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Bacterial gastrointestinal disease is a condition affecting silkworms, primarily caused by the proliferation of intestinal bacteria due to the weakening of the silkworms’ immune system. This disease is particularly prevalent in various silkworm-rearing regions, especially during summer and autumn, where weather conditions and poor leaf quality contribute to its frequent occurrence and severity.
1. Pathogen and Causes of the Disease
The precise pathogen and cause of this disease have been a subject of research for over a century, with no definitive conclusion. There are two main theories about the cause:
- Physiological Theory: This theory suggests that the disease arises from physiological weakness in the silkworms rather than a specific pathogen.
- Bacterial Pathogen Theory: Initially, many scholars believed a specific bacterium caused the disease. However, experiments failed to induce the disease in healthy silkworms by feeding them isolated bacteria, suggesting the bacteria aren’t the primary cause. However, the presence of various bacteria, including streptococci and bacilli, in diseased silkworms, suggests a role in the disease’s progression.
The type of bacteria present can influence the symptoms observed. Early in the disease, when the digestive juice has a higher pH, streptococci are usually present. As the disease progresses, the pH of the digestive juice decreases, and bacilli become more prevalent while streptococci populations lessen.
2. Transmission Routes
Two critical factors must exist for bacterial gastrointestinal disease to occur:
- Weakened Silkworm Physiology: This can be due to congenital weakness or from environmental stressors such as high temperatures, starvation, or poor-quality mulberry leaves. These conditions reduce the silkworms’ metabolic function, lead to physiological disorders, and impair their digestive sterilization capabilities.
- Poor Environmental Sanitation: The presence of bacteria in silkworm rearing environments is crucial. This includes storing wet mulberry leaves for extended periods or using wilted, fermented leaves. These provide a breeding ground for bacteria that can infect silkworms.
3. Symptoms
The symptoms of this disease vary but often include:
- Slow feeding
- Inactivity
- Thin body
- Weakness
- Uneven development
- Irregular or soft feces
- Thin or watery feces
Specific symptoms associated with the parasitic bacteria in the silkworm’s digestive tract include:
| Symptom | Description |
|---|---|
| Shrinking Silkworm | Reduced appetite, slow growth, small body, shrunken skin, gray-yellow body color, sluggishness, and eventual shrinkage leading to death. |
| Hollow-Headed Silkworm | Loss of appetite, lack of mulberry leaf in the front part of the digestive tube, translucent breast, rear-dumped tail corners, and irregular or thin feces. |
| Diarrhea Silkworm | Discharge of unformed soft feces or rosary-like feces, later turning to mucus, which pollutes the tail and causes death. This is often accompanied by spitting before death. |
Further internal changes observed in silkworms with this disease include a decline in eating, digestion, and digestive juice enzyme activity, particularly protease and amylase. The digestive juice changes from translucent to yellow-green and then turbid yellow-brown, with the pH dropping from 9.2-9.8 to below 8.4. The blood pH rises and moves closer to neutral. Severely diseased silkworms can show thickening, brittleness, or melting of the periphagus, cell degeneration in the digestive tube wall, mucus-filled tubes, loss of muscle tension, irregular peristalsis, narrowed lumens, and epithelial cell detachment. Bacteria can then invade the blood, leading to sepsis.
4. Diagnosis and Prevention
Distinguishing bacterial gastrointestinal disease from viral softening disease can be done by disease analysis. If symptoms are improved by eliminating sick silkworms and improving rearing conditions, the disease is likely of bacterial origin. Microscopic examination of the digestive juice of diseased silkworms will reveal a high number of bacteria before death.
Prevention and treatment focus on strengthening silkworms’ overall health through:
- Improved Management Practices: Maintain good mulberry leaf availability, provide proper ventilation, ensure cleanliness in the silkworm rearing area, and avoid storing wet leaves.
- Antibiotics: The use of antibiotics such as chloramphenicol, oxytetracycline, and erythromycin can be effective. A preventive measure could involve starting from the 3rd instar by adding 500 units of chloramphenicol per milliliter once per instar. If a disease outbreak occurs, 500 units of chloramphenicol can be administered three times consecutively.
By implementing these preventative and management strategies, the risk and impact of bacterial gastrointestinal disease can be significantly reduced, ensuring healthier silkworm populations and better silk production.
