Hematological Changes in Nile Tilapia ( Oreochromis niloticus ) Infected with Streptococcus iniae by Nare Inoculation

Tilapia has become an increasingly important food source, and production is predicted to double in the next decade (Stickney 2001). Hematology is a useful tool in monitoring the level of stress in fish (Blaxhall and Daisley 1973; Leamaster et al. 1990). This study was designed to examine the relationship between hematological variables as related to streptococcal disease of tilapia following infection with S. iniae by nare inoculation. Streptococcal disease causes major economic losses to the global aquaculture industry (Boomker et al. 1979; Perera et al. 1994, 1997; Stoffregen et al. 1996; Shoemaker and Klesius 1997). In the U.S. alone the number exceeds $10 million annually (Shoemaker and Klesius 1997). Streptococcus spp. have been shown to gain entry into fish by ingestion (Minami 1979). through injured skin (Rasheed and Plumb 1984), by cohabitation of infected fish and immersion (Robinson and Meyer 1966). Streptococcus iniae has been identified as one of the main etiologic agents of streptococcal disease in cultured tilapia Oreochromis sp. (Perera et al. 1994, 1997; Eldar et al. 1995; Shoemaker and Klesius 1997). Streptococcus iniae is a coccus-shaped, aerobic, Gram-positive bacterium (Pier and Madin 1976). Successful experimental S. iniae infection of tilapia following nare inoculation suggests the nares are a route of S. iniae infection in cultured tilapia (Evans et al. 2000, 2001). Based on this information, it is believed that nare inoculation provides for a more natural route of infection and possibly a more natural response. The objective of this study was to characterize hematological variables of juvenile and adult tilapia as they are affected by S. iniae infection by nare inoculation.