Silkworm grasserie (viral disease)
Grasserie is a severe viral disease that affects the silkworm (Bombyx mori), the insect raised to produce silk. It is caused by a baculovirus that forms small crystals, called polyhedric bodies, inside the cells of the worm. The term "grass...
What is it?
Grasserie is a severe viral disease that affects the silkworm (Bombyx mori), the insect raised to produce silk. It is caused by a baculovirus that forms small crystals, called polyhedric bodies, inside the cells of the worm. The term "grasserie" comes from the greasy, glossy appearance that develops on the skin of the infected worm. This disease is one of the most feared by sericulturists (silkworm breeders) because it can destroy a large portion of an entire colony within days. It primarily affects worms in their final growth stages, just before cocoon formation, resulting in significant economic loss.
Key point
Highly contagious disease that can rapidly decimate an entire silkworm rearing; no curative treatment exists.
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:
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Grasserie is caused by a baculovirus specific to the silkworm, called the Bombyx mori nuclear polyhedrosis virus (BmNPV). This virus infects the worm's cells, particularly those of fatty tissue, and multiplies within them, forming crystalline inclusion bodies called polyhedra, which protect the virus in the external environment.
Insufficient hygiene in the rearing facility strongly favours the disease. High density of worms in the same space increases the risk of contamination. Temperature and humidity conditions that are too high or poorly controlled weaken the worms and promote virus multiplication. Diet consisting of poor-quality, contaminated or poorly preserved mulberry leaves is an important risk factor. Stress from rough handling or sudden changes in rearing conditions can also make worms more susceptible. Reuse of rearing equipment that is inadequately disinfected between cycles promotes virus persistence.
Transmission
The virus is transmitted mainly by oral route: healthy worms become infected by eating mulberry leaves contaminated with droppings or remains of sick worms. Contamination can also occur through contact with rearing equipment, hands or tools soiled by the virus. Transmission from the female moth to her eggs (vertical transmission) is also possible, allowing the virus to persist from one generation to the next. The virus is highly resistant in the environment and can survive for long periods in bedding, rearing frames or the soil of silkworm rearing facilities.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
There is no simple diagnostic test that a breeder can perform directly.
Observation of typical signs (greasy and shiny appearance, abnormal climbing behaviour, rapid mortality) strongly suggests the diagnosis.
A specialist or sericulture laboratory can confirm the presence of the virus by examining under a microscope a tissue or body fluid sample from the worm, looking for the characteristic polyhedral bodies.
In case of doubt, it is advisable to consult a technician or veterinarian specialising in insect rearing or an agricultural service with expertise in sericulture.
Treatment
There is no specific curative treatment for this virus.
Management consists solely of sanitary measures: immediate removal of sick or dead worms, complete disinfection of equipment and premises, and discontinuation of reusing contaminated bedding.
No antiviral medication is available for this disease in silkworms.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention depends primarily on strict hygiene of the silkworm rearing house and breeding equipment.
It is recommended to thoroughly disinfect the premises, rearing trays and tools between each rearing cycle.
Eggs (seed) must come from reliable sources and ideally be tested to confirm the absence of the virus.
Sick or dead worms should be removed quickly from the rearing to limit spread.
Proper management of worm density, ventilation, temperature and humidity in the rearing area reduces the risk of disease.
The quality and freshness of mulberry leaves provided must be monitored, avoiding leaves soiled by earth or contaminated by sick worms.
Possible complications
The disease often causes rapid and massive mortality in the rearing, potentially compromising an entire silk production cycle.
Since the virus persists in the environment, subsequent rearing cycles may become contaminated if disinfection is not thorough.
Economically, grasserie can cause significant losses for silk farmers, particularly for small family operations.
When to see a veterinarian
Sudden appearance of swollen worms, with a shiny or abnormally yellowish colour in the rearing.
Unusual disorderly climbing behaviour towards the top of supports.
Rapid and unexplained mortality affecting multiple worms at the same time.
Leakage of cloudy fluid when the worm skin is touched or ruptures.
Concern about the sanitary quality of a batch of eggs or worms received from another rearing.
Prognosis
The outlook is very poor for silkworms already affected; death occurs in nearly all cases following the appearance of symptoms.
At the farm level, the outlook depends mainly on how quickly the infection is detected and the quality of hygiene measures put in place to prevent spread to other worms.
A well-managed farm, with rigorous disinfection, can limit losses during subsequent cycles.
Frequently asked questions
Can grasserie be transmitted to humans or other animals?
No, this virus is specific to silkworms. It poses no danger to humans, mammals, or other domestic animals.
Can a worm already affected by grasserie be saved?
No, once symptoms appear, the progression toward death is almost inevitable. There is no curative treatment for this virus.
How can I prevent grasserie from returning with each breeding cycle?
Thorough and rigorous disinfection of premises and equipment between each cycle, combined with the use of eggs from reliable sources, significantly reduces the risk of recurrence.
How to identify a silkworm affected by grasserie?
The suggestive signs are a swollen and shiny body, an abnormal yellowish colour, disorderly climbing behaviour, and rapid mortality in the breeding facility.
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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