Disease of Silkworms – Septicemia

One of silkworm bacterial diseases is caused by a variety of pathogenic bacteria invading the blood of silkworms. The most common cases in China’s silkworm areas are black chest sepsis, bleeding sepsis and green head sepsis. In addition to the egg stage, the disease can occur in the three developmental stages of larvae, pupae, and moths. The silkworm stage is more common in summer and autumn, but mostly occurs sporadically, and rarely occurs in outbreaks.

Septica nigricans, bleeding fungus, and septicemia green head are the main pathogenic bacteria of this disease. In addition, Pseudomonas aeruginosa, Streptococcus, Staphylococcus, etc. can also cause sepsis.

1. A large bacterium in the genus Bacillus sp. The length and width of the bacterial body is 1～1.5×3 microns, often two or more connected, both ends are blunt, round flagella, can move, and hemicystis spores. Gram staining is positive, a thick gray-white lawn is formed on the agar medium, and the gelatin can be liquefied in a cylindrical or layered shape on the gelatin medium.
2. Serratia marcescens Bizio is a kind of Brevibacterium of the genus Serratia marcescens. The length and width are 0.6～1.0×0.05 microns, and the flagellum is peripheral, which can move and does not form spores. Gram staining is negative, the agar culture develops into a thick and translucent moist lawn, which can quickly liquefy gelatin on the gelatin medium. The bleeding can produce red pigment. Therefore, the culture medium and diseased silkworm carcasses are red.
3. Aeromanassp. is a small bacterium belonging to the genus Aeromonas, which exists alone. It is 1 to 1.5 microns long, with blunt round ends, extremely monoflagellate, and does not form spores. Gram staining is negative, slimy moss is formed on the gravy agar slant medium, which is translucent and slightly milky white.

Traumatic infection is the only way to spread sepsis. The pathogenic bacteria of septicemia are widely distributed in nature and in the environment of sericulture. Storage of wet leaves under high temperature, fermentation and corruption of mulberry leaves, and dampness of silkworm excrement, etc., if pathogenic bacteria are attached, they will multiply rapidly and become an important source of infection for sepsis.

The incidence of sepsis is different for different silkworm instars. Small silkworms have small bodies, underdeveloped hooks and claws on their stomachs, and are protected by bristles. The flocks of silkworms are not easy to scratch each other, and sepsis is rare. In the large silkworm stage, the pedestal is too dense, the flocks of silkworms scratch each other, and the operation of raising silkworms is extensive, which causes the silkworm body to be injured.

The length of the disease after the silkworm suffers from sepsis is closely related to the type of bacteria infected and the temperature of breeding. Several common septicaemias caused by bacilli are of acute type. Generally, silkworms die within 24 hours after infection at 25°C and normal temperature. Septicemia caused by Staphylococcus sp. and Streptococcus sp. is a chronic type, and the disease can last for 3 to 4 days. The disease course is short during high temperature feeding, and long during low temperature. For example, the time from infection to death of bleeding septicemia is 29 hours at 15°C, 14 hours at 25°C, and about 8 hours at 30°C; another example is the course of blue head septicemia, 34 hours at 20°C. , 25℃ is 20 hours, and it can die within 10 hours at 32℃.

Symptoms

1. Symptoms in the larval stage: Although sepsis has its own characteristics due to different types of parasitic bacteria, the general symptoms are roughly the same. First stop eating mulberry, exercise sluggish, body straight, then the chest gradually enlarged, chest and feet straightened, abdomen and feet tilted back, all parts of the abdomen contracted and the central part was slightly raised. Excretion of soft feces or rosary-like feces, often spit out dirty liquid and discharge dark brown dirty liquid before dying, and finally die on its side. At the first death, there was temporary corpse stiffness, the chest was enlarged, the head and tail were slightly tilted, and the abdomen arched out. After a few hours, the corpse gradually softened and the body stretched. From this time on, various septicemias showed their characteristics due to the different types of pathogenic bacteria. Black chest septicaemia appears on the back of the chest or on the first to third links of the abdomen with dark green corpses, which quickly expands into the front half of the body and turns black (Figure 1). The carcass of bleeding septicaemia silkworms changes color more slowly, and sometimes small brown dots appear on the body wall. With the dissociation and liquefaction of the tissues in the corpse, the whole body gradually turned bright red (Figure 2). The symptoms of green head sepsis vary slightly depending on the time of onset. In the middle and late stages of the 5th instar, green and transparent mass lesions appear on the back of the chest and bubbles appear under the corpse spots shortly after death, so they are commonly known as bubble silkworms. However, the lesions did not change to black. After a few hours, the tissues were corrupted, and the whole body was grayish gray (Figure 3); most of the breasts with onset at the beginning of 5th instar did not show green bubbles. The corpses of septic silkworms eventually dissociated and liquefied, leaving only the chitin outer skin. The body skin is easy to break, and foul-smelling liquid flows out.

Figure 1 Symptoms of black pectoral septicaemia silkworm

Figure 2 Symptoms of hemorrhagic septicaemia silkworm

Figure 3 Symptoms of green head septicemia silkworm

2. Pupa and moth stage: There are two common septic pupae. One is acute septic pupae. The onset is rapid, the pupa body becomes black, liquefied and flat, cannot be picked up, the body skin is easily broken, and a foul-smelling black or red dirty liquid flows out. The other is chronic septic pupae. Mostly caused by coccal infections, the blood vessels on the back of the infected gradually darken to dark brown, and then spread to the whole body, the onset is slow, and the corpse does not quickly decay after death. The scales and hairs of sick moths are dirty, their mobility is extremely poor, their wings are not stretched and they are easily broken and fall off, and their abdomen is swollen first and then collapsed. The speed of onset varies according to the type of infected bacteria. The fastest one will die within a day, and some will die during mating or during spawning. After death, the internal organs of the abdomen are dissociated and corrupted. From the intermembrane or scaly hair loss, the inside of the abdomen can be seen to be dark brown or red, and finally turned into dirty juice, leaving only the head, chest, wings, feet and other chitin parts. Diseased moths rarely or do not lay eggs.

A distinctive feature of septicemia is that the pathogenic bacteria infect the silkworm body and cause the blood disease of the silkworm. Before silkworms die, bacteria generally do not invade other tissues but only flourish in the blood. Then it spreads all over the cavity, taking nutrients from the blood, destroying blood cells and fat bodies, secreting proteases and lecithinases and causing blood degeneration. After the silkworm dies, bacteria invade various tissues and organs to dissociate and liquefy.

Diagnosis and prevention

Diagnosis can be based on characteristic symptoms. In addition, blood lesions can also be observed, and the blood of diseased silkworms before death, such as turbidity and brown, red and other different colors, can be diagnosed when a large number of long or short bacilli are present under the microscope. Prevention and control methods:

1. On the basis of cleaning and disinfecting silkworms and sericulture environment, focus on hygiene management during the breeding process to minimize the pollution concentration of pathogenic bacteria in the sericulture environment. Anti-disease No. 1 can be used to disinfect silkworm body and silkworm seat on a regular basis, and bleaching powder solution can be used to disinfect the mulberry storage room on a regular basis. The water for sericulture and mulberry storage should be clean, the mulberry cannot be accumulated too thick, and the mulberry storage time should not be too long. Avoid feeding fermented mulberry and spoilage mulberry. The sick and dead silkworms in the silkworm seat should be removed in time, and the dead silkworms and dead cage cocoons and dead pupae in the swarms should be removed in time to reduce 2 infections.
2. Appropriate feeding. Avoid excessive accumulation of mature silkworms when picking up mature silkworms and stalks, so as to reduce the chance of the silkworm groups scratching each other. The operations of raising silkworms, collecting cocoons, and producing seeds should be careful to avoid man-made wounds on silkworm, pupa and moth bodies. Seed cocoon breeding should be appropriately postponed to cut cocoons and identify males and females, and try to shorten the exposure time of pupae to reduce the chance of contact and infection with pathogenic bacteria.
3. Chloramphenicol has a good preventive and therapeutic effect on black thoracic sepsis and green head sepsis, and can maintain the antibacterial effect in the silkworm body for about 30 hours, and 14 hours is the critical time for cure. Under normal sericulture conditions, 500 ppm of chloramphenicol solution can be added to the 4th instar silkworm, the 3rd day of the 4th instar, the 5th instar silkworm, the 3rd day of the 5th instar and the mature silkworm. It has a good preventive effect. In the case of more disease, add 500 to 1,000 ppm of chloramphenicol solution every 8 hours for 2 to 3 times, which can basically stop the spread. But chloramphenicol is not effective for bleeding septicaemia.