◾  Several species have been recorded as a causative agent of streptococcal infection including Streptococcus iniae (first isolation from subcutaneous abscess in Amazon freshwater dolphin in 1970s), Streptococcus agalactiae (Ia, Ib and III), Streptococcus dysagalactiae, Streptococcus ictaluri, Streptococcus uberis, Streptococcus parauberis, and Streptococcus phocae, are considered considerable re-emerging pathogens of farmed and wild fishes (Van Doan et al., 2022).

◾  Genus Streptococcus belongs to the order Lactobacillales (lactic acid bacteria) that are Gram-positive, spherical or ovoid arranged in long or short chain, non-spore-forming, non-motile and facultative anaerobic bacteria.

◾  According to the ability to hemolyze blood, Streptococcus has been classified into three groups: α-hemolytic; β-hemolytic, and γ-hemolytic (Foo et al., 1985).

◾  Streptococcosis is a complex multifactorial disease, depending on species, age, immune status, type of pathogen (species and strain), and environmental conditions. Many unfavorable environmental aspects including high water temperature (more than 28°C), low dissolved oxygen, high pH, ammonia, and high organic content, agricultural wastewater, and external parasitic infestation enhance the severity and incidence of infection.