Cholera - The Killing Stool
Over the past few years, the Government of India has started several programmes to eradicate the open defecation system by building public toilets and increasing awareness among people in urban and rural areas. Sometimes, it may seem comical to us, to see our nation’s resources being used to deal with feaces. But today let me tell you what those faeces can do to a nation.
Understanding all the diseases transmitted by the faeco-oral route might not be possible but understanding one disease is incredibly important : Cholera.
Cholera is a profuse, watery diarhhoea.
Now you might think why a few extra runs to the rest room are such a big deal. The problem with cholera is that it drains so much water out of the patients body, that the patient becomes dehydrated and might even eventually die.
So let’s try to understand a little more about the structure and function of the bacteria which causes Cholera.
The bacterium is Vibrio Cholerae. It’s one of the four medically important vibrios. V. Cholera is a gram negative, comma shaped, curved bacilli that is motile by means of a polar flagellum.
On culture, it produces round, convex colonies that appear granular in transmitted light. It grows well at 37°C on media containing mineral salts and a carbon/nitrogen source. A medium called Thiosulfate-citrate-bile-sucrose agar is selective for only Vibrios and produces a distinct yellow colony on a green background. The samples for culture can be obtained directly from the stools or by rectal swabs.
Biochemically, vibrios grown in alkaline nature (high ph) and are oxidase positive. They are halotolerant meaning they can withstand increased amounts of salts.
The classification of Vibrios is unique and developed over the years. It’s called the GARDNER and VENKATRAMAN classification and is based on sero-grouping. V. Cholerae has 206 types of O-liposaccharides that confer serologic specificity. The groups O1 – O139 produce classic cholera and were responsible for most of the pandemics. The other groups produced cholera like diseases. The O1 group can further be classified into two biotypes, El Tor and Classical. These two biotypes can each be further be classified into three serotypes, Ogawa, Inaba and Hikojima.
The pathogenesis of Cholera is highly organised and can be easily segmented for easy understanding. V. Cholerae produces an enterotoxin, that is heat-labile and acts by acting on the cyclic AMP. The disease is transmitted by ingestion of contaminated food. Since V. Cholerae only survive in alkaline nature, to pass through a highly acidic environment like the stomach, a high infective dose of 108 bacilli is required. Once they reach the small intestine, they penetrate into the mucosal layer by the help of mucinases, its motility and an hemagglutinin protease called the cholera lectin. It adheses to the intestinal epithelium with a type-4 fimbriae called toxin co-regulated pili which is regulated by the TOX-R gene.
Once established, it produces a heat labile toxin called the cholera toxin that has a molecular weight of 84Kda. It consists of 2 subunits, A, which is cleaved into A1 and A2 and B, that helps in binding to the GM1 ganglioside receptor. Fragment A1 causes ADP ribosylation of G protien that upregulates the adenyl cyclase activity that further resuslts in the accumulation of CAMP that results in prolonged hypersecretion of water and electrolytes. Also absorption of sodium and chloride by microvilli in the intestine is inhibited. This rapid loss of fluids and electolytes for a prolonged period leads to profound dehydration, anuria which may lead to circulatory collapse. If not treated, the mortality rate varies from 25-50%.
The first line of treatment is immediate replacement of lost fluid and electrolytes.
Oral tetracycline and doxycycline have proved to be effective antimicrobial agents against V. Cholera as they reduce stool output of the bacteria. An attack of Cholera can provide immunity in the future by the action of specific Ig-A antibodies occurring in the intestinal lumen.
Cholera is endemic in developing countries, especially, India and Southeast Asia. It is spread by flies, water and food. Control rests solely on improvement of sanitation and education. Thus let’s fight this tiresome disease, actively as a community, Let’s Swachh Bharat!