGrace

    “humans can expect more such illnesses to emerge in the future, as climate change shifts habitats and brings wildlife, crops, livestock, and humans into contact with pathogens to which they are susceptible but have never been exposed to before.” This reminds me of colonization introducing new life forms.

    I continued, “many host species move into new habitats along with their pathogens.” I hadn’t considered before that there might be a power that drives us to migrate for one reason of another.

    I am urging people to consider ways to reverse climate change, as a result of a short poem that I posted on my blog a few days ago. I hope you will join the conversation.

    1. twynn

      This article mentions a lot about climate change and how it is predicted to increase the spread of infectious diseases. After reading this article, I began to think about ways to help stop (or slow down) climate change. However, the situation will be a difficult one to control. It is sad to say, but there is no way that enough people will take the necessary actions to control climate change or any other factor that contributes to the spread of infectious diseases. With that being said, it is a fact that infectious diseases will continue to increase globally. But, I do not believe that climate change is one of the major causes of the increase spread of infectious diseases. I read an interesting article on the CDC website that said one of the main causes of emerging infectious diseases is the movement of a population from rural areas and travel. Overcrowding cities and human behavior would cause an infectious disease to spread more quickly. For example, in the case of HIV, it was believed that a man (living in rural Liberia) contracted the virus from a monkey (an animal hunted for food) because their strains closely resembled. Now, because of travel and movement, HIV is a global pandemic. Other diseases such as bubonic plague, smallpox and yellow fever were all once localized. These diseases spread through travel and trade. I do agree that climate change will have an affect on the emerging spread of infectious disease. It’s just a newly found component that was added to the list of contributors to infectious diseases.

      1. Gabriel Beltran

        On a micro-scale, reasoning for the survival of species is the first thought that came to mind upon reading the blog post. Whether it is a cognitive relay or unconscious drive, individuals tend to relocate to other parts of the world they deem favorable. On a global scale, it is undeniable that climate change has diverse effects in the environment, which can then manifest potential negative health consequences in humans. As a result, I would deem global climate change to be an indirect culprit for driving infectious diseases since the correlation would undeniably alter other variable parameters such as heat, water, or the infectious agents themselves. Therefore, the drawbacks are interconnected which make it difficult to assess the authenticity of cause and effect. Per the article, the outcomes are in and of themselves expectations and assumptions made on a positive linear growth, the economic and health. As a result, one can only take the account of infectious diseases and climate change as quantitative observations to be speculated with a critical eye.
        Nonetheless, I digress and regard the idea that reverting negative effects of global climate change as a means of altering human lifestyle to be a proactive movement. Although opposition may claim too much damage has been done to revert changes, I believe it is wise to never lose hope. The repercussions of climate change should not be denied despite correlative evidence or lack thereof, the important matter is that infectious diseases are increasing and should be directed. Although we are wired with an immune system that manages to combat many illnesses, children and the elderly are susceptible to infection especially in developing countries where sanitation and food supplies deplete defenses. Therefore, no doubt a favorable environment suitable for the Anopheles species in growing temperatures would be detrimental when regarding malaria. In addition, other agents like cholera or dengue as referenced would cripple countries already suffering from these diseases even more. Although vaccination methods are a possibility, the point that was brought up for diminishing impact of climate change by humans should not be disregarded. In essence, the habitable conditions that promote the infectious diseases would be decreased in turn from improved environmental conditions. This leaves a multiple frontier of defense to individuals; the immune system, thwarting habitable disease agents, and vaccination availability. Although this would be the ideal, it is without a doubt a progressive matter that considers continued efforts from everyone.

  2. Peter M.

    Many tropical diseases will proliferate in a much warmer world. These tropical diseases include Leprosy, Malaria, Tuberculosis, Sexually transmitted infections, African trypanosomiasis, and Chagas disease. The latter two diseases, African trypanosomiasis and Chagas disease are caused by trypanosomatids, which I study in my lab. These parasitic protists are extremely difficult to treat because of their eukaryotic cell structure. Like mosquitoes, trypanosomes enter a dormant state (amastigote) at lower temperatures, thus preventing the parasite from infecting a human. However, rising temperatures could cause trypanosomes to enter different stages of its life cycle which increase its virulence. Promastigote and epimastigote stages, the active forms of the parasite, is marked by the formation of the flagellum, allowing the parasite to become motile. When in the amastigote stage, the parasite can survive dormant for a prolonged period in otherwise unfavorable conditions. An increase in temperatures could awaken a tremendous amount of these amastigotes simultaneously, subsequently causing a sudden spike in African trypanosomiasis and Chagas disease. Both of these diseases cause death if left untreated, and given the lack of resources in areas affected by trypanosomatid diseases, and the predictable increase in other tropical diseases, it would be unlikely that most people will find immediate care for these ailments. I believe that it is important to slow down, or possibly begin developing methods to reverse global warming which will not only conserve the environment but also be friendly towards the economy.

  3. Mercy Adeboye

    The effects of global warming on the economy of different countries cannot be over emphasized. One of the effects of global warming is stress which is connected with other factors that all work together to make global warming a great concern. Stress on the economy, on the people and on the society at large is a result of global warming. Countries that depend mainly on agricultural produce will experience this effect in more ways than one. They will have low produce which results in low harvest. This means that whatever they harvest will not be enough to be sold to gain income for the people and so they feed on what is available. What happens when it is all gone? Starvation sets in. The body starts to use all the stored energy like converting the glycogen to glucose and use all the fat stored in adipose tissues. There will then be loss of weight and they eventually become immunosuppressed due to decrease in antibody production, decrease in circulating lymphocytes, inhibition of cytokines released and their level of receptors available. This is because the hormone glucocorticoid is released which suppresses the immune system. Also, the immune system cannot protect them as much because energy is required for the body to carry out immune functions. People start getting sick and eventually die from starvation and sickness. This reduces the population and the weak ones cannot work. The society gets worse, there is economic setback. Therefore to improve the society and relieve the stress on the people global warming should be addressed. This all has to do with everyone working together to reduce the pollution in the environment and taking up habits like recycling.

  4. M.

    It is an undeniable fact that the effects of Global warming on public health and the constant increase of infectious diseases worldwide can be attributed to human lifestyles. Regardless of proper health infrastructure, “economic growth” and how economically developed countries are the aftermath is incomparable to the global disaster we as humans have already created for ourselves.

    In support of my above argument, Ian Pepper et al., author of Environmental Microbiology purports, “Life on earth depends on biochemical cycles that are microbially driven.” For example plants aid in the removal of carbon dioxide from the atmosphere through the process of photosynthesis. This suggests that human life depends heavily on the ‘microbial world.’ According to Pepper, soil microbes have a great impact on global warming since microbes are a source of “greenhouse gases” such as carbon dioxide, methane, and nitrous oxide. However, Pepper states that this correlation of microbes and global warming is still an ongoing debate.

    The ozone layer is what protects life on earth and all its biodiversity. It absorbs the radiation that has the potential of causing skin cancers. Moreover, the cooling of the earth’s surface is prevented due to a build-up of carbon dioxide that traps heat in the atmosphere. The toxic gases mentioned above greatly affects the ozone layer. Wetlands are a primary source of methane and are also released through landfill gas emission, whereas nitrous oxide is released from both industrial and biogenic sources. Pepper states, “… continued global warming will ultimately have catastrophic impacts on human health via extreme weather events and natural disasters.” Natural selection has ordained that only the fittest will survive and, even though, numerous pathogens threaten human health, we are incredibly responsible for the emergence and re-emergence of infectious diseases. Therefore, one can conclude the number one driver of infectious diseases to be human activity. Another “Environmental Microbiology author, P. D. Sharma mentions that deforestation, use of fossil fuels for domestic cooking and heating, wastes disposal, and landfills contribute to infectious diseases.

    One adverse effect of global warming on the immune system is its inability to combat diseases. The destruction of the ozone leads to the increased radiation from the sun. Even though, a small amount sunlight is beneficial to proving vitamin D for the skin and over exposure to such high radiations consequential increase cancer risks. The Institute of Physics (IOP) issued a publication on a proposed study of “Climate change and the human immune system: Ultraviolet radiation & immunity study, assessing the impact of ultraviolet radiation on the immune response to primary vaccination in Australian adults,” carried out a study to understand how ultraviolet radiation (UVR) can affect the immune system. The study did not report any findings. However, researchers wanted to examine the “humoral immune response baseline” and “cell-mediated response” using “Keyhole Limpet Haemocyanin (KLH) antigen,” which they claim is a safe and reliable “immunogen.” According to the article, “Numerous studies have demonstrated that UVR is locally and systemically immunosuppressive to animals and humans.” The article further went on to state that “90 minutes of mid-latitude, midday, solar exposure would be sufficient to suppress, by 50%, the immune response to infection by Listeria monocytogenes.” Adversely enough, this immunosuppression has the potential to reduce the efficacy of vaccines, as stated in the article.

    McMichael, A. J., et al. in “CHAPTER 8: Stratospheric Ozone Depletion, Ultraviolet Radiation and Health.” in Climate Change & Human Health,” discusses the effects of UVR on the health of human beings such as the suppression of cell-mediated immunity, increased susceptibility to infection, impairment of prophylactic immunization, and activation of latent virus infection. The chapter states that most of the immunological determinants were based on experimental studies done in animals. Thus, increased exposure to radiation is an advent for infectious diseases. McMichael further states that T helper cells mediate in the occurrence of diseases, but cell function can become suppressed. As stated, “In mice, UVR exposure is associated with decreased systemic TH2 as well as TH1 immune responses. UVR leads to increased secretion of the cytokine, interleukin-10 which appears to suppress TH1 and Th2 cytokine responses to external antigens.

    As there seems to be some correlation between high UVR exposure and the suppression of the human host immune responses to infection, studies suggest that this is also evident in the animal host. McMichael states “high UVR exposure in animal decreases its resistance to viruses such as influenza, cytomegalovirus, parasites such as malaria, and other infections such as Listeria monocytogenes and Trichinella spiralis.” Another possible effect of UVR exposure explained in “CHAPTER 8: Stratospheric Ozone Depletion, Ultraviolet Radiation and Health,” is a reduction in vaccine efficacy. The chapter discussed the Bacillus Calmette–Guérin (BCG) vaccine and its low efficacy based on latitudes. Another determinant of whether the vaccine will be effective is if it has to be intradermally administered, as this suppresses the host immune response due to ultraviolet short-wave exposure.

    The WHO states that it seeks to strengthen health systems in response to climatic changes. I beg to differ that, even though, this area that requires much attention, it is in a sense “putting the cart before the horse.” There should be some aim to create programs within communities to educate individuals on what action they can take as individuals within their households to reduce activities that adversely affect the ozone. Industries should consider safer means to prevent the emittance of toxic gases, curb deforestation based on geographical areas, and remediation may serve as a positive enforcer for protecting the ozone. These are means that can suffice until there are immunization breakthroughs as it regards vaccines for emerging and reemerging infectious diseases.

  5. Brian K.

    It’s does make sense that global warming may drive infectious diseases. Longer warmer months promote increased metabolism and proliferation of organisms. Also, people are more likely to be outdoors during warmer climates, increasing the chances of contact with many organisms.

    It seems like people are trying to weight out one major contributing factor causing the increase of infectious diseases. I think it’s a collective of many factors, including climate change, that is causing this phenomenon. Other commenters have mentioned human activity, travel, overcrowding, UV light, immunosuppresion and socioeconomic causes. I believe all of these factors are legitimate. Maybe the increase in the infectious diseases for humans is a nature’s way of trying to balance our ecosystem through those factors mentioned above. All we can do is to fight for survival by alleviating the causes.

  6. S. Henry

    It is known that global warming is linked to the spread of vector-borne diseases such as malaria and an increase in temperature results in an increase in parasite development. Extreme heat in most circumstances kills mosquitoes, but within a survivable temperature range an increase in temperature manifests faster maturation of mosquitoes ultimately resulting in faster parasite development. This increased development means that mosquitoes can transmit diseases faster. An increase in global temperature can also lead to more geographic expansion for mosquitoes to survive. I read an article “Effects of Global Warming on Mosquitoes and Mosquito-borne Diseases and New Strategies for Mosquito Control” where scientists looked at new ways of controlling mosquitoes in an effort to control the spread of malaria, Japanese encephalitis, dengue fever and other vector-borne diseases. The article looked at preventative ways of mosquito control instead of adaptive ways such as pesticides. Awareness to the public and reduction of mosquito breeding places such as stagnant water pools, water filled pots and water filled tires help with residential mosquito control. The use of biological methods such as bacteria and fish that kill mosquito larvae, or the release of genetically modified male mosquitoes which are sterile and mating with them will create no larvae. Use of diesel oils in small water areas causes suffocation of mosquitoes while mosquito traps capture and kill mostly adult female mosquitoes. The use of these different ways of controlling the mosquito population worldwide, using pesticides as a last resort can help to decrease the mosquito population and the spread of multiple vector-borne diseases.

  7. Simon Suarez

    Infectious diseases are known to be climate sensitive, think of cholera, malaria and dengue. However, predicting how climate change will ultimately influence the incidence of infectious diseases is a challenge. The blog editor mentions examples of confounding factors. I would like to stress out the importance of medications. For malaria, effective therapy is available, although we’re now dealing with scary developments, such as the spread of drug resistance. So let’s say you see more malaria coming up when temperatures increase in certain regions. Is it because of the temperature increase, or because of drug resistance? Chloroquine resistance has been spreading quickly through most endemic areas. Clinical and demographic settings vary widely, even within an area affected by increasing temperature. We won’t find definitive answers any time soon.

  8. MVese

    Climate change is a definite concern for the animal species. What isn’t considered is how much of an impact climate change can also have on species in lower tropic levels. Plants, for instance, are repeatedly affected by pathogens during warmer seasons. Winter months kill many of these pathogens and give plants a break from any attacks. With climate changing, however, unreasonably warm winter months give these infectious agents more room to grow and infect, and give them the opportunity to expand to new hosts and regions that they do not traditionally inhabit. This directly affects crop production, limiting food sources to many species of animals. Not only does it influence crop production, but these ill plants can be ingested by animals, such as cattle, and cause adverse effects. The infected plants can produce alkaloid toxins, and when ingested by cattle, can affect their production of milk. In most serious cases, it can cause death in humans when consumed. Climate change does not just reduce population of certain plants due to infectious diseases. It is also possible that it can cause an overpopulation of the plant species. With heavy rainfall, piñon nuts proliferate, and rats, which eat this particular nut, start dominating the region, spreading cases of the
    hantavirus, which causes shortness of breath and liquid-filled lungs in humans. This disease can eventually cause death. Climate change has a direct effect on vectors of different species that can without a doubt induce diseases in humans. Without proper control and moderation of pollution, these diseases can populate and infect regions and hosts that have never been exposed to these pathogens, causing more severe outbreaks in human populations that have no resistance to certain infectious agents.

  9. Diana Liaw

    In addition to mosquitoes and malaria, an increase in climate change also contributes to an increase in waterborne diseases including Cryptosporidium, Campylobacter, and gastroenteritis through heavy rainfall and flooding. The increase in rainfall can also lead to more breeding sites and can force infected waters closer to the human contact. Not only should waterborne diseases be a concern, so should diseases acquired by eating shellfish that acquire cyanobacteria and dinoflagellates that can cause diarrhea, amnesic shell fish poisoning, and dermatitis. Increasing temperatures and water levels encourage the growth of the dinoflagellates and cyanobacteria, which will increase the risk of eating shellfish containing these health harming components.

    Furthermore, humans are also constantly new things about the earth! There may be unknown infectious diseases that have yet to proliferate and are just waiting for the proper conditions. These proper conditions may just be met by the increasing climate change.

  10. Chandni Jivani

    As of today, there are range of diseases which is associated with contemporary environmental issues on earth; such as, warmer temperature, storms, flooding and increase in populations. Change of climate has many undesired side effects. The spread of infectious diseases have many forms; for instance, Cholera is spread through water and this takes place where there is no safe access to drinking water or sanitary conditions. Cholera is known to spread easily by warmer temperature and flooding, in lesser developed area, river is an only example of a source of water. Adaptation in a biological world plays an important role for any organisms to improve its survivability in altering the environment. I believe global warming is occurring due to the adaptation, which we as humans and other species made to live in today’s environment. Mosquitoes are very sensitive to climate change. As we know that mosquitoes increase in the heat and bite more people frequently; however, cold temperature limits and kill their eggs. There are many popular diseases stated when discussed how global climate change affect infectious diseases; among popular, Dengue Fever (known as mosquito-borne infection) is one of them and it is found in areas like tropical regions and it is considered to be lethal complication. As WHO estimates, there is rising up to 50 million dengue infections throughout the world each year. To reduce some of the climate impacts, there should be lifestyle and environmental changes.

  11. P. Gopal

    Climate change has been one of the major factors that lead to the spread of infectious diseases as there at certain temperatures the diseases can rapidly spread. Mosquitos have particularly spread many diseases all around the world, such as chikungunya in parts of Europe. Chikungunya is a disease that is spread by Aedes mosquitos, which results in high fevers, swelling in the hands, feet, legs, arms, joints, knees, and fingers also. Climate change has allowed mosquitoes to increase in population, as in some areas there was no rainfall will help mosquitos breed in the containers that people use to store water, and other areas had heavy rainfall will have large amounts of larval habitats. This increases the spread the diseases, especially chikungunya. If there are higher temperatures, then there will be a faster development of larva, which will increase the spread of the disease. If there is a great amount of humidity in the area, then the adult mosquitoes will have a longer life and increase the spread of the virus that is being carried
    Another disease that is spread due to climate change is leishmaniasis, primarily in Germany. Leishmaniasis is a disease spread by a protozoan parasite that is transmitted by a sandfly There have been nearly 11 cases in Germany since the year 1991 that affected both humans and animals. The climate change has caused the spread of the disease in many areas where the temperature should be 10ºC or higher.

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