Resumen:
This is the first study to isolate, identify and characterize Streptococcus iniae as the
causative disease agent in two tilapia (Oreochromis aureus) populations. The populations
were geographically isolated, of distinct origins, and did not share water
sources. Affected fish showed various external (e.g., exophthalmia and cachexia,
among others) and internal (e.g., granulomatous septicaemia and interstitial nephritis,
among others) signs. All internal organ samples produced pure cultures, two of
which (one from each farm, termed S-1 and S-2) were subjected to biochemical,
PCR and 16S rRNA sequencing (99.5% similarity) analyses, confirming S. iniae identification.
The two isolates presented genetic homogeneity regardless of technique
(i.e., RAPD, REP-PCR and ERIC-PCR analyses). Pathogenic potentials were assessed
through intraperitoneal injection challenges in rainbow trout (Oncorhynchus mykiss)
and zebrafish (Danio rerio). Rainbow trout mortalities were respectively 40% and
70% at 104 and 106 CFU per fish with the S-1 isolate, while 100% mortality rates
were recorded in zebrafish at 102 and 104 CFU per fish with the S-2 isolate. The
obtained data clearly indicate a relationship between intensified aquaculture activities
in Mexico and new disease appearances. Future studies should establish clinical
significances for the tilapia industry.