Disease Vectors

The war on infectious diseases was once believed all but won. In the late 60’s, after the subduing of diseases such as smallpox, polio, and rheumatic fever, Surgeon General William H. Steward declared that it was “time to close the book on infectious diseases and pay more attention to chronic ailments...” (Patlack, 1996). Even as recently as 1978, all UN member nations signed the “Health for All, 2000” accord (Garrett, 1996), which effected measures that they then believed would lead to a future virtually free of infectious disease. Recently, however, a host of pathogens once thought all but eradicated have re-emerged, sometimes in epidemic proportions. Though the responsible factors are diverse, a clear trend has emerged: global environmental change has impacted almost every aspect of infectious diseases, from their life cycles to humans’ susceptibility to them, fostering their virulence and increased transmission. The prognosis is grim: the presence of infectious diseases in our world is a far cry from that envisioned by Steward and the UN, and, considering the accelerating pace of the activities driving such deleterious trends, the situation only stands to worsen.

Although global environmental change may be the factor most often fingered by doomsayers, its role in recent epidemiological trends poses a very real and persisting threat. Over the past century, global mean temperature has increased by .7 degrees Celsius (World Press Review, 1995). Though predictions vary somewhat, several estimates, including that of the UN’s Intergovernmental Panel on Climate Change, project an increase in global mean temperature of about 2.5 degrees Celsius by 2050 (Myers, 1994). Though some still debate the causes of global warming, these are irrelevant to the discussion at hand. Warming has occurred and, according to our best estimates, will continue to occur for some time. The preponderance of evidence indicates that the epidemiological impacts of this trend are many.

Most directly, warmer temperatures and changing climate conditions impact pathogenic organisms themselves. Heat both increases the metabolism of microorganisms, causing more rapid growth and division, and expands the territory they find hospitable (Platt, 1995). But other effects of climate change upon microorganisms are far more insidious. Heat, as well as increased UV-B radiation due to ozone depletion and pollutants, increase the selective pressure incident upon microorganisms (Linden, 1996); because microorganisms reproduce quickly, the effects of these pressures can manifest much sooner than could analogous evolutionary change in a population of higher animals. Furthermore, these same stress conditions can trigger bacterial organisms to express their plasmids (Garrett, 1996). Plasmids are rings of DNA separate from that of the cell; these often code for high-stress coping mechanisms, such as anti-biotic resistance and high temperature tolerance. To make matters worse, even bacteria of different species regularly exchange their plastids; as a result, the bacterial genome is much more dynamic and adaptable than that of higher organisms. Thus, a beneficial mutation in one species of bacteria can spread quickly to others, making a mutation-favoring environment, such as that environmental change is effecting, which is a dangerous problem indeed.

Despite this, the most prevalent effects of environmental change upon pathogens will likely manifest themselves in the organisms that transmit the pathogens. The prospective higher-stress environment affects disease vectors in a manner similar to its effects on pathogens. The ranges of flies, mosquitoes, rodents, and other disease carrying pests are expected to expand as a result of global warming (Platt, 1995). Furthermore, as is the case with microorganisms, a higher-stress environment renders pests more dangerous by selecting for tougher strains (which are often pesticide resistant), and by increasing metabolism. The latter is particularly pronounced in mosquitoes, which transmit several infectious diseases, including malaria and dengue fever (Patlak, 1996). Harvard epidemiologist Paul Epstein has calculated that a warming of only four degrees Fahrenheit (a temperature increase consistent with most estimates of global warming) would more than double the metabolism of mosquitoes (Linden, 1996), forcing them to feed at least twice as often, consequently increasing the transmission of the infections they harbor. Epstein also estimates that the same increase in temperature will “expand malaria’s domain from 42% to 60% of the planet” (ibid.). Furthermore, modeling done by Pim Martens at the Netherlands National Institute of Public Health and Environmental Protection “predicts an extra 100 million cases of malaria worldwide by 2100” (Pennington, 1995). Similar effects have been predicted with other pests and other pathogens (ibid.).

Environmental degradation due to global environmental change and human activities also forces pests into areas of human habitation that they usually avoid (Platt, 1995). This phenomenon is particularly evident in rodents, though hardly limited to them. Recently, rodent invasions as a result of environmental degradation have been linked to an outbreak of hanta virus in the American Southwest in 1993 (Patlak, 1996) and an outbreak of the pneumatic plague in India in 1994 (Linden, 1996).

Finally, the effects of warming on vectors of water-born pathogens are clear-cut; since water is the vector for such diseases, transmission is largely dependent on pathogenic concentration. This, in turn, is dependent on the pathogens’ ability to flourish which, as already addressed, is facilitated by higher temperatures; such temperature increases have been documented in the world’s surface water over the last 50 years (World Press Review, 1995). Though such dangers are largely precluded where adequate water treatment is available, this is not the case in much of the world (Platt, 1996), and recent outbreaks of water-born pathogens attest to that fact. Recent outbreaks of cholera have been linked to unusually warm waters and one in Peru has been directly linked to an El Nino event (Moore, 1998).

Finally, environmental change impacts infectious disease in a third way: by weakening the defenses of their hosts. The same factors which encourage the growth, virulence, and spread of pathogens also tend to suppress host immunity: “Heat, increased ultraviolet radiation...and pollutants like chlorinated hydrocarbons all suppress the disease battling immune systems - both for humans and other animals.” (Linden, 1996). Furthermore, an indirect effect of changing weather conditions, malnutrition, also contributes to the weakening of the immune system (ibid.). And the danger inherent in this is greater than it might at first seem, since once an ordinarily harmless strain of microorganisms begins to evade compromised individuals, that strain can often become virulent enough (with respect to the invaded organism) to infect healthy populations (ibid.).

All of these effects have been exacerbated by societal trends, such as globalization, urbanization and poor health care, but these may not be at the root of the problem. Some experts have expressed concern that the numerous recent outbreaks may be indicative of a flurry of microbial activity in response to environmental factors (ibid.); if this is the case, the real danger may not reside in any particular pathogen, but in a veritable army of emerging and re-emerging microorganisms ready to take the place of any we effectively control (ibid.). However, there is some disagreement among scientists regarding both the existence of climate impacts upon pathogens and the potential dangers those impacts harbor.

Some groups - many of them industry-funded - outright deny that global warming and other climate changes impact pathogens in any significant way (ibid.). Others claim that, though the effects are real, no statistical correlation has yet been established (Moore, 1998). Still others argue that health infrastructures will keep pace with the rising incidence of infection (Linden, 1996). In this scientific clime, only one thing is certain: the microbes are here to stay. Only our existence hangs in the balance.