Diagnosis and Treatment of Luminescence Disease in Shrimp Seedling Rearing and Comparison of Medication Methods
Diagnosis and Treatment of "Luminescence" Disease in Shrimp Seedling Rearing and Comparison of Medication Methods
In the production of shrimp seedling rearing, the "luminescence" phenomenon in shrimp larvae is frequently observed, commonly referred to as "luminescence" disease. The cultured species involved include Chinese shrimp (Penaeus chinensis), Japanese shrimp (Penaeus japonicus), long-haired shrimp (Penaeus penicillatus), Pacific white shrimp (Litopenaeus vannamei), giant freshwater prawn (Macrobrachium rosenbergii), and others. The frequent occurrence of symptoms and delays in diagnosis and treatment not only put seedling rearing operations in a passive position but also cause enormous economic losses. Through years of practice, the author has used various medications to treat "luminescence" symptoms and compared their efficacy, achieving preliminary progress. The findings are summarized below for discussion.
I. Symptoms and Changes of "Luminescence" Disease
During the hatching and rearing of all shrimp species, from the nauplius stage through the mysis stage and postlarval stage, the disease can occur at every stage and is not easily detected. The main reason is that relatively strong natural or artificial light sources inside and outside the hatchery interfere with the diagnosis of "luminescence" disease symptoms in shrimp larvae. After larvae are infected with this disease, mortality occurs progressively, with severe cases reaching 70%-80% mortality within 2-3 days. Symptoms include decreased larval activity, sinking to the middle and lower layers of the pond water, with essentially no shrimp larvae visible at the water surface. By the mysis stage or the middle-to-late postlarval stage, shrimp larvae show weak jumping ability, poor phototaxis, reduced or ceased feeding, and a cloudy white, largely non-transparent body color. In affected shrimp ponds after lights are turned off at night, one can see star-like flickering fluorescence rising and falling within the water body, swimming about in all directions in a myriad of forms. The symptoms change in positive correlation with the intensity of infection in the shrimp larvae.
II. Pathogens and Pathogenesis
1. Pathogens
In production, it has been found that the "luminescence" phenomenon in hatcheries is entirely different from the "luminescence" phenomenon caused by protozoa (such as Noctiluca) or certain luminescent algae in natural sea areas. Through repeated diagnosis, the pathogens are generally bacteria, namely Pseudomonas fluorescens and Vibrio harveyi. The former is a Gram-negative bacterium, while the latter possesses luminescent properties. The salinity and temperature ranges suitable for their reproduction and growth happen to be identical to the physicochemical factors of shrimp seedling rearing water bodies. Within the same physicochemical parameters, they share the same feeding mechanisms and reproductive mechanisms as artificially bred shrimp larvae, making them highly prone to causing disease outbreaks and spread.
2. Pathogenesis
After bacteria infect the shrimp body, they utilize shrimp tissue as nutrients for reproduction and development, and under the action of their intrinsic enzymes, oxidize the luciferin contained in the body, thereby emitting fluorescence. Because this "light" is a weak cold light, it is easily mistaken for reflected light from other light sources and overlooked. Infected diseased shrimp exhibit fluorescence for 2-5 days depending on their physical condition, and eventually die successively. In particular, infection during the molting period when shrimp larvae undergo metamorphosis can easily lead to sudden mass mortality, with severe cases resulting in complete loss overnight.
III. Epidemiology and Harm
Luminescent bacteria have a broad distribution in natural sea areas and are found along the entire coast of China. The disease process accompanies shrimp larvae throughout their entire life to adulthood, with the mysis stage and postlarval stage during seedling rearing being the most susceptible phases, causing the most severe harm, and showing a positive correlation with rising water temperature. In terms of timing, it is most widespread from May to July, as this is when the rainy season begins. Large amounts of land-derived organic matter flow into the sea with runoff, causing eutrophication of coastal water quality, and pathogens rapidly reproduce and grow. Once they enter the shrimp seedling rearing process, they first infect weak and diseased larvae, then rapidly spread, causing infection and severe harm and losses during the Z, M, and P stages of shrimp seedling rearing.
IV. Diagnostic Methods
1. Visual observation. Because the larvae uniquely exhibit luminescence, first, visual detection is possible: when night falls, turn off all lighting in the hatchery, adjust the air flow to individual rearing ponds, and conduct careful observation from multiple angles and depths, which generally yields fairly intuitive conclusions. Second, sampling diagnosis is possible: use a white container to take multiple samples, and observe under dark conditions with a flashlight. It will be found that luminescent spots mostly concentrate on the cephalothorax region of the shrimp body, and in late-stage infections, fluorescent spots can be seen at various parts of the shrimp body.
2. Microscopic examination. To ensure diagnostic accuracy, further microscopic examination with a high-power microscope can be conducted. Under a field of view of 400x or greater, bacterial activity within infected shrimp larvae becomes visibly apparent in relatively transparent areas. Pseudomonas fluorescens appears as short rods with bluntly rounded ends, bearing 1-6 polar flagella, with strong bacterial motility. Vibrio harveyi also appears as short rods, some slightly curved, with polar flagella, and weaker bacterial motility. The optimal temperature range for both in rearing ponds is divided at 27°C: below 27°C, Pseudomonas fluorescens is more prevalent; above 27°C, Vibrio harveyi is more prevalent.
V. Prevention and Control Measures
Rearing ponds that excessively pursue high density and high yield are highly prone to "luminescence" disease. This is because excessively high larval rearing density inevitably requires correspondingly increased feeding amounts, and especially under conditions of poor water exchange and lack of basic biological live feed, it leads to aggravated bottom pollution in the rearing pond, providing conditions for massive bacterial reproduction. In addition, there are many transmission routes for pathogens, such as sea and freshwater sources, exogenous feed, wild-caught or overwintered broodstock, and seedling rearing-related facilities, all of which may harbor pathogenic factors. Therefore, disease prevention and control should start from the source. The main measures are:
1. Use and treatment of water sources. The sea and freshwater sources used for seedling rearing are one of the main sources of luminescent bacteria. Therefore, rearing water should preferably undergo sand filtration and dark treatment. Hatcheries with the capacity may use ultraviolet light or chemicals for disinfection, which will be more effective. Chemicals used include bleaching powder, potassium permanganate, etc. Water bodies treated with chemical disinfectants must wait until the drug efficacy has dissipated before being put into production.
2. Strict feed management. According to observations, an important cause in many hatcheries where this disease occurs is improper treatment or direct use without treatment of biological feed given to broodstock and larvae. Live biological feed such as small shellfish, cephalopods, cladocerans, etc., mostly live in heavily polluted coastal waters or on land, and carry a high probability of pathogens. They should be used after high-temperature sterilization or chemical disinfection. When feeding substitute feed, care must be taken not to overfeed, especially homemade cooked products such as egg yolk and cake-type feeds, which have a single feeding effect and easily cause water quality pollution leading to disease. For other feeds, nutritional balance should be required, with greater use of high-quality fresh live feed and complete formula feed, achieving scientific feeding and maintaining water quality.
3. Selection of broodstock. Currently, there are mainly two sources of broodstock used for artificial seedling rearing: one is wild-caught natural broodstock, and the other is artificially overwintered broodstock. Whether wild-caught or overwintered broodstock, because they live for extended periods in natural environments or artificially controlled environments, the probability of carrying pathogens is relatively high. Comparing the two, overwintered broodstock carry more pathogenic bacteria than wild-caught broodstock. In view of this, when selecting broodstock, first, areas or overwintering ponds with frequent disease outbreaks should be avoided; second, strict chemical disinfection should be carried out before broodstock enter the hatchery; the time for artificial spawning and hatching should be shortened as much as possible; and the living environment should be created and improved to reduce the probability of infection.
4. Facility disinfection and larval rearing. Rearing ponds, overwintering ponds, reservoir ponds, and all auxiliary facilities and tools should be regularly disinfected with bleaching powder at a concentration of 150ppm or above or potassium permanganate at 50ppm or above. After disinfection treatment, they must be repeatedly rinsed with clean seawater to remove chemical residues before use. Larval rearing density should not be too high, generally controlled below 300,000 larvae per cubic meter of water, while high-density rearing ponds exceeding 500,000 larvae per cubic meter of water are highly prone to disease outbreaks.
5. Reasonable water quality regulation. During the seedling rearing process, taking effective measures to create an excellent water environment is an important part of ensuring success. Therefore, residual feed and feces in rearing ponds should be regularly removed, and water exchange capacity should be enhanced. In the middle and late stages of rearing, siphoning and pond transfer should also be properly carried out to minimize the external conditions for pathogenic bacteria survival.
6. Strengthening basic preventive work. On one hand, during the rearing process, through inoculation, unicellular algae such as green algae, diatoms, and golden algae should be cultivated. This not only serves as high-quality biological feed to reduce rearing costs but also inhibits the growth of luminescent bacteria, promoting the smooth progress of seedling rearing. On the other hand, frequent observation and early detection should be emphasized, especially when larvae develop to the late mysis stage or postlarval stage. Once this disease occurs at these two stages, it is not only difficult to treat but also greatly reduces the survival rate of seedling rearing.
VI. Comparison of Chemotherapeutic Efficacy
In recent years, in shrimp seedling rearing production, the author, in response to the characteristics of this disease such as wide occurrence range, rapid outbreak frequency, and great harmfulness, organized multiple hatcheries to conduct comprehensive prevention and treatment, and purposefully compared the efficacy of more than 10 physical and chemical agents. Overall, Pseudomonas fluorescens and Vibrio harveyi show weak sensitivity to antibiotic drugs, with good therapeutic effects on symptoms before the mid-mysis stage but unclear effects in later stages. The combined use of antibiotics and chlorine-containing disinfectants also shows good efficacy.
I. Symptoms and Changes of "Luminescence" Disease
During the hatching and rearing of all shrimp species, from the nauplius stage through the mysis stage and postlarval stage, the disease can occur at every stage and is not easily detected. The main reason is that relatively strong natural or artificial light sources inside and outside the hatchery interfere with the diagnosis of "luminescence" disease symptoms in shrimp larvae. After larvae are infected with this disease, mortality occurs progressively, with severe cases reaching 70%-80% mortality within 2-3 days. Symptoms include decreased larval activity, sinking to the middle and lower layers of the pond water, with essentially no shrimp larvae visible at the water surface. By the mysis stage or the middle-to-late postlarval stage, shrimp larvae show weak jumping ability, poor phototaxis, reduced or ceased feeding, and a cloudy white, largely non-transparent body color. In affected shrimp ponds after lights are turned off at night, one can see star-like flickering fluorescence rising and falling within the water body, swimming about in all directions in a myriad of forms. The symptoms change in positive correlation with the intensity of infection in the shrimp larvae.
II. Pathogens and Pathogenesis
1. Pathogens
In production, it has been found that the "luminescence" phenomenon in hatcheries is entirely different from the "luminescence" phenomenon caused by protozoa (such as Noctiluca) or certain luminescent algae in natural sea areas. Through repeated diagnosis, the pathogens are generally bacteria, namely Pseudomonas fluorescens and Vibrio harveyi. The former is a Gram-negative bacterium, while the latter possesses luminescent properties. The salinity and temperature ranges suitable for their reproduction and growth happen to be identical to the physicochemical factors of shrimp seedling rearing water bodies. Within the same physicochemical parameters, they share the same feeding mechanisms and reproductive mechanisms as artificially bred shrimp larvae, making them highly prone to causing disease outbreaks and spread.
2. Pathogenesis
After bacteria infect the shrimp body, they utilize shrimp tissue as nutrients for reproduction and development, and under the action of their intrinsic enzymes, oxidize the luciferin contained in the body, thereby emitting fluorescence. Because this "light" is a weak cold light, it is easily mistaken for reflected light from other light sources and overlooked. Infected diseased shrimp exhibit fluorescence for 2-5 days depending on their physical condition, and eventually die successively. In particular, infection during the molting period when shrimp larvae undergo metamorphosis can easily lead to sudden mass mortality, with severe cases resulting in complete loss overnight.
III. Epidemiology and Harm
Luminescent bacteria have a broad distribution in natural sea areas and are found along the entire coast of China. The disease process accompanies shrimp larvae throughout their entire life to adulthood, with the mysis stage and postlarval stage during seedling rearing being the most susceptible phases, causing the most severe harm, and showing a positive correlation with rising water temperature. In terms of timing, it is most widespread from May to July, as this is when the rainy season begins. Large amounts of land-derived organic matter flow into the sea with runoff, causing eutrophication of coastal water quality, and pathogens rapidly reproduce and grow. Once they enter the shrimp seedling rearing process, they first infect weak and diseased larvae, then rapidly spread, causing infection and severe harm and losses during the Z, M, and P stages of shrimp seedling rearing.
IV. Diagnostic Methods
1. Visual observation. Because the larvae uniquely exhibit luminescence, first, visual detection is possible: when night falls, turn off all lighting in the hatchery, adjust the air flow to individual rearing ponds, and conduct careful observation from multiple angles and depths, which generally yields fairly intuitive conclusions. Second, sampling diagnosis is possible: use a white container to take multiple samples, and observe under dark conditions with a flashlight. It will be found that luminescent spots mostly concentrate on the cephalothorax region of the shrimp body, and in late-stage infections, fluorescent spots can be seen at various parts of the shrimp body.
2. Microscopic examination. To ensure diagnostic accuracy, further microscopic examination with a high-power microscope can be conducted. Under a field of view of 400x or greater, bacterial activity within infected shrimp larvae becomes visibly apparent in relatively transparent areas. Pseudomonas fluorescens appears as short rods with bluntly rounded ends, bearing 1-6 polar flagella, with strong bacterial motility. Vibrio harveyi also appears as short rods, some slightly curved, with polar flagella, and weaker bacterial motility. The optimal temperature range for both in rearing ponds is divided at 27°C: below 27°C, Pseudomonas fluorescens is more prevalent; above 27°C, Vibrio harveyi is more prevalent.
V. Prevention and Control Measures
Rearing ponds that excessively pursue high density and high yield are highly prone to "luminescence" disease. This is because excessively high larval rearing density inevitably requires correspondingly increased feeding amounts, and especially under conditions of poor water exchange and lack of basic biological live feed, it leads to aggravated bottom pollution in the rearing pond, providing conditions for massive bacterial reproduction. In addition, there are many transmission routes for pathogens, such as sea and freshwater sources, exogenous feed, wild-caught or overwintered broodstock, and seedling rearing-related facilities, all of which may harbor pathogenic factors. Therefore, disease prevention and control should start from the source. The main measures are:
1. Use and treatment of water sources. The sea and freshwater sources used for seedling rearing are one of the main sources of luminescent bacteria. Therefore, rearing water should preferably undergo sand filtration and dark treatment. Hatcheries with the capacity may use ultraviolet light or chemicals for disinfection, which will be more effective. Chemicals used include bleaching powder, potassium permanganate, etc. Water bodies treated with chemical disinfectants must wait until the drug efficacy has dissipated before being put into production.
2. Strict feed management. According to observations, an important cause in many hatcheries where this disease occurs is improper treatment or direct use without treatment of biological feed given to broodstock and larvae. Live biological feed such as small shellfish, cephalopods, cladocerans, etc., mostly live in heavily polluted coastal waters or on land, and carry a high probability of pathogens. They should be used after high-temperature sterilization or chemical disinfection. When feeding substitute feed, care must be taken not to overfeed, especially homemade cooked products such as egg yolk and cake-type feeds, which have a single feeding effect and easily cause water quality pollution leading to disease. For other feeds, nutritional balance should be required, with greater use of high-quality fresh live feed and complete formula feed, achieving scientific feeding and maintaining water quality.
3. Selection of broodstock. Currently, there are mainly two sources of broodstock used for artificial seedling rearing: one is wild-caught natural broodstock, and the other is artificially overwintered broodstock. Whether wild-caught or overwintered broodstock, because they live for extended periods in natural environments or artificially controlled environments, the probability of carrying pathogens is relatively high. Comparing the two, overwintered broodstock carry more pathogenic bacteria than wild-caught broodstock. In view of this, when selecting broodstock, first, areas or overwintering ponds with frequent disease outbreaks should be avoided; second, strict chemical disinfection should be carried out before broodstock enter the hatchery; the time for artificial spawning and hatching should be shortened as much as possible; and the living environment should be created and improved to reduce the probability of infection.
4. Facility disinfection and larval rearing. Rearing ponds, overwintering ponds, reservoir ponds, and all auxiliary facilities and tools should be regularly disinfected with bleaching powder at a concentration of 150ppm or above or potassium permanganate at 50ppm or above. After disinfection treatment, they must be repeatedly rinsed with clean seawater to remove chemical residues before use. Larval rearing density should not be too high, generally controlled below 300,000 larvae per cubic meter of water, while high-density rearing ponds exceeding 500,000 larvae per cubic meter of water are highly prone to disease outbreaks.
5. Reasonable water quality regulation. During the seedling rearing process, taking effective measures to create an excellent water environment is an important part of ensuring success. Therefore, residual feed and feces in rearing ponds should be regularly removed, and water exchange capacity should be enhanced. In the middle and late stages of rearing, siphoning and pond transfer should also be properly carried out to minimize the external conditions for pathogenic bacteria survival.
6. Strengthening basic preventive work. On one hand, during the rearing process, through inoculation, unicellular algae such as green algae, diatoms, and golden algae should be cultivated. This not only serves as high-quality biological feed to reduce rearing costs but also inhibits the growth of luminescent bacteria, promoting the smooth progress of seedling rearing. On the other hand, frequent observation and early detection should be emphasized, especially when larvae develop to the late mysis stage or postlarval stage. Once this disease occurs at these two stages, it is not only difficult to treat but also greatly reduces the survival rate of seedling rearing.
VI. Comparison of Chemotherapeutic Efficacy
In recent years, in shrimp seedling rearing production, the author, in response to the characteristics of this disease such as wide occurrence range, rapid outbreak frequency, and great harmfulness, organized multiple hatcheries to conduct comprehensive prevention and treatment, and purposefully compared the efficacy of more than 10 physical and chemical agents. Overall, Pseudomonas fluorescens and Vibrio harveyi show weak sensitivity to antibiotic drugs, with good therapeutic effects on symptoms before the mid-mysis stage but unclear effects in later stages. The combined use of antibiotics and chlorine-containing disinfectants also shows good efficacy.