The antibiotics most frequently used in aquaculture to combat bacterial diseases are oxytetracycline, florfenicol, sarafloxacin and enrofloxacin. Globally, other antibiotics such as chlortetracycline, quinolones, ciprofloxacin, norfloxacin, oxolinic acid, perfloxacin, sulfamethazine, gentamicin and tiamulin are used.
Even though they are frequently used on shrimp farms, information related to residues of oxytetracycline and enrofloxacin in Litopenaeus vannamei tissue is very scarce.
In an unpublished study performed at the Research Center for Food and Development, Hermosillo, Sonora, México, they administered a diet medicated with enrofloxacin at a level of 200 mg kg-1 for 14 days in an attempt to determine the accumulation and elimination of enrofloxacin and its metabolite ciprofloxacin in a crop of Litopenaeus vannamei. Subsequently, a diet without the antibiotic was administered for 16 days. The study was carried out under controlled laboratory conditions and on a farm. In the lab study, using high-performance liquid chromatography to analyze our results, enrofloxacin reached maximum concentrations in the tail and hepatopancreas of 0.54±0.26 μg g-1 and 3.52±1.9 μg g-1, respectively. For ciprofloxacin, levels of 0.18±0.13 μg g-1 and 1.05±0.20 μg g-1 were reached in the tail and hepatopancreas, respectively.
In the farm study, maximum concentrations of enrofloxacin reached 0.36±0.17 μg g-1 and 1.60±0.82 μg g-1 in the tail and hepatopancreas, respectively. For ciprofloxacin, the numbers were 0.03±0.02 and 0.36±0.08 μg g-1 in the tail and hepatopancreas, respectively. When the feeding of the medicated diets was suspended, enrofloxacin and ciprofloxacin residues in the tissues began to decrease, requiring four to ten days for both antibiotics to reach undetectable levels in the tail and six to fourteen days for elimination from the hepatopancreas.
Oxytetracycline, another of the widely used antibiotics, was studied at a shrimp farm. Shrimp (Litopenaeus vannamei) were treated for 14 days with feed that contained a theoretical oxytetracycline concentration of 5,000 mg kg-1. Next, an antibiotic-free diet was administered for 16 days. The shrimp were sampled every third day. Vibrio bacteria were isolated at the molecular level from the tail and hepatopancreas, counted and expressed as CFUs (colony-forming units).
The results show an average maximum concentration for oxytetracycline in the tail of 31.32±3.44 μg g-1 and in the hepatopancreas of 274.81±62.35 μg g-1. The withdrawal times necessary for the oxytetracycline residues to be eliminated were ten days for the hepatopancreas and sixteen days for the tail. Under lab conditions, a diet was administered to Litopenaeus vannamei shrimp that contained 5,000 mg Kg-1 of oxytetracycline for 14 days. The levels of maximum concentration for oxytetracycline were 33.54±11.19 μg g-1 in the tail, 194.37±16.11 μg g-1 in the hepatopancreas and 18.79±5.87 μg mL-1 in the hemolymph. The elimination time for oxytetracycline was six to ten days for all tissues.
The most frequent administration route for antibiotics is in the feed. It is important that the antibiotic be contained within a pellet to maintain its stability and protect it from leaching and binding to trivalent and divalent cations in the pond. It is very important that the shrimp eat all the medicated feed; otherwise the environment will be contaminated and the emergence of bacterial resistant strains will be favored. Also, water temperature is a critical factor when using medicated feeds because it affects maximum concentration, distribution volume and the rate of elimination. Oxygenation, pH, salinity, stage of disease, the weather and presence of natural food in the ponds are other factors that affect antibiotic therapies.
An estimated 15 to 40 percent of medicated diets are not ingested by the shrimp and remain in the pond. Another fraction of the medication is not absorbed during its passage through the intestinal tract of the shrimp and returns to the environment in fecal matter. The amount of antibiotic transferred to the environment varies from 1% for chloramphenicol to 90% for oxytetracycline. Approximately 70-90% of the antibiotic used in the therapy of shrimp ends up in the environment and sediment, and a high percentage of it exhibits antibacterial activity. It has been reported that residues of oxolinic acid and oxytetracycline are very persistent under certain conditions, with half-lives exceeding 100 days.
International regulations regarding the use of antibiotics in aquaculture have established a list of prohibited products. The strongest restrictions are on the use of chloramphenicol, dimetridazole, furazolidone, nitrofurazone, other nitrofurans and fluoroquinolones, none of which should be used at any stage of the shrimp production process.
Source: Health and Environment in Aquaculture. Editors: Edmir Daniel Carvalho, Gianmarco Silva David and Reinaldo J. Silva. Chapter Eight, The Use of Antibiotics in Shrimp Farming. M.C. Bermúdez-Almada and A. Espinosa-Plascencia. Research Center for Food and Development, Hermosillo, Sonora México