ARS | Publication request: Specific serum antibody responses in channel catfish (Ictalurus punctatus) provide limited protection against Streptococcus ictaluri challenge

Submitted to: Veterinary Immunology and Immunopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 24, 2010
Publication Date: October 21, 2011
Repository URL: http://handle.nal.usda.gov/10113/55493
Citation: Pasnik, D.J., Evans, J.J., Klesius, P.H. 2011. Specific serum antibody responses in channel catfish (Ictalurus punctatus) provide limited protection against Streptococcus ictaluri challenge. Veterinary Immunology and Immunopathology. 144:144-146.

Interpretive Summary: Passive immunization has been shown to provide a spectrum of protection against certain piscine pathogens, and studies were conducted to determine the role of specific antibodies in immunity to Streptococcus ictaluri. Adult Nile tilapia (Oreochromis niloticus) were injected i.p. with tryptic soy broth as control or with 1.5 x 107 colony-forming units (cfu) S. ictaluri/fish, and blood was drawn for serum collection from all fish 90 d after challenge. Fish were passively immunized by i.p. injection with 100 µL of serum from the tryptic soy broth control group (TSB), anti-S. ictaluri serum from 90 d S. ictaluri immunized fish (SSI), or heat-inactivated anti-S. ictaluri serum from 90 d S. ictaluri immunized fish (HISSI). These passively immunized fish were subsequently challenged 72 h later with 1.5 x 108 cfu S. ictaluri/fish. Mortalities occurred between 3 and 19 days post-challenge in all the groups, and the percent survival was 43.3 (TSB), 63.3 (SSI), and 50.0 (HISSI). A significant difference in cumulative percent survival was noted between the TSB and the HISSI groups, and significant differences were noted between these groups and the SSI group. Serum obtained from fish 72 h after passive immunization exhibited increased anti-S. ictaluri OD (optical density) levels, ranging from 0.082 OD (SSI) to 0.088 (HISSI), and were not significantly different. However, the OD levels were significantly higher than that of the control group. Twenty-one days after the challenge, the control anti-S. ictaluri OD levels had increased significantly. The HISSI (0.128 OD) and SSI (0.136 OD) groups also significantly increased above their corresponding pre-challenge OD levels. No significant correlation between increased pre-challenge specific serum antibody levels and survival after challenge was demonstrated when analyzing the control and passive immunization groups. This study indicates that both specific anti-S. ictaluri antibodies and non-specific immune responses are important for protection against S. ictaluri.

Technical Abstract: Passive immunization has been shown to provide a spectrum of protection against certain piscine pathogens, and studies were conducted to determine the role of specific antibodies in immunity to Streptococcus ictaluri. Adult Nile tilapia (Oreochromis niloticus) were injected i.p. with tryptic soy broth as control or with 1.5 x 107 colony-forming units (cfu) S. ictaluri/fish, and blood was drawn for serum collection from all fish 90 d after challenge. Fish were passively immunized by i.p. injection with 100 µL of serum from the tryptic soy broth control group (TSB), anti-S. ictaluri serum from 90 d S. ictaluri immunized fish (SSI), or heat-inactivated anti-S. ictaluri serum from 90 d S. ictaluri immunized fish (HISSI). These passively immunized fish were subsequently challenged 72 h later with 1.5 x 108 cfu S. ictaluri/fish. Mortalities occurred between 3 and 19 days post-challenge in all the groups, and the mean cumulative percent survival was 43.3 ± 3.3 (TSB), 63.3 ± 3.3 (SSI), and 50.0 ± 6.7 (HISSI). A significant difference in cumulative percent survival was noted between the TSB and the HISSI groups (P < 0.0238), and significant differences were noted between these groups and the SSI group (P < 0.0423). Serum obtained from fish 72 h after passive immunization exhibited increased anti-S. ictaluri levels, ranging from 0.082 ± 0.021 OD (optical density; SSI) to 0.088 ± 0.015 OD (HISSI), and were not significantly different. However, the OD levels were significantly higher (P < 0.0001) than that of the control group (TSB: 0.027 ± 0.011 OD). Twenty-one days after the challenge, the control anti-S. ictaluri OD levels (TSB: 0.069 ± 0.009 OD) had increased significantly (P < 0.0001). The HISSI (0.128 ± 0.013 OD) and SSI (0.136 ± 0.019 OD) groups also significantly increased (P < 0.0001) above their corresponding pre-challenge OD levels. No significant (r2 = 0.1283; P < 0.6754) correlation between increased pre-challenge specific serum antibody levels and survival after challenge was demonstrated when analyzing the control and passive immunization groups. This study indicates that both specific anti-S. ictaluri antibodies and non-specific immune responses are important for protection against S. ictaluri.