Development of skeletal deformities in a Streptococcus agalactiae-challenged male Nile tilapia (Oreochromis niloticus) broodfish and in its offsping

Authors: Pasnik, David; Evans, Joyce; Klesius, Phillip

Submitted to: European Association of Fish Pathologists
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2007
Publication Date: 1/18/2008
Citation: Pasnik, D.J., Evans, J.J., Klesius, P.H. 2007. Development of skeletal deformities in a Streptococcus agalactiae-challenged male Nile tilapia (Oreochromis niloticus) broodfish and in its offsping. European Association of Fish Pathologists. 27(5) 2007, 169-176.

Interpretive Summary: Skeletal abnormalities in fish have several causes, including nutritional deficiencies, parasitic diseases, genetic factors, physical and chemical environmental disturbances, toxicoses, and even bacterial diseases. Other authors have described vertebral deformities among Streptococcus iniae-infected fish, but this is the first case report description of vertebral and other skeletal abnormalities in fish infected with S. agalactiae. More specifically, this paper describes skeletal changes in a male Nile tilapia, Oreochromis niloticus, broodfish exposed to S. agalactiae and in its offspring. After injection challenge with Streptococcus agalactiae, a male Nile tilapia, Oreochromis niloticus, exhibited an erratic corkscrew swimming pattern and assumed a “C”-shaped body posture. The fish did not die after challenge but developed a grossly-observable “hunchback” at the level of the cervical spine. Radiographs revealed multiple spinal curvatures along the length of the spinal column. The male fish also developed a cranial cavitation on the dorsal aspect of the head, forming a depression between the eyes. The fish subsequently mated and helped produce fry that initially appeared normal. The fry population soon experienced high mortalities, and sick fish exhibited reddened gills and spun on their longitudinal axis, often with their head pointed down (tail-up swimming). Surviving fry had stunted or absent fins, deformed gill flaps, and/or cranial cavitation, but did not show vertebral deformities. Gram-positive bacteria identified as S. agalactiae were isolated from the fry but not the male broodfish. Streptococcus agalactiae appears to be a causative factor of these skeletal anomalies among the male broodfish and the fry, and this is the first case report description of skeletal abnormalities in S. agalactiae-infected fish.

Technical Abstract: After injection challenge with Streptococcus agalactiae, a male Nile tilapia, Oreochromis niloticus, exhibited an erratic corkscrew swimming pattern and assumed a “C”-shaped body posture. The fish did not die after challenge but developed a grossly-observable “hunchback” at the level of the cervical spine. Radiographs revealed multiple spinal curvatures along the length of the spinal column with vertebral lordosis and kyphosis. The male fish also developed a cranial cavitation on the dorsal aspect of the head, forming a depression between the eyes. The fish subsequently mated and helped produce fry that initially appeared normal. The fry population soon experienced high mortalities, and moribund fish exhibited reddened gills, presumptively over-inflated bladders, and spun on their longitudinal axis, often with their head pointed down (tail-up swimming). Surviving fry had stunted or absent fins, deformed gill opercula, and/or cranial cavitation, but did not show vertebral deformities. Gram-positive bacteria identified as S. agalactiae were isolated from the fry but not the male broodfish. Streptococcus agalactiae appears to be a causative factor of these skeletal anomalies among the male broodfish and the fry, and this is the first case report description of skeletal abnormalities in S. agalactiae-infected fish.