A new ‘dimension’ to infectious disease: Climate change

Blacklegged ticks are migrating northward into zones that were previously too cold for their survival, expanding the threat of Lyme disease to new populations.

Like other disease vectors, blacklegged ticks reproduce and survive better at higher temperatures, according to Ben Beard, PhD, chief of the Bacterial Diseases Branch of the CDC’s Division of Vector-Borne Diseases.

“We see these ticks now in areas of Canada and the northernmost parts of Wisconsin and Minnesota where we never saw them before,” Beard told Infectious Disease News.

The northward expansion of these ticks has been linked to climate change, but the problem is not limited to Lyme disease. According to experts, the rapid warming of the Earth — which is explicitly linked to human dependence on fossil fuels — can increase the reproduction rates of bacteria and viruses like malaria and dengue, as well as the biting rates of the vectors that transmit them. Extreme weather events, intensified by climate change, produce conditions ripe for diarrheal diseases.

Courtesy of Texas State University

“For us to know if and how much the patterns of a certain infectious disease will change, climate change is a dimension that must be plugged into our model,” Yongmei Lu, PhD, professor in the department of geography at Texas State University, told Infectious Disease News.

Diarmid Campbell-Lendrum, DPhil, WHO team leader for climate change and health, said the world is still learning about this relationship, but has enough knowledge to begin addressing the impact humans are having on the environment and the resulting influence on the epidemiology of infectious diseases.

“These are complex relationships — the book is never going to be closed on this,” Campbell-Lendrum said in an interview. “Yet we already know enough to act. We know that these diseases are highly sensitive to weather and climate conditions, and we know we are changing weather and climate conditions.”

Infectious Disease News spoke with several health and climate experts to develop a clearer picture of the relationship between climate change and infectious diseases.

The greenhouse effect

The latest report from the Intergovernmental Panel on Climate Change, or IPCC — an international body established in 1988 to study the effects of climate change — noted that 1983 to 2012 was “likely” the warmest 30-year period in the Northern Hemisphere in the last 1,400 years, and that the rise in global mean surface temperature could exceed 2°C by the end of this century.

According to the World Meteorological Organization, 2016 will likely surpass 2015 as the hottest year ever recorded, meaning 16 of the hottest 17 years since 1880 will have come in the 21st century.

Unsurprisingly, climate change is a threat to global public health, according to James E. Hansen, PhD, adjunct professor and director of the program on climate science, awareness and solutions at Columbia University’s Earth Institute.

Hansen, who has been called the “father of climate change awareness,” was director of the NASA Goddard Institute for Space Studies when he testified to Congress in 1988 that “the greenhouse effect is here,” raising broad awareness of man-made climate change.

He says the level of the Earth’s warming is “right on the money” compared with the model he showed Congress almost 3 decades ago, and warned that failure to reduce the amount of carbon dioxide in the atmosphere will have dire consequences.

“We need to do that to avoid losing all coastal cities,” Hansen told Infectious Disease News.

According to Lu, it is hard to quantify the threat of climate change on public health, especially on a global scale.

“There are uncertainties regarding the exact places, time and extent of the related challenges,” she said.

Mosquito-borne disease

Climate change is likely to increase the risk for infectious diseases spread through food, water and vectors, according to the IPCC report.

“Climate changes are driving the emergence and re-emergence of many infectious diseases, but in particular those transmitted by insects,” Carlos del Rio, MD, infectious disease physician at Emory University, told Infectious Disease News.

Campbell-Lendrum was co-lead author of the section of the IPCC report dealing with human health, which says vector-borne diseases are “some of the best-studied diseases associated with climate change, due to their widespread occurrence and sensitivity to climatic factors.” These factors include temperature and precipitation.

Even a small increase in temperature can affect malaria, for example, because the Plasmodium parasites inside Anopheles mosquitoes move through the life cycle faster at higher temperatures.

“If the temperature gets so high, then the mosquitoes will die,” Campbell-Lendrum said. “But it seems for most parts of the world, an increase in temperature tends to increase the climate suitability for malaria transmission.”

Temperature also can influence mosquito-borne viruses, as well as the mosquitoes themselves, which tend to bite more frequently at higher temperatures, Campbell-Lendrum said.

One study that explored the relationship between temperature and virus survival showed that the spread of dengue infection in Aedes albopictus mosquitoes was reduced when immature aquatic-stage mosquitoes were reared in cool conditions.

Humidity and rainfall also are associated with increased dengue transmission, according to the IPCC report. For instance, heavy rainfall can increase the mosquito population by creating more breeding sites. Drought may have the same effect by providing suitable breeding grounds in the form of stored water.

“Part of the challenge,” Campbell-Lendrum said, “is that the relationship between precipitation and the population numbers of mosquitoes is less easy to characterize than temperature. Precipitation is harder to predict climatically. It’s very local and very variable, so the effect on mosquitoes is more complicated than the temperature effect, but it’s very important.”

While researchers are still learning about Zika virus in the wake of the recent outbreak centered in Brazil, Beard said there are too many factors that have contributed to the outbreak to say that it is being caused by climate change alone.

However, he noted that the outbreak coincided with a strong El Niño event. El Niños, which can create unusually warm conditions, can be made more severe by climate change.

Researchers in England created an epidemiological model showing that the so-called “Godzilla” El Niño of 2015 likely fueled the Zika outbreak in South America. The researchers wrote that temperature conditions related to the El Niño “were exceptionally conducive for mosquito-borne transmission of Zika virus over South America.”

Specifically, the researchers noted that 2015 presented the highest transmission risk for Zika in South America since 1950. The risk, they said, was due to higher biting rates and lower mortality rates and incubation periods for mosquitoes — all related to higher temperatures.

Experts noted that other factors also added to the outbreak, such as Zika being introduced into an immunologically naive population.

“There are so many other factors contributing to this outbreak,” Beard said. “What we do know is that in warmer temperatures, Aedes aegypti mosquitoes complete their development more quickly, and this leads to larger populations throughout the summer. We also know that one of the trends we see with climate change is you have milder winters, earlier springs and longer growing seasons, and this contributes to a longer transmission season.”

Campbell-Lendrum said the continued spread of dengue — now responsible for as many as 400 million infections worldwide every year, according to the CDC — could be a model to predict how climate change will impact Zika.

In a recently published modelling study on dengue virus, Clement N. Mweya, PhD, of the National Institute for Medical Research in Tanzania, and colleagues used climate change predictions to show how the range of A. aegypti — the main vector implicated in the Zika outbreak — will expand, putting more people at risk for infection as dengue outbreaks move farther inland from coastal areas over time.

Similarly, a study in Geospatial Health showed that areas around the globe currently unfavorable for A. aegypti survival, including parts of North America, may become climatically favorable as the Earth warms.

Aedes mosquitoes, which also spread chikungunya, are associated mostly with tropical and subtropical climates, though A. albopictus can live in more temperate climates than A. aegypti and are more widespread in the U.S., according to CDC maps released last year.

“Understanding and at least seeing the coming of the possible changes are important for people in different regions to predict, respond and adapt appropriately,” Lu said.

However, a study in Nature Communications showed that increases in mosquito populations in New York, New Jersey and California — as high as 10-fold in some areas — were not a result of climate change, but the decay of residual DDT in the environment and increasing urbanization.

In fact, Marm Kilpatrick, PhD, assistant professor in the department of ecology and evolutionary biology at the University of California, Santa Cruz, and colleagues did not see any signal of climate change influencing mosquito populations in their study.

“We were quite surprised by this because we examined several measures of temperature and rainfall, and because we did see warming trends in all three regions in our study,” Kilpatrick told Infectious Disease News. “The total warming was about 1°C or so over the last 4 decades in each region, which isn’t a huge amount, and our data indicated that the changes in land use and urbanization and the decay of DDT in the environment were much more important drivers.

“If I had to guess about the future, I’d say that a 2° to 4°C change over the next 50 years would have a larger effect because of the larger increase in temperature. However, I think an even bigger influence will be changes in precipitation and the interaction of precipitation and temperature — for example, drought.”

Tick-borne disease

Similarly, the influence of climate change on the seasons may have contributed to an increase in tick-borne diseases in North America. In the case of Lyme disease, milder winters improve the chances of survival of ticks, which have higher reproductive rates in warmer temperatures, according to the CDC.

In the U.S., most occurrences of Lyme disease are limited to the Northeast and upper Midwest. Beard said climate change has likely led to a “significant northward change in distribution” of Lyme disease. But at the same time, Lyme disease is expanding in other directions, too, raising the question of how much of the overall expansion of Lyme disease is attributable to climate change and how much is attributable to factors like changing land use patterns, reforestation, deer populations and suburban sprawl.

“All of these have likely contributed to people’s exposure to the ticks that transmit Lyme disease,” Beard said.

Diarrheal disease

Compared with vector-borne diseases, the effect of climate change on the spread of diarrheal diseases through food or water is simpler to understand in some ways. All-cause diarrhea is “highly sensitive” to changes in temperature, particularly in poorer countries where water and sanitation services are inadequate, Campbell-Lendrum said.

Multiple studies show that higher temperatures can lead to an increase in the incidence of diarrheal disease.

“The numbers follow each other perfectly,” Campbell-Lendrum said. “You can see the temperatures going up in summer and coming down in winter, and the diarrheal cases do exactly the same.”

In one study, researchers investigating the effect of higher temperatures on childhood diarrhea during the 1997-1998 El Niño in Lima, Peru, noted a 200% increase in the number of daily hospital admissions for diarrhea at an excess cost of $277,000. In the period before the El Niño episode, there was an 8% increase in hospital admissions for diarrhea for every 1°C increase in mean ambient temperature.

About 600,000 children die every year of diarrheal disease, according to Campbell-Lendrum.

“The burden is significant,” he said, “so even a small increase [in temperature] is significant.”

There also is a correlation between precipitation and diarrheal diseases. For example, excessive rainfall and flooding — both products of climate change — can overload water systems, leading to contamination. Conversely, drought may leave people with less access to clean water, leading to poorer hygiene and a higher concentration of pathogens in smaller areas of water, Campbell-Lendrum said.

According to the CDC, the U.S. has already seen an increase in the number of heavy precipitation events over the last several decades, contributing to more flooding in certain regions.

Beyond diarrheal disease, flooding can lead to damp conditions and poor indoor air quality. Both environmental factors have been shown to increase certain health problems, including lower respiratory tract infections such as pneumonia, the CDC said.

According to WHO, flooding increases the risk for water-borne diseases such as cholera, hepatitis A, leptospirosis and typhoid fever; and vector-borne diseases like dengue, malaria, West Nile and yellow fever.

WHO says the risk is low unless the flooding is accompanied by other factors such as population displacement and compromised water sources.

This was the case when Haiti experienced an upsurge in cholera after Hurricane Matthew — a Category 5 storm — damaged the country’s water and sanitation infrastructure in October. An ongoing outbreak of cholera in Haiti has killed thousands since a strain of the disease was reportedly introduced to the country by United Nations peacekeepers in the aftermath of a catastrophic 2010 earthquake.

According to one recent modelling study, by the year 2100, areas suitable for cholera transmission are predicted to increase latitudinally under even conservative climate change scenarios.

Coastal areas of Latin America are suitable environments for the emergence of Vibrio cholerae, the bacterium that causes cholera, because of their ecological similarity to the coast of Bangladesh on the Bay of Bengal, the current epicenter of the cholera epidemic, researchers said. However, using salinity- and temperature-based prediction models, researchers identified several geographic areas that may become suitable for V. cholera in the future due to rising seawater temperatures, including the Pacific coast of Western North America; the Northern Atlantic coast of Quebec, Canada; the Coral Sea and Tasman Sea from Fiji to New Zealand; and the Black Sea in Southeastern Europe.

‘Appropriate warning systems’

While there is a scientific consensus about the validity and seriousness of man-made climate change, current knowledge about the relationship between climate change and infectious diseases is not sufficient to predict and prevent outbreaks, according to Lu and colleagues.

After surveying 131 peer-reviewed articles and government reports published between 1990 and 2015, they said minimizing the effect of climate change on the spread of infectious diseases means being proactive.

“We need better predictions of exact climate change patterns and magnitude across the globe and through time and a better understanding of possible scenarios of a pathogen’s activity level when multiple environmental factors change together,” Lu said.

“Most importantly,” she said, “we should be prepared with appropriate warning systems as well as adaptation strategies in response to health risk changes that are brought about by climate change.”

Recently, a model of predicting zoonotic outbreaks was developed by a team of researchers who said they hoped it can be used to prepare and respond to disease outbreaks related to changes in the environment.

David W. Redding, PhD, research associate in genetics, evolution and environment at University College London, and colleagues tested their model using Lassa fever, which is endemic across West Africa and primarily spread to humans via direct ingestion or inhalation of the urine or fecal matter of an infected rat. According to the model, climate change and a growing human population will more than double the number of people infected with the disease from approximately 195,000 to over 406,000 by 2070.

“Ideally, we need significant progress along two frontiers — accurate prediction of the spatial and temporal patterns of climate change over global and regional scales and a comprehensive explanation of the net outcome of health risk from a pathogen given changes in several environmental factors,” Lu said. “The former is essential for us to know which areas of the world will change by how much in weather variables, including temperature and humidity due to climate change. The latter will enable us to predict the health risk outcome for certain infectious diseases given a particular combination of weather change.”

A positive outcome of climate change

One consequence of climate change is that it forces humans to learn how to respond better to the threat of infectious diseases, Campbell-Lendrum said. There are examples of what can be accomplished with a strong response.

“We know from 100 years of malaria control that as long as we’re putting the right interventions in place, we can often stay on top of it, even in areas which are climatically highly suitable for transmission,” Campbell-Lendrum said. “One of the risks is that, in places where those control measures may break down at the same time we have climate change, making it easier to transmit malaria, the factors may combine to drastically increase transmission in certain locations.”

In Greece, for example, local outbreaks of malaria have coincided with warming in Southern Europe and challenges to its public health system due to austerity measures.

“If you’re not able to maintain control programs,” Campbell-Lendrum said, “then you are still putting people at risk.”

Campbell-Lendrum hopes the interest generated by climate change will lead to an improved global response to infectious diseases.

“Many of the things we need to do to control these diseases in the face of climate change — better provision of water and sanitation; better coverage of basic interventions for vector-borne diseases; more rapid surveillance and response — are good ideas anyway and climate change just makes it more important for us to do these things well,” he said. “We’re now starting to get in the position that we can provide improved surveillance or even early warnings or improved risk maps for these diseases. So, we’re already seeing climate change not only as a driver for infectious disease, but as a stimulus for better control of infectious disease.” – by Gerard Gallagher

• References:
• Alto BW, Bettinardi D. Am J Trop Med Hyg. 2013;doi:10.4269/ajtmh.12-0421.
• Caminade C, et al. Proc Natl Acad Sci USA. 2016;doi:10.1073/pnas.1614303114.
• Cattaneo C, Peri G. Nat Bur Econ Res. 2015;doi:10.3386/w21622.
• CDC. Estimated range of Aedes albopictus and Aedes aegypti in the United States, 2016. https://www.cdc.gov/zika/pdfs/zika-mosquito-maps.pdf. Accessed December 16, 2016.
• Checkley W, et al. Lancet. 2000;doi:10.1016/S0140-6736(00)82010-3.
• Eisen RJ, et al. J Med Entomol. 2015;doi:10.1093/jme/tjv199.
• Escobar, LE. Acta Trop. 2015;doi:10.1016/j.actatropica.2015.05.028.
• IPCC. Climate change 2014: impacts, adaptation, and vulnerability. http://www.ipcc.ch/report/ar5/wg2/. Accessed December 9, 2016.
• International Organization for Migration. Migration and climate change. https://www.iom.int/migration-environment-and-climate-change. Accessed December 9, 2016.
• Khormi HM, Kumar L. Geospat Health. 2014;doi:10.4081/gh.2014.29.
• Redding DW, et al. Methods Ecol Evol. 2016;doi:10.1111/2041-210X.12549.
• Rochlin I, et al. Nat Commun. 2016;doi:10.1038/ncomms13604.
• United Nations. World’s population increasingly urban with more than half living in urban areas. 2014. http://www.un.org/en/development/desa/news/population/world-urbanization-prospects-2014.html. Accessed December 9, 2016.
• Watts N, et al. Lancet. 2016;doi:10.1016/S0140-6736(16)32124-9.
• WHO. Flooding and communicable disease fact sheet. http://www.who.int/hac/techguidance/ems/flood_cds/en/. Accessed December 9, 2016.
• World Economic Forum. How does climate change affect migration? 2015. https://www.weforum.org/agenda/2015/11/how-does-climate-change-affect-migration/. Accessed December 9, 2016.
• Wu X, et al. Environ Int. 2016;doi:10.1016/j.envint.2015.09.07.

• For more information:
• Ben Beard, PhD, can be reached at media@cdc.gov.
• Diarmid Campbell-Lendrum, DPhil, can be reached at campbelllendrumd@who.int.
• Carlos del Rio, MD, can be reached at cdelrio@emory.edu.
• James E. Hansen, PhD, can be reached at jeh1@columbia.edu.
• Marm Kilpatrick, PhD, can be reached at akilpatr@ucsc.edu.
• Yongmei Lu, PhD, can be reached at yl10@txstate.edu.

Disclosures: Beard, Campbell-Lendrum, del Rio, Hansen, Kilpatrick and Lu report no relevant financial disclosures.