Brucella infection in fresh water fish: Evidence for natural infection of Nile catfish, Clarias gariepinus, with Brucella melitensis

Brucellosis is endemic among ruminants in the Nile Delta region of Egypt, where recent reports suggest that the incidence of human infection is increasing. In this region the practice of throwing animal waste into Nile canals is common. As a result, water can be contaminated with potential zoonotic pathogens such as B. melitensis that could infect fish. This study aimed at isolating and characterizing B. melitensis from Nile catfish. Serum samples from 120 catfish captured from Nile canals and 120 farmed catfish were tested for the presence of antibodies against Brucella spp. by using the Rose Bengal Test (RBT) and the Rivanol test (Riv T). Skin swabs from all fish and samples from internal organs (liver, kidney and spleen) from all serologically positive fish were cultured to identify B. melitensis biovar 3 isolates. Polymerase Chain Reaction (PCR) was used to confirm the results. 9.2% and 8.3% of serum samples from Nile catfish were positive by RBT and Riv T, respectively. None of the samples from farmed catfish were seropositive. B. melitensis biovar 3 was isolated from 5.8%, 4.2%, 5.8% and 13.3% of liver, kidney and spleen samples and skin swabs, respectively. To our knowledge this is the first report of isolation of B. melitensis biovar 3 from fresh water fish. Our results suggest that Nile catfish are naturally infected with B. melitensis biovar 3 and this may play a role in the epidemiology of brucellosis. The public should be aware of the consequences of disposing of animal waste into the canals and public health authorities should consider the potential role of catfish as a source of infection.