Fighting Cholera Today

As part of our celebration the 200^th anniversary of the birth of the epidemiologist John Snow, we spoke to Matt Waldor from the Harvard Medical School about how we tackle Cholera in the modern world and find out how the disease could have reached Haiti through relief workers.

Matt - Cholera is a diarrheal disease, but it's not like diarrhoea that you or I have probably ever experienced in our life. It is extremely, extremely severe watery diarrhoea. So, what is most traumatic about cholera is the rapidity with which it can kill its victims. Cholera is caused by a bacterium, a gram negative bacterium, Vibrio cholerae. We usually get infected when we drink contaminated water and a person can be entirely well and then be infected and die from choleric diarrhoea within 7 hours. It may be the most rapidly fatal infectious disease that we know. This organism has afflicted humans really since the beginning of recorded history. There are perfect clinical descriptions of cholera in Sanskrit text that date back at least 3,000 years ago.

Chris - And of the people who succumb to cholera i.e. get infected, what proportion will survive it?

Matt - Well, if left untreated, there's almost a 50% mortality of severe cholera, but in the early '60s, mostly in Bangladesh and in some parts of India, people discovered that they could use what is called rehydration therapy which is just give people salt water with some glucose and that can rehydrate them either orally or with intravenous solution, and then mortality rates can go to below 1%.

Chris - John Snow must be something of a hero of yours as a specialist as you are in cholera.

Matt - Yes, he certainly is and even though Richard Barnett is maybe deflating a little bit the story of Snow's heroism, I think we shouldn't belittle the profundity of his work and his observation which is generally regarded as founding the field of epidemiology by linking a particular vehicle, in this case, contaminated water with a disease that is so far different than the paradigm that ruled the day as the professor said, miasma theory.

But to go forward 200 years, we were inspired by Snow's work, in particular, by tracing how cholera got to London and that was probably via a sailor from Hamburg. The epidemic in 1848 that killed 50,000 people in London, that was probably brought to the city and Snow recognized this and tried to publish it and was rejected by sailor from Hamburg which had previously experienced a big outbreak of cholera. We asked a similar question in late October 2010, how did cholera get to the Island of Hispaniola which includes the nation of Haiti? There had never been, as far as we know, cholera in Haiti ever, before October 16^th, 2010.

Kat - So, in Haiti, to sort of link Snow's work to tracing the modern outbreak today, what happened in Haiti was a massive earthquake and then contamination of the water supply. Can you give me an idea of the scale of the cholera outbreak that happened there?

Matt - Yes, absolutely. So, as far as we know, there was never a cholera in Haiti until this outbreak. The earthquake happened in January of 2010. The outbreak didn't occur until October. So, the cause of the outbreak was a great mystery and that's what we'll talk about in a minute. But the gravity of the outbreak which continues to this day is enormous. To date, something on the order of 700,000 people have suffered severe cholera which is about 7% of the Haitian population and about more than 8,000 people have died so far.

Kat - That's a huge number. So, how are you going about trying to track the routes of this massive outbreak?

Matt - What we did, once we learned of this outbreak in mid-October 2010, some physician colleagues of mine were able to get some samples from the Haiti outbreak and what we did, which I view as sort of a modern parallel of Snow's work was very rapidly, using some very new sequencing technology, determine the complete genome sequences of 3 isolates from different areas in Haiti from a very early part of the outbreak. And then we also sequenced strains from different parts of Asia and also from Latin America. Most of us in the field thought that most likely, the outbreak was caused by some local strain from the Americas. Cholera had been absent from the Americas for about a century before 1991 when it hit Peru and then infected more than a million people.  It never got to Haiti though, but we thought, well, there was a terrible earthquake. Maybe somehow, cholera came from the sea and infected and got into the water supply in Haiti, but that turned out not to be the case?

Kat - So, where did you find the source?

Matt - The full genome sequence of the Haiti outbreak turned out to be nearly identical to the genomes of strains from South Asia and they were distinctly different from the strains from Latin America. So, that tended to strongly argue against an origin from the Americas and suggested instead that they came from Asia.

Kat - So, how did they get there then?

Matt - Yeah, more traditional, real Snow-like shoe leather epidemiology work done by a French epidemiologist Rene Perrow showed that the earliest part of the outbreak began adjacent to a Nepal UN security forces. Those security forces had come from Kathmandu Nepal, just 2 weeks after there was an outbreak of cholera in Kathmandu. Although none of them were symptomatic, what transpired 2 weeks later was the beginning of this epidemic and it was known that the effluent from the toilets of that base actually drained ultimately into one of the main rivers in Haiti, and that's how the epidemic spread. So, I think in a way, Perrow's epidemiology even trumps our modern genomics in making the case extremely strong that the strain actually came to Haiti via human activity, just like Snow realised that a sailor from Hamburg brought cholera to London in 1848.

Chris - Your final thought of cholera and its present situation in the world, Matt.

Matt - Going back to John Snow, even though Snow found that epidemiology and let us understand the spread of cholera, in the past 8 years or so, the WHO has found that cholera is increasing around the world.  So, great challenges still remain on the road that Snow started this on.