In Mexico, industrial-level aquaculture is based on shrimp and tilapia, with an annual growth rate of 17%. Mexico is the fourth largest aquaculture producer in Latin America being Gray GIFT tilapia, the most extended culture. Tilapia aquaculture is actually practiced in the whole 32 states of Mexico as government expects that the activity to contributing to food security. Aquaculture in the states that conform the Yucatan Peninsula relies heavily on tilapia.
The Aquaculture and Aquatic Health Research Laboratory initiated a survey to detect streptococcosis and francisellosis in tilapia farms of Campeche. The state of Campeche had, at some point, about 96 active tilapia farms. Still, the Covid16 pandemic provoked the closedown of most of them, prevailing with only some semi-intensive to intensive production. Therefore, our study sampled only 8 farms, the few still producing. We randomly collected about 240 animals following the sampling scheme recommended by OIE surveillance standards, which assumes a 2% diseased population where analysis includes molecular methods accuracy. We also performed routine bacteriology and demonstrated the infection cycle by fulfilling Koch´s postulates.
Five out of 8 farms produced positive results for streptococcosis, which grew optimally as small white translucent colonies in TSA agar from kidney loop samples (8 isolates). Some adult fish specimens displayed mild exophthalmia, ventral subdermal hemorrhage, and readily visible growth underneath the peduncle skin. When dissected, these grows were also revealed as a widespread confined liquefactive myonecrosis involving the axial musculature. Abnormal behavior was not noted. Staining revealed Gram+ coccoid bacteria, and the isolate was identified as Streptococcus agalactiae by PCR and qPCR. Koch´s postulates were confirmed by experimentally infecting juvenile Oreochromis niloticus tilapia (about 80 g) through abdominal injection (1x106 CFU/mL). The experiment consisted of one control group (SS 0.85% injection) and two infected groups by triplicate. At the end of the experiment, mortality was highly significant (α:0.001) in the infected groups compared with control.
Recently, farmers from Yucatán (bordering state with Campeche) reached out the laboratory concerned by the display of similar signs in their culture. The extent to which Streptococcus agalactiae is widespread in the Yucatan Peninsula is currently unknown, although this study reveals that this bacterium is already present in this region of Mexico