Sunday, September 23, 2012

A Thought About Mosquito Borne Diseases

In his book, 1493: Uncovering the New World Columbus Created, Charles C. Mann describes results of the transfer of the agents of malaria and Yellow Fever to the Western Hemisphere, as well as of the Aedes aegypti vector of Yellow Fever.

Of course, an entire suite of diseases was transferred to the Americas - diseases that had long existed in Europe, Asia and Africa. The epidemics that they caused decimated the indigenous populations, facilitated their conquest by immigrant Europeans, and led to the collapse of Indian land use systems.

Mosquito borne diseases differ in one important way from air borne in an important respect. Mosquito borne diseases seem more easily spread by from supine victims that are easy prey for mosquitoes, while air borne diseases seem more easily spread if the victims are able to move about freely to spread the disease in to people in new places. It has been suggested that as a result, there has been a tendency air borne diseases to evolve to allow victims to be ambulatory and mosquito borne diseases to evolve so that victims are supine.

Mann notes that Malaria and Yellow Fever are diseases that are found most frequently in the tropics, that can persist in temperate zones, but do not become endemic in cold climates. They were terrible in the Caribbean islands and northern South America, bad in the southern United Sates and in a band across South America, and not so much of a problem in the northern American colonies/states. The Mason Dixon line is something of a demarcation between the American malaria zone and the malaria free zone. In South America, these diseases devastated the indigenous population of Amazonia.

Importantly, people from West Africa (have evolved under pressure from these diseases to be) are often immune to one form of malaria and resistant to the other common form. This is not true for the English nor Germans, nor American Indians. Mann suggests that once malaria was endemic to large areas in the Americas, the employment of European indentured servants and/or Indian slaves became less economically advantageous to planters than the employment of Africa chattel slaves, and that this fact helped overcome a British antipathy to slavery.

Mann also mentions the problems that the British forces had with disease in their southern campaign in the U.S. Revolutionary war, when their soldiers who had no resistance to malaria came up against colonial militias formed of locals who had developed such resistance. Malaria may have also played a role in the South during the Civil War.

Today we find it hard to imagine Malaria and Yellow Fever being a problem in the continental United States, but that is certainly true historically. I recall when I lived in Cali, Colombia (1970-73) a physician friend told me that his father, who had also been a physician, had believed that the presumptive diagnosis for any patient that presented with a fever (temperature) was malaria -- the disease was that common even in the 20th century at the 3,000 foot altitude in the Andes.

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