Introduction to Epidemiology 6th Edition by Ray M Merrill

Exercise 11

What were some of the practical problems and barriers that Snow faced that slowed his inquiry of the cholera epidemic of 1854? How might this relate to modern-day epidemiological investigations?

Step 1 of 2

London was free from cholera from 1849 to 1853. Then an important change in the water supply that supplied to several districts of south of London, took place. The Lambeth Company moved from Hungerford Market to Thames Diton. This shift in the company resulted in supply of water free from any London waste. Hence, the houses that were supplied by only Lambeth Company did not report any attacks of cholera.

Cholera is contagious disease that is caused by a gram negative bacterium Vibrio cholerae. It is generally transmitted through contaminated food and water. The main route of entry is the fecal-oral route. Endotoxins released by vibrio cholerae mainly infect the alimentary canal and caused watery diarrhea, muscular spasms and vomiting. Cholera toxin draws water from blood which causes severe diarrhea. As a result of this water and electrolyte imbalance takes place in the body. Treatment includes administration of ORS (Oral Rehydration Solution), Zinc supplements and antibiotics like azithromycin or doxycycline.

During his investigation, Dr. Snow found it difficult to identify the source water supply at each house, partly because most people did not remember the name of companies and could identify it only after reading the receipts and partly because most of them were tenants whose land lords were living far away from the town. This problem was overcome by performing a simple test.

When silver nitrate solution was added to a gallon of water obtained from Lambeth Company, 2.28 grains of silver chloride was obtained. When the same test was performed with water from Southwark and Vauxhall Companies, 91 grains of silver chloride was obtained. This test was helpful, especially when resident was not able to tell clearly about which company’s water supply he had been using. A sample of water was taken from his home, the test was performed and address was noted. These barriers somehow, slowed the rate of investigation.