Environmental Problems, Their Causes, and Sustainability 1

Dengue FeverPainful and sometimes fatal.Carried by four related viruses andstrikes during rainy season.2.5 million people at risk;50 million new cases a year.MalariaEndemic in more than 100 countries.Caused by four protozoa species.270–500 million new cases and1 million deaths per year.Yellow FeverDreaded far more than 400 years.Viral disease that causes symptoms frommild to severe illness and death.200,000 new cases and30,000 deaths a year.Figure 19-9 A few species of mosquito act as vectors to transmit pathogens for a number of infectious diseases,including the three whose normal ranges are mapped here. Female mosquitoes feed on blood and malemosquitoes on plant juices. Thus, only the female mosquito bites people and animals to feed on their blood,and in the process transmits pathogens from one victim to later victims. Throughout human history, diseasetransmission by female mosquitoes has probably killed more people than any other single factor. However,most mosquito species do not transmit infectious diseases and mosquitoes play important ecological roles.Their eggs are a major food source for fish, various insects, and frogs and other amphibians. Adult mosquitoesare an important source of food for bats, spiders, and many insect and bird species. Mosquitoes locate us bythe CO 2 we give off, the odor of lactic acid secreted by our skin, and our body heat, which is why heat-absorbingdark clothes attract mosquitoes more than light-colored clothes do. Once a female mosquito finds us shepierces our skin, injects an anticoagulant mixed with saliva that keeps our blood flowing and also causes anitchy bump to rise, and drinks her fill of our blood. (World Health Organization and the U.S. Centers for DiseaseControl and Prevention)increases populations of pesticide-resistant insects andother carriers of bacterial diseases.An additional factor is overuse of antibiotics. Accordingto a 2000 study by Richard Wenzel andMichael Edward, at least half of all antibiotics used totreat humans are prescribed unnecessarily. In manycountries antibiotics are available without prescriptions,which also promotes unnecessary use.According to a 2001 study by the Union ofConcerned Scientists, nearly 75% of all antibioticsmanufactured in the United States are used mostly infeed additives to boost livestock production. Recent420 CHAPTER 19 Risk, Toxicology, and Human Health