PREVALENCE OF PATHOGENIC Vibrio parahaemolyticus IN Penaeus vannamei SHRIMP FARMS OF TIRUVALLUR DISTRICT, TAMILNADU, INDIA

Vibrios are the Gram-negative, halophilic bacterium that occurs naturally in estuarine environments worldwide. Vibrio parahaemolytics are associated with diseases in fish and food borne infection in human. Disease is the major factor associated with economic and production losses in aquaculture of commercially important species including Penaeus vannamei (Pacific white shrimp). Improper and overuse of antibiotics to control bacterial diseases of shrimp has led to the emergence of antimicrobial resistance in the bacterial population in the aquaculture systems besides leaving residues in the harvested product. This study was carried out with an objective to assess the prevalence of the antimicrobial resistance gene in V.parahaemolyticus from aquaculture systems.

A total number of 228 samples were collected from the P.vannamei farms which included shrimp (88) and water (140) during the period from June 2017 to July 2018 from Tiruvallur District, Tamilnadu, India.Total vibrios were isolated from the samples on Thiosulfate Citrate Bile Sucrose (TCBS) agar. 120 isolates presumptively confirmed as V.parahaemolytics based on the culture characteristics and biochemical tests, were further confirmed by Tox R-based PCR assay. The isolates were subjected to antibiotic sensitivity assay to assess their susceptibility /resistance to ampicillin (10mcg), amoxyllin (10mcg), gentamycin (25mcg), nalidixic acid (30mcg), tetracycline (30mcg), cefotaxime (30mcg), oxytetracycline (30mcg) and kanamycin (30mcg). The results of the resistance of V. parahaemolyticus to various antibiotics tested is presented in Fig.1

Prevalence of antimicrobial resistant genes viz., trh and tdh were assessed by PCR. The PCR screening of isolates (228) from shrimp and water samples showed positive amplification of toxR gene in 52.6% (120/228) there by confirming them as V. parahaemolyticus (Fig.2). To detect pathogenic isolates, virulent genes viz., tdh (thermostable direct hemolysin) and trh (thermostable related hemolysin) were amplified by PCR. No tdh-positive V.parahaemolyticus isolates were detected among the 120 toxR-positive isolates. However, 16 (13.4%) isolates were                   trh-positive (Fig 3). In conclusion, the results indicate that (a) V.parahaemolyticus isolates from P.vannamei shrimp farms in Tiruvallur district are highly resistant to antibiotics. (b) The prevalence of virulent genes viz., toxR (52.6%) and trh (13.4%) demonstrated a high risk of pathogenic V. parahaemolyticus in the P.vannamei shrimp farms in Tiruvallur district, Tamilnadu.