Vibrio cholerae transmission and environmental reservoirs

Cholera Vectors

Cholera is an acute diarrheal infection caused by a bacterium called Vibrio cholerae that infects the intestines of human and causes massive dehydration. Most estimates believe that there are 3-5 million cases of cholera globally each year, with about 100,000 deaths. The infection varies between little or no symptoms to serious and life-threatening cases. In about 5% of the population, infected individuals will have massive watery diarrhea, vomiting, leg cramps, and resulting ancillary issues caused by shock and dehydration (Bjorklund, 2011).

Type of Pathogen

The bacterium that causes cholera, Vibrio cholerae, is a gram negative, non-spore forming, curved rod bacterium that is oxidase positive. From a biological perspective, it is quite mobile with a single point tail (flagellum). It is not toxic itself, but when it enters a host it produces a toxin that is poisonous to humans and causes symptoms. Not all cholera bacteria produce this toxin, however, and there are several sub-strains of the bacteria that vary in toxicity and danger to humans. Many of these subtypes are, in fact, endemic to different parts of the world, with the most toxic living in tropical areas, particularly near brackish or standing waters near ocean coasts. A number of the subtypes of the cholera bacteria do not produce the cholera toxin, but serve as a reservoir for that genome. Many of these, while not producing the full-fledged symptoms of cholera, do produce illness in humans (enteritis, etc.) and are thus serious when ingested. The cholera bacterium seems also to have evolved over time; reports of cholera outbreaks from the Ancient World describe similar symptoms, but there does not appear to have been the large amounts of subgroups, which may have evolved as humans populated certain areas more (Bjorklund, 2011).

Known Reservoirs

Cholera is typically found in water or food sources that have been somehow contaminated with feces from a human already infected with cholera. This is typically in areas with poor sanitation, inadequate water treatment and inadequate hygiene. In some cases, depending on the microenvironment, cholera may life in brackish areas of coastal waters and manifest in shellfish. In fact, eating raw or undercooked shellfish is one vector that transcends economics and occurs in both the developed and developing worlds. Certainly, cholera is more endemic during high stress times (disasters, etc.) and is found in humans, water birds, shellfish, fish and even some herbivores (Krauss, H., et al., 2003).

Transmission and Dosage

The degree to which cholera is transmitted into humans varies based on the amount of acid in the individual's gut, their immune system function and age. Most research notes that the infection dose ranges between 106 and 1011 of ingested bacterium. Cholera is spread by consumption of water or drops of water that have been contaminated by feces, but also through infectious raw fish and seafood. The incubation runs from just a few hours to 5 days after infection, again dependent upon age, amount ingested, acid levels in the gut, and health of the individual (Krauss).

Population Susceptibility and Risk Groups

As noted, populations that are stressed from environmental causes are the most susceptible to cholera. Those living in areas post-disaster that have been flooded or their sanitation facilities damaged are at high risk. Globally, cholera is the second leading cause of death in children under the age of 5; and overall, the fatality rate for cholera remains about 4%. Individuals with compromised immune systems, young children, the aged and pregnant women constitute the highest risk categories. These populations, too, are more likely to live in a more sedentary state, and bathe, work and eat in more close proximities to others, or to potentially infected areas. Children, too, are more likely to lack training in hygiene, and often play near stagnant waterways, putting their fingers into their mouths, and thus potentially transmitting the disease vector. Because the disease may be asymptomatic for several weeks, vibrios are shed in small and intermittent quantities, making the population even more vulnerable. Symptomatic patients may actually shed the bacteria before the clinical signs of the illness and up to 2 weeks after feeling better (Nelson, E., et al., 2009).

Morbidity and Mortality

Cholera may cause symptoms that range in severity to no external symptoms to severe vomiting, diarrhea and cramping. In endemic areas of the disease, 75% of the cases are actually asymptomatic, 20% are mild to moderate and 2-5% are quite severe. The 75% asymptomatic number is quite important, because while these individuals show no outward symptoms, and may even be unaware of their condition, they are infectious to others. Severe symptoms include very watery diarrhea, occasional to projective vomiting, and cramps in the muscles or abdomen. As dehydration gets worse, symptoms occur such as dry mucous membranes, sunken eyes, hypotension, renal failure, seizures, coma and death. Death is primarily due to dehydration and may occur in hours to days, particularly in untreated children and pregnant women, all of whom are less tolerant of severe dehydration. In the cases of extreme cholera symptoms, up to 60% of patients may die, but less than 1% of cases treated with rehydration become fatal. The disease typically lasts from 4-6 days, but may present minor symptoms even after the 6-day period for up to a month. People infected with cholera do not become immune, and may be reinfected at later times (Brock, T., et al., 2000)

Extent of Outbreaks or Threats

Cholera is endemic wherever human populations have less than adequate hygiene. Over the past 200 years, there have been eight major pandemics, typically in subtropical and topical areas (97% in Africa, for instance). There are several million cases each year in these areas, typically in children 2-9 and pregnant women. Epidemics tend to occur in the hot season, after floods or disasters, or when sanitation facilities break. There are fewer threats to the developed world, not simply because of sanitation, but due to a winter period in which the bacteria are killed. Still, the World Health Organization and others consider cholera a reasonable pandemic threat whenever there is a stressed population and/or problems with sanitation (Public Health Agency of Canada, 2013).