A changing climate for disease and death
The combination of climate change and environmental degradation has created ideal conditions for the emergence, resurgence and spread of infectious diseases - diseases which kill more than 17 million people annually. Increased climate change has also altered the functional balance among predators and prey, which is important for controlling the proliferation of pests and pathogens. Warmer and sometimes wetter weather may already be extending the range of infectious diseases beyond regions where they are endemic. These were some of the disturbing conclusions of a study by Dr Paul Epstein of the Centre for Health and Global Environment, Massachusetts, USA.
Martin Jalleh highlights the warning signals of the dangers to human health posed by climate change to which Dr Epstein draws attention in his study.
‘CLIMATE change will have wide-ranging and mostly damaging impacts on human health,’ warns Dr Paul Epstein in a recent study entitled Human Health and Climate Change. ‘There have been periods of uncontrollable waves of disease that radically altered human civilisation in the past, such as when Europe’s population was devastated by bubonic plague in the Middle Ages. That problem was associated with population growth and urbanisation.
‘Now a warming climate, compounded by widespread ecological changes, may be stimulating wide-scale changes in disease patterns,’ Epstein remarks. His study seems to suggest that climate change could have an impact on health in three major ways, by:
(a) creating conditions conducive to outbreaks of infectious diseases,
(b) increasing the potential for transmissions of vector-borne diseases and the exposure of millions of people to new diseases and health risks, and
(c) hindering the future control of disease. ‘There are indications, he notes, ‘that this disturbing change has already begun.’
According to Epstein, extreme weather brings about a drastic increase in pests and hence the spread of diseases. ‘Climate restricts the range of infectious diseases, while weather affects the timing and intensity of outbreaks,’ he observes. Rates of insect biting and the maturation of microorganisms within them are temperature-dependent and both rates increase when the air warms.
‘Warming can also increase the number of insects...Between the limits of too hot and too cold is an optimum range of temperature in which warmer air enhances metabolism and the chances of disease transmission,’ the study reveals.
According to Epstein, ‘Fossil records indicate that when changes in climate occur, insects shift range far more rapidly than do grasses, shrubs and forests.’
Epstein also provides several examples of the strong link between climate change and the increase of pests. He begins with that of heavy rains producing insect-breeding sites, driving rodents from burrows, and contaminating clean water systems. He then goes on to a few specific examples. In southern Africa, rodent populations exploded in 1994, following heavy rainfall in 1993 that had been preceded by a prolonged drought. As a result, the maize crop in Zimbabwe was crippled and plague broke out in Zimbabwe, Malawi and Mozambique.
Both El Nino and La Nina bring climate extremes to many regions around the globe. During the cold phase, from 1995 to 1996, many regions of the world experienced intense rains and flooding, following prolonged drought.
Such rains have been associated with outbreaks of Murray Valley encephalitis and Ross River virus in Australia, and malaria in Argentina, southern Africa and Pakistan.
The dry phases that preceded the wet phase of El Nino also resulted in an increase in the incidence of disease. For example, meningitis epidemics ‘are associated with severe drought conditions, which apparently dry our mucus membranes, making them vulnerable to penetration by colonising organisms’. In sub-Saharan Africa, the 1995-96 outbreak was among the largest ever recorded: over 100,000 people contracted the disease and died.
Extreme climate changes like El Nino can in fact result in disease clusters (see box).
Epstein’s study claims that ‘...other diseases likely to increase and change in connection with the climate include Guinea worm, leishmaniasis, lymphatic filiasis, onchocerciasis, and Chagas’ disease, which altogether affect more than 147 million already.’
Another major conclusion made by Epstein is that ‘the combination of climate change and environmental degradation can create ideal conditions for the emergence, resurgence and spread of disease’.
He quotes a 1996 World Health Organisation report which states that at least 30 infectious diseases new to medicine have emerged in the past 20 years.
Dengue, or breakbone fever, which had essentially disappeared in the Western Hemisphere, has now reappeared in the Americas, infecting over 200,000 people in 1995. Also in 1995, the largest epidemic of yellow fever in the Americas since 1950, struck Peru.
Forms of hantaviruses have resurged in several European nations, particularly in the former Soviet Union and in the war-torn former Yugoslavia. In 1994, plague resurfaced in India following a blistering summer, when temperatures reached 510C (1240F), and an unusually heavy monsoon season.
Epstein also draws attention to another exceptional trend - ‘some infectious diseases are emerging for the first time in developed nations.’
Hantavirus pulmonary syndrome (HPS) and Lyme disease first appeared in the United States. Toxic E.coli 0157 has been a particular problem in the United States, Europe and Japan.
Transitions spur on transmissions
It is also Epstein’s contention that warmer and sometimes wetter weather may already be extending the range of infectious diseases beyond regions where they are endemic and inhabitants have some immunity.
He claims that ‘global warming is predicted to bring warmer winters to many places, and therefore increasing the potential for transmission of vector-borne diseases at higher latitudes and elevations.’ He uses malaria and dengue fever to support his argument.
According to Epstein, malaria is already being reported at unusually high elevations in the mountains of Central Africa as well as Ethiopia and parts of Asia.
‘Highland urban centres, like Harare, Zimbabwe and Nairobi, Kenya, are at increasing risk of outbreaks and are largely unprepared to deal with them.’
Epstein highlights a study which suggests that the proportion of the globe that could sustain malaria transmission would increase from 45% to 60% with the doubling of CO2 emissions.
‘Approximately 270 million are affected by malaria worldwide. Global warming may cause one million additional deaths from malaria each year,’ Epstein warns.
Epstein also believes that climate change is a major cause of the spread of dengue fever, which now occurs regularly in Asia and throughout Latin America. ‘It is projected that global warming will significantly increase the range conducive to the transmission of both dengue and yellow fever,’ says Epstein. ‘As if to confirm these predictions, dengue fever has been reported at higher elevations than before, at 1,240 metres in Central America, 1,000 metres in Mexico and Aedes aegypti was found at 2,200 metres in the Colombian Andes,’ he adds.
The transmission season may also be extended in regions that now lie on the margins of the temperature and moisture conditions that allow disease carriers to reproduce. Epstein uses encephalitis as yet another example of a close and real connection between climate change and the spread of vector-borne diseases.
Mosquitoes can transmit several viruses that cause inflammatory brain diseases in humans. Among these encephalitides are Japanese, eastern equine (in the US), Venezuelan equine, and others. The most common of these infections in the US, for example, is St. Louis encephalitis (SLE). Epidemic outbreaks are strongly associated with periods of a few days when temperatures exceed 300C. Particularly wet late winter months followed by summer drought, may exacerbate the threat. According to Epstein, global warming in the US could result in a more frequent and more northerly occurrence - even up to Canada - of a disease that is currently limited mainly to southern parts of the country.
Perishing predators, pervasive pests
A worrying trend which Epstein reports is the decrease in the number of predators (which also means an increase in pests) as a result of climate change. Epstein emphasises the role of predators. He says healthy ecosystems with preserved predator/prey ratios provide the natural biological controls over infectious diseases and their carriers.
Freshwater fish, birds, reptiles and bats limit the abundance of mosquitoes. Owls, coyotes and snakes help regulate populations of rodents. Some rodents are involved in the transmission of Lyme disease, hantaviruses, arenaviruses (haemorrhagic fevers), leptospirosis and human plague. He provides examples of the drastic consequences of the impact of climate change on predators.
In the south-western region of the United States, a prolonged drought prior to the spring of 1993 reduced populations of rodent predators such as those mentioned above. With the heavy spring rains, rodent populations multiplied 10-fold and hantavirus pulmonary syndrome (HPS), a deadly new disease, emerged.
Over 150 people in the United States have suffered from this viral disease and almost half of them have died. Outbreaks of HPS have also occurred in many Latin American nations since 1995. The heavy rains also provided a crop of grasshoppers and pine nuts which served as nourishment for the deer mice that carry hantaviruses. Thus, HPS may be deemed a 'new disease', the transmission and dissemination of which are mostly attributed to the increased climate variability accompanying climate change.
‘In marine systems, fish, shellfish and sea mammals help to regulate algae - some toxic, others anoxic. Destruction of habitat worldwide is reducing predator populations, and global warming may be increasing the ability of many disease vectors to survive and reproduce,’ Epstein comments. According to him, ‘warming may also compromise the immune system of sea mammals and coral, and encourage the growth of harmful bacteria and viruses in their tissues.’ He adds: ‘Among the possible consequences of disruption in almost any marine ecosystem is an increase in the opportunistic pathogens that can abet the spread of human disease, sometimes to widespread proportions. One example is cholera.’
The climate scenario is likely to take a turn for the worse, warns Epstein. He quotes the Second Intergovernmental Panel on Climate Change (IPCC) Report, which asserts that the frequency of very hot days is likely to increase, resulting in an approximate doubling of heat-related deaths in affected cities. Sea surface temperatures have risen during the last century. Warming has also been detected deep in the Atlantic, Pacific and Indian Oceans, and around both poles. The oceans may turn out to be the long-term repository of this century’s global warming.
Epstein predicts that ‘wide swings in weather patterns may become the norm, as sea surfaces and deeper waters continue to absorb and circulate the heat accumulating in the troposphere. At the same time, abrupt changes in climate - hopefully small enough to provide a warning and without widespread disruption - may be in store.’ He pleads in conclusion: ‘We cannot afford to continue ‘business-as-usual’ (BAU)! Changing course will not be easy, but it is necessary. There are costs associated with acting now to slow global warming. However, in terms of future health care, productivity, international trade, tourism, and insurance costs, the savings could be huge.’
Martin Jalleh is a research officer with the Third World Network.