Epidemics: Reading #4


Epidemics and Climate Change



The Ripple Effect of Climate Change on Epidemic Risk

Patrick Ayscue

Contagion Live | November 3, 2017


[1] The potential impacts of climate change have returned to headlines in recent weeks as scientists, activists, and policy makers try to understand the possible implications of a warming planet during one of the busiest hurricane seasons on record. While rising temperatures and sea levels are often considered, changing climate patterns can have vast implications for epidemic risk as well.


Hot Breeding Ground

[2] Changes in global climate patterns have been widely discussed; however, increasing temperatures also have implications for risk mitigation and management, including impacts on infectious disease epidemics. With 2016 the hottest year ever recorded and 2017 following suit, we anticipate a continued increase in the distribution of disease vectors, like mosquitoes and ticks, which can spread illnesses such as Zika, yellow fever, and dengue to areas where they previously could not be effectively transmitted.


[3] To-date, the continental United States has evaded large-scale outbreaks of Zika, dengue, and yellow fever, since Aedes aegypti, the mosquito species transmitting these viruses, prefer tropical and subtropical climates. Previously restricted to Southern Florida and the Gulf Coast region, weather patterns have caused the mosquito’s range to expand, and isolated populations have been identified as far north as Washington, DC, in recent years.


Weather Alert

[4] Increases in extreme weather events, as predicted by climate scientists, may also lead to increases in infectious disease outbreaks. Epidemics have previously been seen in the wake of natural disasters, which can lead to displaced and crowded populations, hotbeds for infection transmission. Severe rainfall or flooding is particularly effective at creating environments suitable for the transmission and propagation of infectious diseases such as measles or cholera. Conditions, as currently seen on the devastated island of Puerto Rico, are often more amenable for mosquitoes to breed in flood-affected regions and as a result, may increase disease risk in those areas.


[5] Even without rising to the level of a natural catastrophe, increased variation in weather patterns can result in changes in human and animal interactions, increasing the potential for zoonotic pathogens to move from animals into human populations. For example, unusually heavy rains may predispose regions to Ebola outbreaks by creating more favorable environments for bats hosting the virus. Similarly, food scarcity brought about by drought, political instability, or animal disease may lead to more bushmeat hunting, raising the risk for Ebola outbreaks. In the American Southwest, years of drought led to booming rodent populations as predators suffered, resulting in deadly hantavirus outbreaks.


[6] Moreover, melting permafrost and ice threaten to reignite long-dormant threats. In August 2016, melting permafrost in Siberia uncovered a nearly century-old reindeer resulting in multiple people becoming infected with anthrax spores associated with the carcass. Genetic fragments of the 1918 Spanish influenza and smallpox viruses have also been found in graves in Alaska and Siberia, raising the specter of the return of some of humanity’s greatest viral enemies.


Next Steps

[7] While it is important to take note of the impact of climate change on epidemic risk, it is equally important to prepare for its impact on global health and the global economy. In addition to affecting the health and well being of populations worldwide, the financial toll of these epidemics can also be devastating. According to a report from the United Nations, Zika could end up costing Latin America and the Caribbean up to $18 billion. The Ebola outbreak in West Africa in 2014 required over $5 billion in emergency funds to control.


[8] The global health community has largely come to realize that public health preparedness is critical to responding efficiently to infectious disease outbreaks; however, the health community has been slower to recognize that social and economic stability in the wake of epidemics are also needed to ensure community resilience. The World Bank has taken a step to formalize the transfer of funds through risk transfer mechanisms through the Pandemic Emergency Financing Facility, meeting resilience goals by deploying money to affected areas to fight outbreaks, and our work is centered around helping industry and governments manage and quantify infectious disease risk. Because regardless of weather patterns, insights into epidemics and mechanisms for ensuring adequate financial support is critical for managing this risk. 


[9] Since the public health community agrees that the question is not if another outbreak will happen… but when, the steps we take in the coming years to prepare for and mitigate the increasing frequency of outbreaks will determine the broader implications these diseases have on our world.


How Climate Change Is Exacerbating the Spread of Disease

Renee Cho

Earth Institute: Columbia University | September 2014


[1] The accelerating Ebola epidemic in West Africa, which the World Health Organization (WHO) has called “unprecedented,” has so far killed more than half the 3,069 people who contracted the disease in Liberia, Sierra Leone, Guinea and Nigeria. The Centers for Disease Control fears that a worst case scenario could mean 1.4 million cases by January 2015. Ebola can live in animals for years without making them sick; it is transmitted to humans through contact with an infected animal. Once in a human, the disease is spread by direct contact with the bodily fluids of the infected person, and as yet there is no vaccine.


[2] Some scientists think that climate change, with its increase in sudden and extreme weather events, plays a role in ebola outbreaks: dry seasons followed by heavy rainfalls that produce an abundance of fruit have coincided with outbreaks. When fruit is plentiful, bats (the suspected carriers of the recent ebola outbreak) and apes may gather together to eat, providing opportunities for the disease to jump between species. Humans can contract the disease by eating or handling an infected animal.


[3] According to Kris Murray, senior research scientist at EcoHealth Alliance, an organization that researches and educates about the relationships between wildlife, ecosystems and human health, climate change has strong potential to play a role in increasing the risk for ebola.


[4] “With climate change expected to put increasing pressure on food security in Africa, food shortages will push more people to alternative food sources and consumption of bushmeat, like bats, will likely increase,” said Murray. Almost 50 percent of ebola outbreaks have been directly linked to bushmeat consumption and handling (the origin of the current outbreak, however, has still not been determined). Murray added, “Also, some of our computer modeling suggests that with climate change, in parts of central and western Africa, the range of some bat species could expand…this means increased contact between bats and humans.”


[5] According to the United States Agency for International Development, “nearly 75 percent of all new, emerging, or re-emerging diseases affecting humans at the beginning of the 21st century are zoonotic”— meaning they originate in animals. These include AIDS, SARS, H5N1 avian flu and the H1N1 flu. More and more wild animals, which may have carried diseases without effect for years, are coming into contact with humans, often because of deforestation.


[6] Sierra Leone, where ebola is currently raging, lost 96 percent of its forest by the 1920s and may lose the rest by 2018, according to the U.N. Guinea, where the 2014 Ebola outbreak began, has lost 20 percent of its forests since 1990. The human activity that drives deforestation— logging, mining, slash and burn agriculture, demand for firewood and road building— means more and more people are entering the forest, and thus forcing animals like bats to find new habitats closer to human civilization. The EcoHealth Alliance works globally to save endangered species and their habitats and to protect ecosystems. “A really important message that we advocate is that protecting the environment will help protect ourselves from a range of natural disasters, including disease,” said Murray.


[7] The WHO has warned that contagious diseases are on the increase as a result of “the combined impacts of rapid demographic, environmental, social, technological and other changes in our ways-of-living. Climate change will also affect infectious disease occurrence.” A number of diseases well known to be climate-sensitive, such as malaria, dengue fever, West Nile virus, cholera and Lyme disease, are expected to worsen as climate change results in higher temperatures and more extreme weather events.


[8] Malaria killed 627,000 in 2012 alone. According to the Intergovernmental Panel on Climate Change (IPCC), climate change will be associated with longer transmission seasons for malaria in some regions of Africa and an extension of the disease’s geographic range. As temperatures warm, the Plasmodium parasite in the mosquito that causes malaria reproduces faster and the vector (the organism that transmits a disease), i.e. the mosquito, takes blood meals more often. Rain and humidity also provide favorable conditions for young mosquitoes to develop and adult mosquitoes to survive.


[9] Dengue fever infects about 400 million people each year, and is one of the primary causes of illness and death in the tropics and subtropics. The IPCC projects that the rise in temperatures along with projected increases in population could put 5 to 6 billion people at risk of contracting dengue fever in the 2080s. The reproductive, survival and biting rates of the Aedes aegypti mosquitoes that carry dengue and yellow fever are strongly influenced by temperature, precipitation and humidity. In the summer of 2013, Aedes aegypti, usually found in Texas or the southeastern U.S., suddenly appeared in California as far north as San Francisco, though fortunately, none of the mosquitoes tested carried dengue or yellow fever.


[10] The hantavirus broke out in the southwest U.S. in 1993 after a six-year drought ended with heavy snows and rainfall. The precipitation allowed plants and animals to grow prolifically, which resulted in an explosion of the deer mice population. The mice may have carried hantavirus for years, but suddenly many more mice were coming into contact with humans. People became infected through contact with infected mice or their droppings. Hantavirus Pulmonary Syndrome has now been reported in 34 states. Through 2013, 637 cases were reported in the U.S., and approximately 230 people have died.


[11] In 1999, the West Nile virus, transmitted to humans by mosquitoes, made its first appearance in the Western Hemisphere in New York, after a drought followed by heavy rains. Since then, over 1,600 people have died of the disease. This month, the number of reported cases of West Nile virus— 1,993 including 87 deaths— is the highest year-to-date total since it arrived in the U.S., with Texas being hardest hit. A recent study suggests that in the future, higher temperatures and lower precipitation will lead to a higher probability of West Nile cases in humans, birds and mosquitoes.


[12] Extreme weather events can produce a cascade of other effects that influence disease. Heat and droughts create dry conditions, providing fuel for forest fires that end up fragmenting forests and driving wildlife closer to humans. Droughts and floods affect crop yield, sometimes resulting in malnutrition, which makes people more vulnerable to disease while forcing them to find other food sources. Flooding can provide breeding grounds for insects and cause water contamination, leading to the spread of diarrheal diseases like cholera. Moreover, extreme weather can disrupt the finely tuned relationships between predators and prey, and competitors that keep pathogen-carrying pests like mice and mosquitoes in check.


[13] Climate-sensitive diseases are also affected by the shorter-range climate impacts arising from El Niño, which occur when unusually warm sea surface temperatures develop off the Pacific coast of South America. Historically, El Niño events have resulted in drier and hotter than normal conditions in some areas and wetter and cooler than normal conditions in others, effects felt mostly in the tropics. An El Niño is predicted to occur this fall and to last until February 2015, likely bringing with it more rainfall and higher temperatures.


[14] The Earth Institute’s International Research Institute for Climate and Society (IRI) develops tools to predict epidemics, and uses high-level climate information to help societies, particularly in developing countries, understand and manage the impacts of both short and long-term climate. Madeleine Thomson, director of the IRI/PAHO-WHO Collaborating Centre for Early Warning Systems for Malaria and Other Climate Sensitive Diseases, oversees the public health activities at IRI.


[15] One climate contribution to this work is Enhancing National Climate Services (ENACTS), an initiative developed by IRI and its partners. It is making high-quality climate data more available to decision makers in Africa, including those who are dealing with malaria. ENACTS reviews historical climate data to understand natural climate variability over time and monitors current climate, so that decision makers get an “early warning” about when climate effects could trigger malaria outbreaks and when malaria interventions should be implemented.


[16] With a potential El Niño developing, there is an increased likelihood of higher than normal rainfall in East Africa in the next rainy season and malaria control agencies are already getting prepared. Some have shifted stocks of diagnostic tools and medicines out of reserves and made them more readily available, explained Thomson. “Once they get the early warning they can reach out to their donors and the government for more money, they can communicate with the community to use bed nets, to attend clinics if they are sick, and to get their children drugs on time. If there are concerns about an impending epidemic, they can also use insecticide spraying.”


[17] When discussing the likely impact of climate change, Thomson explained that the time frame for decision makers who have to determine budgets and make plans is relatively short, maybe four or five years. Even for global policy, the time frame is at maximum maybe 10 to 20 years. “These are not climate change time frames,” said Thomson. “In terms of decision-making, they have to deal with short-term climate variability— especially for rainfall. The shift to warmer temperatures in highland areas, however, is linked to the larger climate system. It will impact vector-borne diseases, because when it’s warmer, the disease transmission system speeds up.”


[18] Scientists recognize that climate is only one factor in the spread of disease. “Underdeveloped countries are so vulnerable to infectious disease because they are not able to protect themselves with good housing, good infrastructure, education, a sound health system, access to vaccines, and surveillance systems that track cases of disease,” said Thomson. “These vulnerabilities are the first problem— climate impacts on top of them just compound the problems.”  In the U.S., we are better protected against the spread of disease by our high standard of living and strong surveillance systems.


[19] The good news, said Thomson, is that there is a shift in awareness at the global level about the need to understand how climate change is affecting disease, and it is filtering down to local levels. The WHO’s first international conference on health and climate, which took place in Geneva, Switzerland,  at the end of August, is evidence of this growing awareness.


In Zika Epidemic, a Warning on Climate Change

Justin Gillis

NYT | February 20, 2016


[1] The global public health emergency involving deformed babies emerged in 2015, the hottest year in the historical record, with an outbreak in Brazil of a disease transmitted by heat-loving mosquitoes. Can that be a coincidence? Scientists say it will take them years to figure that out, and pointed to other factors that may have played a larger role in starting the crisis. But these same experts added that the Zika epidemic, as well as the related spread of a disease called dengue that is sickening as many as 100 million people a year and killing thousands, should be interpreted as warnings.


[2] Over the coming decades, global warming is likely to increase the range and speed of the life cycle of the particular mosquitoes carrying these viruses, encouraging their spread deeper into temperate countries like the United States. Recent research suggests that under a worst-case scenario, involving continued high global emissions coupled with fast population growth, the number of people exposed to the principal mosquito could more than double, to as many as 8 billion or 9 billion by late this century from roughly 4 billion today.


[3] “As we get continued warming, it’s going to become more difficult to control mosquitoes,” said Andrew Monaghan, who is studying the interaction of climate and health at the National Center for Atmospheric Research in Boulder, Colo. “The warmer it is, the faster they can develop from egg to adult, and the faster they can incubate viruses.”


[4] Already, climate change is suspected— though not proved— to have been a factor in a string of disease outbreaks afflicting both people and animals. These include the spread of malaria into the highlands of eastern Africa, the rising incidence of Lyme disease in North America, and the spread of a serious livestock ailment called bluetongue into parts of Europe that were once too cold for it to thrive.


[5] In interviews, experts noted that no epidemic was ever the result of a single variable. Instead, epidemics always involve interactions among genes, ecology, climate and human behavior, presenting profound difficulties for scientists trying to tease apart the contributing factors. “The complexity is enormous,” said Walter J. Tabachnick, a professor with the Florida Medical Entomology Laboratory, a unit of the University of Florida in Vero Beach.


[6] The epidemics of Zika and dengue are cases in point. The viruses are being transmitted largely by the yellow fever mosquito, Aedes aegypti. That creature adapted long ago to live in human settlements, and developed a concomitant taste for human blood. Cities in the tropics, the climate zone most favorable to the mosquito, have undergone explosive growth: Humanity passed a milestone a few years ago when more than half the population had moved to urban areas. But spending on health care and on basic public health infrastructure, like water pipes and sewers, has not kept pace. Mosquito control has also faltered in recent decades.


[7] The mosquito lays its eggs in containers of water, of a sort that are especially common in the huge slums of Latin American cities. With unreliable access to piped water, people there store water in rooftop cisterns, buckets and the like. Old tires and other debris can also become mosquito habitat. Water storage near homes is commonplace in areas where Zika has spread rapidly, like the cities of Recife and Salvador in northeastern Brazil, and where dengue experienced a surge in 2015, like São Paulo, Brazil’s largest state. Altogether, dengue killed at least 839 people in Brazil in 2015, a 40 percent increase from the previous year. Worldwide, dengue is killing more than 20,000 people a year.


[8] Several experts said in interviews that a main reason for the disease outbreaks was most likely the expansion of the number of people at risk, through urbanization, population growth and international travel. They see the changing climate as just another stress on top of a situation that was already rife with peril.


[9] While they do not understand to what degree rising temperatures and other weather shifts may have contributed to the outbreaks, they do understand some of the potential mechanisms. The mosquitoes mostly live on flower nectar, but the female of the species needs a meal of human blood to have enough protein to lay her eggs. If she bites a person infected with dengue, Zika or any of several other diseases, she picks up the virus.


[10] The virus has to reproduce in the mosquito for a certain period before it can be transmitted to another person in a subsequent bite. The higher the air temperature, the shorter that incubation period. Moreover, up to a point, higher temperatures cause the mosquitoes to mature faster. With rising temperatures, “You’re actually speeding up the whole reproductive cycle of the mosquitoes,” said Charles B. Beard, who heads a unit in Fort Collins, Colo., studying insect-borne diseases for the Centers for Disease Control and Prevention in Atlanta. “You get larger populations, with more generations of mosquitoes, in a warmer, wetter climate. You have this kind of amplification of the risk.”


[11] Aedes aegypti is present across the southern tier of the United States. Brief outbreaks of dengue have occurred recently at the warmest margins of the country, and one is underway in Hawaii. But with pervasive window screens and air-conditioning, the risk of disease transmission is far less for most Americans than for people in poorer countries. The mosquito does not thrive in areas with cold winters. Some research suggests that continued climatic warming could allow the mosquito to colonize more of North America in coming decades, though how much of a disease risk that would represent is anybody’s guess.


[12] The yellow fever mosquito competes with a cousin, the Asian tiger mosquito, that has also colonized the United States, and is more tolerant of cold weather. Whether one would beat out the other in a hotter climate is unclear. Likewise, it is unclear how effective the Asian tiger mosquito might become at transmitting Zika or dengue viruses. In principle, the risk from continued global warming applies not just to temperate countries, but to cities at high altitude in tropical countries. Researchers are keeping a close eye on Mexico City, for instance.


[13] With 21 million people in the city and its suburbs, Mexico City is the largest metropolis of the Western Hemisphere. While the lowlands of Mexico are plagued by yellow fever mosquitoes and the viruses they transmit, the country’s capital sits on a mountain plain that has— up to now— been too cold for the mosquitoes.


[14] But temperatures are rising, and the mosquitoes have recently been detected in low numbers near Mexico City. “The mosquito is just down the hill, literally,” Dr. Monaghan said. “I think all the potential is there to have virus transmission if climatic conditions become a bit more suitable.”


Climate change will fuel the rapid and unpredictable spread of deadly epidemics in Europe

Martha Henriques

International Business Times | August 2, 2017


[1] Climate change is expected to bring a large-scale and sometimes unpredictable change to the spread of infectious diseases in Europe, a new study finds. The large-scale review has mapped how 100 human infectious diseases are likely to spread or change their range due to climate change in Europe. Nearly two-thirds of the pathogens studied were sensitive to climate.


[2] The authors also studied 100 animal infectious diseases and found that a similar proportion were likely to be altered by climate change. Zoonotic diseases— those that jump from animals to humans, such as SARS, HIV and Ebola— were more sensitive to change than those that infect either only humans or only animals.


[3] Across the board, those most likely to be affected were diseases transmitted by ticks or insects, such as mosquitoes. Food, water and soil-borne diseases were the next most climate-sensitive. "Although there is a well-established link between climate change and infectious disease, we did not previously understand how big the effects will be and which diseases will be most affected," said Marie McIntyre, who led the project at the University of Liverpool's Institute of Infection and Global Health


[4] "Climate sensitivity of pathogens is a key indicator that diseases might respond to climate change, so assessing which pathogens are most climate-sensitive, and their characteristics, is vital information if we are to prepare for the future." Some pathogens had many climate 'drivers', so that their response to a changing climate would be particularly large or unpredictable. The diseases with the most drivers were cholera, liver fluke, anthrax and Lyme disease.


[5] "Currently, most models examining climate effects only consider a single or at most two climate drivers, so our results suggest that this should change if we really want to understand future impacts of climate change on health," McIntyre said. Climate change already appears to be increasing the range of several diseases beyond their historic ranges. Zika in South America is one example of this, as are the livestock diseases bluetongue and Schmallenberg disease in Europe.


[6] It's hoped that the findings could feed into policies to focus on surveillance of these diseases to anticipate climate-driven outbreaks. The research was published in the journal Scientific Reports.