Disease of Silkworms – Viral Flacherie

One type of silkworm virus disease is an infectious silkworm disease caused by the parasitism of a softening disease virus. It often occurs in the summer and autumn silkworm seasons in various silkworm areas, commonly known as short-head disease. For a long period of time, it was believed that the disease was mainly caused by the proliferation of non-specific intestinal bacteria due to the weakness of silkworm bodies. By 1960, it was confirmed that the viral softening disease was caused by a virus.

Pathogen

Bombyx mori infectious-flacherie virus belongs to the Picornaviridae family, and its scientific name is Bombyx mori infectious-flacherie Virus, which is abbreviated as FV or IFV. The virus particles are spherical, with a diameter of 25-30 nanometers, with an average of 26±2 nanometers, and a sedimentation coefficient of 180S. The nucleic acid is single-stranded ribonucleic acid (ssRNA) with a molecular weight close to 2×106. The code is R/1: 2.4/29: s/s: I/0 (see figure).

This virus does not form polyhedrons in silkworm cells, so it is a non-closed naked particle. Purified FV can be used as an antigen and injected into goats, rabbits and other animals to form antibodies and obtain immune serum. The virus can agglutinate the red blood cells of mice. This agglutination reaction is hindered when anti-FV serum is present, but it cannot produce agglutination reactions with the red blood cells of goats, sheep, chickens and humans.

The stability of silkworm softening disease virus is significantly different due to its existing state and environment. FV exists in diseased silkworm carcasses or diseased silkworm feces, and is still pathogenic when stored indoors for 2 to 3 years. The FV in silkworm manure was not inactivated after dry heat treatment at 100°C for 30 minutes. Although stored at -18°C, it still maintains strong pathogenicity even after 500 days. FV is still pathogenic in the feces excreted after being eaten by livestock and poultry. Since this virus is very infectious and pathogenic, special attention should be paid to silkworm rearing. Softening disease virus, exposed to direct sunlight at 35.7°C for 29 hours, 100°C temperature for 3 to 5 minutes, 2% formaldehyde solution at 20 to 27°C for 3 to 10 minutes, 0.3% available chlorine bleaching powder solution for 3 to In 5 minutes, 0.5% lime slurry can lose its pathogenicity in 3 to 4 minutes.

Mode of transmission

Viral softening disease is mainly transmitted through ingestion. After FV enters the digestive tract, it may also be partially inactivated by the action of antiviral substances.

FV mainly infects goblet cells in the midgut and does not form polyhedrons. After goblet cells are infected, the cytoplasm is hypertrophy and mitochondria are reduced. Later, the cells shrink and degenerate into spheroids. After FV invades the midgut, it infects the goblet cells at the front end of the midgut and gradually expands to the back end. Broken cells and viruses are scattered into the intestinal lumen, so silkworm feces contain a lot of viruses.

Midgut smears stained with arsenic red-methyl green, optical microscopy or slices can show that there are peach red type A in the cylindrical cytoplasm, as well as the free type B spheroids, this kind of basophilic Spheroids are degenerated goblet cells. When the goblet cells are destroyed, the new cells continue to differentiate and replenish. If the rate of proliferation of new cells can make up for the damaged goblet cells, the disease course of silkworms can be prolonged. This is a disease tolerance phenomenon.

The proliferation of FV in the midgut is basically the same as that of nuclear polyhedrosis and cytoplasmic polyhedrosis. After inoculation with FV, the virus proliferation in the midgut of diseased silkworms can also be divided into four stages: latent period, slow proliferation period, high speed (logarithmic) proliferation period and stable (deceleration) proliferation period. The proliferation speed of FV is affected by the feeding temperature. The proliferation speed is faster in the range of 25℃～30℃; it is very slow at 16℃; 37℃ has a certain inhibitory effect on the proliferation of the virus. Under these conditions, uninfected or newly formed goblet cells can be protected from virus infection. FV mainly infects goblet cells in the midgut. Because the function of the goblet cells is to secrete digestive juice, they not only digest and decompose mulberry leaves, but also have the functions of bacteriostasis and detoxification. After infection, goblet cells degenerate and fall off, which affects digestion and sterilization functions. Therefore, bacteria in the intestinal tract can easily multiply. The co-infection of viruses and bacteria accelerates the death of silkworms. Under the conditions of aseptic breeding with artificial feed, the death period of silkworms was significantly prolonged when inoculated with FV. This may be due to the lesser influence of bacteria in the aseptic state, the shedding of goblet cells and the ability to regenerate the midgut are easily balanced.

Symptom

The symptoms of this disease vary depending on the period of onset. In the early stage of the disease, only the reduction of nibbling mulberry, stunted growth, and uneven sleep rise were seen. Large differences in size between individuals. Symptoms such as shrinking, short head, shrinking, diarrhea and vomiting. After death, the corpse collapses. This disease and the diversity of the course are mostly related to the type and quantity of intestinal bacteria that multiply with the virus infection. The course of FV infection alone is relatively long. Symptoms of shrinking occur more frequently within 1 to 2 days after the food of each instar, especially in the 5th instar. Diseased silkworms rarely eat mulberry or stop eating mulberry altogether. In the group, the body color does not turn blue, the body shrinks, and the body wall is wrinkled. Sometimes vomiting liquid, discharging yellow-brown thin feces or dirty liquid, shrinking and death. Short-head symptoms appear in the full-eating period of all ages, especially during the silkworm period. Diseased silkworms rarely eat mulberry, their body color loses the original bluish white (mulberry color), their breasts are slightly enlarged, translucent and slightly dark red. Gradually, the whole body is translucent, and the feces or dirty liquids are discharged. Spit liquid before death, and soften the corpse after death. When severely ill, the silkworm seat and silkworm chamber will have an abnormal odor.

The number of days after diseased silkworms became ill varies with the instar of the silkworms, the number of infected viruses, and the reproduction of bacteria. However, they usually die after 5-12 days. The symptoms are similar to bacterial gastrointestinal diseases. However, viral softening disease has the characteristics of strong contagion, serious disease and continuous spreading. Its symptoms also have many similarities with cytoplasmic polyhedrosis. The main difference is the disease of the midgut. The midgut of this disease is not milky white; feces There is no milky white but dark brown dirty liquid; there is no polyhedron and a lot of bacteria in microscopic examination.

Diagnosis and prevention

Diagnosis is the most accurate using serological agarose two-way immunodiffusion, convective immunoelectrophoresis, and fluorescent antibody method. The difference from bacterial gastrointestinal disease is mainly judged from the condition of the group. The condition of this disease is more serious, and the condition continues. If the disease is mild, the disease will no longer occur after the diseased silkworms are removed, and it is mostly a bacterial gastrointestinal disease.

Microscopic examination of the diseased silkworm intestinal juice showed a large number of diplococci, cocci, and no polyhedrosis. A small piece of diseased silkworm midgut tissue (length and width each 2 ~ 3 mm) can also be cut into a smear specimen, which is fixed and washed with Carnot’s fixative. Cover the coverslip with the arsenic red-methyl green stain when wet. Microscopic examination under a microscope at 400 to 600 times. In the center of the cylindrical cell, there is a purple-red nucleus, and the cytoplasm near the nucleus has A-type spheroids stained with arsenic red and B-type spheroids that exist alone.

When the diagnosis cannot be made by naked eye and microscopic examination, an infection test can be performed. Use a microscope to examine the midgut homogenate of diseased silkworms without polyhedrons, dilute with 10 times sterile water, and separate the heart at 1,000 rpm for 10 minutes. Take the supernatant and apply it to the mulberry leaves to feed the silkworms. When the same symptoms appear, it can be diagnosed as viral softening disease.

The prevention and treatment of viral softening disease is to eliminate pathogens and prevent infection opportunities, improve feed quality and improve feeding conditions to enhance physical fitness.

1. The silkworm house and silkworm utensils should be disinfected according to the requirements of the disinfection specification.
2. Strictly raise greens in batches, isolate or eliminate stunted and diseased silkworms to prevent the infection of silkworm sacs.
3. Control the temperature and humidity during feeding, pay attention to the selection and storage of mulberry leaves, strengthen the body constitution and improve the disease resistance of the silkworm body.