Background:

Shrimp farms in northwestern Mexico have had outbreaks since 2013 with losses of more than 80%. The only background is the acute hepatopancreatic necrosis diseases (AHPND) originated in Asia for a new strains of Vibrio parahaemolyticus (VP-AHPND+) that produce toxins PirA and PirB of the plasmid pVA1.

Goals:

To characterize the causative agent of epizootics in farms; to perform genomic comparisons of sequencing of the 16S rDNA region to identify the variability that occurs in the strains and evaluate the resistance developed against antibiotics in vitro.

Methods:

Estuaries, drains, and farms of southern Sonora were monitored; samples of water, sediment and shrimps were aseptically sampled. Bacterial cultures were performed in TCBS and the green colonies were streaked in the selective culture media CHROMagar Vibrio medium. Bacteria were identified by PCR-AP3 specific for the strains VP-AHPND+. Polymorphism of strains were analyzed by sequencing the 16S rDNA region and the rep-PCR (GTG)5 technique. Antibiotic resistance was examined for oxitetracicline, florfenicol, enrofloxacin, magnacin, sulfacloropiridacin-trimetroprim and enrofloxacin.

Results:

Throughout 2013-2014, 444 strains were isolated (82 from 2013, and 362 from 2014). Eighty-three were selected, from which 16 were PCR-positives. Sequences from 16S rDNA region were compared with those deposited at GenBank, and a high homology was found with sequences from VP-AHPND+ from Sinaloa with differences of those from Thailand and China, polymorphism occur mainly in 15 nucleotide sequences. Regards to antibiotic resistance tests, strains with resistance to oxytetracycline and sulfachloropyridazine-trimethoprim were found.

Conclusions:

VP-AHPND+ was identified as the causative agent of AHPND in shrimp farms from Sonora and the polymorphism among the strains was confirmed, also antibiotic resistance was found against antibiotics, regularly applied to shrimp farms.