Densovirus Disease in Silkworms: Causes, Symptoms, and Prevention

by Elizabeth / Sunday, 25 July 2021 / Published in Sericulture

Densovirus disease is a significant threat to silkworm rearing, caused by a highly infectious virus. This article delves into the specifics of this disease, including its cause, transmission, symptoms, diagnosis, and prevention strategies.

1. The Pathogen: Densovirus

Densovirus (DNV) belongs to the Parvoviridae family. Here’s what makes it distinct:

Shape and Size: The virus particles are spherical, measuring between 20 to 23 nanometers in diameter.
Sedimentation Coefficient: It has a sedimentation coefficient of approximately 100S.
Nucleic Acid: The viral nucleic acid is single-stranded deoxyribonucleic acid (ssDNA). Its content and molecular weight can vary slightly among different strains.
Code: The code is D/1: 1.5～2.2/19～32: S/S: I/O.

2. Transmission Routes of Densovirus

Densovirus can spread through several means:

Environmental Contamination: Trace amounts of the virus can persist in the sericulture environment, leading to infection.
Mulberry Pests: Pests in the mulberry fields, particularly the larvae of mulberry borers, can act as carriers, spreading the virus.

3. The Impact of Densovirus on Silkworms

Once ingested, densovirus targets the silkworm’s midgut:

Affected Area	Changes Caused by Densovirus
Midgut	Virus replicates in the nucleus of cylindrical cells, causing enlargement and degeneration.
Periphagus	Disappears due to the infection
Cytoplasm	Collapses, releasing degenerated cells into the gut lumen.
Tissue	Eventually, most of the midgut tissue is replaced by goblet cells.

4. Silkworm Resistance and Genetics

A key aspect of densovirus is the silkworm’s variable resistance:

Resistant Varieties: Some silkworm varieties possess strong resistance and will not become ill even when exposed to high concentrations of the virus.
Recessive Major Gene: Resistance is primarily controlled by a pair of recessive major genes.
Complete Resistance: Certain silkworm varieties possess fixed disease resistance genes, providing complete immunity, while others can isolate infected individuals, effectively stopping the spread of the disease.
Unique Resistance Mechanism: There are two distinct systems of infection and complete resistance with no cases in between. This discovery aids in the prevention and understanding of the disease mechanism.

5. Symptoms of Densovirus Infection

The external symptoms of densovirus in silkworms are generally straightforward:

Loss of Appetite: Diseased silkworms stop consuming mulberry leaves.
Abnormal Movement: Affected silkworms will crawl erratically.
Posture: The head and chest of the silkworm may fall backward.
Digestive Tract: Upon dissection, the digestive tract will be found lacking mulberry leaf fragments, containing only translucent digestive juices.

6. Diagnosis and Prevention Strategies

Accurate diagnosis and effective prevention are crucial for managing densovirus outbreaks:

Diagnostic Methods	Prevention Strategies
Serological tests like agarose two-way immunodiffusion	Routine disinfection before raising silkworms.
Convective immunoelectrophoresis	Selection of disease-resistant silkworm varieties.
Fluorescent antibody method	Prevention of mulberry pests and elimination of insects in the wild.

By understanding the mechanisms of densovirus infection and implementing these diagnostic and preventative measures, sericulturists can effectively protect their silkworm populations and ensure silk production.