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Hurrying Toward Disaster?
by C. J. Peters
 By far the most common underlying causes of emergence can be found in changes in the ecology of the infectious disease or its hosts. A classic example is Bolivian hemorrhagic fever, which emerged in the 1960s. Machupo virus, which causes this dangerous disease, is carried by rodents, and settlements in new areas of Bolivia's department of Beni led to its emergence as a concentrated human health problem. The cause of the disease was unknown, but it invaded small towns, causing high death rates and often prompting the abandonment of entire settlements. When authorities discovered that the disease was caused by a rodent-borne virus, the animals were eliminated and the towns were safe again. However, rodents continue to live in clearings where isolated houses are built and in fields where crops are planted to sustain nearby towns. The well-fed animals in the fields remain a constant threat to the rural people who work there.
The need to provide increasing amounts of food and water for the world's burgeoning population also contributes to the emergence of new diseases. Intensive agricultural methods often mean that a single, genetically homogeneous species is raised in a limited area, creating a perfect target for emerging diseases, which proliferate happily among a large number of like animals in close proximity. The growing impoundment of water and expanding irrigation are important elements in the emergence of mosquito-borne diseases, and the lack of potable water for consumption and washing in poorer populations leads to increasing transmission of intestinal diseases such as cholera and typhoid. The problem is not only that these ecological changes are occurring, but that they are occurring at an accelerated rate. Indeed, the rate of increase is itself increasing. In this sense, it is very possible that we are hurrying toward an appointment with disaster.
Drug resistance Up to now, we have successfully battled many of the classic bacterial infectious diseases through the use of antibiotics such as penicillin to treat them. Yet many pathogens are now coming back to haunt us. Plague, pneumococcal meningitis and pneumonia, tuberculosis and typhoid are only a few of the once-subjugated bacteria now reemerging as significant pathogens that we can no longer successfully deal with. Future generations may be impressed with our discoveries of powerful drugs to fight lethal bacteria, but they are likely to be less impressed with the way we squandered these drugs life-saving qualities by using them in ways that predictably lead to resistance.
The problem is clear in evolutionary terms: using antibacterial drugs selectively and with proper controls minimizes the emergence of resistant organisms. But their widespread use for trivial infections or against viruses that are not inhibited by the drugs destroys the sensitive bacteria in hosts and assures that the next infections will be with bacteria that have evolved ways to escape the drugs' action. Some very important infections fall into this category, among them typhoid fever. Thanks in large part to the widespread use of inexpensive antibiotics for uncertain indications, the typhoid bacterium has developed resistance to all three drugs that have been used successfully to treat the disease in the past. Typhoid is also rapidly developing resistance to a newer, expensive class of drugs that is currently the last bastion of defense. In poorer countries, the effect on people who lack access to clean water will be devastating; but the impact will also flow over to the more affluent in our global society who travel or otherwise come in contact with carriers of resistant typhoid bacteria. A similar situation is evolving with tuberculosis and many other bacteria.
Meanwhile, the pharmaceutical industry faces declining profits from the production of antimicrobials, and both science and industry are lagging in the development of new drugs to replace those that have been lost. We can save the remaining ones only by improving doctors prescribing practices and patients attitudes, by controlling over-the-counter sales and by eliminating the most important drugs from use in livestock production.
If the antibacterial drug situation is bleak, antiparasitic drugs present an even more distressing problem. Profit margins for critical compounds used to treat parasitic diseases in tropical countries are even lower than for antibacterial drugs. Malaria--which kills 1 million people annually--economically justifies a small continuing effort by the pharmaceutical industry, but lesser killers, such as trypanosomiasis and schistosomiasis, do not provide sufficient incentive.
As for antivirals, few drugs are available other than for HIV because market forces are too weak to spur development and production. This is also true for new pesticides that could curb disease vectors in the world's poorest countries.
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