The scourge of silk and Pasteur's cure

In 1845, a strange disease infecting silkworms broke out in France, Italy, Spain, Greece and Turkey, like wildfire. The flourishing silk industry of France was almost wiped out because of this havoc.

The disease that slaughtered the silk industry was called Pebrine, as the diseased worms were covered with black  spots that looked like pepper (pebre in French means pepper).

It was at this point in time that Louis Pasteur was asked to attend to the problem. Pasteur soon found out that the wee globules (corpuscles) seen under the microscope were related to the disease.

He called the agricultural committee and advised them to check the fatty tissue of the father and mother moths below their skin for such spores. “If such spores are not found, you can use these eggs for a healthy generation,” he declared. But Pasteur could not convince them to do so. Meanwhile, he started touring the sericultural areas, convincing the sericulturists, giving lectures, working in the lab and answering hundreds of queries of sericulturists.

Where are the springs?

For results, he had to wait till the next rearing season. He was quite hopeful about a positive outcome. But during spring, the big bubble of hope burst. The disease had resurfaced like a phoenix.

So for the next time, with great difficulty, Pasteur procured two batches of worms, one healthy and the other infected with Pebrine, for further study. He observed the behaviour of Pebrine and came to the conclusion that the disease was hereditary, but it was difficult to judge at what stage the symptoms appeared. The disease could be confirmed only at the moth stage.

The next time, he crushed the moths to check for Pebrine spores. He used the eggs of such disease-free moths for the next rearing. As expected, the eggs produced from these “disease-free” pairs gave healthy worms. That year, he reared this healthy stock and distributed the disease-free layings to the farmers. Thus, he concluded that Pebrine could be broadly classified into two categories: ‘hereditary’ and ‘contaminative’.

The ‘hereditary’ one exists due to the infection of seeds obtained by diseased moths. The larvae hatch out with the disease already in them. The ‘contaminated’ Pebrine was caused due to an infected material. In Pasteur’s method of preparing ‘disease-free layings’ to prevent Pebrine, each of the female moths is crushed after oviposition and examined under microscope for the presence of Pebrine spores.

It may be mentioned here that Pebrine spores can easily enter the egg (trans-ovarian), but they are too large to be carried by sperms. That is why inheritance of the disease is entirely maternal. The eggs which show the presence of Pebrine spores are destroyed.

However, care should be taken to keep the seeds away from the source of infection like spore laid faeces in the silkworm bed, infected rearing appliances etc. The history of seed ‘stock’ is also a must. This time, he advised the sericulturists to crush the entire moths for checking the disease causing organisms. Thus came the concept of mother moth examination.

Next year, in February 1867, he started receiving letters from those seri-farmers to whom he had distributed the disease-free laying. As expected, most of them were getting healthy crops. But with a few farmers, the crops were again ‘spoilt’. A disappointed Pasteur soon realised that it was some other disease, but not Pebrine.

Moreover, it was not very difficult to keep away this disease called ‘Flacherie’, the main symptom of which was the worms going inactive and dying during spinning. He suggested sorting out such worms separately. During the next year, the farmers followed Pasteur’s advice and harvested healthy crops.

There were always controversies and criticism about Pasteur’s work, especially about Pebrine. However, his contributions cannot be discounted. The beauty of Pasteur’s work lies in complete solution to the problem.