| Climate and Weather |
Arthropods, important in the spread of many diseases, are particularly sensitive to meteorological conditions. Anopheles mosquitoes, for example, only transmit malaria where temperatures routinely exceed 60° F. Temperature influences the proliferation rate of the mosquito, as well as the maturation rate of the parasite within the insect. Mosquitoes live only a few weeks; warmer temperatures raise the odds that the parasites will mature in time for the insect to spread the protozoans to humans.
Global climate change has already altered the species ranges of a number of animals and plants. (See the Biodiversity unit.) Further change may increase the range of the mosquito vectors that transmit disease. This could expose sixty percent of the world's population to malaria-carrying mosquitoes. (Forty-five percent of the human population now reside in a zone of potential malaria transmission.) In fact, malaria is reappearing in areas north and south of the tropics, including the Korean peninsula and areas of Europe. During the 1990s outbreaks of locally transmitted malaria occurred in Texas, Florida, Georgia, Michigan, New Jersey, New York, and Ontario. Although these incidents probably started with a traveler or stowaway mosquito, conditions were such that the infection could be transmitted to individuals who had not been traveling.
Cholera and Global Climate Change
Global climate change may also bring flooding. In addition to creating breeding grounds for insects, this could increase the incidence of water-borne diseases such as cholera. The bacterium Vibrio cholerae causes seasonal outbreaks of intestinal infection so severe that individuals can lose as much as twenty-two liters (six gallons) of fluid per day. The intestinal lining becomes shredded so that white flecks, resembling rice grains, are passed in feces. Without adequate fluid replacement, death can occur in hours. During a 1991 epidemic in Bangladesh 200,000 cases were counted in only three months.
Historically, cholera (caused by V. cholerae) has been a problem in coastal cities, especially those where the quality of the water supply is poor. In a 1849 groundbreaking study, John Snow mapped cholera deaths in London and realized that victims had been drinking from the same well. The association between cholera and contaminated water was established, and appropriate water treatment seemed to bring the threat under control. Yet, especially in areas where water treatment is unaffordable, cholera epidemics continue.
Where does Vibrio cholerae go between epidemics? This question intrigued Rita Colwell and her associates. Surprisingly, they found the bacterium in Chesapeake Bay in a dormant, spore-like form that was difficult to culture in the laboratory. Colwell used antibodies, directed to a component of the bacteria's cell membrane, and was able to detect the dormant organism. In this form V. cholerae survives in a range of habitats, including seawater, brackish water, rivers, and estuaries. Colwell also found that wherever tiny crustaceans known as copepods were abundant so were the bacteria, which cling to the copepod and colonize its gut.
Understanding the reservoir for cholera may be important to unraveling the periodicity of epidemics. Colwell turned her attention to locations where cholera outbreaks were common, such as in Bangladesh. By reviewing data from satellite monitors, she noticed that seasonal peaks in sea-surface temperatures in the Bay of Bengal correlated with the number of cholera admissions in nearby hospitals. Similar correlations existed between sea-surface temperatures and South American cholera epidemics in the 1990s. It is possible that the rise in temperature raises sea-surface height, driving seawater into estuaries. Alternately, rising temperatures might provide the right set of environmental conditions to boost copepod populations, perhaps by increasing populations of the photosynthetic plankton, which copepods feed upon. In either case, recognizing the association between sea-surface temperature and cholera incidence may make epidemics easier to predict. The relationship between climate and epidemic also increases the concerns raised by global climate change.
Climate and Hantavirus
Weather patterns can also influence the numbers of vertebrate animals serving as reservoirs for human pathogens. In 1993, in the Four Corners area of the United States (where New Mexico, Arizona, Utah, and Colorado meet), researchers tracked an outbreak of pulmonary illness that killed half of those infected. The causative agent, hantavirus, was not a new threat but was endemic in the rodent population of the area. Researchers were able to find the deadly virus in mouse tissue archived years earlier. Hantavirus spreads to humans by rodent urine and droppings. During the mild, wet winter of 1993, piñon nuts, a favored food for the deer mouse, flourished. As rodent populations soared, the opportunities for mouse-human interactions increased. Native American legend describes an association between piñon nut abundance and illness. Scientists found an association between the periodic climate pattern El Niño-Southern Oscillation and outbreaks of hantavirus.
Medical practices, the adaptability of microbes, global travel, crowding, human susceptibility, alternate vertebrate hosts, insect vectors, and climate are just some of the factors that influence the emergence of disease. In most cases the interplay between multiple factors must be understood. Not the least of these is deteriorating public health systems in many countries where substandard water and waste management continues. War and famine also set up conditions that lead to the emergence of disease and, especially in poor nations, the political impetus to implement prevention and control strategies is often lacking.