Ichthyophthirius multifiliis Fouquet (Ich) and Streptococcus iniae are 2 major pathogens of cultured Nile tilapia Oreochromis niloticus (L). Currently there is no information available for the effect of coinfection by Ich and S. iniae on fish. The objective of this study was to determine the effects of parasite load and Ich development size on fish mortality following S. iniae infection. Low mortality (< or =20%) was observed in tilapia exposed to Ich or S. iniae alone. Mortalities increased from 38% in tilapia exposed to Ich at 10,000 theronts fish(-1) to 88% in fish at 20,000 theronts fish(-1) following S. iniae exposure. The median days to death were significantly fewer (7 d) in fish exposed to Ich at 20,000 theronts fish(-1) than fish exposed to 10,000 theronts fish(-1) (10 d). A positive correlation (correlation coefficient = 0.83) was noted between tilapia mortality and size of Ich trophonts at the time of S. iniae challenge. Fish parasitized with well-developed trophonts (Day 4, 2 x 10(7) microm3 in volume) suffered higher mortality (47.5%) than fish (10.0%) infested by young trophonts (Hour 4, 1.3 x 10(4) microm3 in volume) after S. iniae challenge. The results of this study demonstrated that both parasite load and trophont size increased susceptibility and mortality of tilapia to S. iniae infection.
Calcein marking and cohabitation challenges have not been investigated in fish parasite research. This study evaluated a cohabitation challenge method in immunization trials against Ichthyophthirius multifiliis (Ich) using calcein, a fluorescent dye, to mark channel catfish Ictalurus punctatus (Rafinesque). Fish were marked by calcein immersion at 0, 500, and 1500 mg l(-1), and then challenged with 15 000 theronts fish(-1). No difference was noted in fish infection levels, mortality, and mean days to death (MDD) caused by Ich between unmarked and marked fish or between fish marked with high (1500 mg l(-1)) and low (500 mg l(-1)) concentrations of calcein. After ensuring that calcein marking had no effect on the susceptibility of fish to Ich theronts, 2 immunization trials were conducted to evaluate the cohabitation challenge model using calcein-marked catfish. Fish mortality, relative percent survival (RPS), and MDD were compared between cohabitation-challenged fish and fish challenged by non-cohabitation. No significant difference was observed in RPS for cohabitation-challenged fish and fish challenged by non-cohabitation. A cohabitation challenge can be used as an alternative challenge method in parasite studies, since it closely mimics natural exposure.
A specific and rapid PCR detection method for Flavobacterium columnare based on the 16S-23S rDNA intergenic spacer region (ISR) of the ribosomal RNA operon has been developed. The ISR of 30 F. columnare strains and other Flavobacterium species was amplified using universal primers and sequenced. Once F. columnare specific sequences within the ISR were recognized, specific PCR primers were designed against them (FCISRFL and FCISRR1). The primers were sensitive and able to detect as low as 7 colony forming units from pure culture by PCR. The new PCR detection method was applied to experimentally infected channel catfish. Two different experiments in which channel catfish fingerlings were infected by intramuscular injection or by immersion bath showed the advantage of the PCR method over standard culture techniques. F. columnare was detected by PCR in both tank water and catfish tissue samples with a higher frequency and in less time than standard microbiological methods. Furthermore, PCR detection confirmed that F. columnare can be transmitted horizontally indirectly through the water column without fish-to-fish contact. The newly developed PCR detection method for F. columnare was more sensitive and rapid than standard culture on bacteriological media for detection of F. columnare in channel catfish tissues and in tank water.
