In southwest Mexico, mortality of farmed tilapia O. niloticus is recurrent. In 2018, 2019, 2020, and 2022, samples of internal organs and lesions of farmed tilapia were taken to detect the most crucial fish pathogens. First, real-time PCR protocols were standardized to obtain a reliable qualitative detection of the selected bacteria using fixed tissues. 599 farmed tilapias in rural ponds and cages from Guerrero, Oaxaca, and Chiapas Pacific belonging to southwest Mexico were analyzed. This Mexican region is characterized by tropical weather, where, in general, the water temperature of the cultured tilapia was optimal for the growth of Nile tilapia. Still, sometimes extreme temperature values were registered. Most of the sampled tilapia showed a normal appearance, but some displayed hemorrhage areas, desquamation, bulging at the base of the caudal fin, exophthalmia, etc. No Francisella sp. was detected in any sample, Staphylococcus sp. was the bacterial genus more prevalent over time (from 0 to 64%) (figure 1), the low prevalence was found in Aeromonas sp. (4.3%), and Streptococcus iniae had 1.4% prevalence in Chiapas (2019).
Streptococcus agalactiae, the most dangerous bacterial pathogen for tilapia, was detected at high prevalence (0 to 59 %) in the three Mexican states. This is the first report of bacterial pathogens in farmed tilapia in Mexico using the real-time PCR assay. Detection of S. agalactiae in high prevalence constitutes a significant health risk for tilapia aquaculture in Mexico and a potential spread of these pathogens to other aquaculture areas.