Subscribe to the OSS Weekly Newsletter!

Register for the OSS 25th Anniversary Event

Contact Tracing is the Key To Solving the Spread of Infectious Diseases

The process that epidemiologists are now using to shed light as to how SARS-CoV-2 is spreading is not new. One can look to history, back to John Snow and his mapping of the cholera epidemic for the first example, a lifesaving one, of contact tracing.

Today it is COVID-19 that is sowing panic around the world but in 19th century England the fear was of cholera, a dreadful disease. It causes diarrhea so severe that a victim can lose as much as ten liters of water in a day. If untreated, it can lead to rapid dehydration and death within a few days. The disease first appeared in Europe in 1831, an undesired import from the Indian subcontinent where it was endemic. Over 50,000 Britons died within a year, causing widespread panic. Physicians didn’t know what to do. They plied their patients with arsenic and strychnine, they administered tobacco enemas, they wrapped them in flannel soaked in turpentine, they bled them with leeches and they blistered them with nitric acid. All of course to no avail. No one knew what caused the disease, but prevailing opinion was that cholera was somehow transmitted by bad air, or a “miasma,” that emanated from the sick and from garbage. A London dentist claimed that the solution was to fire cannons every hour to disperse the bad air!

Dr. John Snow, a young apprentice doctor at the time, attended to many of the cholera victims. He didn’t buy the idea of poisoned air. It seemed clear to him that cholera was due to some poison that acted directly on the intestines and therefore was most likely introduced through the mouth. His suspicion turned to water when he noted that the city of Birmingham had been spared of cholera. What was the difference here?  Nobody drank river water! The water of the river Tain was so foul that nobody could stomach it. People in Birmingham drank only well water. By 1849 Snow had published a pamphlet suggesting that cholera was spread by water, but his ideas were largely ignored. Having been trained through apprenticeship rather than through one of the Royal Colleges, Snow was not a member of the establishment and his views were looked down upon. Most prominent physicians did not want their theories about miasmas disturbed, and in any case, by 1849 the epidemic had waned.

In 1854, Snow got his chance to prove his theory. A terrible cholera epidemic broke out in London, centered in the Soho district where more than 500 people died within ten days in an area of a few city blocks. Dr. Snow then took a map of the city and laboriously plotted on it the location of houses where someone had come down with cholera. An amazing pattern was revealed. The red dots, signifying cholera cases clustered around a major thoroughfare then known as Broad Street. What on this particular street could be causing cholera, Snow wondered? He soon found the answer. 

A water pump that supplied the neighborhood sat right in the area pinpointed by the dots on his map as the focal point of the epidemic. He quickly discovered that seven men who lived outside Soho but had worked in the area around the pump all died of cholera and that a widow who had just moved away from the neighborhood to Hampstead but had sent for some of the water she had been accustomed to drinking also died. But perhaps the most convincing observation Snow made was that in a nearby brewery, (where the workers never drank water) not a single worker came down with cholera!

At this point Snow approached city officials, showed them his map, and suggested that the handle of the pump in Broad Street be removed as a public health measure. Legend has it that this immediately stopped the epidemic. Actual fact is that by the time the pump handle was removed, the number of new cholera cases had slowed to a trickle. Snow did not single-handedly stop the cholera epidemic as the myth suggests, but that in no way diminishes his accomplishment. He clearly showed that cholera could be transmitted by polluted drinking water and went on to demonstrate that sewage was the culprit.

The Broad Street epidemic was eventually traced to a baby with diarrhea whose mother had washed diapers in water that was dumped in a cesspool that leaked into the well supplying the pump. Snow went on to show that cholera was far more prevalent in homes supplied by a water company that took its water from near London Bridge than one that drew it from further up the river. By 1855 he had clearly shown that the disease was caused by some infectious agent in the water and his book published that year, “On The Mode of Communication of Cholera,” stands as the seminal work that eventually led to the chlorination of drinking water and the saving of millions of lives.

Dr. Snow’s solving the mystery of the transmission of cholera can be attributed to his pioneering effort at contact tracing, the process that epidemiologists are now using to shed light on how the SARS-CoV-2 virus is spreading.


Leave a comment!

Back to top