Vibrio cholerae (Cholera)

Cholera is a contagious infection caused by
the bacteria Vibrio cholerae, which can in turn cause severe gastroenteritis and excessive
watery diarrhea for several days. Rapid dehydration and electrolyte imbalances
can be fatal as suspected in the deaths of James K. Polk, the 11th President of the United
States; and Charles the Tenth, King of France. V. cholerae is a gram-negative, curved bacteria
which looks like little red or pink comma-shapes on a gram stain. It’s positive for oxidase and grows in alkaline
media. It has pili and a single polar flagellum,
kind of like a tail, at one end which it uses for movement through the gastrointestinal
tract. It’s a facultative anaerobe so that means
it can undergo respiratory and fermentative metabolism. Transmission of V. cholerae typically occurs
through a fecal to oral route. This includes consuming untreated sewage water,
and anything that comes in contact with it, like raw or undercooked fish including shellfish;
and improper hygiene, like a lack of hand washing after a bowel movement. Cholera tends to be more common in developing
countries and places lacking advanced sanitation and sewage treatment facilities, with high
rates in some locations in Africa and South America. People who have low gastric acidity or have
an O-blood type are particularly at risk for a severe infection. Now, when V. cholerae enters the stomach it
shuts down protein production to conserve energy and nutrients, and to survive the acidic
environment. But once V. cholerae is in the intestines,
it uses its flagella to move toward the intestinal walls; propel through the mucous layer on
top of the epithelial cells lining the intestines; and attach to the finger-like cellular projections,
called villi, on the surface of the epithelial cells. There, V. cholerae can begin to multiply and
produce toxins. And though V. cholerae does not enter the
epithelial cells itself, the toxins do and they can cause a lot of trouble. Now, the exact toxins produced can depend
on the strain of V. cholerae. Some strains produce toxins that won’t cause
any, or maybe just mild clinical symptoms. But some strains produce cholera enterotoxin,
also called choleragen, which is most often the cause significant clinical symptoms. When cholera enterotoxin enters the epithelial
cells, it leads to the ADP-ribosylation of the Gs alpha subunit of G-protein. So, this causes the G-protein to becomes permanently
activated and it keeps activating a membrane-bound protein called adenylate cyclase. This protein in turn leads to an overproduction
of the intracellular secondary messenger protein, cyclic adenosine monophosphate, or simply
cAMP. Increase in cAMP causes chloride channels
on the cells to increase the secretion of chloride into the lumen while inhibiting the
channels that let sodium and chloride back into the cell. So, the abnormally high levels of luminal
sodium and chloride disrupt the osmotic balance between the intestines and surrounding tissue;
and water, bicarbonate, and potassium rush into the lumen of the intestines as a result. This can cause symptoms like vomiting and
voluminous amounts of watery diarrhea containing extremely high concentrations of sodium, potassium,
chloride, bicarbonate; as well as high levels of live V. cholerae ready to infect another
individual. There could also be flecks of the mucous layer
of the intestine which looks similar to rice in water. Typically, there is no fever, pain or cramping. And while the incubation time for V. cholerae
can take hours, to 2-3 days; severe dehydration and depletion of electrolytes can happen within
4 to 12 hours of the first bout of diarrhea or vomiting. These imbalances can lead to symptoms like
disorientation, dry mouth, swollen tongue, sunken eyes, cold clammy skin, or shriveled
and dry hands and feet. Other, even more severe, fatal complications
can come from depleted electrolytes and water in the blood. Like low levels of bicarbonate can cause metabolic
acidosis with deep and labored breathing called Kussmaul breathing. Or low levels of potassium can cause muscle
dysfunction including leg cramps, weakness and abnormal heart rhythms. And low chloride and sodium can cause headaches,
poor balance, disorientation, seizures and coma. Finally, severe dehydration can cause hypovolemic
shock with a significant decrease in blood pressure. Diagnosis is typically made by a stool sample
growing V. cholerae on a thiosulfate-citrate-bile salts-sucrose agar. But even before a definitive diagnosis can
typically be made, the first line of treatment is to rapidly replace the lost water and electrolytes
with rehydration salts either orally or intravenously. In cases of mild to moderate diarrhea, this
type of supportive therapy is enough of a treatment course and the diarrhea may resolve
on its own in about 3 to 7 days. But more extreme cases may require antibiotics. In this case, the stool culture can be helpful
in deciding which antibiotics would be most effective against the infecting strain, and
may include tetracyclines, ciprofloxacin, ofloxacin, furazolidone, or trimethoprim-sulfamethoxazole. So, to recap: Cholera is a contagious, severe
gastroenteritis caused by the gram-negative Vibrio cholerae bacteria. It’s route of transmission is fecal to oral
through consumption of contaminated sources like water and shellfish. Some strains produce cholera enterotoxin at
the surface of intestinal epithelial cells which leads to the over-activation of cAMP
by permanently activating Gs alpha. Patients most often present with voluminous
“rice-water” diarrhea that quickly leads to dehydration and electrolyte imbalances
that can be fatal if not rapidly treated with rehydration salts.

13 Replies to “Vibrio cholerae (Cholera)”

  1. "Learning medicine is hard work" Tell me about it ! especially when learning a whole other language along with it

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