Sick or Silk: How Silkworms Spun the Germ Theory of Disease

Dec. 6, 2019

In the early 1800s, many top scientists ardently believed that disease occurred spontaneously from bad smells or “miasmas” (known as miasma theory). We know today that some microbes can produce heady odors while thriving in decay, so associating 'bad smells' and disease makes sense in hindsight. The miasma theory made perfume the Purell of the early 19th century and gave the plague doctor his distinctive potpourri nose. 

The beak of a plague doctor mask allowed physicians to carry aromatic items to “purify” the air, in accordance with the prevailing to miasma theory at the time.

Truly, there could be no modern microbiology without a theory of “germs.” At the time germ theory was discovered in 1835, it had the monumental task of toppling the prevailing miasma theory. Revolutions start small, and the miasma theory upheaval began with a germinating spore on the back of a silkworm in Italy. 

The Rise of the Precarious European Silk Industry 

Like the diseases they can cause, germ theory did not spontaneously occur once Antonie van Leeuwenhoek first described microbes as “animalcules.” It took serious motivation for scientists to link microbes to disease after discovering microorganisms; that motivation was based in economics. Specifically, the desire to produce fine silk finally prompted the association between microbes and infection. The historical passion of Europe and the Middle East for silk was so strong that their trade route to China was called “the silk road.” The silk industry (also called sericulture), imported certainly from China and perhaps in the hair of an intrepid princess, was blossoming throughout Europe by the 1100s. However, the production of silk requires large numbers of silkworms—and large numbers of any population become vulnerable to infectious disease. 

If you’ve never thought about where silk comes from, here is a short primer: silkworms are caterpillar “cows” that spin silk cocoons in exchange for piles (and piles) of mulberry leaves. Fortunately, silkworms have been domesticated for at least 5,000 years (spanning into legend), so these caterpillars are fairly cooperative given the right conditions.

A freshly emerged silkworm beating his ineffectual wings for the first time.
Source: Photo courtesy B. Lovett.

Maintaining enough caterpillars to produce the miles of silk required for cloth breeds conditions that are ideal for caterpillar diseases. Scientists and silk farmers were initially ignorant to the battle between silkworms and their pathogens and the success of silkworm farms was thus precarious. It was a common tragedy for silkworm farms to be overrun with disease with few interventions the farmers could apply.

This led to superstitious silkworm rearing practices, such as burning incense, based on tradition and the belief at the time that bad smells would spontaneously make silkworms sick. Sick silkworms meant less silk, which resulted in less profit for the silk industry. As was the case in many countries, Italy was facing a stark, silkless future with their silkworm industry. These silkworm farming failures caused the industry to look to science for an answer. 

The Germ Theory of Disease in Italian Silkworms

Agostino Bassi was an insect scientist from Lodi who set to work to devise practical solutions to keep silkworms healthy, and thus save the waning silk industry. Scientists know today that the silk industry was up against multiple microorganisms: various viruses, bacteria and fungi have a taste for caterpillars. In Bassi’s time, though the microbial etiology was unknown, the diseases caused by these different microbes were classified by their distinctive symptoms.

A sketch of the Italian entomologist Agostino Bassi, who developed the germ theory of disease in silkworms.

Silkworm diseases have had many different names throughout the history of silk production, but modern scientists use the French names: pebrine, muscardine, flacherie, and grasserie. Bassi zeroed in on white muscardine, which turns caterpillars into little mummies that subsequently sprout brilliant white fungi. This symptom was so striking that the French named it “muscardine,” after an eponymous bonbon. To Bassi, this disease was called calcinaccio (Itallian for “rubble”). The other name adopted by Italian silk farmers was “mal del segno,” because noticing this disease among your caterpillars was definitely a “bad sign.”

After choosing a disease that was both conspicuous and a pressing problem for the Italian silk industry, Bassi spent the next 25 years carefully studying silkworm bonbons. He began by asking simple questions: Which caterpillars succumb to the disease? How are diseased caterpillars related? How does the disease progress? Ultimately, he was looking for a way to predict which caterpillars would get sick, so silk farmers could intervene before one sick caterpillar became an epizootic.

Bassi’s research led him to understand that the disease could be reliably transmitted by moving fungal spores from a dead silkworm bonbon onto a healthy caterpillar. Imagine the candlelight reflecting off of a cluster of fungal spores as this Itallian entomologist carried them through an intentional arc onto an unsuspecting silkworm. This was a paradigm-shifting moment in our understanding of disease: the dawn of germ theory. Bassi had realized that if silk farmers want more silk, they need fewer infective bonbons.

The Interwoven Threads of Basic and Applied Microbiology

Bassi distilled his realization into a manuscript entitled “Del mal del segno, calcinaccio o moscardino,” with a subtitle translated into English as “a disease that affects silkworms and on the means of freeing therefrom even the most devastated breeding establishments.” Herein, the fundamental discovery that microbes (namely, “an extraneous germ”) can cause disease is interwoven with practical tips for rearing silkworms.

Bassi’s theory had such an impact on our understanding of disease that his silkworm breeding tips are obvious to a modern reader: among them are 1) use disinfectants to clean contaminated insects and equipment, 2) separate caterpillars to prevent disease transmission and 3) remove infected caterpillars before they become contagious. Witnessing this phenomenon in insects led him to propose that we humans are similarly susceptible to diseases caused by germs.

Freshly hatched silkworm caterpillars that are hungrily leaving their white eggs in search for some mulberry leaves.
Source: Photo courtesy B. Lovett.

Inspiring the Next Generation of Scientists

The importance of Bassi’s new theory was recognized immediately. Since his evidence-based silkworm breeding tips offered a way forward for the disease-stricken silkworm industries throughout Europe, it was translated into many languages, including French, so it could be widely distributed. The French translation of Bassi’s work landed on the desk of Louis Pasteur, who was heavily influenced by this newfangled germ theory of disease.

Pasteur went on to work on pebrine in silkworms (a microsporidian disease caused primarily by Nosema bombycis), and he championed the germ theory of disease to help topple miasma theory for good. This story represents science at its best: international, collaborative, fundamental and then applied. Today, Bassi is memorialized in the name of the white muscardine fungus Beauveria bassiana that helped him draw his monumental conclusions. This honor recognizes that this Italian scientist traced the thread of our desire for the finer things in life down to a deeper understanding of the world around us.

Further reading

This insect pathology story and many others, including Louis Pasteur’s first encounter with a silkworm pupa, are more fully and eloquently told in Disease in a minor chord by Edward A. Steinhaus.

Author: Brian Lovett, Ph.D.

Brian Lovett, Ph.D.
Brian Lovett is a postdoctoral researcher working on fungal biology and biotechnology at West Virginia University.