Prevalence of pathogenic bacteria detected by qPCR from cultured Nile tilapia (Oreochromis niloticus Linnaeus, 1758) in southwest Mexico

Nile tilapia (Oreochromis niloticus) is one of the most important aquaculture species in the world. When bacteria are present in cultured tilapia but do not cause a declared disease, it makes them asymptomatic carrier organisms. Once environmental or nutritional conditions change, an outbreak may occur. This is why it is so important to detect pathogens before outbreaks occur. This is the first study that use molecular techniques based on PCR to estimate prevalence of fish pathogens in southwest Mexico. During 2018, 2019, 2020 and 2022 samples of internal organs and lesions of Nile tilapia were taken and analyzed for detection of the main bacterial tilapia pathogens using one-step PCR or qPCR. A total of 2396 samples from the internal organs of Nile tilapia pond and cage cultured come from the Mexican Pacific southwest states of Guerrero, Oaxaca and Chiapas were analyzed. Most of the sampled tilapias were apparently healthy and had no relation between the clinical signs and the pathogens detection was found. No Francisella sp. was detected in any sample, Staphylococcus sp. was the most prevalent bacterial genus from the three states over time (from 0 to 64 %). Prevalence of Aeromonas sp. was from 0 to 4.3 %, although the fish pathogen A. dhakensis was not detected. Meanwhile, S. iniae was only detected in Chiapas in 2019 at low prevalence (1.4 %) and S. agalactiae was detected in the three sites at high prevalence (from 0 to 59 %). Both Streptococcus can cause streptococcosis, the most dangerous re-emergent disease to cultured tilapia, which means a great risk for tilapia farming in Mexico.