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Submodule 6: Microbes and Fungi -- the Pathogens
Historical Consequences of Invasive Pathogens in the U.S.
Historians tell us that the movement of pathogens around the globe has had a significant impact on humans, animals, and plants in the U.S. Overall, for example, it is estimated that about 56 million people died due to European exploration of the New World, with the vast majority of these deaths attributed to the introduction of disease.
The transport of slaves into the Western Hemisphere brought the viruses that cause yellow fever and dengue fever in addition to the mosquito that transmits these diseases, Aedes aegypti. After relatively successful mosquito elimination campaigns in the 1950s and 1960s, the population of A. aegypti has greatly increased in recent years causing an increased fear of the spread of yellow fever and dengue fever.
Yellow fever endemic zones in Americas and Africa and number of yellow fever cases reported to
World Health Organization, 1980-1987 (image courtesy CDC).
Several members of the Brucella species of bacteria are known to cause brucellosis. This disease can infect humans causing fever, malaise, and muscle pain; but it is primarily known for infecting animals. Brucellosis was probably introduced to the U.S. by way of imported cattle hundreds of years ago and has been known for causing spontaneous abortions in cows and orchiditis (testicular inflammation) in bulls. It has more recently moved into some wild populations of bison and elk causing miscarriages. The bacteria were once developed as a biological weapon in the U.S. Since the adoption of pasteurization as a common practice in this country, the incidence of the disease of drastically decreased.
Another pathogenic microbe was introduced when rats, particularly Rattus norvigcus, an animal now considered invasive around the world, arrived in this country from Europe in the 16th century. Thriving in this new environment, they reached the west coast in the late 1800s. With the rats came the bacterium that causes plague, Yersinia pestis. Plague has become endemic in the western U.S.; several cases are still reported each year, with a mortality rate of over 10%.
More recently, the introduction of the Asian tiger mosquito, Adedes albopictus, is posing a significant health risk to people in the southeastern U.S. It is thought that the pathway of entry was via some imported used tires from Japan. Not only is this mosquito a nuisance species, it is also a vector of encephalitis viruses and the dengue virus; it has been shown to be the potential vector to other human viruses as well.
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