Non-fiction: Studies on the Disease of the Silkworm

Overview
Louis Pasteur's 1865 Studies on the Disease of the Silkworm (Études sur la maladie des vers à soie) addresses a catastrophic series of epidemics that devastated European sericulture. The work combines careful observation, systematic microscopy, and controlled experiments to identify the cause of the most destructive affliction, known as pébrine, and to propose practical measures that breeders could use to limit its spread. The study stands out for translating laboratory insight into field-applicable procedures that restored confidence and productivity in silkworm breeding.

Background and problem
Mid-19th-century silkworm breeders faced repeated losses that threatened local economies dependent on silk. Two superficially similar maladies, pébrine and flacherie, produced heavy larval mortality and poor yields, but their origins were disputed. Some blamed climate, diet, or husbandry; others suspected living agents. Pasteur entered the controversy as an analytical observer determined to combine pathological description with reproducible experiments to settle whether a transmissible agent was responsible and to identify reliable diagnostic signs.

Investigations and findings
Using meticulous microscopic examination of affected larvae, pupae, and adult moths, Pasteur discovered characteristic dark bodies and spores associated consistently with pébrine. He demonstrated that these corpuscles were not merely the effect of disease but were present in healthy-looking breeders and their eggs, providing a mechanism for transmission across generations. Experiments that transferred material from diseased to healthy insects produced the same disease, establishing contagiousness. Pasteur contrasted pébrine with flacherie, showing that flacherie had different signs and likely resulted from digestive disturbance and secondary infection rather than the same parasitic agent.

Methods and diagnostics
A central contribution was a practical diagnostic technique accessible to sericulturists: the microscopic inspection of adult moths and eggs before using them for breeding. Pasteur described how to detect the telltale corpuscles in the bodies and on the surface of eggs and recommended discarding any stock showing infestation. He emphasized selection of healthy parents, isolation of cohorts, and hygienic rearing conditions to prevent cross-contamination. Experimental hygiene measures and the systematic culling of infected breeding stock provided a workable protocol that cut transmission by eliminating infected maternal lines.

Impact and legacy
Pasteur's study had immediate economic and scientific consequences. Adoption of his diagnostic and selection methods sharply reduced pébrine outbreaks and revived silk production in affected regions. More broadly, the work reinforced the emerging germ theory by showing a microscopic parasitic organism could cause an epidemic in an economically important animal. The methodology, combining microscopy, controlled transmission experiments, and pragmatic preventive measures, became a model for applied microbiology and veterinary science, foreshadowing later advances in disease control and the professionalization of sanitary practices in agriculture.