The little Worm That Changed The History of Medicine

If we carried out a poll and asked a few pedestrians the name of 3 domestic animals, it is sure that the silkworm would not be amongst the most popular. However; this insect provides us with a unique fabric, it has had a tremendous impact in the history of humankind (remember the silk route) and it’s the most domestic animal out of all of them since there are no wild populations of it.

Figure 1: Silk worms (Bombix mori)s

These larva made a unique contribution to medical science:

Let’s travel in time to 1865 to the South of France: breeders of silk worms were completely desperate. A disease was killing millions of caterpillars and their production was decreasing sharply.

They called it the evil pebrin (from pebre, pepper in French) since caterpillars would be covered with small black spots, as if they had been sprinkled with ground black pepper. The affected worms would stop growing, would languish, and end up dying.

The silk industry was very important to France back then.

Figure 2: Worms affected by pebrin

Some counties had a revenue of up to 16 million of francs yearly thanks to sericulture. An real fortune. All of this fell apart with the arrival of pebrin. The production fell over 50% and the silk that would arrive to the looms was low quality. The producers ruined, bankruptcies multiplied. The boldest people were able to go to Japan to bring eggs and larvae resistant to the disease, but nothing seemed to work.

In 1865, Jean Baptiste Dumas, native of the infected region and then minister of commerce as well as of agriculture, seeing the debacle that was falling upon the silk industry, called one of his friends and old disciple: Louis Pasteur.

He was a famous chemist and could to try to find the cause and solution for the disease of the silk worms. Pasteur was reluctant at the beginning but couldn’t fail his old professor. He couldn’t have believed then that he would spend 5 years researching this ailment.

The observations that Pasteur conducted –along with his helper Gernez- with the microscope allowed them to conclude that sick caterpillars had tiny globules in their interior, and those tiny globules would multiply themselves in an exponential way as the disease advanced in its development.

Figure 3: Microscope used by Pasteur in his investigations about pebrin along with some pupae he used in his studies

Some butterflies would lay eggs that would also present these formations. From these contaminated eggs, the larva that was born would get sick later on and die.

He also proved that healthy caterpillars would get sick when eating infected mulberry leaves with feces of infected larvae, confirming that the pathology was contagious.

Pasteur attributed the disease to the globules and proved that, if silk producers selected for the coming up spring season eggs examined by a microscope free from those globules; the hatching worms would be healthy and able to build their cocoon –the silk source- normally. Sericultors added microscopes as one of their main  working tools and started to select healthy eggs and were able to hatch healthy worms.

Pasteur had already proved previously that some microscopic organisms were responsible for some biological processes like wine or beer fermentation.

But to discover that it was a germ who caused this disease was an absolutely transcendent discovery that changed medicine history forever. This finding, which was called the microscopic theory of disease was contested by a lot of people since many could not believe that such tiny living organisms, invisible to the human eye, were able to kill not just worms but also people (in fact, ironies of destiny, Pasteur lost two of his daughters due to diseases produced by microorganisms: the older one died due to typhoid fevers and the second one after getting sick of typhus).

Despite the doubts of some; previous studies of Pasteur, Koch and others ended up proving irrefutably this new reality and contributed to the development of vaccines as well as understanding the importance of hygiene in operating rooms to avoid the transmission of diseases.

It was like this how a small domestic animal and parasitic fungus (Nosema bombycis) –that we now know are the cause of pebrin- changed medical health forever.



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  1. Prescott-Allen, R. Genes from the Wild: Using Wild Genetic Resources for Food and Raw Materials. 2013
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  1. Jean Baptiste Dumas (1800-1884)

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