Characterisation of a low pathogenic form of Gyrodactylus salaris from rainbow trout

Abstract

Gyrodactylus salaris was isolated from rainbow trout in a Danish freshwater trout farm, and a laboratory population of this particular parasite form was established on rainbow trout. Challenge infections were performed using different salmonid strains and species, including East Atlantic salmon Salmo salar (from the Danish River Skjernå), Baltic salmon S. salar (from the Swedish River Ume älv) and rainbow trout Oncorhynchus mykiss (from the Danish rainbow trout farm Fousing). These were compared to infection studies on the Norwegian Laerdalselva parasite form kept under exactly the same conditions in the laboratory. The Danish G. salaris form had low virulence towards both Atlantic and Baltic salmon, whereas rainbow trout proved susceptible to the parasite. The Danish G. salaris form was able to maintain a very low infection on East Atlantic salmon, but not on the Baltic salmon, which eliminated the infection within 2 wk. Rainbow trout developed infection intensities ranging up to several hundred parasites per host. The host colonization patterns of the parasite differed clearly from those of previous studies on microhabitats of the Norwegian form of G. salaris. A comparative study on morphological characters (opisthaptoral hard parts) from the Danish parasite form and Norwegian G. salaris showed no significant differences. Selected genes comprising internal transcribed spacers 1 and 2 (ITS), ribosomal RNA intergenic spacer (IGS) and cytochrome c oxidase subunit I (COI) regions were cloned and sequenced. Five sequenced ITS clones from 5 individuals of the Danish strain consistently revealed a single base substitution compared to ITS sequences from all other known species and strains of Gyrodactylus. Mitochondrial COI gene sequences demonstrated that the Danish G. salaris form is closely similar to the Laerdalselva parasite form found in Norway. The IGS sequences were highly variable, but very similar to those obtained from German isolates of G. salaris.

Response of rainbow trout (Oncorhynchus mykiss) in skin and fin tissue during infection with a variant of Gyrodactylus salaris (Monogenea: Gyrodactylidae)

“…The present challenge infection was only sustained for 33 days and consequently a decline in parasite numbers was not observed. However, the study by Jørgensen et al (2007) showed that rainbow trout responded and controlled infection with the Danish strain of G. salaris when infection progresses over a longer period (42 days). A clear trend for an initial colonisation of the ventral fins was observed but translocation to the body region was evident at days 22 and 33.…”

“…In contrast, spreading of the pathogenic parasite form to uninfected countries such as Denmark and the UK is of great concern due to the presence of highly susceptible salmon in these countries. The Danish rainbow trout form of G. salaris has recently been isolated by Jørgensen et al (2007) and the susceptibility of East Atlantic salmon, Baltic salmon and rainbow trout to infection with the parasite was tested. The study showed that the two salmon strains were resistant and able to control the infection caused by this particular strain of G. salaris.…”

Assessment of exotic fish disease introduction and establishment in the United Kingdom via live fish transporters

“…strains may achieve high infection rates (Jørgensen et al 2007 (Soleng et al 1999, very effective at killing iodophors); efficacy affected by Poléo et al 2004), and most rhabdoviruses (Smail 1999).…”

Demonstrating freedom from Gyrodactylus salaris (Monogenea: Gyrodactylidae) in farmed rainbow trout Oncorhynchus mykiss

“…Sensitivity of morphometric methods depends on the skill of the parasitologist but can approach 100% in separating G. salaris from G. derjavini 2006, Jørgensen et al 2007). The microhabitat selection of G. salaris on Atlantic salmon may vary with abundance and with the presence of other gyrodactylid parasites.…”

“…G. salaris has been found on farmed RBT in Denmark, Sweden and Finland (see Bakke et al 2007 for details). A RBTadapted strain of G. salaris which is not pathogenic for salmon and can achieve infection rates of several hundred parasites per fish in RBT has been found in Denmark (Jørgensen et al 2007). Recently, a strain of G. salaris has been found on Arctic charr, which has not affected Atlantic salmon in the same river.…”