Francisella sp. infection in tilapia fish

This case report describes an outbreak of Francisella sp. infection in a group of roughly one-year old hybrid tilapia, Oreochromis aureus x O. niloticus. Presenting complaints from the producer included red “sores” on the bodies of the fish, lethargy, and mortalities. The cumulative number of mortalities during the two to three weeks prior to presentation was estimated at 200 fish. Initial examination disclosed external and internal parasitism; focal hemorrhage on the fish’s bodies; pale, brown-tinged gills; necrotic/”ragged” fins; enlarged, pale, and nodular spleen and kidneys; enlarged gall bladders; and pale, mottled livers with petechiae. Further diagnostics revealed sections from spleen and kidney to be strongly positive for Francisella sp. Additional diagnostic investigations were negative for secondary causes of mortality. Formalin treatment for external parasitism was administered and water quality was improved through increasing the flow through volume in the producer’s system. Mortalities decreased in response to these treatments