Fecal Strings Associated with Streptococcus agalactiae Infection in Nile Tilapia, Oreochromis niloticus

This study provides the first description of long fecal strings in Streptococcus agalactiae-infected fish. Nile ti- lapia (Oreochromis niloticus) were administered an intraperitoneal injection with 0.1 mL solution containing a 10-fold di- lution from 10 1 to 10 6 cfu S. agalactiae/fish. While infected fish developed clinical signs commonly associated with S. agalactiae infection, up to 40% of infected fish in each group also produced considerably longer (up to 20 cm in length) fecal waste strings than unchallenged tilapia. Fecal strings from these infected fish were observed trailing from the anus and often became increasingly opaque and white over time. Though S. agalactiae was not isolated from the fecal strings themselves, all S. agalactiae-challenged fish sampled were culture-positive. Histologic examination of the intestines of infected fish exhibited the presence of sloughed intestinal mucous cells and enterocytes in the feces and the absence of normal fecal matter. The presence of long brown and/or white, opaque fecal strings may suggest a clinical sign of bacte- rial disease and the need for further clinical examination. Streptococcus agalactiae affects numerous wild and cul- tured fish species worldwide and causes disease involving septicemia and colonization of numerous organs (1-4). Clini- cal signs of S. agalactiae infection include lethargy or excit- ability, going off feed, 'C'-shaped body posturing, erratic swimming and whirling, opercular clearing, spinal curvature, and death (2, 3, 5). Nile tilapia (Oreochromis niloticus) are susceptible to S. agalactiae infection (3-5). In a variety of previous S. agalactiae challenge studies at the Aquatic Ani- mal Health Research Laboratory, Nile tilapia (n > 500; weight ranging from 3 to 100 g) were injected intraperito- neally with doses ranging from 10 1 to 10 9 colony-forming units (cfu) S. agalactiae/fish and maintained at approxi- mately 30˚C. Though these studies were not intended to ex- clusively examine S. agalactiae-related clinical signs, it was nonetheless noted that numerous challenged fish produced long fecal strings. Since fecal strings have not been previ- ously associated with S. agalactiae infection, a study was performed to examine the presence of fecal strings after ex- perimental S. agalactiae challenge. Nile tilapia (O. niloticus) with a mean weight of 13.9 ± 0.6 g were housed at the USDA/ARS Aquatic Animal Health Laboratory in Chestertown, Maryland, USA. The fish were kept in 57 L glass aquaria supplied with flow-through dechlorinated tap water, maintained at a mean temperature of 30.5 ± 0.6oC, mean dissolved oxygen of 5.3 ± 0.9 mg/L, and mean ammonia concentration of 0.2 ± 0.8 mg/L. The fish were also fed daily to satiation with Aquamax feed (Bren- twood, Missouri, USA) and maintained on a 12 h : 12 h light :