ARS | Publication request: INFLUENCE OF NATURAL TRICHODINA SP.PARASITISM ON EXPERIMENTAL STREPTOCOCCUS INIAE OR Streptococcus AGALACTIAE INFECTION AND SURVIVAL OF YOUNG CHANNEL CATFISH ICTALURUS PUNCTATUS (RAFINESQUE)

Submitted to: Aquaculture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 15, 2007
Publication Date: June 1, 2007
Citation: Evans, J.J., Klesius, P.H., Pasnik, D.J., Shoemaker, C.A. 2007. Influence of natural trichodina sp. parasitism on experimental streptococcus iniae or streptococcus agalactiae infection and survival of young channel catfish ictalurus punctatus (rafinesque). Aquaculture Research. Volume 38, Issue 6:664-667.

Interpretive Summary: Although Trichodina spp., Streptococcus iniae, and S. agalactiae are individually major causes of morbidity and mortality in fish, the interactions between concurrent infections with this parasite and each of these two streptococcal species are unknown. External Trichodina spp. parasitism can be responsible for mortalities among heavily-parasitized fish and in young fish. Streptococcus iniae has been reported to cause morbidity and mortality in hybrid striped bass, Morone chrysops x M. saxatilis, and in tilapia, Oreochromis spp., but not in channel catfish, Ictalurus punctatus. Streptococcus agalactiae caused morbidity and mortality in Klunzingeri mullet, Liza klunzingeri (Day), seabream, Sparaus auratus L., and Nile tilapia, O. niloticus. However, Streptococcus iniae and S. agalactiae are not usual pathogens of channel catfish. Synergism between the external parasites and either S. iniae or S. agalactiae infection may reduce survival of channel catfish fry. Therefore, we conducted a concurrent infectivity study to determine whether S. iniae or S. agalactiae may decrease survival in non-parasitized or Trichodina spp.-parasitized channel catfish fry. Non-parasitized or naturally-parasitized channel catfish fry were exposed to either S. iniae or S. agalactiae to determine whether the streptococcal organisms could cause decreased survival in non-parasitized or concurrently-parasitized catfish. Fish were observed for behavior changes, morbidity, and mortality for 7 d after challenge. Cumulative percent survival was high (80 to 100%) among non-parasitized catfish but lower (0 to 73%) among parasitized fish. These results of the present study indicate that both S. iniae and S. agalactiae alone are not usual pathogens of channel catfish fry. However, these streptococcal organisms can cause decreased survival in young channel catfish externally parasitized with Trichodina spp.

Technical Abstract: Streptococcus iniae and S. agalactiae are usually not considered pathogens of channel catfish, Ictalurus punctatus, though concurrent infections may decrease catfish survival when infected with streptococcal organisms. Non-parasitized or naturally-parasitized channel catfish fry were challenged with either S. iniae or S. agalactiae to determine whether the streptococcal organisms could cause decreased survival in non-parasitized or concurrently-parasitized catfish. Fish were immersion-challenged with 1 ´ 109, 108, 107, 106, or 105 CFU S. iniae or S. agalactiae/mL in 50 mL water baths and observed for behavior changes, morbidity, and mortality for 7 d after challenge. Cumulative percent survival was high (80 to 100%) among non-parasitized catfish, irrespective of challenge species or dose. Mortalities among parasitized fish began within 1 d after streptococcal challenge and most occurred within 3 d after challenge. The LD50 values in these Trichodina spp.-parasitized fish 7 d after streptococcal challenge were 1.10 x 107 CFU/mL for S. iniae and 3.80 x 107 CFU/mL for S. agalactiae. These results of the present study indicate that both S. iniae and S. agalactiae alone are not usual pathogens of channel catfish fry. However, these streptococcal organisms can cause decreased survival in young channel catfish externally parasitized with Trichodina spp.