Diseases And Their Control

Aquatic animals are susceptible to a variety of diseases, including those caused by viruses, bacteria, fungi, and parasites. A range of chemicals and vaccines has been developed for treating the known diseases, although some conditions have resisted all control attempts to date, and severe restrictions on the use of therapeutants in some nations has impaired the ability of aquaculturists to control disease outbreaks. The United States is a good example of a nation in which the variety of treatment chemicals is limited (Table 6). In many instances when a drug is cleared for use on aquatic animals in the United States, the species on which the drug can be used is limited. Clearing a drug for catfish, for example, does not necessarily mean it can be used on trout. The cost of obtaining clearance for drug and chemical use can be in the millions of dollars, and it is often uneconomical to attempt gaining clearance for species that are not major contributors to overall aquaculture production.

Maintenance of conditions in the culture environment that minimize stress is one of the best methods of avoiding diseases. Vaccines have been developed against several diseases and more are under development. Selective breeding of animals with disease resistance has met with only limited success. Good sanitation and disinfection of contaminated facilities are important avoidance and control measures. Some disinfectants are listed in Table 6. Pond soils can be sterilized with burnt lime (CaO), hydrated lime (Ca[OH]2), or chlorine compounds (14).

When treatment chemicals have to be used, they may be incorporated in the food; dissolved in water for use in dips, flushes, and baths that allow relatively short exposure periods; or allowed to remain in the water until the chemical breaks down. Higher dosages can be used in dips, flushes, and baths than with chronic exposure. Since one of the first responses of aquatic animals to disease is reduction or cessation of feeding, treatments with medicated feeds must be initiated as soon as development of an outbreak is suspected and before the animals quit feeding. Antibiotics, such as oxytetracycline, can be dissolved in the water but may be less effective than when given orally.

Vaccines can be administered through injections, orally, or by immersion. Injection is the most effective means of vaccinating aquatic animals, but it is stressful, time-consuming, and expensive. The time and expense may be acceptable for use in conjunction with broodfish and other valuable animals. Oral administration of vaccines may be ineffective, because many vaccines are deactivated in the digestive tract of the animals the vaccines are intended to protect. Dip treatment by which the vaccines enter the animals through diffusion from the water are not generally as effective as injection but can be used to vaccinate large numbers of animals in short periods of time. Vaccines have been developed for a number offish diseases, but scientists have not achieved much success with invertebrates, which have a primitive immune system.

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