Cholera causes

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editors-In-Chief: Priyamvada Singh, MBBS [2]

Overview

Cholera is a severe diarrheal disease caused by the bacterium Vibrio cholerae.. Persons infected with cholera have massive diarrhea. This highly liquid diarrhea, which is often compared to "rice water," is loaded with bacteria that can spread under unsanitary conditions to infect water used by other people.

Causes

• Cholera (or Asiatic cholera or epidemic cholera) is a severe diarrheal disease caused by the bacterium Vibrio cholerae.
• V. cholerae is a Gram-negative bacteria which produces cholera toxin, an enterotoxin, whose action on the mucosal epithelium lining of the small intestine is responsible for the characteristic massive diarrhea of the disease.
• The major reservoir for cholera was long assumed to be humans, but some evidence suggests that it is the aquatic environment.
  • Vibrio cholera is not dependent on humans for its propagation.
  • Vibrio cholera can grow inbrackish estuaries, coastal seawaters (in close association with copepods or other zooplankton), and water of lower salinity when it is warm and adequate organic material is available.

Overview

Vibrio cholerae is a gram negative bacterium with a curved-rod shape that causes cholera in humans. V. cholerae and other species of the genus Vibrio belong to the gamma subdivision of the Proteobacteria.  There are two major strains of V. cholerae, classic and El Tor, and numerous other serogroups.

V. cholerae was first isolated as the cause of cholera by Italian anatomist Filippo Pacini in 1854, but his discovery was not widely known until Robert Koch, working independently thirty years later, publicized the knowledge and the means of fighting the disease.

Habitat

V. cholerae occurs naturally in the plankton of fresh, brackish, and salt water, attached primarily to copepods in the zooplankton. Coastal cholera outbreaks typically follow zooplankton blooms. This makes cholera a typical zoonosis.

Pathogenesis

V. cholerae colonizes the gastrointestinal tract, where it adheres to villous absorptive cells via pili, and secretes a binary toxin, called cholera toxin (CT). The two CT subunits are named A and B, and are synthesised in a 1:5 ratio. B subunits bind and internalize A subunits, which are processed to A1. The A1 form catalyses ADP ribosylation from NAD to the regulatory component of adenylate cyclase, thereby activating it. Increased adenylate cyclase activity increases cyclic AMP (cAMP) synthesis causing massive fluid and electrolyte efflux, resulting in diarrhea.

CT is encoded by the ctxAB genes on a specific filamentous bacteriophage. Transduction of this phage is dependent upon bacterial expression of the Toxin Coregulated Pilus (TCP), which is encoded by the V. cholerae pathogenicity island (VPI). VPI is generally only present in virulent strains and is laterally transferred. VPI was originally thought to encode a filamentous phage responsible for transfer. This theory was discredited by a study of 46 diverse V. cholerae isolates which found no evidence of VPI phage production. The generalized transduction phage CP-T1 has been shown to transduce the entire VPI which is then integrated at the same chromosomal location. Also, VPI has been shown to excise and circularize to produce pVPI via a specialised mechanism involving VPI-encoded recombinases. It is not known whether pVPI is involved in CP-T1 transduction or if it is perhaps a component of an alternative VPI mobilization mechanism.

Additionally, it produces two different proteases called chitinase and mucinase. Chitinase is responsible for the ability of Vibrio cholerae to enter copapods. Mucinase is a non-specific protease that assists entry into the human gastro-intestinal tract.

Finally, Vibrio cholerae produces what is called a ZOT toxin, termed as "Zona Occludans Toxin". This toxin specifically attacks the zona occludans or "tight" junctions joining epithelial cells.

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  Crabs have been a repeated source of cholera in the United States and elsewhere, even though they are rarely eaten raw. From Public Health Image Library (PHIL).

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  Typical Vibrio cholera contaminated water supply. From Public Health Image Library (PHIL).

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  Scanning electron micrograph (SEM) depicts a number of Vibrio parahaemolyticus bacteria; Mag. 19058x. From Public Health Image Library (PHIL).

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  Scanning electron micrograph (SEM) depicts a grouping of Vibrio vulnificus bacteria; Mag. 13184x. From Public Health Image Library (PHIL).

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  Scanning electron micrograph (SEM) depicts a flagellated Vibrio vulnificus bacterium; Mag. 26367x. From Public Health Image Library (PHIL).

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  Scanning electron micrograph (SEM) depicts a grouping of Vibrio vulnificus bacteria; Mag. 13184x. From Public Health Image Library (PHIL).

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  Scanning electron micrograph (SEM) depicts a number of Vibrio cholerae bacteria of the serogroup 01; Magnified 22371x. From Public Health Image Library (PHIL).

References

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