Renibacterium salmoninarum and Mycobacterium spp.: two bacterial pathogens present at low levels in wild brown trout (Salmo trutta fario) populations in Austrian rivers

Abstract

Background: Renibacterium salmoninarum and Mycobacterium sp. are important bacterial pathogens of fish. R. salmoninarum is the causative agent of bacterial kidney disease, a Gram-positive bacterium mostly known for causing chronic infections in salmonid fish, while multiple species belonging to the Mycobacterium genus have been associated with mycobacteriosis in fish as well as in human. The objective of this study was to determine the prevalence of these two bacterial pathogens in populations of wild brown trout (Salmo trutta fario) in four rivers (Kamp, Wulka, Traun and Ybbs) in Austria. Results: A total of 457 kidney samples were examined for both bacterial agents using nested and conventional PCR as well as bacterial cultivation on KDM-2, histological examination and immunohistochemistry. Molecular evidence showed an estimated prevalence level of 0.94% for R. salmoninarum in 2017 while the bacterium could not be detected in 2018 and histology showed signs consistent with a low-level chronic inflammation in the kidney of infected fish. Similarly, no fish were found positive for Mycobacterium in 2017 but in 2018, the prevalence was found to be 37.03% in the Kamp river (4.08% across all rivers). The sequencing data confirmed that these fish carried Mycobacterium sp. although the precise species of Mycobacterium could not be ascertained. Conclusions: This survey constitutes the first insight into the prevalence rate of R. salmoninarum and Mycobacterium sp. in wild brown trout (Salmo trutta fario) populations in Austria. Both of these pathogens were only detected in the summer months (June and July), which might suggest that the stress linked to increased water temperature could act as stressor factor and contribute to the outbreak of these diseases. The age of the fish might also play a role, especially in the case of Mycobacterium sp. as all the infected fish were in their first summer (June).

Mycobacteriosis and Infections with Non-tuberculous Mycobacteria in Aquatic Organisms: A Review

“…Moreover, RT-qPCR procedures are available to detect M. marinum based on this sequence [ 84 ]. It is noteworthy that these sequences are quite similar throughout the Mycobacterium genus and, therefore, these PCRs do not generally allow one to identify the bacterium at the species level [ 85 ]. Other genes have been suggested for the development of PCR primers, including the heat shock protein 65kD gene ( hsp 65 ) [ 86 ], as summarized in Table 3 .…”

Identification Mycobacterium spp. in the Natural Water of Two Austrian Rivers

“…In total, 45 water samples were collected from the Kamp and 25 samples from the Wulka River. Genomic DNAs were extracted from these samples, and PCRs were performed for each sample in order to detect the presence of NTM based on three different PCR protocols by Delghandi et al [ 34 ], Talaat et al [ 37 ], and Telenti et al [ 38 ] ( Figure 1 ). Notably, the results were identical for all three PCR protocols, and Mycobacterium sp.…”

“…in water samples in order to maximize our confidence in the results. Initially, a PCR assay was performed according to the protocol developed at the University of Veterinary Medicine and published by Delghandi et al in 2020 [ 34 ], using Myco 16F1 (5′-AGCTCGTAGGTGGTTTGTCG-3′) and Myco 16R1 (5′-CCACCTTCCTCCGAGTTGAC-3′) for the detection of the 16S rRNA gene [ 34 ]. The total volume of amplification was 25 µL, comprising 12.5 µL Dream Taq Green PCR Master Mix, 1 µL of each primer (10 pmol) and 4 µL eDNA solution.…”

“…Because Mycobacterium sp. are known pathogens of wild fish, notably isolated in 2018 from brown trout originating in the Kamp river in Austria [ 34 ], we decided to estimate the prevalence of these organisms in the Kamp and the adjacent Wulka river.…”

Renibacterium salmoninarum—The Causative Agent of Bacterial Kidney Disease in Salmonid Fish

“…Horizontal transmission via water is considered to be limited since R. salmoninarum is not able to survive outside the host for longer than a week [ 2 , 4 , 24 ]. Nonetheless, transmission has also been reported between cages in farm and farm to farm as well as from wild or escaped fish that play a role as a reservoir [ 2 ], a recent survey conducted in Austria showed that the bacterium was present in low numbers even in externally healthy wild fish populations [ 47 ]. Moreover, sea lice ( Lepeophteirus salmonis ) have been suspected to act as a vector; however, this has not been confirmed, even if treatment against sea lice is known to have a significant impact on the occurrence of BKD [ 4 , 20 ].…”