Points for attention in the treatment of fish diseases

Points for attention in the treatment of fish diseases

In the prevention and control of diseases of fish, shrimp and shellfish, the efficacy of the same drug is reduced or even ineffective due to long-term and repeated use of the same drug. the emergence of this phenomenon is often called drug resistance. In recent years, all over the world have attached great importance to this phenomenon, and the United Nations World Health Organization and FAO have also set up special agencies to study its countermeasures. According to the statistics of relevant departments, more than 400 kinds of agricultural, fishery and health diseases and insect pests in the world have developed serious drug resistance (not including microbial drug resistance.) Under natural conditions, the resistance of some diseases and insects to organophosphorus drugs is more than 2700 times, and the drug resistance under indoor conditions is as high as 10, 000 times. In recent 40 years, the resistance of diseases and insect pests in fish culture in China is also quite serious. In the early 1960s, 2.5% trichlorfon powder 0.5 mg / L was effective in killing crustacean parasites, but in the 1980s, more than 90% crystal trichlorfon was not effective in some areas. Chloramphenicol and oxytetracycline were used in some areas to control bacterial diseases of fish, shrimp and shellfish. However, in recent years, in some areas, the efficacy of these antibiotics in the prevention and treatment of fish, shrimp and shellfish diseases is low or even ineffective. Fishermen all over the country also reflected that the dosage of these antibiotics per unit area increased year by year, while the efficacy decreased year by year. The author has done some discussion in this aspect in recent years.

I. the causes of drug resistance

Drug resistance is the result of the interaction between biological and chemical drugs, and the degree of drug resistance is generally expressed by the comparative determination of the virulence of sensitive strains and resistant strains. The results of investigation in Jiangsu, Fujian, Hunan, Hubei, Anhui and other places show that:

1. Due to the long-term and repeated use of the same drug. All kinds of organisms have the instinct to defend against the harsh environment of the outside world. under the condition of constant drug attack, the pathogens of fish, including bacteria and parasites, will naturally produce a drug resistance response.

In a pathogen strain or pest population, due to individual differences, there are different responses to fish drugs. For example, the fish Gill disease parasite Zhonghuo, individuals who are sensitive to trichlorfon or a mixture of copper sulfate and ferrous sulfate generally die within 48 hours after application; insensitive individuals can endure 144 hours of death; but some individuals will recover from paralysis after poisoning and continue to survive and reproduce. Offspring derived from insensitive individuals are basically insensitive. As a result, insensitive individuals gradually form advantages and produce insensitive populations, that is, drug-resistant populations. The formation of this population is generally the result of the reuse of the same drug over the years. It is particularly obvious for diseases and insect pests that occur for many generations in a year, such as Chinese mackerel, Anchor head finger ring worm, third-generation insect and so on. For this part of people try to increase the dose or increase the rate of application to solve the problem, and achieved certain results, but this effect can only be short-lived. For example, in Jiangsu and Zhejiang provinces, trichlorfon is usually used every two months or even half a year to kill Chinese oysters and to prevent and cure the disease of "getting water" in fish. But in recent years, every 10 days or half a month, some drugs have to be used only a week, and the dose is gradually increasing, otherwise it will be ineffective. As a result of such application, the number of remaining individuals of parasites decreased, but these individuals were more resistant to drugs. The populations formed by their reproductive offspring are more resistant to drugs, which will eventually make the drug ineffective.

2. it is caused by the technology of pesticide application. Incorrect application technology will not only waste drugs and reduce efficacy, but also easily lead to drug resistance, such as uneven application, generally more drugs are applied on the surface and edge of the water, the drug concentration is high, and pathogens are less likely to receive drugs. In addition, the solubility of some drugs such as furazolidone and allicin is very low, even bleach is slightly soluble in water, so it is easy to produce uneven distribution, which provides conditions for the survival of drug-resistant populations, especially in the egg and spore stage of pathogens. Then there is the dose, because of the concern for the safety of fish (such as cupric sulfate, mercurous acetate, mercurous nitrate, etc.) or lack of experience in application, the dosage is often lower than the prescribed dose, especially the use of antibiotics, such as chloramphenicol, oxytetracycline, erythromycin, etc., it is easy to induce mutations of drug-resistant strains, which not only fails to achieve the purpose of prevention and control, but also easy to induce drug resistance of pathogens. Drug resistance will occur in the selection of drug dosage forms, methods of drug use, diseases and insects, biology of bacteria and so on.

3. The influence of feeding system. Some fish breeding systems, such as auxiliary release, restocking and other measures, are easy to change the parasitic conditions of viruses, bacteria, diseases and insects, and expand the spread of drug-resistant populations. According to the investigation, many pathogens in adult fish ponds are mostly transferred from fish ponds (including local or non-local transferred fish species), especially when rotational fish culture or common use of fish nets are used. In addition, in the aspects of feeding water plants and water injection and drainage, it is also easy to bring drug-resistant varieties of pests.

II. Preventive measures of drug resistance

1. Carry out comprehensive prevention and control. First of all, the ponds should be cleared thoroughly and mixed culture reasonably in order to change the parasitic host of pathogens; to strengthen breeding and management, and to strictly disinfect fish species (including restocking), feeding and water quality; and to regularly check the monitoring of pathogens and drug resistance, so as to formulate countermeasures such as rotation or replacement of drugs before the existing drugs fail to certain pathogens.

2. Alternate use of drugs. Do not use a single variety of drugs for a long time, which can cut off the formation of pathogen resistant populations. Rotation varieties should choose drugs with different mechanisms as far as possible, such as Chinese oyster, water centipede, etc., and trichlorfon, dichlorvos or sodium chloride caused by rotifer. Chantrin and other drugs can be used to cause Gill pain caused by ring worms, third-generation worms and so on.

3. Apply Chinese herbal medicine. Chinese herbal medicine has a wide range of sources, easy to obtain materials, and easy to produce drug resistance, so it is an ideal disease control method, such as pine leaves bundled in the water can control fish lice, anchor head fish, etc., bitter can control parasitic Gill disease, Anchor head fish, third generation worms, ring worms, etc., Polygonum polygonum can control Chinese oysters, fish lice, etc., but no drug-resistant population has been found.

4. Mixed drugs. Rational mixed use of drugs can not only treat diseases and insect pests, increase efficacy and reduce cost, but also overcome and restrain the development of drug resistance. For example, 28 of the 190 new drugs recommended in agriculture in the United States are mixed preparations. The mixed use of copper sulfate and ferrous sulfate commonly used in fishery in China is an example. However, it should be noted that the combination of drugs used in mixed use must be carefully studied and should not be blindly mixed; the confirmed mixed drugs should not be used for a long time, and should be used in rotation as necessary to prevent multiple resistance to pests, that is, to produce resistance to a variety of drugs at the same time.