Emerging diseases in a warmer world

Posted: 19 February 2001

Author: Paul Epstein

Author Info: Dr Paul Epstein is Associate Director of the Center for Health and the Global Environment at Harvard Medical School, Cambridge, USA.

Environmental and social changes are leading to the outbreak of new diseases and a resurgence of the old ones warns Dr Paul Epstein, a leading expert in health and the global environment.

According to The World Health Report 1996, 30 new diseases have emerged in the past two decades. The resurgence of old diseases is equally concerning: drug-resistant tuberculosis, exacerbated by HIV/AIDS, causes three million deaths annually, while diphtheria, whooping cough and measles ­ also transmitted person-to-person ­ are increasing, particularly where social systems have deteriorated.

Malaria, dengue ('breakbone') fever, yellow fever, cholera and rodent-borne viruses are also appearing with increased frequency. These diseases, transmitted by animals or water, reflect environmental and social change.

Global change, ranging from global warming to ozone depletion, deforestation and coastal pollution, are together altering biological diversity. And environments experiencing multiple stresses are showing increased susceptibility to the invasion and emergence of opportunistic species.

Floods, BangladeshTrygve Bolstad/Panos Pictures

Weeds, rodents, insects and micro-organisms are opportunists: they reproduce rapidly, have huge broods, small body sizes, wide-ranging appetites and are good at dispersal and colonization of new environments. In stable environments large predators fare well and keep opportunistic species under control. But in degraded environments, opportunists can seize the upper hand; just as opportunistic infections take advantage in patients with weakened immune systems.

Owls, coyotes and snakes, for example eat rodents; and rodents can devour grains and carry Lyme disease ticks, hantaviruses, arenaviruses (South American hemorrhagic fevers), human plague and leptospirosis bacteria. Control of mosquito populations is naturally performed by reptiles, birds, spiders, dragonflies, damselflies and bats ­- and fish that consume their larvae in ponds. Mosquitoes provide nourishment for these animals - but some carry malaria, yellow fever, dengue fever and several types of encephalitis.

Several aspects of global change tend to reduce predators disproportionately, releasing prey from their biological controls. Among the most widespread of these are: habitat loss and fragmentation; monocultures in agriculture and aquaculture; excessive use of toxic chemicals; excess ultraviolet radiation; and climate change and weather instability. Fragmentation of wilderness into smaller patches, compounded by "edge effects," reduces the habitat for large predators, favouring pests. Monocultures, with reduced genetic and species diversity, show increased vulnerability to infections and invasions of exotic species. Excessive use of pesticides harms birds and "helpful" insects. Rachel Carson writing in 1962 in Silent Spring referred to the absence of the chorus of birds in spring, and the resulting resurgence of herbivores that had evolved resistance to the pesticides.

Population explosions of nuisance organisms may thus be viewed as signs of failing ecosystem health ­ of systems removed from equilibrium, where the ratio of animal groups performing essential functions is altered. Such multiply-stressed systems exhibit reduced resilience and resistance in the face of new stresses.

Distress syndrome

Some ecologists have begun to describe what might be considered as generalized Environmental Distress Syndrome. The symptoms of this syndrome would include:

  • Emerging infectious diseases.

  • Loss of biodiversity.

  • The growing dominance of "generalists" (such as crows, Canadian geese and gulls ­ that have wide-ranging diets) over "specialists" (like plovers ­ with disappearing, localized niches).

  • The decline in one type of specialists ­ the pollinators (bees, birds, bats, butterflies and beetles) ­ whose niches and activities fit with, and are indispensable for, the preservation of flowering plants.

  • The proliferation of harmful algal blooms and "dead zones" in semi-enclosed seas.

Emerging diseases

Outbreaks of vector-borne diseases are compounded by a range of social, biological and environmental factors. Peri-urban sprawl, poor sanitation and proliferating water containers, and social inequities in general, provide the setting for the resurgence of dengue fever in Latin America, for example.

But meteorological factors also play a role. In general terms, climate circumscribes the range at which such diseases can occur, while weather influences the timing of outbreaks. In the tropics, rain is the limiting factor; in the extra-tropics and at high altitudes, temperature and precipitation are key parameters.

The likely impact of climate change, in this respect, is discussed in the interview with Dr Rudi Slooff in this site, but already insects and insect-borne diseases, such as malaria and dengue fever, are being reported at higher altitudes in Africa, Asia and Latin America. Highland malaria is becoming a problem for rural areas in Papua New Guinea and for urban centres in Central Africa. In 1995, dengue fever blanketed the Americas, crossing mountain ranges that previously presented barriers to spread.

At the same time, extreme events such as floods, storms, droughts and un-contained fires are expected to accompany global warming ­ events which can be devastating for agriculture, for human settlements and for health. Heatwaves and winter storms both usher in cardiac deaths. Floods spread bacteria, viruses and chemical contaminants, foster the growth of fungi and favour insect breeding. Prolonged droughts interrupted by heavy rains favour population explosions of insects and rodents.

Extreme weather events (most often associated with El Niño/La Niña changes in Pacific surface ocean temperatures) have been accompanied by malaria outbreaks in Asia and water-borne diseases like typhoid, hepatitis A, bacillary dysentery and cholera, in Latin America and in Asia.

Nasty synergies

Rodents are a growing problem in the United States, Latin America, Africa, Europe, Asia and Australia. Believed to be the fastest-reproducing mammal, rodents eat everything humans do, thrive on contaminated water and food, and are even great swimmers. Rodents consume 20 per cent of the world's growing and stored grain; 13 per cent in the United States, and up to 75 per cent in some African nations. Rodents also can carry diseases.ratRats and rodents thrive in degraded environments© Mark Edwards/Still PicturesRodent-borne hantaviruses have resurged in several European nations, particularly in former Yugoslavia; and rodent-borne diseases like leptospirosis are increasingly reported in urban centres in America, where sanitation has declined. In late 1996, hantavirus infection emerged in western Argentina.

In Southern Africa, in 1994, rodent populations exploded in the aftermath of 1993 and 1994 rains; and plague reappeared in India in 1994, following a blistering summer, leaving animals prostate across the north and creating furnaces for fleas in houses with stored grains. The unusually heavy monsoons following the heatwave led to population crowding in Surat; and an apparent outbreak of pneumonic (person-to-person) plague. Malaria and dengue fever upsurges also followed in the wake of flooding. Current land-use practices and the overuse of chemicals to control pests may increase the chances for such nasty synergies. A disturbance in one factor can be destabilizing; multiple perturbations can affect the resistance and the resilience of a system.

West Nile virus

In the spring and summer of 1999, mid-Atlantic US States baked under intense drought, until torrential end-of-August rains - capped by Hurricane Floyd - drenched the coast. A plausible scenario of how droughts amplify diseases such as West Nile virus, that cycle among urban mosquitoes, birds and humans is as follows. First, droughts boost the populations of city-dwelling mosquitoes (Culex pipiens)in foul water that concentrates in catch basins where they breed. (Another mosquito - Aedes vexans - breeds in standing water following rains. Aedes transmit Eastern Equine encephalitis, and can carry WNV, though not as efficiently as Culex). Second, mosquito predators like frogs, darning needles and dragonflies, decline in droughts. Third, birds congregate around shrinking water sites, enhancing viral circulation among birds and mosquitoes; and finally, heatwaves speed up the maturation of the viruses within mosquitoes.

Coastal ecosystems

Coasts throughout the world are subject to increasing pressures, among which is a rate of human population growth double the global average. Among other pressures are excessive nutrients from sewage, fertilizers and aerosolised acid (nitrogen) precipitation; a reduced acreage of wetlands, which act as nature's kidneys to filter nitrogen and other wastes; overfishing, that can reduce predation; and chemical pollutants and excess UV-B penetration that may increase mutation levels.

In addition, global warming increases algal growth and photosynthesis, and can help shift algae to more toxic species.

In fact, all these factors favour the growth of coastal algae. And warming may also reduce the immune systems of sea mammals and coral and encourage the growth of opportunistic infections.

Cholera is more widespread today than ever before, and there is evidence that it can be harboured in marine plankton. In 1991, cholera reached the Americas. During the first 18 months over 500,000 cases occurred in Latin America, with 5,000 deaths. In 1991, Peru lost $770 million in seafood exports and another $250 million in lost tourist revenues.

Ocean warming

Recent analysis of ocean temperature data by the National Oceanic and Atmospheric Administration indicates that all oceans warmed in the past half century - down to 3 kilometres. Warmed ocean temperatures in the North Atlantic have helped thin the North Polar sea ice and warm air is reducing Greenland ice sheets. Freshwater intrusions from melting ice have apparently generated two cold tongues: one accentuating the Labrador Current that hugs the Northeast US coast, and another joing the transaltantic Gulf Stream that has moderated weather in Northern Europe.

Global warming is not uniform over the globe, and its pattern will change over time. Disproportionate warming at northern latitudes in the past several decades may have created a flicker in the climate system, which may or may not be temporary. But we can expect greater variance and more erratic weather, and more extreme extremes.

There is evidence, too, that deep ocean warming may be harming marine plankton. It may also be associated with a shift in marine flora and fauna occurring along the California coast since the 1930s.

Warming - in the presence of sufficient nutrients - may also be contributing to the proliferation of coastal algal blooms. Harmful algal blooms of increasing extent, duration, intensity - and involving new species - are being reported from nations throughout the globe. Indeed, "the worldwide increase in coastal algal blooms may be one of the first biological signs of global change" according to Theodore Smayda, a leading authority in this field.

Counting the cost

The impacts of disease on humans, agriculture and livestock can be costly. While the 1991 cholera epidemic cost Peru over $1 billion, airline and hotel industries lost from $2 to $5 billion from the 1994 Indian plague. Cruise boats are turning away from Asian islands racked by dengue fever, threatening that region's $12 billion tourist industry (employing over 500,000 people). The global resurgence of malaria, dengue fever and cholera ­ and emergence of relatively new diseases like Ebola, toxic E. coli and Mad Cow disease ­ can affect eating habits, trade, tourism and politics.

More generally, we are using Earth's resources, and generating wastes at rates beyond which biogeochemical systems can adequately recycle them. Practices affecting forestry, fisheries, petrochemicals and fossil fuels must all be examined in light of their impacts on biodiversity and the global resurgence of infectious diseases across a wide taxonomic range.

To take just one example, the impacts of oil extraction in Ecuadorian forests at the headwaters of the Amazon may be felt throughout the pathway of that great basin, with untold consequences for water quality and marine biodiversity. Fossil fuel combustion, in turn, is having enormous local and global impacts.International governance is a key concept in dealing with such global threats. The Montreal Protocol for eliminating CFCs is an example; the United Nations Convention on the Laws of the Seas (UNCLOS) is another.

The Framework Convention on Climate Change (FCCC) is also essential, for carbon and heat budgets are key to living systems. Significant positive incentives (financial instruments) are needed in the FCCC to drive development of renewable energy sources and energy-efficient technologies, and stimulate markets to purchase their products. Encouraging the manufacture and distribution of renewables, of environmental restoration and reclamation could provide the stimuli for global and national economies well into the next century.

The global resurgence of infectious diseases in the latter quarter of the twentieth century is one of the consequences of compounding global-scale changes in physical, chemical, biological and social systems. We may be vastly underestimating the true costs of "business-as-usual" and underestimating the benefits to society as a whole of using the resources we have inherited efficiently.

Finding solutions

We have embarked on a global experiment that cannot be repeated. We have apparently underestimated just how sensitive biological systems are to small changes in average temperature and to the wide swings in extremes associated with warming.

But, there are solutions. Public health budgets must be substantially fortified; around the world. Forecasts of severe weather can help target surveillance, and can catalyze early, environmentally-friendly responses. And global warming can be tamed; but only by significantly reducing air pollution.

Burning carbon fossils powered the industrial revolutions of the 19th and 20th centuries. But the extraction, mining, refining and combustion of oil and coal now threaten precious river deltas, forests and human health. Alternative energy sources, such as solar and fuel cells, must be employed, and economic incentives must be restructered to rapidly "push" and "pull" these new technologies into world markets.

Clean energy sources and energy-efficient technologies can be the engines of economic growth in this century. Indeed, most oil companies have begun to shift priorities, and cities, towns and states and the nation can all play parts, beginning with clean transport and clean utilities.