Pebrine of Silkworms (Microsporidiosis)
Pebrine is a serious parasitic disease affecting the mulberry silkworm, the caterpillar used to produce silk. It is caused by a tiny parasite called a microsporidium, a single-celled organism that invades the silkworm's cells. This disease...
What is it?
Pebrine is a serious parasitic disease affecting the mulberry silkworm, the caterpillar used to produce silk. It is caused by a tiny parasite called a microsporidium, a single-celled organism that invades the silkworm's cells. This disease holds major historical significance: in the 19th century it triggered a devastating crisis in silk-producing regions, particularly in Europe. Today it remains a concern for sericulture operations wherever silk production occurs.
The parasite responsible is called Nosema bombycis. It forms small resistant structures called corpuscles, visible under the microscope, which give the disease its name—"pebrine" comes from a word meaning peppercorn, referring to the spotted appearance of infected caterpillars.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Infected silkworms show marked growth retardation compared to healthy caterpillars.
Their appetite decreases and they become listless, moving little and feeding poorly.
The silkworm's skin may develop small dark spots resembling peppercorns, which is how the disease got its name.
Infected caterpillars are often smaller and weaker, with irregular development within the same batch.
In severe cases, silkworms die before reaching the chrysalis stage, or they produce poor-quality cocoons or no cocoons at all.
Moths from infected caterpillars can transmit the disease to their offspring without necessarily showing very obvious signs themselves.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Pebrine is caused by Nosema bombycis, a microsporidium. It is an obligate intracellular parasite, meaning it can only live and multiply inside the cells of its host—in this case, the silkworm.
The parasite forms highly resistant spores in the environment, capable of surviving for long periods outside the caterpillar.
Sericulture operations using eggs (known as seeds) from uncontrolled moths carry the highest risk.
Insufficient hygiene in silkworm rearing facilities—the buildings where silkworms are raised—promotes the buildup of spores in the environment.
Overcrowding of silkworms in the same space, poor air circulation, and failure to replace rearing materials also increase risk.
Mulberry leaves contaminated with residual spores represent another significant risk factor.
Transmission
Transmission occurs in two main ways. First, vertical or transovarial transmission: an infected female moth passes the parasite to her eggs, contaminating the entire next generation from the start.
Second, horizontal transmission: healthy silkworms become infected by ingesting mulberry leaves soiled by parasite spores from the droppings or carcasses of diseased caterpillars in the rearing facility.
Good sanitation practices in the facility and control of breeding stock are therefore essential to limit spread.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis relies on microscopic examination. Technicians or veterinarians specializing in sericulture examine homogenized samples of moths, suspect caterpillars, or eggs to look for the characteristic spores of the parasite.
This microscopic examination, though straightforward, requires experience and allows confirmation or exclusion of the disease before any decision is made regarding a rearing batch.
Treatment
There is no effective curative treatment for a silkworm already infected with pébrine.
Control therefore relies entirely on prevention and removal of contaminated individuals or batches, rather than on medication.
Disinfection measures in the rearing environment, using appropriate disinfectant products, can reduce the spore load and protect subsequent rearing cycles.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies primarily on systematic microscopic examination of breeding moths, known as seed examination. Each female is examined after laying to confirm the absence of parasite spores; if she is infected, all corresponding eggs are destroyed.
This method, developed in the 19th century, remains the foundation of pébrine control worldwide.
Thorough disinfection of rearing facilities, equipment, and the rearers' hands between each production cycle is essential.
Immediate removal of sick or dead silkworms limits contamination of the rearing environment.
Use of clean, dust-free mulberry leaves that are free from soil and debris completes these measures.
Possible complications
The disease can cause significant mortality in caterpillars before cocoon formation, which severely reduces silk production.
Cocoons produced by infected silkworms are often inferior in quality, with silk thread that is more fragile or available in reduced quantity.
Across an entire operation, uncontrolled contamination can lead to loss of several successive generations, as the parasite is transmitted to eggs.
When to see a veterinarian
Presence of silkworms that are smaller, weaker, or showing uneven growth in the rearing facility.
Appearance of unusual dark spots on the skin of caterpillars.
Abnormal mortality of caterpillars before cocoon formation.
Absent, malformed, or visibly degraded cocoons at the end of the cycle.
Before using any eggs or breeding stock from an uncontrolled rearing operation.
Prognosis
For worms already infected, the outlook is poor, with mortality potentially high before cocoon formation.
However, at the breeding operation level, the outlook is much better if breeder selection and hygiene measures are applied strictly, allowing healthy batches to be maintained from one generation to the next.
Frequently asked questions
Can pebrine affect other animals or humans?
No. Nosema bombycis is specific to the silkworm and a few closely related insects. It poses no danger to humans or domestic animals.
Can a silkworm that is already ill be cured?
No, there is no curative treatment. The only effective solution is to eliminate infected individuals and prevent contamination of other livestock.
How to tell if your silkworm eggs are healthy?
Only a microscopic examination performed by a technician or sericulture specialist makes it possible to verify the absence of parasite spores in breeding moths before using their eggs.
Why does this disease have historical significance?
In the 19th century, pébrine destroyed a large portion of silk production in Europe, causing a major economic crisis. It was through studying this disease that microscopic inspection methods for breeding stock were developed, methods still used today.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 24 August 2026
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