This paper briefly reviews the use of chemicals to prevent and treat bacterial diseases in aquaculture, and provides a detailed summary of the current state of knowledge on the development of bacterial resistance to antimicrobial agents in fish and shellfish. The topics covered include mechanisms of resistance, resistance of bacterial fish pathogens, resistance to antibacterial agents associated with use in aquaculture, and factors causing selection of resistant variants. Emphasis is placed on avoiding and solving problems related to bacterial resistance in aquaculture, and recommendations on antibiotic usage in aquaculture are made.

Chemotherapeutic agents have been developed for treating bacterial infections and have been widely used for cultured fish for the last 30 years in Japan. The extensive use of chemotherapeutants has resulted in an increase in the occurrence of drug resistance in fish-pathogenic bacteria and also in the bacterial flora of the intestinal tract of cultured fish. The kinds of chemotherapeutants used are correlated with the occurrence of the corresponding drug-resistant genes in fish-pathogenic bacteria. Almost all multiple-drug resistant strains are carried on the transferable R plasmid, although resistance in fish pathogens to nitrofuran derivatives and pyridonecarboxylic acids is associated with a chromosomal gene. The DNA sequences of R plasmids generally differ depending on the species of fish pathogen. Exceptions are the R plasmids of Aeromonas hydrophila and A. salmonicida, which have the same resistance markers as chloramphenicol, streptomycin, and sulfonamides (SA); and the R plasmids of A. hydrophila and Edwardsiella tarda, which have the same resistance markers as SA and tetracycline. The fish pathogens A. hydrophila, A. salmonicida, E. tarda, Enterococcus seriolicida, Pasteurella piscicida, and Vibrio anguillarum are all widely distributed in fish farms in various areas, and within each species the R plasmid has an identical DNA sequence. The chloramphenicol resistance (cat) gene of the R plasmid from Gram-negative bacteria was classified into CAT I, II, III, and IV according to the DNA sequence. The cat gene of P. piscicida was classified as CAT I, those of A. salmonicida and E. tarda were classified as CAT II, and that of V. anguillarum was classified as CAT II or IV, depending on the time the strains were isolated. The tetracycline-resistance determinants (Tet), which occur in six classes (Tet A through Tet G), were class D in the R plasmids obtained from strains of V. anguillarum that were isolated from 1989 to 1991. The Tet for strains of V. anguillarum isolated from 1973 to 1977 was classified as Tet B, while for strains isolated from 1980 to 1983 it was classified as Tet G.

Safe and wholesome food is essential for good health. Therefore, when one considers health issues related to unsafe foods, recorded morbidity and mortality as well as economic losses in a population must be included. Due to their presence in unsafe food, micro-organisms are generally considered to pose a major risk to human health. In aquaculture, chemicals are used mainly in the treatment and prophylaxis of disease problems, which constitute the largest single cause of economic losses. However, the increasing use of chemicals in aquaculture has led to wide-spread public concern. The concerns related to human health due to chemical use in aquaculture are repeatedly found in the published literature. They include allergic reactions in previously sensitized persons triggered by chemical residues, and the potential impacts on human health resulting from the emergence of drug-resistant bacteria caused by the use of sub-therapeutic levels of antibiotics and by antibiotic residues persisting in the sediments of aquaculture environments. This paper discusses the risk evaluation principles, data requirements and the concept of maximum residue limit. The uncertainties inherent in the process include, but are not limited to, the state-of-the-art of toxicological evaluation, the level of understanding of the environmental transport process of chemicals, the exposure data available, and any assumptions and extrapolations.

This paper provides an overview on the use of chemicals in seven countries in Asia (Bangladesh, Cambodia, Nepal, Laos PDR, Pakistan, Sri Lanka and Viet Nam), with an emphasis on coastal shrimp aquaculture and inland carp farming systems. The data come primarily from a recently completed survey of aquaculture farms in Asian countries conducted under the ADB/NACA Regional Study and Workshop on Aquaculture Sustainability and Environment. The issues discussed include the types and uses of chemicals in shrimp and carp culture, farm management practices and use of chemicals, hazards and adverse impacts associated with chemical use, alternative approaches to chemical use, and research recommendations. In inland carp farming, apart from lime and fertilizers, which are unlikely to give rise to any significant negative environmental impact, the overall use of chemicals is extremely low. Piscicides are used in some countries to control predators prior to stocking of ponds, but the use of antimicrobials and disease-control chemicals is limited to a small percentage (<5%) of producers. Most small-scale producers, who dominate aquaculture production in these countries, simply do not have the resources or need for such chemicals. The situation is similar in shrimp culture, with lime and fertilizers, followed by piscicides, being the most common chemicals used. The use of antimicrobials increases with intensification in shrimp culture, and these chemicals are mostly used in more intensive shrimp farming. In both shrimp and carp culture, promotion of “primary” health management practices probably offers greatest scope for prevention of aquatic animal disease outbreaks and the need for chemical use.

Aquaculture systems in Indonesia have developed toward intensive culture. As a result of intensification of fish culture, increased outbreaks of disease have occurred. Various chemotherapeutic agents like antibiotics and other chemicals have been widely used for treatment and prevention of infectious diseases in fish and shrimp farms. Antibiotics such as oxytetracycline, chloramphenicol, neomycin, streptomycin, erythromycin, prefuran and enrofloxacin are used in the treatment of bacterial diseases. Other chemicals such as malachite green oxalate, potassium permanganate, formalin, methylene blue, chlorine and teaseed have been used for the treatment of various diseases. Organic fertilizers, such as chicken manure, and inorganic fertilizers like urea and trisodium phosphate are often applied by shrimp farmers to improve primary productivity in ponds. Bacterial products with trade names like “Multi bacter,” “Enviro star” and “Super NB” have recently been used by shrimp farmers to decompose organic matter resulting from excessive feeding. Feed additives such as vitamin C, “Protec Plus,” and “Super Embak” are used for disease prevention.

Aquaculture is an increasingly important force in both the Malaysian as well as the Singaporean economies. In recent years, Singapore has focused on the aquarium fish trade, making it one of the largest ornamental fish production and transhipment centers in the world. Similarly, the Malaysian aquaculture industry has made rapid strides in the last few years and is poised to become a major contributor to the national fish supply by the early part of the next century. A significant trend in both countries has been the growing intensification of culture systems to achieve higher production per unit area. This has led to a greater occurrence of disease, particularly among aquarium fish, shrimp and marine fish farms. To obviate and control these diseases, there has been a concurrent increase in the use of chemotherapeutants. The three major groups of commonly used chemotherapeutants are: topical disinfectants, antimicrobials and probiotics. There is a wide range of topical disinfectants used by aquafarmers. The most common of these include lime, teaseed cake, formalin, benzalkonium chloride, acriflavine, malachite green, hypochlorite and poly-vinyl pyrrolidine. Of these, lime and teaseed cake are used exclusively in ponds, and acriflavine and malachite green only in hatcheries, while the others are used in both systems. Antimicrobials being used include sulfonamides, tetracyclines, nitrofurans, chloramphenicol, oxolinic acid and virginiamycin. A number of other chemotherapeutants are also used, albeit on a limited basis. The current concerns surrounding the use of chemotherapeutants and the legislative framework surrounding their sale and distribution are also discussed.

In Sri Lanka, the active promotion of chemical products to prevent disease in shrimp hatcheries has led to an increase in the use of drugs and chemicals without much emphasis on understanding their efficacies. A survey was carried out to evaluate trends in the use of drugs and chemicals as therapeutic treatments for shrimp-hatchery diseases. A wide range of chemicals and drugs are being used, both for prophylactic treatment and to prevent or control parasitic, fungal and bacterial diseases in hatcheries. Without proper scientific investigation into treatment regimes, there has been a tendency for individual hatcheries to select their own treatment regimes and to do their own experimentation. Little knowledge exists among hatchery operators as to the hazardous effects of the chemicals in use. Lack of legislation on the use of chemotherapeutants in aquaculture has led to the uncontrolled use and improper selection of chemicals for use in shrimp hatcheries.

In Thailand, many chemicals are used to treat diseases of cultured aquatic animals and to improve water quality in culture facilities. Along with the intensification of aquaculture practices that has occurred in recent years in Thailand, chemical use has also increased, particularly in marine shrimp culture. This paper summarizes information on the types of chemotherapeutants commonly used in Thailand, their sources and costs, the treatment regimes used, the adverse impacts that have resulted and the hazards posed. Also included is information on national regulations, a summary of on-going research, and recommendations to aquaculturists, producers and suppliers of chemicals, government agencies and scientists. It is concluded that although chemicals and drugs will continue to play an important role in the development of Thai aquaculture, they must be used with caution to avoid adverse effects such as environmental damage and the development of resistant strains of pathogens. To minimize chemical usage, additional emphasis needs to be placed on developing good management practices for aquaculture systems.

Wild-caught mangrove red snapper, Lutjanus argentimaculatus, reared for a year in 4.0 m diameter by 1.5 m deep circular concrete tank spontaneously matured and were used for induced spawning trials. On 19 August 1992, a sexually mature female (4.6 kg BW) and male (3.2 kg BW) fish were given a single intramuscular injection of 1500 IU human chorionic gonadotropin (hCG)/kg BW. Spawning occurred 27 h after injection with total egg collection of 1.3 M. Hatching occurred 16 h after spawning at 28 °C and 32 ppt. On 18 March 1993, the same male and female together with a newly-caught spermiating male (6.3 kg BW) were injected intramuscularly with 100 µg luteinizing hormone-releasing hormone analogue (LHRHa)/kg BW. Spawning occurred 44 hours after injection with the total egg collection of 0.7 M. Hatching occurred 16 hours after spawning at the same temperature and salinity as the first trial. The successful spawning trials encourage further research to determine the effective minimum dose of hCG and LHRHa.

Changes in steroid hormone levels in the serum and ovarian fluid were studied during overripening in goldfish. Ovulated eggs retained in the ovarian cavity become overripe at around 12 h after ovulation and completely overripe 24 h after. Blood and ovarian fluid were taken at 0, 3, 6, 12, 18, and 24 h after ovulation. Estradiol-17ß (E 2) , testosterone (T), progesterone (P) and 17α,20ß-dihydroxy-4-pregnen-3-one (17α,20ß-P) in the serum were extracted directly with a solvent while those in the ovarian fluid were separated by HPLC before radioimmunoassay. Both serum and ovarian fluid P showed a highly significant decline at 18 h with a further decline at 24 h; P levels were higher in the ovarian fluid. Serum 17α,20ß-P showed a progressive and more rapid decline, decreasing significantly at 12 h with further decreases at 18 h and 24 h; the level was five-fold lower at 24 h compared to the 0 h level. Serum T increased significantly at 3 h which was maintained until 18 h, when it declined to 0 h level. No significant changes in E2 were observed in the serum, except for a significant difference between 6 and 24 h. There were no significant changes in E2, T and 17α,20ß-P in the ovarian fluid. Of the four steroids measured, only 17α,20ß-P and P showed changes which bear some correlation with the time course of overripening. The declines in the mean ratios of 17α,20ß-P/E2 in the serum and P/E2 in the ovarian fluid also appeared to have a good correlation with the time course of overripening. The postovulatory follicles (POFs) showed degenerative features which likewise correspond to the decline in P and 17α,20ß-P.