Studies were carried out to examine the biology, pathogenicity and treatment of Vibrio spp. in cultured shrimp (Penaeus monodon Fabricus). The phenotypic characteristics and the sensitivity to Oxytetracycline (OTC) of bacteria Isolated from diseased shrimp in two areas of Thailand were compared. The areas were Samutsakhon province where shrimp have been cultured for many years and Chanthaburi province, an area in which shrimp culture started after 1988. Thirty-one Vibrio spp. isolates wore obtained from Samutsakhon and hwenty-two Vibrio spp. and one Aeromonas spp. were obtained from Chanthaburi. The most numerous species from both areas was Vibrio parahaemolyticus. Minimal Inhibitory Concentration and Minimal Bacteriocidal Concentration studies were conducted to determine the sensitivity of the isolates to OTC. The isolates from Samutsakhon were found to be more resistant to OTC than those from Chanthaburi.
Batches of shrimp were fed with pellets containing different quantities of OTC and their muscle was subsequently analysed to determine its OTC content. In shrimp fed with pellets containing 3 or 5 gOTC/kg of feed, for up to 5 days, the maximum drug in the muscle was approximately 1 ppm. As the OTC was administered for longer periods the concentration and persistence in the muscle increased.
A study was conducted to examine the interaction between prophylactic administration of in fed OTC and an Injection challenge with a pathogenic OTC sensitive Vibrio parahaemolyticus. Histological examination revealed that the prophylactically treated shrimp produced a less marked cellular response and healed more rapidly when injected with bacteria.
The implication of all these results for the treatment of vibriosis in cultured shrimp is discussed