DAO 89:117-123 (2010)  -  DOI: https://doi.org/10.3354/dao02192

ABSTRACT: Streptococcus iniae poses a serious threat to finfish aquaculture operations worldwide. Stringent regulatory standards limit the use of antibiotics to treat S. iniae infections; improved vaccination strategies are thus of great interest. We investigated the potential for efficient, non-injectable batch vaccination via the use of live attenuated vaccines. Three attenuated S. iniae strains with genetic mutations eliminating the production of virulence factors—capsular polysaccharide (ΔcpsD), M-like protein (ΔsimA), and phosphoglucomutase (Δ pgmA)—were evaluated in parallel with an adjuvanted, formalin-killed, whole-cell S. iniae bacterin. Juvenile hybrid striped bass (HSB; Morone chrysops × M. saxatilis) were vaccinated through intraperitoneal (i.p.) injection or bath immersion and held for 800 degree-days prior to challenge with a lethal dose of the virulent wild-type (WT) S. iniae parent strain. The ΔcpsD, Δ pgmA, and bacterin vaccines provided the highest level of vaccination safety (0% mortality), whereas the ΔsimA mutant, although it caused 12 to 16% vaccination-related mortality, was the only vaccine candidate to provide 100% protection in both i.p. and immersion delivery models. Our studies demonstrate the efficacy of live attenuated vaccines for prevention of S. iniae infection, and identify immersion delivery of live vaccines as an attractive option for use in commercial aquaculture settings.

KEY WORDS: Streptococcus iniae · Live attenuated vaccine · Hybrid striped bass · M-like protein · Capsular polysaccharide · Phosphoglucomutase