Abstract
Streptococcus iniae and Gyrodactylus niloticus are two common pathogens of cultured Nile tilapia, Oreochromis niloticus . We studied concurrent infection of tilapia by G. niloticus and S. iniae and evaluated whether parasitism in tilapia with Gyrodactylus increased susceptibility and mortality following immersion infection with S. iniae . Results showed that death mainly occurred in fish with G. niloticus and challenged with S. iniae (G–S group). The accumulative mortality (42.2%) was significantly higher in the G–S group than in fish not infected by the parasite (6.7%), but exposed to S. iniae . Bacteriological examination revealed S. iniae from ≥92% of dead or moribund fish challenged with S. iniae . Gyrodactylus not only damaged fish epithelium and provided entry for invasive bacteria but also was found to harbour viable cells of S. iniae for 24 and 72 h. Streptococcus iniae was isolated from 60% and 40% of G. niloticus collected from fish infected by intraperitoneal injection or immersion, respectively, at 24 h post‐challenge. The present study confirms that parasitism of tilapia by G. niloticus increased host mortality following exposure to the bacterial pathogen S. iniae .