Streptococcosis, Lactococcosis and Enterococcosis are potential threats facing cultured Nile tilapia (Oreochomis niloticus) production

Streptococcosis, lactococcosis and enterococcosis are among the most important bacterial diseases affecting tilapia farms in Kafr Elsheikh governorate, Egypt. A number of clinically diseased fish were collected and submitted to our laboratory during disease outbreak in 2018. They were characterized by nervous swimming behaviour, skin darkness, exophthalmia, ocular opacity and haemorrhages. Necropsy findings were splenomegaly, congestive hepatomegaly, liquefied brain and enteritis. The phenotypic and molecular characterizations of the bacterial strains isolated from naturally infected fish identified three genera of Gram-positive cocci:Streptococcus agalactiae,Enterococcus faecalisandLactococcus garvieae. Infectivity trials were conducted in four groups of Nile tilapia inoculated withS. agalactiaeEgy-1,E. faecalisEgy-1 andL. garvieaeEgy-1 strains and saline. Mortalities, clinical signs and pathological findings were recorded daily 14 days post infection. Experimentally infected tilapia showed similar clinical signs, postmortem lesions, but varied in the severity and experienced high mortalities up to 70% in case ofS. agalactiaeandL. garvieaeinfections and 30% in case ofE. faecalisinfection. Pathological examination of infected tissue sections stained with modified Brown-Brenn and immunohistochemistry revealed an important direct correlation between the distribution of each bacterial isolate and the lesions developed in different organs. Furthermore, the isolates were subjected to profiling against 11 antibiotics, and they showed resistance against several types of antibiotics, which implicate potential risk to human health and emphasize the urgent need for alternate bio-control strategies to prevent the diseases and the problem of multidrug resistance in aquatic environment.