The Impact of the Environment on Global Patterns of Cholera

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Research continues to reveal the many contributions climate has on controlling ecosystems at scales ranging from growth, reproduction, and even the survival of individual organisms (Wang); so who’s to say the same cannot be said about infectious bacteria living in the same environments? This question/idea is not recent, around 415 BC Hippocrates was one of the first to contemplate that diseases were caused naturally (not as a punishment from the gods) and that that there was a possible connection between them and the climate (Lipp).

In the 20th century concerns began to circulate around the impact of humans and our alterations to terrestrial and aquatic habitats; this coupled with an obvious changing global climate, resulted in a combination of disciplinary efforts to understand how such changes might affect human health. These alterations outcomes are now thought to change the geographic range of many pathogens, potentially increasing exposure and risk of infection for humans (Lipp). For example, the slow warming of Earth by green house gasses and related environmental changes are commonly linked to algae growth and associated plankton blooms, these events could considerably influence the dispersal of one of the most common water-borne diseases, cholera (Sattenspiel).

Cholera is a water bourn disease (transmitted through the ingestion of contaminated water) caused by the bacterium Vibrio cholerae. It infects the intestine “causing acute intestinal illness with watery, profuse diarrhea and rapid dehydration, followed over half the time by death, sometimes within a few hours of the first symptoms,” (Sattenspiel). “In 2004, 70% of all cholera cases identified were in the regions of Southeastern Africa, Northern Africa and Central Africa (figure 3). Mozambique had the highest count of cholera cases, at twenty thousand,” (Worldmapper). The most important treatment for cholera is quick rehydration of fluids and electrolytes. There is an antibiotic for this disease that can help reduce the large amount of fluids needed to cure a person, but in most cases of sudden outbreak its availability at the quantity needed is usually inaccessible (Butler).

Only in 1997, was V. cholerae officially recognized as a natural occurring member of the aquatic microbial community; commonly found in river, estuary, and coastal water environments in almost every part of the world (Lipp). These naturally occurring hot spots can some times be the onset of epidemics, but usually only when some environmental or socioeconomic cue occurs. Instances like this usually occur when the bacteria comes in contact with infected stool, poor sanitary conditions, or a natural disaster that exacerbates bacterial population numbers past a threshold point, resulting in outbreaks. However despite the natural worldwide occurrence of this causative bacterium, only certain regions of the world (tropics and subtropics) can maintain endemicity of the disease (Lipp).

A survival trait that cholera has acquired is the ability to go dormant for long periods of time. Before this trait was revealed, scientists and epidemiologist were puzzled at how this disease could disappear for decades and then suddenly return to places that it was once thought to be eradicated from and in places that it had never been seen before. It is though that during periods between epidemics V. cholerae was able to adopt and enter a state of low metabolic activity where no dividing of the cells occurred, but they are still alive. This gave the bacteria a huge advantage, allowing it to survive in extreme conditions that would normally kill it. Only when favorable environmental factors return, can reversion back into a metabolic state happen (Reyburn).

Another hypothesis as to why cases of cholera can turn up any time, anywhere is that it has a symbiotic relationship with zooplankton. The bacteria frequently are found on the hard upper shell of zooplankton as well...
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