Diagnostic tools for rapid detection and quantification of Weissella ceti NC36 infections in rainbow trout.

Weissellosis of rainbow trout is caused by the Gram-positive bacteria Weissella ceti and has been reported in China, Brazil and the United States. This disease can result in high mortality in market-sized fish and thus can cause significant economic loss. Thus far, phenotypic characterization and 16S rRNA sequencing have been used to confirm a Weissellosis diagnosis. Here, we present the development of PCR-based diagnostic tools for the rapid identification and quantification of W. ceti within bacteriological culture and infected tissues. A duplex PCR, which amplifies both genus- and strain-specific targets, positively identifies isolates as W. ceti NC36. A qPCR assay was also developed to quantify pathogen load from infected tissues, using a W. ceti NC36 unique locus. A proof of concept study was performed to demonstrate that quantification using traditional plate count methods and qPCR were significantly correlated when assessed from infected brain and spleen tissue. These tools were also used to confirm diagnosis of Weissellosis in a commercial rainbow trout farm during an outbreak investigation. These are the first diagnostic tools developed for identification and quantification of W. ceti infection within rainbow trout, contributing to rapid Weissellosis diagnosis, enhanced pathogen surveillance and epidemiological studies.

Thyroid hormones in brown trout (Salmo trutta) reproduction and early development.

Gravid brown trout (Salmo trutta) females were injected with various doses of a synthetic gonadotropin-releasing hormone analog (GnRHa), given with or without an injection of triiodothyronine (T3), in order to investigate the potential of T3 (a) to enhance the stimulatory effect of GnRHa on ovulation, and (b) to enhance the growth and survival of the produced progeny. From the time the hormonal treatments were initiated until ovulation was detected 5-38 days later, endogenous plasma T3 levels increased from an average of 3.6 to 11.6 ng ml(-1). Injection with 20 mg T3 kg(-1) body weight, further elevated plasma T3 levels at ovulation (16.0 ng ml(-1). Mean time to ovulation was reduced significantly in fish injected with 10 µg kg(-1) of GnRHa, whereas treatment with lower doses was ineffective. Injection with T3 did not enhance the ovulatory response of brown trout to GnRHa. Unfertilized eggs obtained from T3-injected females had a higher T3 content, suggesting a transfer of T3 from the maternal circulation into the oocytes. Maternal T3 injection had no effect on egg fertilization rates, embryo survival to eyeing and hatching, or the prevalence of abnormal larvae at the time of hatching. Length and weight gain of the progeny during yolk absorption was also not influenced by maternal T3 treatment. At the completion of yolk-sac absorption, progeny from females injected with T3 had a higher prevalence of skeletal abnormalities than controls. The results suggest that in teleosts like brown trout, which have high endogenous circulating T3 levels, treatment of females with T3 does not enhance responsiveness to GnRHa and it has the potential for deleterious effects on their offspring.