Streptococcosis on a red tilapia, Oreochromis sp., farm: a case study E Hernµndez, J Figueroa and C Iregui Department of Science for Animal Health, Veterinary Pathobiology Research Group, Universidad Nacional de Colombia, Bogota ´, Colombia Abstract A commercial red tilapia farm was diagnosed with Streptococcus agalactiae infection using histopathol- ogy, microbiology and immunohistochemistry. One hundred fish were randomly taken from different weight/age groups including broodstock, market fish (larger than 150 g), on-growing fish between 20 and 150 g, juveniles and larvae. Fish were clinically examined, anaesthetised and necropsied. Samples were taken from brain, liver, spleen, eyes and kidney for microbiology. All organs were processed for histopathology and an indirect immunoperoxidase test (IIP). Organs from wild fish and birds found in close proximity to the farm were also sampled for microbiology and IIP. The prevalence of lesions or infection found by IIP, histopathology and micro- biology was 16%, 29% and 7% respectively. Clin- ical disease, lesions or infection were not seen in larvae or juveniles. By contrast, infection and disease were found in fish larger than 20 g, suggesting that the condition was linked to the intensive culture conditions of broodstock, on-growing and market fish. S. agalactiae was not found in wild fish, or in birds, by microbiology and IIP. Keywords: culture, diagnosis immunoperoxidase, histopathology, Oreochromis sp., Streptococccus aga- lactiae. Introduction Disease caused by streptococci has become a major problem in cultured freshwater and saltwater fish species worldwide. Streptococcus and Enterococcus spp. are commonly associated with mortality, whereas Lactococcus and Vagococcus spp. appear to be less pathogenic (Nieto, Devesa, Quiroga & Toranzo 1995). Initially reported in Dolphin iniae by Hoshina, Sano & Morimoto (1958), S. iniae and other species of streptococci have been described in numerous outbreaks of disease in several fish species including tilapia. Amongst the streptococci, S. agalactiae has a broad host range, infecting both terrestrial and aquatic animals. This bacterium causes neonatal meningitis and mastitis in humans and cattle, respectively. However, other animals such as mice, cats, dogs, hamsters, camels and frogs can also be infected (Evans, Klesius, Gilbert, Shoemaker, Al-Sarawi, Landsberg, Durem- dez, Al-Marzouk & Al-Zenki 2002). Streptococcus agalactiae has been isolated from numerous fish species in natural outbreaks of disease and has been shown to be pathogenic to several fish species in experimental trials using different routes of infec- tion such as cohabitation, immersion, intraperito- neal and intra-muscular injections (Evans et al. 2002). The pathogenesis of the disease remains unknown. In the present study we used microbio- logical, histopathological and immunohistochemi- cal tools in an attempt to understand the source and spread of disease induced by S. agalactiae in a red tilapia farm naturally infected by this bacterium. Materials and methods Study farm The red tilapia farm selected for this study contained all life stages of fish, including brood- stock, market fish (larger than 150 g), on-growing Journal of Fish Diseases 2009, 32, 247–252 doi:10.1111/j.1365-2761.2008.00981.x Correspondence C Iregui, Laboratorio de Patologı ´a Veterinaria, Faculty Medicina Veterinaria y Zootecnia, Universidad Nacional de Colombia, Bogota ´ , Colombia (e-mail: caireguic@unal.edu.co) 247 Journal compilation Ó 2009 Blackwell Publishing Ltd. No claim to original US government works.