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4. Infectious Diseases

Published onApr 10, 2020
4. Infectious Diseases

The great progress that has been achieved could be undone in some places in a single transmission season.

—Dr. Margaret Chan, WHO Director-General, World Malaria Report 2013

It is not difficult to see why heat-related illnesses will become more common and pervasive on a warming planet. The evidence that climate change will increase the risk of certain infectious diseases is more complex but just as compelling.1 Many diseases that are of greatest concern have multiple facets that are susceptible to the effects of climate change. These include the physical properties of ecosystems that affect diseases and disease vectors, such as temperature and moisture, and the numbers and types of species in the system, including species extinction. Human activity has already had and will continue to have an enormous impact on ecosystems and the species that depend on them. Understanding how climate change affects each of these factors is critical to developing and implementing efforts to mitigate and adapt to climate change and to control diseases such as malaria, dengue, West Nile fever, and others.

Changes in the locations where insect vectors live is perhaps the most obvious factor affecting vector-borne diseases. For example, changes in precipitation impact the breeding cycle of mosquitoes, such as the females in the genus Anopheles, carriers of malaria, and Aedes aegypti, the principal mosquito vector for dengue fever. Changes in temperature may affect the reproductive cycle of malaria parasites and the mosquito vector.

The need to integrate all of these variables has given rise to a new scientific specialty known as disease ecology.2 Disease ecologists study the relationships between climate and its effects on the physiological state of the pathogen and the population of disease carriers, or vectors, including the balance of species in an ecological system. Disease ecologists are concerned increasingly about a phenomenon known as the dilution effect. When the diversity of intermediate host species increases (e.g., the number of mosquito species increases), there is a reduction in the exposure to disease(s) carried by a specific species. This is known as dilution. In other words, diluting the species with multiple species reduces risk. Risks increase when there is less diversity among species, as has been shown to be the case for Hanta virus exposure and Lyme disease. The risk of both diseases has been shown to rise as species diversity falls. Similarly, the risk of West Nile virus exposure rises as diversity among bird species falls.

Application of the principles of disease ecology is beginning to have important implications for public health. A case study of malaria in Botswana illustrates a successful application of the disease ecology approach.3 Malaria is a major public health problem in Botswana in spite of the fact that it is a semiarid nation, and Botswana suffered a major malaria epidemic in 1996. The country’s malaria control strategy was developed from a retrospective analysis of the seasonal variability of malaria and a multimodal approach to climate prediction based on sea-surface temperatures. Variations in the temperature of the ocean are, in turn, the result of predictable fluctuations in prolonged oceanic warming, known as El Niño, and the corresponding atmospheric pressure component, known as the Southern Oscillation. Together, these components are referred to as the El Niño–Southern Oscillation (ENSO).

From ENSO data, disease ecologists were able to make more accurate climate predictions for southern Africa. Improved climatological predictions in turn lead to better forecasts for malaria risk. Using these combined data, public health officials were better able to allocate resources needed to combat an impending malaria epidemic. This process involved judicious use of insecticides, administration of drugs to individuals during periods of high risk to prevent malaria (chemoprophylaxis), and the early detection and management of the disease in individual patients.

This ability to reference changes in disease prevalence to distant climatological phenomena, such as ENSO, is referred to as making a teleconnection—literally, connecting distant events. Other recent evaluations of teleconnections have led to the discovery of links between Rift Valley fever and periods of heavy rainfall and between the disease known as chikungunya and high temperatures coupled with drought.4 Both of these diseases are carried by mosquitoes. In the case of Rift Valley fever, high rainfall amounts allow the vector to flourish. In contrast, chikungunya outbreaks are thought to be due to in-home water storage that is increasingly common during drought. The common denominator is an increase in the mosquito vectors, but the mechanisms that underlie the increases are quite different.

Contemporary strategies for controlling diseases carried by insects rely increasingly on combining multiple techniques known collectively as integrated vector management (IVM). The World Health Organization defines IVM as “a rational decision-making process to optimize the use of resources for vector control. IVM requires a management approach that improves the efficacy, cost effectiveness, ecological soundness and sustainability of vector control interventions with the available tools and resources.”5

As implied by this definition, the first step in the process is to understand local factors that affect vector ecology and then select the control measures from a range of available options. These include environmental management (e.g., draining standing water), the use of biological controls (e.g., fish that eat larvae or bacteria that kill larvae, such as Bacillus thuringiensis [Bt]), chemical controls such as insecticides, and personal protective measures such as avoiding outdoor activity when mosquitoes feed, wearing long-sleeved shirts and long pants, and using insecticide-impregnated sleeping nets. Integrating these methods is designed to have a maximum effect on disease prevention with a minimum of risks to health and/or environmental effects of the control mechanisms.

Arthropod-Borne Diseases

“Arbovirus” is an acronym for “arthropod-borne virus.” These viruses are carried by arthropods, invertebrate animals with an external skeleton or cuticle (made of chitin with or without calcium carbonate) and with jointed appendages. Examples include insects, spiders, ticks, other arachnids, and crustaceans. (Try not to think of bugs when sitting down to a tasty lobster or crab dinner!) Insects are arthropods with three body parts, a head, a thorax, and an abdomen. They also have six legs, compound eyes, antennae, and often wings. Mosquitoes are the most important vectors for diseases caused by arboviruses. These diseases include dengue, St. Louis encephalitis, Zika, and West Nile fever. Ticks are also arthropods. However, they have two body parts and eight legs and do not have antennae or wings. Lyme disease is the most important tick-borne disease.

Malaria parasites are protozoa, single-celled organisms containing a nucleus and other organelles. This section includes malaria along with the arboviruses because malaria parasites are carried by mosquitoes. Thus, factors that affect mosquitoes affect the diseases caused by arboviruses and malaria.


Dengue, also known as breakbone fever, is a leading cause of death and disease in tropical and subtropical regions. A 2011 comprehensive review concluded that dengue’s prevalence had increased by a factor of thirty during the preceding five decades and was now endemic in 112 nations.6 Estimates conclude that in 1990 around 30 percent of the world’s population had a greater than 50 percent chance of contracting the disease.7 The authors of this study used predictions of the future climate and the ecology of its mosquito vector to estimate that by 2085 between five and six billion people will be at risk for this disease. Their model predicts that the probability of transmission will be greater than 0.2 for large portions of the south-central and southeast portions of the United States, with the probability rising to 0.9 or more in sections of the Texas gulf coast and Southern Florida. (Note: Probability is expressed as a number ranging from 0.0, impossibility, to 1.0, certainty.)

Currently, dengue is relatively rare in the United States. Most verified cases have been found among those who have returned to the United States after travel to countries where the disease is common. For example, a June 4, 2014, report in Time described twenty-four confirmed cases of dengue in Floridians who had traveled to regions where dengue is prevalent.8 A more serious outbreak of dengue occurred in Hawaii in February 2016, leading Governor David Inge to declare a public health emergency.

Dengue is a viral disease. It is caused by RNA viruses in the genus Flavivirus. This is a particularly nasty genus; other members of the group cause yellow fever, West Nile fever, St. Louis and Japanese encephalitis, and others. Collectively, they are known as arboviruses because they are typically transmitted by arthropods. Several species of the mosquito genus Aedes, principally A. egypti, transmit the disease. These mosquitoes are well adapted to urban living and thrive in regions that are warm and humid—that is, tropical and subtropical regions at relatively low elevations. Although there is hope on the horizon that vaccines for the virus will be forthcoming, none are available at present. (In chapter 10, I discuss the progress toward developing a vaccine.)

Therefore, preventive measures designed to control mosquitoes through the techniques of integrated vector management are needed. These measures include eliminating standing water and preventing mosquito bites. As the planet warms and atmospheric water increases as a result, it is expected that conditions that favor increases in the ecological niche will promote the propagation and spread of Aedes mosquitoes. Thus, regions where dengue is rare or virtually nonexistent will shrink as the latitude and elevation boundaries that favor mosquito-breeding change.

There are four strains or serotypes of the dengue virus (DEN-1, -2, -3, and -4) that differ in their virulence, which creates the possibility of multiple infections in the same person by different strains of the virus. Dengue is usually asymptomatic or mild, and the patient may not associate the fever with the disease. However, some patients (fewer than 5 percent) develop low blood pressure and shock due to leakage of blood products from blood vessels. The most severe infections are associated with hemorrhage—hence the name dengue hemorrhagic fever. Unfortunately, contracting a mild case of dengue does not confer immunity on the host. To the contrary, reinfection with a virus of a different serotype may cause disease that is much more severe than the first time around. The reasons for this immunologically based phenomenon are complex and incompletely understood.

The hemorrhagic form of dengue is most common in infants and children but may occur in adults. Children with excellent nutrition are paradoxically more likely to develop severe manifestations of the disease than those who are undernourished. The protection associated with undernutrition may be due to the poor immunological response associated with protein-calorie deficiency.9 This may be the only silver lining associated with the dark cloud of undernutrition.

West Nile Fever

West Nile fever is caused by another of the mosquito-borne viruses in the Flavivirus genus. As noted previously, other members of this genus cause dengue and other diseases. Although rare cases were identified before the mid-1990s, West Nile fever became relatively common in Algeria after that time. The first US case was identified in 1999 in New York City. It spread rapidly across North America during the next five years.10

West Nile virus has several host reservoirs. It infects humans after the bite of a mosquito that carries the virus. These mosquitoes are primarily in the genus Culex, with regional variations in the species.11 In the United States, there are approximately sixty different species in the Culex genus, but fewer than ten are thought to be the main vectors, which vary regionally. Cx. pipiens (the northern house mosquito) is responsible for more than half of the isolates in the Northeast, Cx. quinquefasiciatus (the southern house mosquito) predominates in the South, and Cx. tarsalis predominates west of the Mississippi River. The latter is the most efficient transmitter of the disease. Crows and robins act as reservoirs for the disease and are partially responsible for spreading the disease from one location to another.12 Monitoring deaths of these birds has proven to be a valuable means of detecting spread of the disease.

Fears have arisen that globalism and the ease of air transportation will lead to rapid disease transmission. This was one theme of the 2011 film Contagion. In the film, Gwyneth Paltrow’s character, Beth Emhoff, dies after flying home to the United States after contracting a disease in China. A global epidemic ensued, spread initially by Emhoff’s fellow passengers on her flight. In the real world, travelers to the 2014 FIFA World Cup (soccer or football) were warned about dengue transmission when they traveled to Brazil, the nation with the largest number of cases of dengue. Brazil was gripped by an epidemic of the disease during the games. There are similar fears surrounding the 2016 Summer Olympics, scheduled for Brazil.

Although most cases of West Nile fever are unreported, because the symptoms are generally mild or unnoticed, the Centers for Disease Control reported that 2012 was the worst year for West Nile fever in the United States. In that year, there were 5,674 confirmed cases and 286 deaths. Similar numbers were reported in 2002 to 2003. An investigation of the 2010 European outbreak led to the conclusion that elevated temperatures were largely responsible for the epidemic. Similar conclusions were drawn for the 2012 US outbreak.

Climate change seems virtually certain to affect the prevalence of West Nile fever. However, we cannot draw a straightforward link between temperature and the spread of the disease. A 2013 study of the effects of climate change illustrates this point.13 Using the IPCC A2 climate change scenario, which predicates an approximately fivefold increase in carbon dioxide emissions by 2100, and the so-called Dynamic Mosquito Simulation Model, investigators concluded that the population of Cx. quinquefasiciatus will not be homogeneous and will depend on variables such as temperature and precipitation. For example, in some parts of the country the temperature will be too high for mosquitoes to breed, and in others it may be too dry. In South Florida and the Texas Gulf Coast, the study authors predict bimodal disease peaks at about weeks 20–25 into the year, and then later at weeks 40–45. In the arid southwest, they predict a larger single peak at about week 40. Some of the projected regional differences are based on the knowledge that when it is too hot, Culex mosquitoes fail to reproduce in large numbers. Thus, latitude, altitude, temperature, and precipitation all affect these mosquitoes and therefore the spread and prevalence of the disease.


Chikungunya is caused by a virus in the Alphavirus genus. The name chikungunya is reportedly derived from the Makonde language, meaning that which bends up, a reference to the posture assumed by some victims. Its symptoms include headache, fever, a skin rash (petechial or maculopapular), and joint pain that typically last for two days but may persist for many days or even months. The mortality rate is around one in one thousand. The virus can be recovered from some patients months after the initial infection. It is spread by mosquitoes in the Aedes genus, usually A. egypti.

About fifty years after the initial description of the disease, an outbreak occurred in Italy that was linked to A. albopictus.14 This change in vector is thought to be due to a mutation in the virus that enables A. albopictus to carry the disease. This is worrisome, as the albopictus mosquito is a more aggressive biter and therefore may be a more efficient transmitter of the disease. A review indicates that chikungunya is quite widespread, with as many as 244,000 cases on the island of La Reunion, a French island east of Madagascar, and a million cases in India.15 The high prevalence on this island nation was thought to be due to a change in the vector and importation of the disease by travelers. There is concern that chikungunya may pose a threat in the United States. Although there are no FDA-approved vaccines for the disease, promising trials have been reported.16 The development of a safe and effective vaccine would be a major step forward in controlling this disease.

Zika Disease

In January 2016, a widely publicized climate-linked threat to health emerged in the form of the Zika virus.17 The Zika virus belongs to the Flavivirus genus, the same genus as the virus that causes dengue. Like dengue, it is spread by Anopheles mosquitoes. Zika virus infections are likely to become more common as climate change increases the range of the insect vector. Monitoring the prevalence of dengue and chikungunya is likely to foreshadow the spread of Zika virus disease. The symptoms of Zika virus disease are usually mild and similar to those of dengue. Zika virus infections are thought to have severe adverse effects on developing brains, however, causing microcephaly, or small head and brain size. The risk of microcephaly is likely to be greatest among pregnant women infected during the first trimester but fetal infection may occur at any time. Microcephaly causes severe, life-long mental retardation. As a result, the Centers for Disease Control and Prevention issued a travel advisory urging pregnant women to avoid travel to regions where Zika virus has been isolated. There are no specific treatments for the disease nor is there a vaccine. Avoiding mosquito bites is the only effective preventive measure.

Lyme Disease

Lyme disease is caused by Borrelia burgdorferi, a corkscrew-like bacterium also known as a spirochete. The Centers for Disease Control and Prevention (CDC) website is a rich source of information about this disease.18 It is the most common of the vector-borne diseases in the United States and is on the rise. In 2004, there were 19,804 reported cases. In 2013, this number grew to 27,203 confirmed and 9,104 probable cases. Most reported cases are from the northeastern part of the country or the upper midwest. Data from 2013 show that the incidence was highest in Maine, with one hundred cases per one hundred thousand people, and lowest in southern states, many of which reported no cases that year. As suggested by the number of cases, surveillance data show clearly that Lyme disease is spreading as well as increasing in prevalence in states where it already exists.

The disease is contracted after being bitten by an infected tick. Three to ten days later, between 70 and 80 percent of infected persons develop a characteristic rash called erythema migrans, a red, target-shaped rash centered on the site of the bite. Blood tests, when done properly, are diagnostic for the condition. Treatment with oral antibiotics almost always is curative, if given at an appropriate time after the infection is contracted. Lyme disease may have other manifestations that occur days or weeks after the bite, including Bell’s palsy (drooping of one side of the face due to involvement of the seventh cranial nerve), rashes appearing on other parts of the body, severe headaches due to infection and inflammation of the meninges (membranes that cover the brain and spinal cord), arthritis (especially involving large joints, such as the knees), or irregularities of the heartbeat that cause palpitations or occasionally dizziness.

Relatively simple steps to avoid ticks may prevent the disease. These include avoiding areas where ticks are prevalent (woods, grassy areas), application of repellents—especially those containing DEET (N, N-diethyl-m-toluamide) or permethrin—conducting a whole-body search for ticks, and bathing after a potential exposure. Ticks may also be present on pets or other objects, such as clothing. Ticks should be removed using fine-tipped tweezers. After removing the tick, apply alcohol or an iodine scrub to the affected area or wash carefully with ordinary soap and water. Public service announcements by the media may be an effective and inexpensive method to teach individuals how to deal with ticks and prevent Lyme disease.

In the northeastern, mid-Atlantic, and north-central parts of the United States, the disease is usually spread via the blacklegged or deer tick (Ixodes scapularis). On the Pacific Coast, the western black-legged tick is the vector (Ixodes pacificus). Unfortunately, these are tiny ticks, and the immature nymph stage of development typically is the culprit. The nymphs are about the size of a poppy seed. Adults that have not had their blood meal are about the size of a sesame seed. Most bites occur during summer months.

Several studies have shown that migrating birds carry the nymph form of Ixodes scapularis as unwelcome passengers.19 The tick climbs aboard the birds as they search for food on the ground. After birds complete their migration, the ticks detach themselves, complete their transformation into adults, and then spread the disease to other species, including humans or some other intermediate hosts. In this way, these tiny creatures are able to move into new territories where conditions are favorable for proliferation of the tick and the spirochetes they may harbor.

Based on what is known about the conditions necessary for survival of the ticks, it seems highly likely that Lyme disease will continue its spread northward into Canada from locations in the United States. Modeling studies that project the northern boundaries where the temperature will remain warm enough during the winter to allow the ticks and thus the spirochetes to survive predict that Lyme disease may spread northward by as much as 1000 km by the 2080s.20 It seems likely that warming that has already occurred has contributed to the spread of Lyme disease already observed.


Malaria has long been and remains one of the great scourges of mankind. Estimates of the actual number of individuals with malaria worldwide vary substantially. The Global Burden of Disease project reports an almost 20 percent increase in malaria mortality between 1990 and 2010, with the disease responsible for 1.17 million deaths in 2010.21 The World Health Organization (WHO) paints a more optimistic picture, reporting a 45 percent reduction in all age groups and a 51 percent reduction in the 2000 to 2012 time interval.22 In yet another time interval, global malaria deaths were estimated at 995,000 in 1980, rising to peak of 1,817,000 in 2004, then decreasing to 1,238,000 by 2010.23 The deaths in 2010 in one report were estimated to be twice the number reported by the WHO for the next year (2011). In its 2010 Millennium Development Goals Progress Report, WHO touts a 20 percent reduction in childhood malaria deaths as progress toward its 2015 goal.

Some of the discrepancies in these numbers are the result of differences in the time intervals included in the various reports. In spite of the failure of experts to agree on the numbers, all of these reports emphasize severity of the problem and the need for increased financial support for malaria eradication efforts. Reduced financial support for malaria eradication efforts due to the recent worldwide financial crisis is thought to be the cause of setbacks in the control of the disease. The fragility of the attempts to control malaria was emphasized by Dr. Margaret Chan of the WHO in the World Malaria Report 2013 quotation at the beginning of this chapter: “The great progress that has been achieved could be undone in some places in a single transmission season.”

References to malaria date back about four thousand years. It was known to Hippocrates and is thought to be responsible for the decimation of several Greek city states and rural areas during the age of Pericles (461–429 BCE). There are references to the disease in Sanskrit and in Chinese writings. The Romans linked the disease with swamps. The properties of the Qinghao plant were described in Chinese manuscripts from around 340 CE. Derivatives of the active ingredient of this plant, known as artemisinins, are still a mainstay in the treatment of malaria. From these writings, we can conclude that malaria has been a worldwide problem for a great many years.

The Nobel Committee has recognized the importance of malaria research with the award of four prizes. The first went to Ronald Ross in 1902 for his description of malaria parasites in the gastrointestinal tract of mosquitoes and the elucidation of the complete life cycle of the parasites. Charles Louis Alphonse Laveran was awarded the 1907 prize for much earlier work that described malaria parasites in the blood of a malaria patient. Camillo Golgi found that the fevers that are characteristic of the disease coincided with the rupture of parasite-laden red blood cells; he was awarded the 1906 prize. In the summer of 2015, the prize was awarded to Youyou Tu for her development of artemisinin (also known as qinghaosu) and dihydroartemisinin. Her initial work was done in a secret laboratory and was designed to aid North Vietnamese soldiers during the war with the United States. She and her assistants reviewed more than two thousand recipes for traditional Chinese medicines before developing an effective strategy for extracting the drug from sweet wormwood (Artemisia annua), a common plant in China.24

Important advances in the methods needed to control malaria were made during the US occupation of Cuba, after the Spanish-American War, and during the construction of the Panama Canal. The beginning of the end of malaria in the United States came when malaria control was integrated into the Tennessee Valley Authority’s mission. As a result, the disease is now uncommon in this country.

Malaria still poses a low-level threat. In its 2014 Malaria Surveillance Report, the CDC reported that there were 1,687 cases in the United States during 2012.25 All but four of these were imported into the country. Although rare, transmission in the United States has been reported to occur due to infected mosquitoes; the potential is the highest in the South due to the abundance of potential vectors, so-called airport malaria caused by infected mosquitoes that hitchhike aboard aircraft, congenital malaria due to transplacental spread, and transfusion transmission.

All malaria parasites belong to a single genus, Plasmodium. According to the CDC malaria website, there are approximately one hundred different species in this genus; however, only five routinely cause human disease.26 The most common species affecting humans are P. falciparum, found worldwide in subtropical and tropical areas, and P. vivax, found mainly in some parts of Africa, Asia, and Latin America. P. ovale is found mainly in Africa and islands in the western part of the Pacific. These are known as tertian forms of malaria, because fevers recur every forty-eight hours. P. malariae is found worldwide and is the only one of the group that has a three-day cycle, with fevers occurring every seventy-two hours (quartan malaria). P. knowlesi occurs in macaque monkeys, but it may infect humans and progress rapidly because of its twenty-four-hour replication cycle.

Malaria parasites exist in three stages: a mosquito stage, a human liver stage, and a human blood stage. Disease transmission occurs when an infected female mosquito bites a human, inoculating him or her with the parasites. The parasites travel to the liver and enter a liver cell, where they replicate and mature. Eventually, the infected liver cell ruptures, releasing the mature form of the parasite into the blood. The parasites then undergo asexual multiplication in red blood cells. The diagnosis of malaria can be made at this stage by examining appropriately stained red blood cells with a microscope. In time, the infected red cells rupture, releasing the parasites into the blood. Most clinical manifestations of the disease occur during this stage. The release of hemoglobin into the bloodstream can be massive and may cause severe anemia and kidney damage or failure. When hemoglobin is released into the blood stream, it colors the urine, a condition known colloquially as blackwater fever. (Note: Other disease states that cause red blood cells to rupture also can cause the urine to become dark in color.) When an uninfected mosquito bites an infected human, the mosquito ingests parasites and becomes infected. Another stage of the life cycle occurs in the mosquito, where parasites eventually mature into the form that can infect a human host during a blood meal.

This is a simplified description of the very complex malarial life cycle. Each stage in the cycle consists of several steps, and each of those steps is susceptible to modification. For those who are gluttons for punishment, more detailed descriptions, complete with the technical names for the parasite at various steps in each stage, can be found on the CDC malaria website mentioned earlier.

The mosquito stage is particularly susceptible to climate change and the dilution effect, described earlier. As the number of mosquito species decreases, possibly as the result of a changing climate, the probability of transmission rises.27

To better understand malaria and the factors affecting its transmission, it is necessary to be familiar with mosquitoes and some aspects of their life cycle. According to Wikipedia, there are over 3,500 different species of mosquitoes. Not all mosquitoes consume blood, and of those that do, pressure gradients between the host and the insect determine in part whether the mosquito has the capability to transmit disease. Only female mosquitoes consume blood. Mosquitoes have four stages of development: eggs, larvae, pupae, and adults. Most mosquitoes lay their eggs in water. Some are quite fussy and have a limited range of choices, and others are generalists and will lay their eggs almost anywhere. The elimination of breeding grounds has been of singular importance in malaria eradication. Therefore, an understanding of any preference or requirement is likely to be a critical element in mosquito control.

Anopholes mosquitoes, the vectors for malaria, breed in water and usually bite during the night. The introductory chapter referred to the life cycle of moths that live in the fur of sloths and the potential for many factors to disrupt this complex cycle: the same potential exists for Anopholes mosquitoes. Water preferences vary, including any shallow collection, such as rice paddies, puddles, or even the hoofprints of animals. Thus, rainfall patterns, temperature, and humidity play major roles, which are all subject to the effects of climate change.28 For most species of Anopholes mosquitoes, warm tropical temperatures and high humidity favor development and maturation of the parasites in the female insects. The risk of transmission is greatest at the end of rainy seasons, when mosquito eggs hatch and the larvae that live and develop in water mature into adults. The highest risk areas are those where weather or climate conditions change to favor mosquito reproduction and where the human population has a low immunity to the parasite.

Improvements in the control and reduction in the number of cases of malaria is one of the WHO Millennium Development Goals. Curiously, the 2013 progress report, while detailing many aspects of malaria control and threats to further progress, fails to address climate change as a threat.29 However, the IPCC Working Group II report points out a number of climate-related events and circumstances that are likely to lead to increases in the risk of the development of malaria.30 These challenges include large-scale disruption of populations due to the consequences of floods, rising sea level, and changes in precipitation; food insecurity, leading to undernutrition; and violence and the subsequent disruptions of social systems. Violence has been a severe problem in many parts of Africa, and undernutrition is a particular problem among children. Pregnancy is also cited as a risk factor for malaria. This increase in risk is thought to be due to changes in the immune system, formation of the placenta, and anemia. Disruption of the public health infrastructure due to the global financial crisis of the first part of this decade appears to have contributed to the decline in progress toward the eradication of malaria during the past several years, as fund-raising goals went unmet.31

In its analysis of vector-borne illnesses, the IPCC reports that there is high confidence that there is a positive association between temperature and humidity and malaria at a local level (where high confidence is a function of agreement among reports, their type, amount, quality, and consistency of the evidence).32 In looking to the future, it is likely that malaria will spread into regions where it is not already present, while in other regions it may diminish. In some regions, malaria is already so prevalent that there is little or no room for an increase. Thus, it is possible that the global disease burden for malaria could change little but that there could be major changes in the distribution of risk.

Water-Borne Illnesses

Many of the diseases in this category are transmitted by exposure to or the ingestion of contaminated drinking water or water used for bathing, washing, or swimming. Infections may also occur after exposure of a cut or other open wound or contact with eyes or ears. The oral–fecal route is particularly important in the transmission of these diseases, which include cholera. Other diseases are due to infections with the Salmonella and Campylobacter species of bacteria. Outbreaks of the latter two are frequently associated with warming weather.


Cholera is caused by the bacterium Vibrio cholerae. The disease manifestations are caused by toxins produced by the bacteria and not the bacteria themselves. There are over two hundred identifiable strains of the bacterium, but only two produce the toxin responsible for manifestations of the disease. Vibrios typically exist among phytoplankton. Small crustaceans known as copopods feed on plankton, and even though they may be only a few millimeters in size, a single copopod may contain a large enough number of bacteria to cause disease if ingested. Usually, however, the cholera bacteria are in an inactive state in the plankton. The threat of a disease eruption rises when algae bloom (proliferate). A single case of the disease can then give rise to many others, triggering an epidemic.

Because of links between temperature, precipitation, and time of year, several groups have been successful in defining relationships between the incidence of cholera and climatological variables. In Southeast Africa, the incidence of the disease increased by an annual factor of 1.08 between the years 1971 and 2006, an increase attributed primarily to global warming.33 In a more complex study of cholera in Bangladesh, investigators found relationships between temperature and the number of hours of sunlight per day.34 Increases in both were associated with increases in the number of new cases per month. However, since the cloud cover increased during the warmer summer, the synergistic effect was blunted. In another study of cholera in East Africa, the investigators found a significant interaction between temperature and rainfall amounts.35 Each of these studies shows a significant relationship between climate and cholera, and, as a group, they help pave the way for predicting outbreaks. Armed with these data, public health officials may be able to better prepare for the future.

After a steady rise in the number of cases of cholera reported to the WHO in the beginning of this century, the number fell substantially in 2012.36 During the intervening time, there was a shift in the burden of the disease from Asia to Africa. In 2012, two-thirds of all cholera deaths occurred on this continent. The Democratic Republic of Congo was hit the hardest that year with 33,611 cases and 819 deaths, a case fatality rate of 2.4 percent. The death rate in all of Africa was 1.7 percent, compared to a rate of 0.4 percent in Asia and 0 percent in Europe. Many of the deaths in the second decade of this century were the result of an epidemic in Haiti. This epidemic serves as an excellent example that illustrates the increased risk of cholera after natural disasters such as tropical storms and flooding, which are likely to occur more often due to climate change.

Cholera is characterized by nausea, vomiting, and severe diarrhea that may take on a milky appearance, so-called rice water diarrhea. In severe cases, victims may produce as much as five gallons of diarrheal fluid in a day, giving rise to severe dehydration, circulatory collapse, and death. The disease often progresses very rapidly, with only hours separating the onset of symptoms and death. Untreated severe cholera typically has a 50 percent mortality rate. However, with the widespread introduction of oral-rehydration therapy, the death rate can be below 1 percent. So-called cholera cots help prevent the spread of the disease in a healthcare facility by channeling feces into a receptacle. Measuring fecal volume and vomitus guides rehydration efforts and prevents volume depletion and shock. Antibiotics usually are not needed. This disease is most likely to occur in areas with poor sanitation and unsafe drinking water.

The Haitian epidemic began suddenly and explosively. The nation had been free of cholera for almost one hundred years.37 That changed on October 18, 2010, when nine patients with diarrhea were hospitalized at the Mirebalis Hospital. Two days later, another nine were hospitalized. Three days later, thirty-five required hospitalization. A similar series of events unfolded at the Albert Schweitzer Hospital, where twenty-four patients were hospitalized on October 20. A total of five had been hospitalized during the preceding three days. Things were even worse at St. Nicholas Hospital, where 404 patients were hospitalized on October 20. There were forty-four deaths that day. On the day before, there were no hospitalizations for diarrhea. The Haitian National Public Health Laboratory rapidly determined that these patients were suffering from cholera. Within days, virtually the entire nation was at risk.

Although cholera persists in Haiti, the number of new cases has declined substantially. In a report issued at the end of June 2014, the Pan American Health Organization, part of the World Health Organization said there had been 703,510 cases of cholera in Haiti, including 393,912 hospitalizations and 8,562 deaths that were attributed to the epidemic.38

The conditions that led to the epidemic followed the devastating magnitude 7.0 earthquake that struck the nation on January 12, 2010. In spite of international aid from many nations, the infrastructure of this already poor nation was damaged severely, contributing to the conditions that led to the epidemic. To make matters worse, Hurricane Tomas struck the country on November 5. The ensuing flooding and damage to an already weakened country exacerbated the epidemic. In a final coup de grace, many of the climatic conditions in Haiti—including warm moist weather with critical mixing of seawater and freshwater at the mouths of rivers—favored spread of the disease.

Although there was a considerable amount of controversy concerning the origin of the outbreak, it seems virtually certain that it arose from a United Nations Stabilization Mission in Haiti (MINUSTAH) camp along the Artibonite River, where sanitation conditions were wretched and insufficient to protect river water. Molecular biological techniques were used to evaluate the bacterial isolates. These studies showed that the serotype of the bacteria was typical of the South Asian strains, proving that the bacteria did not originate in Haiti. The MINUSTAH camp in question was staffed by Nepalese; blaming them touched off a political storm. This appears to be an example of a disease being transported from one part of the world to another due to globalization.

Many of the recommendations of the Independent UN Committee that investigated the Haitian epidemic apply to preparing for a world affected by global warming and the expected increase in the severity of extreme weather events, such as tropical storms and coastal flooding. First, improving the infrastructure associated with delivering reliable, safe water supplies is essential. There also must be appropriate mechanisms in place for the disposal of human waste to prevent contamination of water used for drinking, bathing, and washing. Emergency responders must be prepared and screened appropriately to prevent the introduction of disease, as is suspected to have occurred in Haiti, and to prevent the outbreak of disease among aid workers.

Hantavirus Diseases

Hantaviruses are a relatively new discovery. These viruses are carried by but do not cause disease in rodents. According to the hantavirus site maintained by the CDC, the rodents most likely to be carriers in the United States include deer mice, cotton rats, rice rats, and white-footed mice. Hantavirus disease gained attention during the Korean War, when around 3,200 UN soldiers developed a hemorrhagic fever that led to the isolation of the virus.39

There are two conditions linked to infection by the virus: Hemorrhagic Fever with Renal Syndrome (HFRS) and Hantavirus Pulmonary Syndrome (HPS). HFRS typically begins with relatively nondescript symptoms, including fever with chills, headache, back and abdominal pain, and nausea. The full-blown illness consists of five phases: the febrile phase; a hypotensive phase characterized by low platelet counts, low blood pressure, and low amounts of oxygen in the blood; an oliguric phase, with reduced urine output due to renal failure; a diuretic phase in which urine output may reach a gallon or more daily; and a convalescent phase. Death or permanent kidney damage may occur.

HPS, the form occurring most commonly in the United States, begins like HFRS. Those with the most serious manifestations rapidly develop shortness of breath, with chest x-ray evidence of adult respiratory distress syndrome. In spite of supportive therapy with oxygen and mechanical ventilation, the mortality rate is around 40 percent. HPS was identified near the Four Corners area of the United States when a man died after developing respiratory failure while on his way to his fiancée’s funeral. She had died of a similar illness. Subsequent evaluations determined that he had a hantavirus infection.

These illnesses are much more common than many people appreciate. The most recent IPCC report estimates that about two hundred thousand patients are hospitalized each year due to hantavirus infections. Between 1951 and 1986, 14,309 individuals were hospitalized in Korea with the disease, although it is much less common in the United States: According to the CDC, there were 606 confirmed cases between 1993 and the end of 2013.

The 1993 outbreak and subsequent cases identified more recently among visitors to Yosemite National Park led to an intensification of efforts to study the disease. Once again, disease ecologists have made important contributions to such studies.40 Their investigations began with the observation that the rodents carrying the disease were much more common after periods of above-normal precipitation in desert areas. They then combined data from vegetation maps derived from satellite data, hydrological data from weather stations, and geological information (such as elevation and the slope of the terrain) to make highly accurate predictions of the rate of infection in deer mice. It also seems likely that, once again, patterns of precipitation in the southwest desert are dependent on El Niño and the Southern Oscillation and are related to increases in the risk of hantavirus diseases.


Leishmaniasis is a disease caused by an infection by any one of approximately twenty species of protozoa from the genus Leishmania. WHO estimates that there are around 1.3 million new cases and between 20,000 and 30,000 deaths each year. There are three main forms of leishmaniasis. The most serious, visceral leishmaniasis, kala-azar, is characterized by fever, enlargement of the liver and spleen, and anemia. It is fatal if untreated. The vast majority of new cases (about 90 percent) occur in Bangladesh, Brazil, Ethiopia, India, Sudan, and South Sudan. Cutaneous leishmaniasis is the most common form and causes ulcers on the skin that leave scars and may cause serious disability. About two-thirds of the cases occur in Afghanistan, Algeria, Brazil, Columbia, Iran, and Syria. Mucocutaneous leishmaniasis causes destruction of mucous membranes in the nose, mouth, and throat. Bolivia, Brazil, and Peru harbor most of these cases. It is rare in the United States, with most cases contracted during visits to areas where it is endemic.

The disease is transmitted by the bite of infected female sand flies (more specifically, phlebotomine flies). A number of animals act as reservoirs for the parasite. The range of the reservoirs and flies is affected by temperature, rainfall, and humidity. Thus, the disease is likely to be affected by climate change. This issue has been addressed. The risk of it spreading to North America, where increases in the range of the disease are predicted as climate change becomes more severe, was discussed in a 2010 report.41 Recent research has shown that sand flies benefit from the leishmanial infection by gaining resistance to pathogens that affect them.42

The disease can be diagnosed by observing the parasites with a microscope in samples of white cell–enriched blood or bone marrow. It can be treated with antibiotics, particularly lysosomal amphotericin B. Preventive measures include sleeping under insecticide impregnated nets.

The Trajectory toward the Future

Large numbers of people worldwide have diseases, as described in the preceding sections, that are likely to be affected by climate change. Many, but not all, are tropical in their distribution, and their range may expand in a warmer world. Many of these diseases are zoonotic; that is, they are transmitted from other animals to humans. Thus, understanding the future impacts of these diseases depends on an understanding of the effects of climate on the animal host and the organism responsible for causing the disease.

Disease ecology is likely to become increasingly important, particularly for diseases with limited or poor treatments. The study of the interactions between climate variables, such as temperature and precipitation; disease vectors, such as mosquitoes; pathogens, such as malaria parasites; and how changes in climate affect disease patterns will become increasingly important. These studies have been aided enormously by the development of remote sensing equipment, often carried by satellites. These data, combined with mathematical models that link climate variables to disease patterns, have already led to improvements in public health. For example, armed with the results of these models it should be possible to mobilize the resources needed to combat predicted outbreaks or increases in the risk of contracting a specific disease. Seemingly remote changes in local conditions that are associated with El Niño and the Southern Oscillation can be used to predict changing patterns of disease.

Using this knowledge effectively may be more difficult than acquiring the knowledge in the first place. As discussed in the first chapter, lack of political will, reluctance to mobilize resources to help distant populations, adverse local conditions due to political considerations, and violence are almost certain to create barriers to developing measures that are needed to prevent and adapt to climate change.

Copyright © 2016 Massachusetts Institute of Technology. (All rights reserved.)

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