BACTERIAL PATHOGENIC DIAGNOSIS OF TILAPIA Oreochormis niloticus IN THE EL GALLO DAM, MEXICO

El Gallo dam in the state of Guerrero Mexico, is a water body utilised by small-scale rural tilapia producers to grow out fingerlings in floating cages. Over the last 2 years outbreaks of disease with moderate to high levels of morbidity and mortality have been affecting the fish from post deployment (3g) to near harvest (700g) size causing severe adverse effects to the local economy. The clinical signs observed during the outbreaks include erratic swimming, darkened or pale colour, unilateral or bilateral exophthalmia, skin haemorrhages skin ulcers, frayed fins and hyporexia. Local producers report noteworthy reduction in mortalities when treating fish with broad-spectrum antibiotics however clinical signs and mortalities re-appear within the first work post treatment. The present study aimed to isolate and characterise potential bacterial pathogens presented in ceololomic and cranial organs of moribund fish at El Gallo Lake. In October 2017 six moribund fish were euthanised, necropsied and the kidney, spleen, brain, liver and heart were aseptically collected for bacteriological analyses. Subsamples of the tissues showing gross pathological lesions were swabbed onto selective and general media and incubated at 27 C for 3 days. Dominant colonies were picked and purified before being tested using laboratory diagnostic methods. From this 10 isolates were chosen for testing by 16S rRNA PCR and sequencing analysis to identify the bacterial species. This allowed identification of Streptococcus agalactiae, S. iniae, and bacteria belonging to the genera Vibrio and Aeromomonas. The later were further characterised by gyrB sequence similarity and phylogeny and confirmed as V. cholerae and A. veroni while the S. agalactiae isolates were typed according to their capsule into 3 different biotypes (Table 1). Based on the results here obtained, it appears that Streptoccosis and Motile Aeromonas Septicaemia (MAS) are the main diseases causing mortalities in the lake however further samplings i.e. surveillance programmes with continues monitoring should be established to follow-up our findings and to alert the presence of novel emergent pathogens.