Diagnosis, treatment and Drug comparison of "luminous" Disease in Shrimp breeding

In prawn breeding production, the phenomenon of "light emission" of prawn larvae is often found, which is often called "light emission" disease, involving cultured species such as Penaeus chinensis, Penaeus japonica, Penaeus penicillatus, Penaeus vannamei, Macrobrachium rosenbergii, etc. The frequent occurrence of symptoms and delay in diagnosis and treatment not only bring passive to breeding work, but also cause huge economic losses. The author in many years of practice, in view of "luminous" symptoms using a variety of drugs for treatment and efficacy comparison, made preliminary progress, summarized as follows, for discussion.

Symptoms and changes of "luminous" disease

During the incubation period of all shrimp, from fleas to mysis and larvae, the disease occurred in all stages and was not easy to be found. The main reason was that the strong natural or non-natural light sources in and out of the nursery interfered with the diagnosis of the "luminous" disease symptoms of shrimp larvae. After larvae infected with this disease, they will die one after another. The mortality rate is as high as 70%--80% in 2--3 days. The symptoms are that the larvae have decreased vitality and sink to the middle and lower layers of the pool water. There are basically no shrimp larvae on the surface of the pool water. At the mysid stage or the middle and late stages of the shrimp, the shrimp larvae show weak bounce, poor phototaxis, weakened or stopped feeding, and their body color is white and basically opaque. After the lights were turned off at night, the fluorescent lights of stars could be seen fluctuating up and down in the water body, swimming around and showing different states. The symptoms were positively correlated with the intensity of infection.

II. Pathogens and Pathogenesis

1. pathogens

During production, it was found that the phenomenon of "luminescence" in the nursery room was completely different from that caused by protozoa (such as noctilucent insects) or partial luminescent algae in natural sea areas. After repeated diagnosis, the pathogens were generally bacteria, namely Pseudomonas fluorescens and Vibrio harveyi. The former is Gram-negative bacteria, the latter has luminescent characteristics, the salinity and temperature range suitable for reproduction and growth of both are exactly the same as the physical and chemical factors of shrimp nursery water, in the same physical and chemical index, and in the same feeding mechanism and breeding mechanism with artificial breeding shrimp seedlings, so it is easy to cause disease occurrence and spread.

2. pathogenesis

After bacteria infect shrimp, they use shrimp tissues as nutrients for reproduction and development, and oxidize fluorescein contained in the body under the action of its inherent enzymes, and then emit fluorescence. Because this "light" is weak luminescence, it is easily considered to be caused by reflected light from other light sources and ignored. Infected shrimp, depending on their physical strength, showed fluorescence phenomenon for 2 - 5 days, and finally died one after another. Especially shrimp larvae metamorphosis and molting during infection, easy to lead to a large number of shrimp larvae suddenly died, serious will be destroyed overnight.

III. Epidemic and hazard

Luminescent bacteria are widespread in natural sea areas and distributed along the coast of China. The disease process follows shrimp larvae to adult life, and the mysid stage and juvenile stage of shrimp larvae are easy to occur. The damage caused by Luminescent bacteria is also the most serious, and there is a positive correlation with the increase of water temperature. In terms of time, the epidemic is most widespread from May to July, because it enters the rainy season, a large amount of terrestrial organic matter flows into the sea with runoff, which makes the coastal water eutrophication, and the pathogen quickly reproduces and grows. Once entering the prawn breeding process, it will first infect the weak and residual seedlings, and then spread rapidly, causing serious damage and loss in Z, M and P stages of prawn breeding.

IV. Diagnostic methods

1. Visual inspection. Because larvae have unique luminescence phenomenon, one can be visually discovered, that is, when night falls, turn off all lights in the nursery room, by adjusting the gas volume of a single nursery pool, multi-angle, multi-level careful observation, generally can draw a more intuitive conclusion; the other is to take samples for diagnosis, that is, use white utensils, multiple sampling, under dark light conditions, using flashlight irradiation observation, will find that the luminous points are mostly concentrated in the head and chest of shrimp, shrimp in the later stage of infection will see fluorescent points in different parts.

2. Microscopic examination. To grasp the accuracy of diagnosis, can be further examined with a high power microscope. Under 400-fold visual field, bacterial activity in infected shrimp larvae was dominant in relatively transparent areas. Pseudomonas fluorescens was short rod shape, two ends blunt round, terminal flagella 1 - 6, strong cell activity; Vibrio harveyi was also short rod shape, some slightly curved, with polar flagella, cell activity is weak. The optimum temperature range of the two species in the nursery pond was 27℃, Pseudomonas fluorescens was more common below 27℃, Vibrio harveyi was more common above 27℃.

V. Prevention and control measures

The "luminous" disease is very easy to occur in the nursery ponds that pursue high density and high yield too much. Because the density of larvae cultivation is too large, it is inevitable to increase the feeding amount accordingly, especially in the case of unfavorable water exchange and lack of basic biological food, which will lead to increased pollution of the bottom of the cultivation pool and provide conditions for the mass reproduction of bacteria. In addition, there are many transmission channels of pathogens, such as sea and fresh water sources, exogenous bait, sea catch or overwintering parent shrimp and nursery facilities, etc., all of which have pathogenic factors. Therefore, disease prevention and control should be started from the source. The main measures are:

1. Water use and treatment. The sea and fresh water sources used for seedling are one of the main sources of luminescent bacteria. Therefore, it is best to treat the seedling water with sand filtration and darkness. If there are conditions, the seedling farm can be disinfected with ultraviolet rays or drugs, and the effect will be better. The drugs used include bleaching powder, potassium permanganate, etc. The water body disinfected by drugs can only be put into production after the efficacy disappears.

2. Strict bait feeding. According to observation, the main reason for the occurrence of this disease in many nursery grounds is that the biological food fed by parent shrimp and larvae is not treated properly or directly used. Living biological bait, such as small shellfish, cephalopods, cladoceras and other living environments are mostly in the polluted coastal waters or land, with a high probability of carrying germs, and should be used after high temperature sterilization or drug disinfection measures. Feeding substitute bait should pay attention not to excessive, especially self-made egg yolk, cake and other cooked products, because of its single feeding effect, easy to cause water pollution and disease. For other bait, nutrition balance should be required, high-quality fresh bait and complete feed should be used more, scientific feeding should be achieved, and the quality of water should be maintained.

3. Selection of parent shrimp. At present, there are two main sources of parent shrimp used in artificial breeding, one is natural parent shrimp caught in the sea, and the other is artificial overwintering parent shrimp. Both parent shrimp and overwintering parent shrimp lived in natural environment and artificial control environment for a long time, so the probability of carrying pathogens was higher. In view of this, in the process of selecting parent shrimp, firstly, we should avoid the bacteria-prone sea area or overwintering pool, secondly, we should do a good job of drug disinfection before the parent shrimp enters the room, shorten the artificial spawning and incubation time as much as possible, create and improve the living environment, and reduce the probability of infection.

4. Facility disinfection and larval rearing. Nursery pool, wintering pool, storage pool and all auxiliary facilities, tools, etc. shall be disinfected regularly with bleaching powder with concentration above 150ppm or potassium permanganate with concentration above 50ppm. After disinfection, they shall be washed repeatedly with clean seawater to remove drug residues before use. The density of larvae cultivation should not be too high, generally controlled below 300,000 fish/cubic meter of water, while the high density cultivation pond with more than 500,000 fish/cubic meter of water is easy to cause diseases.

5. Reasonable regulation of water quality. In the process of seedling raising, effective measures should be taken to create a good water environment, which is an important link to ensure the success of seedling raising. Therefore, the residual bait and feces in the seedling pool should be removed regularly to enhance the ability of changing water. In the middle and later stages of seedling, we should also do a good job in matters such as sewage absorption and pouring into the pool, and try our best to reduce the external conditions for the survival of pathogenic bacteria.

6. Strengthen basic prevention work. On the one hand, in the process of breeding, through inoculation, cultivation of unicellular algae, such as green algae, diatom and golden algae, not only can be used as high-quality biological bait to reduce the cost of breeding, but also can inhibit the growth of luminescent bacteria, promote the smooth progress of breeding work; on the other hand, attention should be paid to frequent observation and early detection, especially when larvae develop to the late stage of mysis or juvenile shrimp. Once this disease occurs in these two stages, it is not only difficult to treat, but also the survival rate of seedlings will be greatly reduced.

VI. Comparison of the effects of drug therapy

In recent years, in the prawn breeding production, the author organized many breeding farms to carry out comprehensive prevention and treatment according to the characteristics of wide occurrence range, fast attack frequency and great harm, and compared the curative effects of more than 10 kinds of physical and chemical drugs. In general, Pseudomonas fluorescens and Vibrio harveyi showed weak sensitivity to antibiotics, and had good effect on the disease treatment before the middle stage of mysis, but the effect was not obvious at the later stage. Antibiotics mixed with chlorine-containing disinfectants also have better curative effects.