There have always been reported some predispositions for particular diseases which makes specific groups of people more susceptible to that disease. Cholera, an acute gastrointestinal disease which is responsible for the deaths of thousands of people all around the world each year is one of those diseases infecting specific individuals in the community. Besides other predispositions, it has been found that O blood group persons are more at risk of being infected by severe cholera disease than individuals with other blood group phenotypes. Here, the question which may arise in one's mind is "What makes an individual with blood group O more susceptible to cholera? And what is the explanation for this susceptibility? To answer this question, this piece of writing will explore several hypotheses that have been proposed by scientists considering the relationship between particular properties of the organism responsible for the disease and the characteristics of the host organism. In order to understand the factors and mechanisms that might contribute to the resistance of specific blood group to cholera disease, this article will first give a brief overview of the aetiology, pathogenesis, diagnosis, treatment and clinical features of the cholera disease with an emphasize on the association between the different blood group antigens and their susceptibility to the disease. Next, this writing will have a close look at the two journal articles related to predispositions of cholera infection and briefly summarise their contents. Finally, the text would be concluded with stating the major findings out of this investigation.
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Cholera is an acute, diarrheal disease caused by the intestine infection with the Vibrio cholera bacteria. The infection is usually transmitted by polluted water or food. It occurs especially in the temperate months among the low socio-economic communities mostly because of their unhygienic behavior. Even though people in any age group can be affected by cholera, but children between the aged 1 to 5 years are more susceptible to this disease. The majority of people infected with V. cholera do not normally produce any symptoms. Only a few percentage of people who are infected by the V. cholera will develop serious illness mainly characterized by vomiting, watery diarrhoea and stage cramps.
The organism responsible for the development of cholera in human is identified as Vibrio cholera. This is a Gram-negative bacterium capable of producing an enterotoxin, whose activity on the lining epithelia of the gut is found to be accountable for the process of developing diarrhoea. Out of many V. Cholerae strains, the majority of infection with cholera in human is found to be caused by the two well known O1 and O139 serogroups.
Once in the host body, Vibrio cholera employs its deadly strategy by means of either colonization of the intestine mucosa or the releasing of a powerful enterotoxin that forces the small intestine to secrete fluid and electrolytes. Since no association between different blood group phenotypes and the colonization of bacteria has been reported, this step of infection has been ruled out as one of the factor contributing to the dependency of cholera severity to blood-group, pointing to the significance of cholera toxin in building this association.
After entering the cell by endocytosis, the cholera toxin is activating adenylate cyclase, leading to an increased level of cAMP inside the cell. Ultimately, this prevents the absorption of sodium and chloride by the intestine's microvilli as well as opening of the ion channels, leading to the electrolytes and water outpour into the lumen of intestine
As early as 1950s, studies conducted by researchers revealed an increased risk of severe cholera infection among the individuals with blood group O. So far, several hypotheses have been proposed by researchers regarding this association. proposes the hypothesis that secreted blood group antigens might be acting as receptor analogs or inhibitors of cholera toxin, with blood group A and B glycopeptide antigens inhibiting the cholera toxin from binding to the cell surface. Another hypothesis proposed by seeks the idea behind this association in different level of mucous secretion by the intestine cell, induced by the cholera toxin. ( argues that mucus has a protective role in intestine surface during the infection, preventing the bacterium and its toxin from reaching the mucosal cell. If this hypothesis is true, than the Cholera toxin induces cells with A, B or AB antigens more than other cells to produce mucus and protect the cells from bacterial invasion.
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Cholera can be diagnosed be examining the diarrheic stools of the infected person and looking for the presence of the V. cholera itself. Normally, severe diarrhoea causing rapid progression to dehydration suggests that cholera is the cause in severe cases. Milder cases will not be diagnosed unless a stool is cultured or microscopy of a fresh faecal is examined.
Infection with V. Cholerae can be simply treated by an immediate replacement of the electrolyte, fluid and salts which are lost through diarrhoea. Patients can be treated by drinking oral rehydration solution (ORS) recommended by (n case of severe dehydration, patients need intravenous infusions to restore blood volume. Antibiotics with tetracycline as a drug of choice may diminish the severity of the disease.(Moreover, there is an oral vaccine available which provides a better immunity with less unfavorable effects but here one should notice that the cholera vaccine is less protective in O blood group individuals than any other groups. Proper hygienic behavior, adequate clean water supply and the safe disposal of faecal waste would do much to prevent the occurrence of cholera.
In this article, Swerdlow DL, Mintz ED, Rodriguez first give a brief history of the cholera epidemic with emphasize on the South East Asia regions. Next, this article investigates the association between blood group phenotype and its susceptibility to V. Cholerae through a survey in the Chicago neighbourhood in March and April, 1991 after the cholera outbreak in the region.
After evaluating the collected data, the authors find a strong link between blood group O and the severity of infection with V. Cholerae 01 serotype. Even though the authors don't propose any exact mechanisms involved in this association but they hypothesize that secreted blood group antigens by intestine cells in group O phenotype may act as inhibitors to the cholera toxin while these antigens in individuals with blood group phenotypes other than O act as receptor analogues of the cholera toxin.
Next, the authors argue that as the frequency of O blood group individuals in South America is higher than the rest of the world, the health authorities should be aware that there would be a higher rate of cholera infection during cholera outbreaks in the future. Moreover the authors conclude that the efficacy of cholera vaccine may be lower in South America compared to the areas with a lower frequency of blood group O. This conclusion is based on an investigational study conducted by researchers in Bangladesh saying that the vaccine is relatively less protective against diarrhoea in O blood group individuals.
The technical language and terms used in this article indicate that it is primarily written for the medical professionals and students who have previous knowledge in the area in some extent.
Finally, the authors conclude that even though it is not yet clear what mechanism are involved in susceptibility of O blood group individuals to cholera disease, but they suggest that both evolutionary and molecular explanations should be taken in account when investigating the nature of this relationship.
Besides reliability and relatedness to the research question, this article contributed to a better understanding of the different mechanisms employed by microorganisms during the process of host organism infection.
To identify the factors contributing to susceptibility of O blood group to V. Cholerae, undertake an observational study based on the patients infected with in Bangladesh capital city, Daka.
They select their sample from patients aged 6 months and over, presented to the local hospital with a severe diarrhoea for inclusion in this study. After typing their blood for ABO antigen and collecting anti-cholera toxin antibody titres, they start evaluating the result.
Besides observing a severe cholera infection in patients carrying blood group O, they find that O blood group people are primarily protected against V. Cholerae O1 infection, despite the severity of the illness once infected. Next, they discuss the importance of both innate host factors and adaptive immune responses in susceptibility to cholera infection.
The technical language and terminology used in this article suggests that it is primarily written for medical students and professionals with some background knowledge in the area.
Finally, the authors suggest that considering the way cholera toxin discriminates between blood-group antigens as well as other predispositions, the medical community should minimize the risk of V. Cholerae infection by conducting more investigational studies in the future.
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The detailed and evidence based information presented in this article was very useful in seeking to answer the research question as this text had argued not only the evolutionary side of the issue but the different possible reasons behind it too.
The media article is taken from a social network website, known as BUKSIA in which, people can share their ideas with other members in the internet community. This article is aiming to aware the public of the facts associated with V. Cholerae and its influences to the public health. It does not go beyond listing these facts to evaluate the nature of the associations and the underlying mechanisms involved in shaping these facts. Even though the media article did not contribute to answering my research question, but it gave me the idea to think about these facts and if possible, find the answer for them.
On the other hand, even though the journal articles did not end up with a definite answer to the research question, but the hypotheses discussed in them were quite useful in term of knowing the different possible mechanisms employed by V. Cholerae in the process of infecting human body. The issue discussed in both of these articles was very relevant to my research question and contributed to a major part of answer to it. The authors had effectively communicated the blood group antigen behaviour during the infection as well as the other mechanisms which may be involved in the susceptibility of individuals to V. cholera.
Besides helping me with creating the research question, these articles specifically broadened my understanding of the way cholera disease selectively pick up O blood group individuals in the society. Thinking even further, these texts and the idea discussed in them made me think of the process of natural selection, with O blood group individuals being selected against. It is true that at this stage it is just a hypothesis, but conducting more researches in the future would help us to say the final words regarding the natural selectivity of O blood group individuals.
The journal articles chosen were discussing this issue and were proposing hypothesis regarding the mechanisms involved in the association between blood group and cholera susceptibility. They never come to the point where they would be able to conclude the topic with an exact and evidence based mechanism regarding this association, instead they just made hypothesis and left the issue unresolved. To further investigate the issue, these hypothesise are really helpful in considering the different possibilities which may be the answer for the question.
In summary, it is no doubt that there is a strong relationship between being an O blood group individual and the severity of infection with the bacterium V. Cholerae. Even though researchers are still struggling to find out the exact underlying mechanisms involved in this association, but one should consider that it is the nature of the interaction between cholera toxin and the different types of antigenic molecules present in the intestinal cell surface which is contributed to this selectivity. Some of the possible explanations so far suggested by researchers regarding this issue include acting of the secreted blood group antigens as inhibitors or receptor analogues of cholera toxin and the protective mucous secretion by intestine cells of non O blooded individuals.
Further developments on the understanding of the susceptibility of O blood group individuals to V. Cholerae requires a better understanding of the biochemical and evolutionary responses of the host organism to the V. Cholerae as well as the ecology and biochemical properties of the cholera bacterium and its enterotoxin. Until there is no intensive researches carried out in the field, the reason behind this association would remain undiscovered which will make it difficult for the researchers to introduce vaccines and other new protective measures against V. Cholerae. This in turn will leave a large group of the population unprotected from the deadly cholera disease, especially in the areas with an increasing poverty, population and poor sanitation.