Cholera

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Sari water filtration technique, designed by Rita Colwell. From: Downtoearth.org [1]

V. cholerae with flagellum. From: Uaidintl.org [2]

Etiology/Bacteriology

Taxonomy

Domain: Bacteria | Phylum: Proteobacteria | Class: Gammaproteobacteria | Family: Vibrionaceae | Order: Vibrionales | Genus: Vibrio | Species: V. cholerae

NCBI: Taxonomy Genome: Vibrio cholerae

Pathogenesis

Transmission

The first recorded outbreak of cholera occurred in India in 1817, specifically near the mouth of the Ganges River [3]. Vibrio cholerae naturally attach to the chitinous exoskeleton of zooplankton in fresh, brackish rivers and coastal salt water. Because of this, cholera outbreaks often coincide with zooplankton blooms [4]. The transmission of V. cholerae is usually by water and food contaminated with zooplankton and/or fecal matter. Because the pathogen is ingested, it is also transmitted through oral-fecal transmission. The transmission of V. cholerae is often related to the inadequate sanitation and water treatment procedures of an area [5].”

Infectious dose, incubation, and colonization

V. cholerae lacks the acid resistance genes found in many other intestinal pathogens and therefore has a high infectious dose. One must ingest over one million microbes in order to contract cholera because many of the bacteria will die in the stomach due to its highly acidic environment. The incubation period is dependent upon how many organisms successfully passed through the stomach into the small intestine where V. cholerae can colonize. Therefore, the incubation period before showing symptoms ranges from a few hours to five days, typically taking two days before causing symptoms. Once in the small intestine, studies show that some of the bacteria use their flagella to swim towards the epithelial cells and adhere in the crypts of the intestine using their toxin-coregulated pilus to keep from being flushed out [6].

Epidemiology

As Cholera occurs when one comes in contact with contaminated fecal matter, outbreaks typically occur in areas with inadequate sanitation. Poorer areas without access to proper sanitation or water sources are at higher risk of infection. Cholera also becomes in issue in displacement camps, slums, and disaster zones when few sources of water are available. Water can often be contaminated with Vibrio cholerae after a natural disasters such as a flood, so cholera epidemic can be a concern during a recovery period [7].

Broad Street Pump, discovered by John Snow to be the source of a cholera epidemic. From: Secret-Cities.com [3]

Epidemiological studies of cholera typically focus on determining the common contaminated water source. John Snow first established this method of evaluation when studying London neighborhood water sources after an outbreak in 1854. In fact, much of what we now know about cholera came from John Snow's study on the communication of cholera [8] Often referred to as the first epidemiological study, Snow visited the infected neighborhoods, asking people about their habits, and marking infected homes on a map. From his conversations and speculation as to the transmission of the infection, Snow deduced the site of initial contamination to be a well located on Broad Street, a common water source for the infected neighborhoods. [9]

Virulence factors

The virulence factors of V. cholerae include toxin co-regulated pillus, cholera toxin, and motility. All of these factors contribute to the pathogenicity of the bacteria. Without one or more of these factors, V. cholerae would lose much of its ability to infect and cause symptoms in the host.

Motility

V. cholerae have flagella, which make them motile. This flagella allows the bacteria to swim through the lumen of the small intestine in search of nutrient rich cells areas and places to adhere to the epithelial cells that make up the lining of the small intestine, specifically the crypts of the small intestine. This motility could also be an important factor in the bacteria breaking through the mucosal layer that coats the inside of the small intestine [6].

Adherence

V. cholerae toxin mechanism. From: epi.ac.uk [4]

The virulence factor that contributes to the adherence of V. cholerae in the small intestine Toxin Co-regulated Pilus (TCP). The expression of TCP is linked with the production of Cholera Toxin (CT). The TCP is a Type IV pili, which are found on all gram-negative bacteria [14]. It is possible that V. cholerae could use this immune response like Salmonella does in order to establish its own niche in the small intestine, but scientists are not entirely sure. These pili are located on the polar ends of the cell and likely contribute to colonization via direct adherence to the epithelial cells lining the small intestine or through other pilus-mediated bacterial interactions. The host cell receptors for TCP have yet to be identified, and the mechanism for attachment is still largely unknown [15].

Toxin Mechanism

Cholera toxin is an A-B toxin secreted by V. cholerae that causes the host cell to expel large amounts of water and ions. The toxin is made up of five identical B subunits linked to one A subunit. The B subunits bind to monosialosyl ganglioside present on the surface of intestinal mucosal cells, allowing the A subunit to enter the cell. Once inside the cell, the A subunit stimulates heightened formation of cAMP by transferring an ADP-ribosal moiety of NAD to a protein that regulates the adenylate cyclase system [15]. The regulation protein inactivates the system when GTP is hydrolyzed. The A subunit essentially keeps GTP hydrolysis from occurring by affecting other proteins.

The high levels of cAMP in the cell activates mucosal membrane pumps to push Cl- ions into the lumen of the small intestine. This creates an ionic gradient between the host cell and the lumen. This gradient causes the cell to expel large amounts of water and positively charged ions (electrolytes) into the lumen to counteract the negative charge of the Cl- ions [15]. The water and electrolytes expelled are replaced by the bloodstream and subsequently pumped out of the cell once more. This can lead to severe dehydration in the host if left untreated.

Clinical features

Symptoms

Most cholera infections are asymptomatic in nature. Only 5-10% of those infected will display the severe symptoms that are characteristic of cholera (CDC). Cholera can still be spread from asymptomatic carries, as the bacteria will be shed for 7-14 days (Mayo clinic). The typical symptoms of cholera include diarrhea, nausea and vomiting, and dehydration.

Diagnosis

As the excrement of infection with V. cholerae is so distinct, diagnosis typically can be made upon obtaining and culturing a stool sample. Typical clinical manifestations, such as the "rice water" stool and vomiting, as well as recent consumption of a contaminated water or food source, can lead to investigation into cholera by the physician. A rectal swab or fecal sample when cultured will correctly indicate the source of the infection [10]. Cholera diarrhea is typically self-limiting, as the species is not native to human microbiota and will be expelled after several days [3].

Immune Response

Host Response

The human mucosal immune system is thought to act against cholera infection by the development of immunological memory to the pathogen. In studies of vaccination of humans with a strain of V. cholerae, localized and peripheral immune responses differed from initial infection, suggesting memory of the adaptive immune response induces a response to reinfection. Vaccination proved to invoke antibody responses in the intestine, as well as stimulate mucosal immunological memory. IgA antibodies were found in the highest proportion locally, suggesting IgA as the dominant isotype in intestinal tissue. In addition, in the research subjects who were not vaccinated yet were exposed a second time to V. cholerae, IFN-y secretion dramatically increased. The same was found in all but one individual in the vaccination group after a second round of vaccination, thus determining the vaccine stimulated a similar response [11].

Typically, antibody responses to infection may require large amounts of the bacteria, and the response can be short-lived. With cholera, however, the toxin is so potent that it seems to stimulate a stronger antibody response. Thus, protection from disease return in cholera can last for several years, compared to the few months associated with other infections with coproantibody stimulation [11]. Immunity to cholera has both antitoxin and antibacterial components. Both aspects of the host response appear to be necessary for immunity. One study shows that vibriocidal antibody titer presence alone is not an indication of protection against reinfection with the disease. As well, several pathogenic strains exist, and antibody protection against one strain does not confer protection against an alternate strain [12].

Bacterial Evasion

As aforementioned, the human immune system relies heavily on IgA antibodies to suppress infection by V. cholerae [11]. In order to combat this response, V. cholerae appears to disguise itself from IgA receptors. By failing to secrete surface adhesion, a type IV mannose-sensitive hemagglutinin (MSHA) pilus, the pathogen is able to be unrecognized and thus can evade the host immune response. ToxT controls the down-regulation of MSHA in order to use gene repression as a mode of evasion [13].

References

1 "Stopping cholera with a sari". Down to Earth - Science and Environment Online. 2013, July 12.
2 " Kogan, Nicole. "Exploring World Involvement in Haiti". United Against Infectious Diseases. 2013, July 12.
3 Despommier D, Chen S. "Cholera". Medical Ecology. 2004.
4 News-Medical "Cholera Transmission". 2013.
5 Centers for Disease Control and Prevention (CDC). Cholera - Vibrio cholerae infection. General Information. "Where is Cholera Found". 2013, July 9.
6 Spagnuolo AM, DiRita V, Kirschner D. "A model for Vibrio cholerae colonization of the human intestine.". J Theor Biol. 2011, November 21; 289; pp. 247-258.
7 World Health Organization (WHO). Cholera - Fact sheet. 2012, July.
8 Snow, John. On the Mode of Communication of Cholera. Wilson and Ogilvy, London, 1849. Accessed for free on UCLA Department of Epidemiology website.
9 Summers, Judith. Soho - A History of London's Most Colourful Neighborhood, Bloomsbury, London, 1989, pp. 113-117. Accessed for free on UCLA Department of Epidemiology website.
10 Centers for Disease Control and Prevention (CDC). Cholera - Vibrio cholerae infection. "Diagnosis and detection". 2013, July 12.
11 Quidling, Marianne, et al. The Journal of Clinical Investigation (JCI). "Intestinal immune responses in humans. Oral cholera vaccination induces strong intestinal antibody responses and interferon-gamma production and evokes local immunological memory." 1991, July. 88(1). pp. 143-148.
12 Saha, Debasish, et al. Journal of Infectious Diseases. 2003, May. 189(12). pp. 2318-2322.
13 Hsiao, Ansel, et al. Proceedings of the National Academy of Sciences of the United States of America (PNAS). "Vibrio cholerae virulence regulator-coordinated evasion of host immunity". 2006, September 26. 103(39). pp. 14542-14547.
14 Bose N, Payne SM, Taylor RK. "Type 4 Pilus Biogenesis and Type II-Mediated Protein Secretion by Vibrio cholerae Occur Independently of the TonB-Facilitated Proton Motive Force" Journal of Bacteriology. 2002. vol 184.
15 Todar K, PhD. "Vibrio cholerae and Asiatic Cholera" Todar's Online Textbook of Bacteriology. 2008.

Created by Bhumi Patel, Dehra McGuire, and Gracen Conway, students of Tyrrell Conway at the University of Oklahoma.