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John Snow Had First 'Mapped the Source' of Cholera in 1854, It's Now Being Used for Covid-19 Tracing

Representative image.

Representative image.

An epidemiologist named Dr John Snow, had first used mapping to trace the source of Cholera in a neighborhood in London back in 1854. This mapping technique is a precursor of contact tracing.

Before you begin reading this, it is important to let you know that we are not talking about Jon Snow, the Lord Commander of the Night’s Watch from HBO’s popular series, Game of Thrones.

We are, instead, discussing an epidemiologist named Dr John Snow, who first used mapping to trace the source of Cholera in a neighborhood in London back in 1854.

This mapping technique is a precursor of contact tracing which is currently being used by countries across the world to detect source cases of COVID-19 so that they can identify the affected cluster and isolate those who have come in contact with virus-hit individuals.

In a recent book, written by Dr Swapneil Parikh, a practising physician in Mumbai, Maherra Desai, a clinical psychologist and medical researcher, and Dr Rajesh Parikh, the director of medical research and honorary neuropsychiatrist at the Jaslok Hospital and Research Centre, titled ‘The Coronavirus, WHAT YOU NEED TO KNOW ABOUT THE GLOBAL PANDEMIC’, the authors delve into how Dr John Snow was perhaps the only one who knew something about Cholera’s mode of transmission, although, initially no one was willing to believe him.

In the book the authors write: ‘In 1846, a cholera pandemic started in the Ganga delta in India and, despite the absence of commercial air travel, rapidly spread through Asia, Europe, Africa and North America. This was the third cholera pandemic and it killed at least 1 million people before petering out in the 1860s. It was believed that cholera was caused by airborne ‘miasma’ that originated from decomposing waste and if someone inhaled the miasma, they would get cholera. Contrary to the prevailing miasma theory, Dr John Snow suspected that the contamination of drinking water by sewage caused cholera. Snow published his theory in 1849 but his colleagues, believing in the miasma theory, thought Snow was wrong. They probably taunted him, ‘You don’t know, John Snow.’

In August 1854, a mother washed her sick baby’s diaper in a town well on Broad Street, London, and sparked a cholera outbreak that killed over 600 people.

John Snow set out to investigate the Broad Street cholera outbreak. He wrote: ‘Within 250 yards of the spot where Cambridge Street joins Broad Street there were upwards of 500 fatal attacks of cholera in 10 days. As soon as I became acquainted with the situation and extent of cholera, I suspected some contamination of the water of the much-frequented street-pump in Broad Street.’

Pioneering a new epidemiological technique, he used a grid map of the area to plot the deaths due to cholera. Each death was marked as a dot at the house of the deceased. On the map, most deaths were clustered around a water pump at the intersection of Broad Street and Cambridge Street. Armed with this evidence, Snow convinced reluctant town officials to turn off the Broad Street water pump.

The cholera outbreak stopped. Over the next few months, Snow tracked almost every single case back to the water pump but no one could figure out how the pump was contaminated. It was later discovered that a mother had discarded her cholera-afflicted baby’s soiled diaper in a cesspool just three feet from the Broad Street pump.

In 1855, Dr Snow published an essay, ‘On the mode of Communication of Cholera’, including his original drawing plotting the deaths around the Broad Street pump. This is perhaps the most famous map in epidemiology and the methods he developed laid the foundations for the investigation of every subsequent major epidemic.

In the 1860s, Dr Louis Pasteur conducted several experiments proving the germ theory of disease and in 1883, Dr Robert Koch isolated the bacterium Vibrio cholerae. Dr Koch proved that cholera was transmitted by unsanitary water or food supply sources, finally validating Snow’s theory. John Snow, it turned out, did know something.

The methods pioneered by Snow are currently in use to investigate COVID-19. Koch’s postulates, the criteria established by Dr Robert Koch way back in 1883, were used to establish that the novel coronavirus, SARS-CoV-2, is the agent causing COVID-19.

A Certain Tyrrell

Apart from John Snow, a certain Tyrrell had also made significant contributions to COVID-19 research. His name was Dr David Arthur John Tyrrell, (no, he wasn’t from House Tyrell), a British virologist.

In the book, the authors write that Tyrrell had joined the Common Cold Research Unit (CCRU), which was founded by the United Kingdom’s Medical Research Council, right after finishing his college education (in 1957) with the hope of finding a cure for the common cold, and his discoveries had put down the framework for COVID-19 research.

The book says: ‘At the CCRU, he (Dr Tyrell) experimented on volunteers to study viruses responsible for the common cold. Newspaper advertisements offered a unique ten-day stay at CCRU where volunteers would be infected with preparations of a cold virus.

They would be paid £1.75 per day and would be housed in small groups, strictly isolated from one another. In 1965, Dr Tyrrell’s team isolated an unusual virus from a young boy with the common cold. They exposed several volunteers to his nasal washings and they developed

a cold. Interestingly, the virus, initially named B814 after the number of nasal washings, grew exclusively on human embryonic tracheal cultures.’

In 1966, Dr Dorothy Hamre and Dr John Procknow identified a similar virus in medical students sick with a cold. Later that year, Dr Tyrrell demonstrated under an electron microscope that the new virus resembled the bird bronchitis virus and the mouse hepatitis virus.

With this new information, scientists around the world identified related viruses, giving them similarly unimaginative alphanumeric names. Shortly thereafter, these new viruses were collectively named coronaviruses (CoVs) for their crown-like appearance.

Dr David Tyrrell’s brilliant discovery laid the groundwork for modern research on coronaviruses. Scientists have since identified numerous coronaviruses in a multitude of animal species. These include alpha, beta, gamma and delta coronaviruses. Gamma and delta coronaviruses infect birds and have not been proven to cause human infections.

Alpha and beta coronaviruses have infected humans and many other mammals, especially bats. The seven coronaviruses that have infected humans are called human coronaviruses (HCoVs).

Of these, HCoV-229E and HCoV-NL63 are two human alpha coronaviruses, while HCoV-HKU1 and HCoV-OC43 are two human beta coronaviruses. Last, there are the three notorious human beta coronaviruses that have caused SARS, MERS and the ongoing COVID-19 pandemic. The WHO has declared COVID-19 as the most dangerous threat to world public health, the first pandemic due to a coronavirus.

The following excerpts have been published with permission from Penguin Random House, India.

first published:May 04, 2020, 17:35 IST