Pasteur and the attenuated vaccine

Last week I detailed how in the 18th century Edward Jenner used inoculation of a related cowpox virus to impart immunity against the smallpox virus.  It took nearly a century later for another important advance in the development of vaccines.

Louis Pasteur is one of the greatest scientists who ever lived, based on the scope and importance of his different accomplishments.  For example, when he published his famous germ theory of disease in 1861 (ridiculed at the time by his contemporaries), in which he proposed that illnesses were caused by microscopic pathogens, it forever changed the course of medicine.

In the early 1870s, already a world-famous scientist, Pasteur became fully engaged in the study of diseases and would become a major contributor to vaccine development.  He reasoned that if Jenner could find a vaccine for smallpox, then vaccines were possible for all known diseases.  In 1877, he was studying chicken cholera, a nasty bacterial disease that was decimating the breeding chicken population.  He succeeded in isolating the causative virulent bacterium of chicken cholera and growing it in culture.  He then started inoculating chickens with the bacterium and observed a very high death rate.  Nevertheless, he kept working to find safer inoculation methods.

One day before leaving on holiday, Pasteur instructed an assistant to inject chickens with a fresh culture of bacteria.  The careless assistant forgot to carry out the injections and left for a vacation himself.  Upon his return, Pasteur was none too happy to find out about his negligence and immediately sent his assistant to carry out the injections (using the cultures that had been set aside pre-vacation for that purpose).  The chickens only showed mild signs of the disease and all of them survived.  Intrigued by this and feeling the bacteria his assistant used were somehow compromised, Pasteur injected the chickens again when they returned to health, this time with freshly grown bacteria.  He was befuddled when they showed little sign of distress and didn’t die.

He soon realized that the older cultures had become so weakened that their virulence was significantly reduced, but yet, at the same time, instructed the immune system to effectively fight the infection.  He concluded that just like the situation with a weaker cowpox imparting immunity against smallpox, artificially weakening (or attenuating) the cholera bacteria could safely protect the chickens from cholera.

History is full of stories of important discoveries through serendipity, or chance.  Pasteur was smart enough to understand the significance of his findings and adapted the concept of pathogen attenuation to the creation of other vaccines such as anthrax and rabies.  His rabies vaccine saved countless patients from certain death.