Pébrine: The Silkworm Disease That Changed Medicine

A Silkworm Disease Rewrote Germ Theory

Pébrine was first recorded in France in 1845, spread through Italy, Spain, Syria and Romania, and by 1865 had collapsed the French and Italian silkworm industries. The French government sent Louis Pasteur to find out why.

What he found did more than save an industry.

What Pébrine Is

The name comes from the tiny dark spots that appear on infected caterpillars, like grains of pepper. The cause is Nosema bombycis, a microsporidian — a spore-forming parasite — first described by Nägeli in 1857.

What Pasteur Established

He spent more than two years on it, and the conclusions were not obvious at the time. He showed the disease spread by contact between worms, and through contaminated food. Then he showed something nobody had demonstrated before:

The parasite passed from an infected mother moth to her offspring, through the egg.

That was the first demonstration that a pathogenic microbe could be transmitted vertically — from parent to child rather than between neighbours. It is a foundational idea in infectious disease, and it was established in a silkworm.

The Practical Fix

Pasteur's answer was not a treatment. It was a method: examine the mother moths, discard the eggs of any that were infected, and rebuild the stock from clean lines only.

That technique — still used in sericulture today, where it is called mother-moth examination — is why the industry survived. It also demonstrated something that outlived silk entirely: you can break an epidemic by selection and hygiene, without ever curing a single patient.

Why It Still Matters to Anyone Keeping Silkworms

Pébrine has not gone away, and the reason commercial silkworm eggs are certified disease-free traces directly back to Pasteur's method. When a batch of worms fails inexplicably, this is part of the history behind why the supply chain works the way it does.

Related: the four silkworm diseases · raising silkworms at home