Antibiotics are defined as substances produced by microorganisms which have the capacity of inhibiting the growth, or destroying other microorganisms. A broader definition includes chemical compounds derived from, or produced by living organisms and their synthetic analogues which even in small concentrations are capable of inhibiting the life processes of microorganisms.

Aquaculture is a rapidly expanding industry in the world today. In developing nations, such as India, a large proportion of the industry comprises semi-intensive finfish and shrimp culture, and especially in semi-intensive culture systems, disease outbreaks are recognized as an important factor limiting. Chronic disease can affect the growth rate and feed efficiency and mortalities contribute directly to a loss of investment in time, labor, and feed, etc.

Most bacteria that infect fish / shrimp fall into one of two groups ­ gram-positive or gram-negative. These groups are named based on their response to a protocol called gram staining. Gram-positive bacteria stain blue, and gram-negative bacteria stain pink. They stain differently because each group has a different type of outer structure known as the cell wall. This difference is important for the producer and aquaculturist because some antibiotics work better against gram-positive bacteria and others work better against gram-negative bacteria. We at Tropical Biomarine Systems provide you the wide range of antibiotics to combat bacterial diseases causing fish / shrimp mortalities.

2. PHARMACOKINETICS

Although selecting the correct antibiotic is an important first step in controlling bacterial disease, proper administration of any antibiotic for the recommended number of days is equally important. Your fish / shrimp health specialist should provide you with instructions on the amount of antibiotic to use (dose), the frequency (how often) and duration (how long) it should be given (administered), withdrawal time (if applicable - this is the length of time required, after the last dose of antibiotic has been given, that a producer must wait before selling the fish / shrimp and any other relevant information. The technical term for the branch of medical science that determines all of these variables is 'pharmacokinetics'; defined as the study of how drugs are absorbed by, distributed within, chemically altered within, and eventually excreted by the body (in this case, the body of a fish / shrimp).

3. PERCENT ACTIVE INGREDIENT

Many antibiotics commonly used for fish / shrimp aquaculture are with percent of active ingredient vary from product to product. This means that you may not have 100% of pure antibiotic required for disease control. You need to check the actual percentage of antibiotic that is active in the product you have purchased and calculate your dosage accordingly. For example, if the product contains less that 100% of active ingredient, your dosage needs to be increased to bring the level up to the equivalent 100% active ingredient. If you are still unsure how to do this calculation, call your local fish / shrimp health specialist.

4. MAJOR ROUTES OF ADMINISTRATION

Mixed in food

In aquaculture production, the most cost effective and commonly used method to deliver antibiotics, is orally by mixing them into food. The proper dose of antibiotic is mixed into the feed using fish oil or other binders as a binding agent, and the mixture is then fed to the fish / shrimp for the prescribed number of days. Oral administration of antibiotics requires that most of the fish / shrimp are still eating, so every attempt should be made to catch bacterial diseases early, before the majority of fish / shrimp stop eating. It is a good idea to train fish / shrimp to eat a prepared diet (i.e., one that can be used to incorporate antibiotics), so that, should it becomes necessary to use antibiotics, the fish / shrimp will be more likely to eat the medicated food as it is familiar to them.

Bath treatments

Although bath treatments are a popular method of administering antibiotics, much more drug is required when compared with oral treatments to achieve the desired result. In many cases, even a large amount of antibiotic in the water does not guarantee that enough of it will get into the fish / shrimp to be an effective treatment. At the same time, excessive amounts of antibiotic in the water can increase the likelihood of water-borne bacteria developing resistance to that drug. Furthermore, to avoid poor water quality and any potential toxicity, between 70% and 100% of the water should be changed at the end of each daily treatment and also prior to redosing. Finally, bath treatments are not recommended in recirculating systems or in any aquarium system where the treated water will contact the biological filter, because the antibiotics will kill or inhibit the nitrifying bacteria in the biological filters. If treating fish / shrimp in a bath, ideally a separate container should be used, or tanks and vats should be taken off-line during treatment. In summary, bath treatments should be considered only when the majority of the fish / shrimp are not eating, or when treating primarily external bacterial infections and fish / shrimp should be switched to oral medications as soon as they resume eating.

5. CONSEQUENCES OF IMPROPER DOSAGE AND TREATMENT TIME

If the dose is too high or treatment times are too long, there is a danger of toxicity to the fish / shrimp, frequently causing liver, kidney, or other organ damage that may or may not be reversible.

On the other hand, if the dose of antibiotic is too low or treatment time is too short, the bacteria will not be killed or weakened enough for the immune system of the fish / shrimp to remove them, and this greatly increases the risk of the bacteria developing resistance to the antibiotic. When bacteria become resistant to a specific antibiotic, even high concentrations of that drug will not be effective.

Antibiotic resistance can also occur when antibiotics are used improperly, such as the 'shot-gunning' method. 'Shot-gunning' involves administering one antibiotic after another to a population of fish / shrimp, frequently at improper dosages, for shortened treatment times, and without the aid of proper diagnosis (i.e., without culture and sensitivity tests). If shot-gunning is used frequently at a facility, resistance is almost certain to occur. In some cases, the problem may not be due to bacteria, but rather poor water quality or other management issues that have not been properly investigated.

While shot-gunning may work occasionally, over time, it introduces too great a risk of producing populations of bacteria that are resistant to multiple antibiotics, which can result in the very real possibility of a 'superinfection,' where the bacteria cannot be controlled with antibiotics. Once a system has a superinfection, it is usually necessary to sacrifice the entire affected population, completely break down and disinfect the affected system, and start over. This is obviously not a desirable outcome. The importance of using antibiotics wisely (running culture and sensitivity tests, using proper dosages and adhering to proper treatment times) cannot be overstressed.