The threatened plague

Posted: 15 March 2003

Author: Paul Epstein

As an unknown pneumonia virus threatens to spread from Asia, experts warn that environmental and social changes are leading to the outbreak of new diseases and a resurgence of the old ones. In this special report Dr Paul Epstein, a leading expert in health and the global environment, reviews the evidence.

According to The World Health Organisation, 30 new diseases have emerged in the past quarter of a century.

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. Others, like West Nile virus (see below), are undergoing redistribution.

Global changes

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.

Now diseases of trees, coral reefs, crops and wildlife – outcomes of the same set of driving forces – have, themselves, become drivers of global change as they alter habitat and threaten species that are endangered or of key importance.

Floods, Bangladesh© Trygve 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, ladybugs 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 generalised 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 along coastlines worldwide.

    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.

    Climate change is expected to have a growing impact. 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 Pictures

    Rodent-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.

    As the potential risks from pesticides for disease control must be weighed against the health risks of the disease, an early warning system of conditions conducive to amplification of the enzootic cycle could help initiate timely preventive measures, and potentially limit chemical interventions, throughout the Americas.

    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

    There is evidence, too, that deep ocean warming is occurring and 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.

    Insurance costs associated with extreme weather events, some have which have increased in intensity; some of which have also increased in frequency. Averaging about $4 billion/yr in the 1960s, the rose an order of magnitude in the ‘90s (~ $40 billion/yr). UNEP projects that they could rise to $150 billion losses annually within a decade if current trends continue.

    Climate stability

    Stability of the current climate regime (the “Holocene”) is the primary concern. Past records indicate that climate change can occur abruptly. Some minor changes appear to have already occurred in the past few decades.

    In 1976 Eastern Pacific Ocean temperatures warmed significantly, still further in 1990, and cooled in 2000; while those in the East remain excessively warm and the patterns of have changed. Meanwhile, recent warming in the Northern Hemisphere has melted a lot of North Polar ice.

    Since the 1970s the floating North Polar ice cap has thinned by almost half. A second source of cold fresh water comes from Greenland, where continental ice is now melting at higher elevations each year. Some melt water is trickling down through crevasses; lubricating the base, accelerating ice 'rivers', and increasing the potential for sudden slippage.

    A third source of cold fresh water is rain at high latitudes. Overall Ocean warming speeds up the water cycle, increasing evaporation. The warmed atmosphere can also hold and transport more water vapour from low to high latitudes. Water falling over land is enhancing discharge from five major Siberian rivers into the Arctic, and water falling directly over the ocean adds even more fresh water to the surface.

    The cold, freshened waters of the North Atlantic accelerate transatlantic winds, and this may be one factor driving frigid fronts down the eastern US seaboard and across to Europe and Asia in the winter of 2003. The North Atlantic is also where deep-water formation drives thermohaline circulation, the “ocean conveyor belt” considered key to climate stabilization. In the past few years the northern North Atlantic has freshened, and since the 1950s the deep overflow between Iceland and Scotland has slowed by 20 per cent.

    Slowing pump

    The ice itself and pollen and marine fossils reveal that cold reversals have interrupted warming trends in the past. The North Atlantic Ocean can freshen to a point where the North Atlantic deep water pump -- driven by sinking cold, salty water that is, in turn, replaced by warm Gulf Stream waters – can suddenly slow down.

    Labrador current

    Some 13,000 years ago, when the globe was emerging from the last Glacial Maximum and continental ice sheets were thawing, the Gulf Stream abruptly changed course and shot straight across to France. The Northern Hemisphere refroze - for the next 1,300 years - before temperatures jumped again in just several years, warming the world to its present state. In the past few years the northern North Atlantic has freshened, and since the 1950s the deep overflow between Iceland and Scotland has slowed by 20 per cent.

    Calculations (of orbital cycles) indicate that our hospitable climate regime was not likely to end due to natural causes any time soon. But due to the burning of fossil fuels, atmospheric levels of carbon dioxide are now greater than at any time in the last half million years. The recent build-up of heat-trapping greenhouse gases is forcing the climate system in new ways and into uncharted seas.

    These changes are plausibly contributing to a cold tongue from Labrador across to Europe and enhancing the Labrador Current that hugs the U.S East Coast. Such “paradoxical cooling” from warming and ice melting could alter projections for climate, weather and disease for Northern Europe and the Northeast of the U.S. It is the instability of weather patterns that is of most concern for public health and society.

    The task ahead

    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 central to all 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 (22nd)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. Emerging diseases are now threatening coastal marine habitat, forests and wildlife and together pose new threats to the life support systems upon which we all depend.

    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. West Nile virus

    West Nile virus (WNV) was first reported in Uganda in 1937. WNV is a zoonosis (animal disease), with “spill-over” to humans, which also poses significant risks for wildlife, zoo and domestic animal populations.

    While it is not known how West Nile virus (WNV) entered the New World in 1999, anomalous weather conditions may have helped amplify this Flavivirus that circulates among urban mosquitoes, birds and mammals.

    Analysis of weather patterns coincident with a series of urban outbreaks of St. Louis encephalitis (SLE) (a disease with a similar life cycle) in the Unites States, and recent large outbreaks of WNV in Europe, reveal drought as a common feature. Culex pipiens, the primary mosquito vector (carrier) for WNV, thrives in city storm drains and catch basins, especially in the organically rich water that forms during drought and the accompanying warm temperatures.

    WNV is primarily a disease of wildlife and there is also evidence of infection in birds (120 species) and other animals, primarily horses. Raptors (owls, kestrels) have been particularly affected; West Nile virus likely caused thousands of birds of prey to die in Ohio and other states in July 2002. Some zoo animals have died (e.g., 8 Humboldt penguins in the Milwaukee Zoo and macaques.)

    Note: The population impacts on wildlife and biodiversity have not been adequately evaluated. The impacts of declines in birds of prey could ripple through ecological systems and food chains, and could in itself contribute to the emergence of disease. Increases in rodents, for example, could increase the potential for hantavirus, Lyme disease, leptospirosis and plague.


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

    The mission of the Center for Health and the Global Environment is to study and promote a wider understanding of the human health consequences of global environmental change, believing that people will be motivated to protect the natural environment when they recognize that their health depends on it.