Diseases and Human Evolution

17: Easy Route to Fame and Gripe: Cholera, the Salmonella Gang, and Other Prominent Gut Bugs

CHAPTER 17 Easy Route to Fame and Gripe Cholera, the Salmonella Gang, and Other Prominent Gut Bugs The early stages of globalization facilitated the spread of various diseases throughout the world. No longer confined to specific regions, many diseases found new havens as they traveled far and wide on board ships that increased in spread and range. They began to appear as if out of nowhere, in places where they had never been before. Progress marched forward, and many geographically localized endemic diseases began to evolve into global pandemic diseases. European industrialization made possible globalization, and led to the development of new commercial centers throughout the world. Their rapid growth attracted large numbers of people who hoped to benefit from wage work and opportunities to improve their lives. The newly mechanized industries required greater numbers of workers for increasing production. Industrialized towns and cities grew rapidly throughout the world with little regard to basic infrastructure, such as housing and drinking water, sewage and waste disposal. People often dump factory wastes and raw sewage into rivers and streams, polluting the very sources of water used for drinking. Wells dug to 279 obtain safe drinking water do not always escape pollution. Shallow wells placed near overused public latrines are in line for sewage runoff. The urban poor working classes proved to be the most vulnerable to unsafe drinking water, since they were often crowded into clusters of poor housing units dependent on a single latrine and a single water source (Cartwright and Biddiss 2000). Contaminated drinking water can serve up unsuspected deadly cocktails loaded with water-borne infectious diseases. Diarrhea, sometimes mild, sometimes deadly, commonly follows. Humans have always been susceptible to water-borne diseases. The development of agriculture, followed by the process of civilization, created greater opportunities for intestinal (enteric) pathogens to attack. Epidemics often waxed and waned, depending on the type of infectious agent and source. Whenever drinking water became contaminated with human or animal waste, opportunities arose for water-borne agents of disease. Also, undercooked or raw foods contaminated by polluted water or feces carried the risk of infection. The most notorious disease arising from contaminated waters made its formal global debut in the early nineteenth century— cholera. Before industrialization and European expansion, cholera had been a disease of the flood-prone Ganges delta region known as Bengal on the Indian subcontinent (modern-day Bangladesh). The Story of Cholera Cholera is caused by bacteria belonging to the genus Vibrio that inhabit estuaries , marshes, and coastal waters. Vibrios require salty waters for growth. They move about by way of a hair-like tail known as a flagellum. Over thirty species exist, but only about ten of them can accidentally infect humans. Cholera has become the most widespread and best known of the pathogenic vibrios, having more than eighty variants (Sakazaki 1992). Cholera does not naturally infect humans, but instead operates as an accidental infection caused by opportunistic bacteria with pathogenic potential (McNicol and Doetsch 1983). Unlike many other infectious agents of the gut that depend on passage from one host to another by way of fecal contaminated water, vibrios can survive independently when they return to their natural environment. They don’t need to invade a human or animal host to survive, but they can adapt to the gut microenvironment of a host when accidentally ingested. Once they escape from a host, they can either return 280 CHAPTER 17 to nature or pass through another host by way of fecal contaminated waters (Colwell and Spira 1992). “Stick-to-itiveness” is a way of life for vibrios. The bacteria attach to the surface area of a source likely to bring them into contact with minute food particles . They prefer to attach to chitin, the major substance forming the outer shells of crustaceans, particularly the chitin of tiny copepods found in plankton . Vibrios may actually play a role in the life cycle of these tiny creatures by promoting the release of their eggs from the egg sac through their ability to disrupt proteins. Vibrios also attach to the chitin-lined hind gut of the blue crab and other large crustaceans. Vibrios can stick to the roots of water plants, and even to trickling filters used to treat raw sewage (Colwell and Spira 1992). The evidence points strongly to the Ganges delta as the origin of the pathogenic vibrio that causes cholera; scattered outbreaks of the disease and...