Cholera is a diarrheal illness caused by the bacteria Vibrio cholerae. This species is not endemic to humans, and its presence in the human digestive system is not part of the natural life cycle of the bacteria. Normally found in an estuary ecology, the Vibrio cholerae bacteria life cycle naturally shifts between various reservoir species such as small snails and crustaceans, free-floating planktonic forms and static forms resident in the silt and muck of the estuary.
The Vibrio cholerae bacteria is an accidental tourist in humans, though once present in a human community it can become widely spread. There are several strains of Vibrio cholerae bacteria, some toxic and some non-toxic. The non-toxic strains do not cause the classic symptoms of cholera and seldom survive in the human gut for more than a few days at best, though they can be spread through the human population when poor sanitation is a problem. Toxic strains, however, are deadly. Vibrio cholera triggers intense diarrhea, leading to pain, fever and worst of all severe and sudden dehydration. Modern medical treatments can reduce cholera fatalities by stopping dehydration--the true killer in most cases of cholera.
Natural Life Cycle
Vibrio cholerae bacteria naturally live as rod-shaped bacteria existing primarily in plankton populations in shallow, brackish water. Attaching themselves to microscopic crustaceans called copepods that exist as part of the planktonic ecosystem, they move naturally through several environments. Colonies of the bacteria can exist on the surface of the copepods, flourishing during times when temperature, low salinity and high nutrient levels cause algal blooms in the estuary, explaining why cholera has traditionally been associated with monsoon conditions. The bacteria also, however, exist as colonies of biofilms coating the surface of various natural features of the estuary, covering the water surface but also plants, stones, shells and similar items. They can take non-active form and survive in the silt of the estuary. Finally they have been found resident in the egg masses of native midges, which serve as a reservoir for cholera bacteria. In all of these instances the bacteria is a natural inhabitant, not associated with damage to the ecology or the organisms with which it comes in contact.
Abnormal Life Cycle
When Vibrio cholerae enters the human ecology, however, it can quickly cause severe epidemics. Toxic strains of cholera bacteria produce poisons that trigger violent diarrhea in humans. The result, in terms of life cycle, is to pour thousands of bacteria into human water supplies. As durable and adaptable a bacteria as Vibrio cholerae is able to survive in spite of the non-ideal conditions that usually apply to human water sources. Bacteria multiply, particularly in situations where water is filled with nutrients that encourage the same growth that nutrient-enriched estuaries promote. Each newly infected human adds waste and bacteria to the environment, bringing about rapid spread of the disease until such time as a change in environment ends the expansion of the bacterial population. Weather changes, population loss and improved sanitation can all end an outbreak.
Entering The Human Ecosytem
Vibrio cholera bacteria enter the human ecosystem through a variety of routes. The most common entry is through contaminated food or water. When humans eat seafood--in particular shellfish native to estuary environments such as oysters or crabs--and fail to cook them completely or even eat them raw, they can ingest the large amounts of bacteria necessary to cause a case of cholera. Poorly cleaned vegetables irrigated by contaminated water sources is another common source. In situations where sanitation is severely challenged, such as in refugee camps or communities with highly limited water resources, a single affected victim can contaminate all water for an entire population.
The Spread Of Cholera
Cholera is not contagious from human to human. Unlike bacteria naturally resident in the human population, which depend on human transmission as part of their natural life cycle, cholera bacteria need some rough approximate of their natural estuary environment to spread and reproduce. Unfortunately, unsanitary water sources serve as an acceptable substitute for the natural ecological niche. Thus the spread of cholera is dependent on poorly managed water resources, on poor cooking habits and on poor sanitation.