Vertical transmission of Renibacterium salmoninarum in cutthroat trout (Oncorhynchus clarkii)

Vertical transmission of Renibacterium salmoninarum has been well-documented in anadromous salmonids but not in hatchery-reared inland trout. We assessed whether the bacterium is vertically transmitted in cutthroat trout (Oncorhynchus clarkii) from a Colorado, USA hatchery, and assessed the rate of transmission from male and female brood fish. Adult brood fish were killed, tested for R. salmoninarum in kidney, liver, spleen, ovarian fluid, blood and mucus samples, then stripped of gametes to create 32 families with four infection treatments (MNFN, MNFP, MPFN, MPFP; M: male, F: female, P: positive, N: negative). Progeny from each treatment was sampled at 6 and 12 months to test for the presence of R. salmoninarum with an enzyme-linked immunosorbent assay and quantitative polymerase chain reaction. Our study indicated that vertical transmission was high and occurred among 60% of families across all infection treatments. However, the average proportion of infected progeny from individual families was low, ranging from 1% (MNFP, MPFN and MPFP treatments) up to 21% (MPFP treatment). Hatcheries rearing inland salmonids would be well suited to limit vertical transmission through practices such as lethal culling because any amount of transmission can perpetuate the infection throughout fish on a hatchery.

Prevalence and distribution of Renibacterium salmoninarum, causative agent of bacterial kidney disease, in wild trout fisheries in Colorado

Detections of Renibacterium salmoninarum in Colorado USA fish hatcheries have become more frequent in recent years, including one disease outbreak that originated with a wild broodstock. Our objectives were to document the prevalence and distribution of R. salmoninarum in Colorado's wild trout fisheries, investigate variables that influence that distribution, and evaluate the effectiveness of common testing methods on non-anadromous trout. We sampled wild trout across Colorado and tested kidney tissue with enzyme-linked immunosorbent assay (ELISA), quantitative polymerase chain reaction (qPCR), nested polymerase chain reaction (nPCR), and direct fluorescent antibody test (DFAT). Screening with ELISA showed high prevalence of R. salmoninarum among fish populations, but antigen levels were low. No clinical disease was observed in any of the fish sampled despite the antigen of R. salmoninarum being common. Antigen levels measured by ELISA increased in smaller streams with lower historic fish stocking rates. Brook trout Salvelinus fontinalis had the highest prevalence of the bacterium among fish species and highest ELISA antigen levels. The distribution of brook trout in the smallest streams may help explain the patterns of R. salmoninarum across the landscape. The most effective assays for screening wild trout were qPCR and ELISA; DFAT was inconsistent for bacterial levels encountered in wild trout and generally uninformative. Additionally, qPCR and ELISA can provide quantitative information about bacteria levels. The bacterium R. salmoninarum is ubiquitous in Colorado trout fisheries but is generally found at low levels. Active infections are rare and overt bacterial kidney disease appears more common in Colorado hatcheries than in wild fish.

Evidence for the Use of Mucus Swabs to Detect Renibacterium salmoninarum in Brook Trout

Efforts to advance fish health diagnostics have been highlighted in many studies to improve the detection of pathogens in aquaculture facilities and wild fish populations. Typically, the detection of a pathogen has required sacrificing fish; however, many hatcheries have valuable and sometimes irreplaceable broodstocks, and lethal sampling is undesirable. Therefore, the development of non-lethal detection methods is a high priority. The goal of our study was to compare non-lethal sampling methods with standardized lethal kidney tissue sampling that is used to detect Renibacterium salmoninarum infections in salmonids. We collected anal, buccal, and mucus swabs (non-lethal qPCR) and kidney tissue samples (lethal DFAT) from 72 adult brook trout (Salvelinus fontinalis) reared at the Colorado Parks and Wildlife Pitkin Brood Unit and tested each sample to assess R. salmoninarum infections. Standard kidney tissue detected R. salmoninarum 1.59 times more often than mucus swabs, compared to 10.43 and 13.16 times more often than buccal or anal swabs, respectively, indicating mucus swabs were the most effective and may be a useful non-lethal method. Our study highlights the potential of non-lethal mucus swabs to sample for R. salmoninarum and suggests future studies are needed to refine this technique for use in aquaculture facilities and wild populations of inland salmonids.

The cost of travel: How dispersal ability limits local adaptation in host-parasite interactions

Classical theory suggests that parasites will exhibit higher fitness in sympatric relative to allopatric host populations (local adaptation). However, evidence for local adaptation in natural host-parasite systems is often equivocal, emphasizing the need for infection experiments conducted over realistic geographic scales and comparisons among species with varied life history traits. Here, we used infection experiments to test how two trematode (flatworm) species (Paralechriorchis syntomentera and Ribeiroia ondatrae) with differing dispersal abilities varied in the strength of local adaptation to their amphibian hosts. Both parasites have complex life cycles involving sequential transmission among aquatic snails, larval amphibians and vertebrate definitive hosts that control dispersal across the landscape. By experimentally pairing 26 host-by-parasite population infection combinations from across the western USA with analyses of host and parasite spatial genetic structure, we found that increasing geographic distance-and corresponding increases in host population genetic distance-reduced infection success for P. syntomentera, which is dispersed by snake definitive hosts. For the avian-dispersed R. ondatrae, in contrast, the geographic distance between the parasite and host populations had no influence on infection success. Differences in local adaptation corresponded to parasite genetic structure; although populations of P. syntomentera exhibited ~10% mtDNA sequence divergence, those of R. ondatrae were nearly identical (<0.5%), even across a 900 km range. Taken together, these results offer empirical evidence that high levels of dispersal can limit opportunities for parasites to adapt to local host populations.

Comparison of direct and indirect techniques for evaluating endoparasite infections in wild-caught newts (Taricha torosa and T. granulosa)

Studies of amphibian parasites have increased over the past 20 yr, in part because of their role in amphibian population declines and deformities. Such patterns underscore the importance of non-lethal methods for detecting and quantifying endoparasitic infections. The goal of this study was to compare results of indirect methods (fecal smears and fecal floats) with quantitative necropsies to detect endoparasitic infections in adult newts. In 2015, we collected fecal samples from 68 adult newts (Taricha granulosa and T. torosa) in the East Bay region of California and used fecal smears, sodium nitrate fecal flotation solution, and Sheather's sugar flotation solution to assess infection (i.e. the presence and/or abundance of a parasite). Across all methods, we detected 3 protozoans (Eimeria tarichae, Tritrichomonas sp., and Balantidium sp.) and 3 nematodes (Rhabdias tarichae, Cosmocercoides variabilis, and Chabaudgolvania sp.). Based on generalized linear mixed models, the likelihood of detection varied between hosts (with T. torosa showing more overall infection relative to T. granulosa) and by assessment method: while fecal smears were more sensitive in detecting protozoans, comprehensive necropsies were the most reliable for quantifying infections of R. tarichae. Nonetheless, both the likelihood of R. tarichae detection within fecal samples as well as the number of infectious stages observed correlated strongly with infection intensity from necropsy, highlighting the utility of non-lethal assessment methods. The overall congruence between indirect methods and gross necropsy helps to validate the use of less-invasive methods for parasite detection and abundance, especially for sensitive or protected host taxa such as amphibians.

Chance or choice? Understanding parasite selection and infection in multi-host communities

Ongoing debate over the relationship between biodiversity and disease risk underscores the need to develop a more mechanistic understanding of how changes in host community composition influence parasite transmission, particularly in complex communities with multiple hosts. A key challenge involves determining how motile parasites select among potential hosts and the degree to which this process shifts with community composition. Focusing on interactions between larval amphibians and the pathogenic trematode Ribeiroia ondatrae, we designed a novel, large-volume set of choice chambers to assess how the selectivity of free-swimming infectious parasites varied among five host species and in response to changes in assemblage composition (four different permutations). In a second set of trials, cercariae were allowed to contact and infect hosts, allowing comparison of host-parasite encounter rates (parasite choice) with infection outcomes (successful infections). Cercariae exhibited consistent preferences for specific host species that were independent of the community context; large-bodied amphibians, such as larval bullfrogs (Rana catesbeiana), exhibited the highest level of parasite attraction. However, because host attractiveness was decoupled from susceptibility to infection, assemblage composition sharply affected both per-host infection as well as total infection (summed among co-occurring hosts). Species such as the non-native R. catesbeiana functioned as epidemiological 'sinks' or dilution hosts, attracting a disproportionate fraction of parasites relative to the number that established successfully, whereas Taricha granulosa and especially Pseudacris regilla supported comparatively more metacercariae relative to cercariae selection. These findings provide a framework for integrating information on parasite preference in combination with more traditional factors such as host competence and density to forecast how changes within complex communities will affect parasite transmission.