<i>Streptococcus iniae</i> challenge induces minimal acute histopathological changes in Nile tilapia (<i>Oreochromis niloticus</i>) selectively bred for disease resistance

Streptococcus iniae is a causative agent of streptococcal disease that negatively impacts tilapia ( Oreochromis spp.) production worldwide. Previously, our collaborative research team demonstrated substantial genetic variation for resistance to S. iniae and confirmed the ability to improve resistance through selective breeding. Although positive gains in resistance have been achieved, the underlying mechanism(s) involved in the observed resistance are unknown. To address this knowledge gap, the objective of this work was to describe differences in mortality and pathology between S. iniae resistant and susceptible Nile tilapia. Marker assisted selection was used to produce tilapia from resistant and susceptible parents. The tilapia were challenged with S. iniae and organ samples were collected from challenged individuals for histology prior to and 24 h post-challenge. The resistance phenotype was confirmed as demonstrated by 100% survival in the resistant tilapia and only 10% survival in the susceptible tilapia following S. iniae challenge. Lesions observed in the anterior kidney, spleen, and brain of susceptible tilapia were consistent with classic S. iniae infection including histiocytic meningitis, splenitis, nephritis, and the presence of gram-positive cocci. The severity of these lesions was greatly reduced in the resistant tilapia and no S. iniae were observed in the tissues. Changes described through histological assessment of S. iniae challenged tilapia provide a foundation for subsequent research to elucidate the mechanisms governing the improved survival and reduced lesion formation in the resistant tilapia.