Silkworm flacherie
Flacherie is a serious disease affecting silkworms—the caterpillars of Bombyx mori—raised for silk production. It primarily affects the digestive system of the caterpillar. The term "flacherie" comes from old French and refers to the soften...
What is it?
Flacherie is a serious disease affecting silkworms—the caterpillars of Bombyx mori—raised for silk production. It primarily affects the digestive system of the caterpillar. The term "flacherie" comes from old French and refers to the softening of the body of an infected caterpillar, which becomes limp and flaccid. This disease is one of the major causes of economic loss in silkworm farms (sericulture) worldwide. It may be caused by viruses or bacteria, which sometimes makes precise identification difficult without laboratory testing.
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:
Affected caterpillars lose their appetite and stop feeding on mulberry leaves. Their body becomes soft and flaccid to the touch, losing the normal firmness of a healthy caterpillar.
Vomiting of intestinal fluid and diarrhea are often observed, with soft or liquid droppings that soil the bedding. The abdomen may appear swollen or distended.
The caterpillar becomes increasingly lethargic, moves little, and eventually dies. The dead body darkens rapidly and decomposes with an unpleasant odor, sometimes liquefying almost entirely. The skin may wrinkle and take on a dull appearance before death.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Flacherie can have several infectious origins. It is most often caused by viruses specific to silkworms, particularly viruses that attack the midgut cells of the caterpillar. Some forms of flacherie are due to opportunistic bacteria that take advantage of an already weakened intestine to proliferate.
In many cases, multiple infectious agents act together, making the disease more severe. Poor-quality, contaminated, or wet mulberry leaves also promote disease development by irritating the caterpillar's digestive system.
Several conditions increase the risk of flacherie in a silkworm farm. Insufficient hygiene of the breeding facility (magnanery) and equipment is a major factor, as infectious agents spread easily through contact with droppings or soiled surfaces.
Unfavorable climatic conditions such as excessively high temperature, excessive humidity, or sudden temperature fluctuations weaken caterpillars and make them more vulnerable.
Poor-quality mulberry leaves—too young, too old, wet, or contaminated by pesticides or molds—is also an important risk factor. Excessive caterpillar density on the rearing trays promotes rapid disease spread between individuals.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Precise identification of flacherie requires careful observation of the caterpillars and their behaviour by the breeder, but a definitive diagnosis must come from examination in a laboratory specialising in sericulture. This laboratory can examine intestinal tissues under a microscope or carry out analyses to detect the virus or bacterium responsible.
If there is any doubt about the health status of a rearing facility, it is recommended to contact an agricultural technical service or a sericulture specialist who can direct you to the appropriate contacts.
Treatment
There is no effective curative treatment for flacherie once the disease has developed in a caterpillar. Management consists mainly of isolating and removing sick individuals to protect the rest of the rearing facility.
In some cases, immediate improvement of rearing conditions (cleanliness, ventilation, leaf quality) can limit spread to healthy caterpillars. No specific antiviral treatment is available in standard practice.
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 strict hygiene in the rearing facility. It is essential to thoroughly disinfect the rearing room, the rearing frames and all equipment before each new generation of silkworms.
Sick or dead caterpillars must be removed immediately and disposed of away from the rearing facility to prevent any contamination. Bedding should be changed regularly to limit the buildup of droppings.
The quality of the mulberry leaf supplied must be closely monitored: it should be fresh, clean and not wet. It is also important to maintain stable temperature and humidity levels suited to the caterpillars' stage of development, and to avoid overcrowding on the rearing frames.
Possible complications
Flacherie can result in the loss of much or all of a silkworm rearing if it is not controlled quickly. Dead caterpillars contaminate the bedding and equipment, perpetuating the infectious cycle for future generations if the facility is not properly disinfected.
Economically, a flacherie outbreak in a silk farm represents a substantial loss of income for silk producers.
When to see a veterinarian
Caterpillars suddenly refusing to eat mulberry leaves.
Caterpillar body that is abnormally soft or flaccid to the touch.
Presence of diarrhoea, liquid droppings or vomiting on the rearing frames.
Unusual or grouped mortality of several caterpillars in a short period of time.
Unusual odour or rapid decomposition of dead caterpillars.
Prognosis
The prognosis is guarded to poor for caterpillars already affected, as the disease generally progresses to death within a few days once symptoms are well established.
For the rearing facility as a whole, the prognosis depends on how quickly the breeder reacts: early detection and strict hygiene measures often allow losses to be limited to individuals already affected and the rest of production to be preserved.
Frequently asked questions
Can flacherie affect animals other than the silkworm?
No, flacherie as described here is specific to the silkworm and its particular rearing methods. It does not affect other insects or domestic animals or humans.
Can a caterpillar with flacherie be saved?
Once symptoms are clearly established, there is no treatment that can cure the caterpillar. The best approach is to remove it quickly from the breeding colony to protect the others.
How can I tell if my breeding is at risk of flacherie?
Monitor the quality of mulberry leaves, the cleanliness of drying frames, and the behavior of silkworms. A refusal to feed or abnormal limpness of the body should raise concern.
Is disinfection sufficient to eliminate the risk for the next generation?
Thorough disinfection of the premises and equipment significantly reduces the risk, but it must be combined with good quality bedding and consistent hygiene throughout the rearing period.
References
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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