Counting cholera
My most recent post to significance magazine concerned the under-reporting of disease outbreak cases. In particular, as of Nov 9th the current numbers for the cholera epidemic in Haiti is 11,125 and 724. Given these numbers it is hard to get a handle on how many cases there really are.
As an estimate, we can use the numbers from a recent Science Friday segment where the CDC's Eric Mintz was interviewed. In the iterview Dr. Mintz said that in an epidemic typically 75% of individuals who are exposed to cholera have essentially no symptoms. Of the other 25%, 2/3 have mild diarrhea and most of the rest of only moderate diarrhea. If I understood the interview correctly, of all of those exposed about 2% will have severe diarrhea and vomiting which can quickly cause dehydration and stroke and then lead to death. If we assume these are the cases that get hospitalized then, as of Nov 9th, we would have had 556,250 people exposed which would act as an upper bound. If instead, everybody with moderate diarrhea or worse was hospitalized then we would have had 133,500 exposed which acts as a lower bound.
Also included in the Science Friday segment is the natural protection we have against cholera. As can be expected, the severity of the disease is linked to the amount of the bacterium Vibrio cholerae that is ingested, but also linked to severity is the pH of the stomach and blood type. A more acidic stomach will kill more of the bacterium before it can enter the small intestine and cause cholera symptoms. So children and those taking antacids will be more susceptible to the disease. According to the interview, those with a blood type of O are more susceptible to cholera although the reasons for this are unclear at this point.
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