February 2006


Controlling disease-causing germs is a constant challenge in hospitals, nursing homes, farms and veterinarians' offices. Chlorine-containing disinfectants are important allies in the war against germs. One such disinfectant, chlorhexidine, can be used on hard surfaces, such as medical examining tables, and also (in a more diluted form), on skin surfaces. Chlorhexidine kills bacteria, fungi (fungi, pronounced "fun guy," is the plural of fungus) and viruses that can spread sickness among people and animals.

An Antiseptic for People and Animals

Chlorhexidine is called an antiseptic when it is used on the skin. An antiseptic destroys germs on skin without harming skin tissue. Surgeons wash their hands with chlorhexidine before performing operations to prevent the transfer of germs from their hands to their patients. Similarly, chlorhexidine is used to clean and disinfect small wounds.

Timber, the author's yellow Labrador
Retriever, enjoys having his ears
cleaned with a solution containing  
Veterinarians use chlorhexidine to treat animal skin conditions, gum disease and ear infections. If you have a dog with long, floppy ear flaps like Timber, pictured at left, he or she may be prone to developing ear infections. Long ear flaps limit the amount of air that circulates around the ear opening, contributing to dark, moist ear environments--perfect living conditions for certain germs. Periodic cleanings with chlorhexidine can keep these germs under control, and dogs comfortable.

Chlorhexidine and Bird Flu

You have probably heard news reports about the threat of "bird flu." Bird flu is a sickness caused by a virus, called H5N1, that has infected birds in some countries and some people who live in close contact with those birds. Scientists are keeping a watchful eye on bird flu, anxious that small changes to the bird flu genes, known as mutations, could cause it to spread rapidly from person to person. Products containing chlorhexidine may provide valuable help in the fight against bird flu in poultry farms and other infected areas.

The Science Center Salutes:
Dr. John Snow, Disease Detective and Germ Theory Champion

Dr. John Snow, the Father of Modern Epidemiology*

The Germ Theory of Disease states that most infectious diseases (those you can "catch") are caused by germs. That theory is a fairly recent one in human history. People used to think diseases arose spontaneously, or that they were caused by evil spirits, or bad air known as "miasma." Dr. John Snow, a London doctor who in the summer of 1854 witnessed a deadly outbreak of cholera (pronounced: KOL-eh-ra) in his neighborhood, was convinced the disease was caused by germs found in the public drinking water. Just about everyone else blamed miasma.

Around this time, a handful of scientists, including the famous Frenchman Louis Pasteur, were convinced of the existence of germs but struggled to convince others, especially surgeons, of this fact. Snow gave the world dramatic evidence that contaminated drinking water caused the widespread sickness and helped stop the outbreak. For his work on the London cholera epidemic, John Snow is known as the "Father of Epidemiology." Epidemiology is defined as the study of diseases in a population.

Modern science tells us that cholera is indeed caused by a germ--the bacterium Vibrio cholerae. The germ lives in water contaminated with the waste of infected people. Before it was common to destroy germs in public drinking water using chlorine disinfectants, cholera and other waterborne diseases routinely sickened and killed tens of thousands of people every year. The unsanitary conditions of cities in the past, in which wastewater often mixed with drinking water, made these areas breeding grounds for disease.

John Snow was as much detective as physician and scientist. When the epidemic struck, he walked around his neighborhood interviewing the families of the victims. Using a street map to mark the locations of cholera deaths, he found that almost all of the victims died within a short distance of a community water pump on Broad Street. It was there that all the victims had obtained drinking water. Snow was not surprised to find that there were no deaths among 70 workers at the nearby Broad Street brewery; the workers never drank the local water because they were given a daily allowance of free beer.

A replica of the original Broad Street Pump, close to the original pump site in London 

Two cholera deaths, miles from the pump, puzzled Snow until he rode out to the location to investigate. There he was told that the victims, a woman and her niece, had drunk water from the Broad Street pump recently. Snow found out that because the woman had preferred the taste of the Broad Street water to her local water, she had sent her servant to Broad Street daily to draw water from the pump into a bottle. The servant then carried the water back to the woman by horse-drawn cart. The woman had shared that water with her visiting niece.

Snow examined a sample of the Broad Street pump water under a microscope and found white particles that he thought were the disease-causing germs. Presenting all of his evidence, he convinced the doubtful local authorities to remove the handle of the pump. Soon after, the cholera epidemic ended.

If you travel to Dr. Snow's old London neighborhood today, you can see a replica of the Broad Street pump-minus its handle. The pump is a tribute to a man who, by unraveling the mysteries of a cholera epidemic in his neighborhood, laid the groundwork for the science of epidemiology. 

Follow-up Activities

  1. Bird flu is caused by a virus, and cholera by a bacterium. Compare viruses and bacteria. How are their sizes different? Can viruses and bacteria live and reproduce on their own? What are helpful bacteria?

  2. Which dog breed is more likely to have problems with ear infections, German Shepherd or Beagle? (Hint: See the American Kennel Club picture list of Recognized Breeds at: http://www.akc.org/breeds/breeds_a.cfm)

  3. You are John Snow. Based on your investigation of the 1854 cholera epidemic, write a report to the London city government recommending it remove the handle of the Broad Street pump.

  4. Explore waterborne disease germs at the Water Germs Busted by Chlorine and the Swimming Pool Germs Busted by Chlorine coloring and activity books.

*John Snow, Photograph, 1857. Wellcome Historical Medical Museum and Library, London in Gordis L. Epidemiology, WB Saunders, Philadelphia, 1996.

For a list of previous "Chlorine Compound of the Month" features, click here.


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