by Charles Mann is that diseases from Eurasia and Africa decimated the native American populations after 1491. (There was also a disease component of the experience of the European involvement with west Africa. We hear about the lethality of the tropical diseases of west Africa to Europeans who sought to live there, but diseases that had not crossed the Sahara came with European ships and decimated African societies as they were being raided to kidnap people for the slave trade.)The "native American" population in 1491 was descended from a relatively small number of people who arrived from Siberia making the journey at a time when the northern regions were much colder than they are today. They brought relatively few diseases with them. Because of the evolutionary bottleneck that the population had passed through there was relatively little genetic diversity among Native Americans -- perhaps even less among South Americans than among North Americans.
Many human diseases have been acquired from domestic animals. In Europe and Asia there was a long history of humans living with domestic animals and many diseases had been so introduced and spread. The native Americans had domesticated few animals and had picked up few diseases from them.
Among the diseases introduced to the Americas from the Eastern Hemisphere were smallpox, measles, mumps, malaria, yellow fever, influenza, whooping cough, typhus, chicken pox and the common cold. One might add to that list tuberculosis, cholera, typhoid fever, and plague. Europeans also introduced chickens, cattle, pigs and horses all of which carried diseases that could and would be shared with native Americans.
Since native Americans had not been previously exposed to these diseases, they had not developed immunity to them.
Different communicable diseases spread by different means. Diseases such as malaria and yellow fever are carried by mosquitoes. The agents of these diseases are complex, and people tend if at all to develop resistance rather than immunity to them. In the low lands of the tropics where mosquitoes can be very numerous, these diseases are a huge problem. Indeed, it was not possible to complete the Panama Canal until Yellow Fever was controlled. These diseases decimated the humid American tropics after they were introduced to this hemisphere. Other vector born diseases such as dengue fever are perhaps less lethal, but very debilitating.
Other diseases such as typhoid are water born, and their control depends on washing and the provision of clean, potable water. I suspect that many of the native American populations depended on surface waters near their villages and might well have been quite vulnerable to water born diseases.
Other diseases spread through the air. Often there is a period of days or even longer in which an infected person can transmit the disease before symptoms of the disease occur. These diseases spread more rapidly where populations are dense, as was the case in the Inca and Aztec empires. They spread more rapidly when people are in small enclosed spaces, as was probable frequently the case in the cold highlands of the Inca empire, or indeed in the houses of the colder areas of North and South America.
Think about the road system of the Incas, in which runners would carry messages by running in relays from dawn to dusk. Running at six miles an hour for 12 hours a day, a message could more 72 miles a day, 500 miles a week. So could an air born communicable disease. The runner, having completed his service or having fallen ill would of course go into a nearby village where he could share the condition with the locals.
As Mann points out, native American populations have fewer Human Leukocyte Antigens (HLA) than do European populations. HLAs determine the antigens that the immune system can recognize. If a disease agent gets into a person and does not present an antigen that the person's immune system can recognize, the person is likely to be infected. The population of the agent will then grow and evolve. Both population growth and evolution can progress rapidly. Clearly the disease agent population in the seriously ill patient will be composed largely of strains of the agent (virus) that the patient's immune system does not fight. When that patient coughs and sends virus to another person, if there is little HLA diversity in the population, then it is more likely that the new person will be infected and that the strain of the virus with which s/he is infected will reproduce rapidly in his/her body. Thus these epidemics may have been both more widely spread and more lethal in native American populations than in Eurasian (or African) populations.
One supposes that the native Americans also quickly adopted new species of livestock, and thus were quickly subjected to zoonotic diseases from their livestock.
All of these problems might well have been exacerbated by the lack of social mechanisms such as those which had evolved in Europe and Asia to limit the spread of communicable diseases. While Europeans had developed systems of quarantine and isolation, they probably were not part of American cultures. Surely no one at the time understood the processes of contagion or the nature of communicable diseases, and there were no effective preventive nor curative treatments.
It has been estimated that native American populations decreased by 90 or 95 percent. Of course, complex irrigation, land terracing and food distribution systems fall apart when the people who run them get sick and die. When there is less food, people become malnourished. The malnourished are more susceptible to disease. There is a very viscous cycle!
No comments:
Post a Comment