Co-Evolution of Disease and Living Conditions

CASE 2
Infection and Evolution: Malaria

Infections occur as microbes try to find a niche to survive and replicate. To do this they need a host organism and a vector (or carrier) organism. In the case of malaria, humans and various other animals have been the hosts, and insects (mosquitoes, for example) have been the vector. One of the first associations noted between a genetic trait and a disease was that of sickle-shaped red corpuscles (leading to sickle cell anemia) and malaria. In 1949, J.B.S. Haldane noticed an overlap of malaria and sickle cell anemia in regions of Africa.

Malaria is an ancient, prehistoric disease caused by a single-celled protozoan of the genus plasmodium. This organism is believed to have originated several hundred million years ago, first living inside marine creatures. Eventually the branch called Haemosporidia evolved to spend part of its life in a host blood stream. The malaria plasmodium are of two types and have settled on the mosquito as the main vector for human disease transmission. Stagnant water that breeds mosquitoes is the usual environment in which malaria thrives. Hippocrates, the Greek physician, considered father of modern Western medicine gave an account of the disease as being of environmental origin, occurring near low-lying swamps and bad air (mal-aria).

Malaria, a disease accompanied by intense fever, chills, and sweating, causes more than 300 million acute illnesses and at least 1 million deaths (primarily of children) each year. The World Health Organization reports that 90% of deaths from malaria occur in Africa, where a child dies every 30 seconds due to the disease.

The parasite attempts to feed and reproduce by attaching to the red blood cell. The abnormal sickle-shaped red blood cell occurs in people with an abnormal S-shaped hemoglobin. The shape hampers the parasite's attempts to attach to the red blood cell. So people with this trait have survived in the malaria-infected regions in Africa and Papua New Guinea (where a different deformity of the red blood cell occurs). The sickle shaped cell is however less efficient in iron and oxygen metabolism so that a person with a pair of sickle cell genes dies at an early age of anemia. Thus the coevolution of parasite and human reaches a balance point in some areas. But when descendants of the original population with the sickle cell mutation migrate to another place with no threat of malaria, this evolutionary advantage is merely a disease or risk of disease.

The story of malaria is long and complex. Global temperature rise, coupled with the possibility of swampy conditions, the increase of malaria is a possibility. Malaria is originally believed to have come to humans from monkeys in central Africa and spread northward. Egyptian documents describe malaria as early as 1500 BCE. The increase of sea trade in the 1600's helped spread malaria all over the world.

Following World War II, the World Health Organization started a large campaign to use the new wonder-pesticide DDT (Dichloro-diphenyl-trichloroethane). The property of DDT as an insecticide was discovered by Paul Muller in 1939, although DDT had been first synthesized in 1874. Because of its insecticidal properties whose discovery earned Muller the Nobel Prize, DDT became the pesticide of choice for all "pests" varying from insects that destroyed crops, mosquitoes that cause malaria, to body and hair lice. DDT was freely used as a pesticide and even applied directly to people's hair.

The eventual discovery that DDT and other pesticides accumulated in the human and animal bodies and cause numerous long-term effects was the basis of the warnings of Rachel Carson's Silent Spring. After the initial decline of malaria to some extent by extensive dustings of DDT, however, the plasmodium has mutated to become resistant to DDT. This is the evolutionary survival pathology for many microbes as they have short lifecycles and can mutate to develop resistance to various chemicals used to eliminate them.

PREV | NEXT