Epidemiology of Gyrodactylus salaris (Monogenea) in the River Tornionjoki, a Baltic wild salmon river

Geographical distribution of Gyrodactylus salaris Malmberg, 1957 (Monogenea, Gyrodactylidae)

BACKGROUND

Gyrodactylus salaris Malmberg, 1957 is an OIE (Office International des Epizooties)-listed parasitic pathogen and had until the current study been reported from 19 countries across Europe, although many of these records require confirmation. The last comprehensive evaluation regarding the distribution of G. salaris, however, was made in 2007, although some of the states identified as being G. salaris-positive were ascribed this status based on misidentifications, on partial data resulting from either morphological or molecular tests, or from records that have not been revisited since their early reporting. It is thus important to go through the reports on G. salaris to obtain a status for each country.

METHODS

To provide a revised update of the G. salaris distribution, a literature review was necessary. This literature, however, was not always readily accessible and, in certain cases, the article only made superficial reference to the parasite without providing details or data to support the identification. In most cases, the original specimens were not deposited in a national collection. Additional Gyrodactylus material for the current study was obtained from selected salmonid populations with the aim to contribute to current understanding regarding the distribution of G. salaris. Additional parasite material collected for this study was processed following standard procedures for species identification in Gyrodactylus [1].

RESULTS

From the work conducted in the current study, G. salaris is reported from a further three regions in Italy, alongside three other species, and appears to occur extensively throughout central Italy without causing significant mortalities to its rainbow trout, Oncorhynchus mykiss (Walbaum), host. The analysis of archive material from G. salaris-positive farms would suggest that G. salaris has been in this country since at least 2000. Material obtained from rainbow trout from Finland and Germany are confirmed as G. salaris, supporting existing data for these countries. No specimens of G. salaris, however, were found in the additional Gyrodactylus material obtained from rainbow trout reared in Portugal and Spain. A morphologically similar species, Gyrodactylus teuchis Lautraite, Blanc, Thiery, Daniel et Vigneulle, 1999, however, was found.

CONCLUSIONS

Following the present review, Gyrodactylus salaris is reported from 23 out of 50 recognised states throughout Europe; only records from 14 of these states have been confirmed by either morphology and/or by an appropriate molecular test and are considered valid, while only nine of these records have been confirmed by a combination of both methods.

A large wild salmon stock shows genetic and life history differentiation within, but not between, rivers

Anadromous salmonid fishes frequently exhibit strong geographic population structuring. However, population genetic differentiation of Atlantic salmon (Salmo salar) at fine geographic scales differs across equivalent spatial extents in different regions. So far, fine-scale genetic differentiation has not been assessed in rivers of the Baltic Sea, a region that contains an evolutionarily distinct Atlantic salmon lineage. Thus, Baltic salmon are currently managed on the river level, without focus on potential genetic structure and diversity within rivers. Here, we used microsatellites to characterize the genetic structure of wild juvenile salmon sampled throughout the interconnected, northern Baltic Tornio and Kalix Rivers. We found genetic differentiation within the two rivers, but not between them: salmon in the upper reaches differed from individuals in the lower reaches, regardless of river system. Further, examining smolts migrating from the river to the sea and adults returning from the sea to spawn, we found an association between the genetic structure and seasonal migration timing. Out-migrating smolts genetically assigned to upper river reaches were older and tended to reach the sea later in the season than smolts from the lower reaches. In contrast, mature adults originating from the upper reaches returned to the river early in the season. Our observation of genetic population structuring between downstream and upstream reaches of the large Tornio and Kalix rivers, and its association with migration timing, implies that careful temporal management of the northern Baltic fisheries would help to preserve the diversity and sustainability of the wild salmon stocks of these rivers.

Phylogeography and demographic history of Gyrodactylus konovalovi (Monogenoidea: Gyrodactylidae), an ectoparasite on the East Asia Amur minnow (Cyprinidae) in Central China

Gyrodactylus konovalovi is an ectoparasite on the Amur minnow (Rhynchocypris lagowskii) that is widely distributed in the cold fresh waters of East Asia. In the present study, the phylogeography and demographic history of G. konovalovi and the distribution of its host in the Qinling Mountains are examined. A total of 79 individual parasites was sequenced for a 528 bp region of the mitochondrial NADH dehydrogenase subunit 5 (ND5) gene, and 25 haplotypes were obtained. The substitution rate (dN/dS) was 0.068 and indicated purifying selection. Haplotype diversity (h) and nucleotide diversity (π) varied widely in the Qinling Mountains. Phylogenetic trees based on Bayesian inference (BI), maximum likelihood (ML), and maximum parsimony (MP) methods and network analysis revealed that all haplotypes were consistently well-supported in three different lineages, indicating a significant geographic distribution pattern. There was a significant positive correlation between genetic differentiation (F st) and geographic distance. The results of mismatch distribution, neutrality test and Bayesian skyline plot analyses showed that whole populations underwent population contraction during the Pleistocene. Based on the molecular clock calibration, the most common ancestor was estimated to have emerged in the middle Pleistocene. Our study suggests for the first time that a clearly phylogeography of G. konovalovi was shaped by geological events and climate fluctuations, such as orogenesis, drainage capture changes, and vicariance, during the Pleistocene in the Qinling Mountains.

Genetic gradient of a host-parasite pair along a river persisted ten years against physical mobility: Baltic Salmo salar vs. Gyrodactylus salaris

The Atlantic salmon, Salmo salar L., in the Tornio River in the Northern Baltic Sea basin accommodates a monogenean ectoparasite, Gyrodactylus salaris. The aim of the study was to understand the population structure of apparently co-adapted host-parasite system: no parasite-associated mortality has been reported. The parasite burden among salmon juveniles (parr) was monitored along 460km of the river in 2000-2009. Among the parr, 33.0% were infected (n_fish=1913). The genetic structure of the parasite population was studied by sequencing an anonymous nuclear DNA marker (ADNAM1, three main genotypes) and mitochondrial CO1 (three clades, six haplotypes). During the ten years, the parasite population was strongly and stably genetically differentiated among up- and downstream nurseries (n_ADNAM1=411, F_ST=0.579; n_CO1=443, F_ST=0.534). Infection prevalence among the smolts migrating to sea was higher than in the sedentary parr populations (82.2%, n_fish=129). The spatial differentiation observed among the sedentary juveniles was reflected temporally in the smolt run: parasite genotypes dominating the upper part of the river arrived later than downstream dwellers (medians June 4 and June 2) to the trap 7km from the river mouth. The nuclear and mitochondrial markers were in stable disequilibrium which was not relaxed in the contact zone or among the smolts where the parasite clones often met on individual fish. Only five parasite specimens on smolts (n_worms=217) were putative recent sexual recombinants. The contribution of extant salmon hatcheries into the infection was negligible. The host salmon population in Tornio River is known to show significant spatial differentiation (F_ST=0.022). The stable spatial genetic structure of the parasite against the high physical mobility suggested a possibility of local co-adaptation of the host-parasite subpopulations.

Killing of Gyrodactylus salaris by heat and chemical disinfection

Background

Gyrodactylus salaris is a monogenean, which has collapsed tens of wild Atlantic salmon populations. One of the means of preventing the spread of the parasite is the disinfection of the fishing equipment, which is used in the rivers having susceptible salmon populations. Little is known about the dosage of disinfectants against G. salaris. There are not standards for the testing of disinfectants against multicellular parasites. The present investigation developed a method to test disinfectants and examined the effectiveness of heated water and a commercially available disinfectant (Virkon S) in killing G. salaris. Individual G. salaris worms were followed under the microscope during treatment with heated water or Virkon S disinfectant blend. The logarithm of the time needed to kill the parasite was used as a dependent variable in linear regression. The upper 99.98 % prediction line for the dependent variable was used to obtain a value resembling the time needed for a 4 log reduction of the microbial pathogen, which is commonly used as a criterion for disinfectants. Also 6 log reduction was applied.

Results

Exposure to a relatively low temperature was found to kill the parasite. Even 5–50 min treatment (=10–100 times the 99.98 % upper prediction value) with heated water at 40 °C might be used. This would enable the utilisation of hot tap water in the disinfection of fishing gear. The present practice of 1 % Virkon S for 15 min was also found to kill the parasite.

Conclusions

The follow-up of single parasites of a test population and the use of the calculated upper predictive line in the regression analysis offers a method to analyse the effects of disinfectants on parasites like G. salaris. The results of our tests give possibilities for using disinfection methods, which may be more acceptable by the fishermen than the present ones.

Genetic population structure of Gyrodactylus thymalli (Monogenea) in a large Norwegian river system

The extent of geographic genetic variation is the result of several processes such as mutation, gene flow, selection and drift. Processes that structure the populations of parasite species are often directly linked to the processes that influence the host. Here, we investigate the genetic population structure of the ectoparasite Gyrodactylus thymalli Žitňan, 1960 (Monogenea) collected from grayling (Thymallus thymallus L.) throughout the river Glomma, the largest watercourse in Norway. Parts of the mitochondrial dehydrogenase subunit 5 (NADH 5) and cytochrome oxidase I (COI) genes from 309 G. thymalli were analysed to study the genetic variation and investigated the geographical distribution of parasite haplotypes. Three main clusters of haplotypes dominated the three distinct geographic parts of the river system; one cluster dominated in the western main stem of the river, one in the eastern and one in the lower part. There was a positive correlation between pairwise genetic distance and hydrographic distance. The results indicate restricted gene flow between sub-populations of G. thymalli, most likely due to barriers that limit upstream migration of infected grayling. More than 80% of the populations had private haplotypes, also indicating long-time isolation of sub-populations. According to a molecular clock calibration, much of the haplotype diversity of G. thymalli in the river Glomma has developed after the last glaciation.

Local patchiness of Gyrodactylus colemanensis and G. salmonis parasitizing salmonids in the South River watershed, Nova Scotia, Canada

Prevalence and intensity of Gyrodactylus colemanensis and G. salmonis (Monogenea) parasitizing juvenile/adult brook trout Salvelinus fontinalis, rainbow trout Oncorhynchus mykiss, brown trout Salmo trutta, and Atlantic salmon Salmo salar at 3 localities over an 8 km stretch in the South River, Nova Scotia, Canada, were calculated 4 times over a 9 mo period (October 2009, December 2009, March 2010, June 2010). G. colemanensis was on all 4 salmonids (endemic and non-endemic), while G. salmonis parasitized mostly S. fontinalis (endemic) and occasionally S. trutta (non-endemic). At an upstream locality, beyond a waterfall barrier, in a small tributary of the main river, G. colemanensis was more common than G. salmonis. In the main river, 7 km downstream, prevalence of G. colemanensis on S. fontinalis was comparable, or higher, than that of G. salmonis, while intensity of G. salmonis was higher than that of G. colemanensis. Downstream a further 1 km, in a tributary of the main river, both prevalence and intensity of G. salmonis on brook trout were higher than those of G. colemanensis. Stocks at a local trout hatchery had only G. colemanensis. The present study reports on a method by which exit water from such farms can be monitored for gyrodactylid parasites through a simple settling procedure. We estimated that up to 230,000 dislodged, live G. colemanensis exit the hatchery daily in discharge water entering the river. It is suggested that such systems are ideal for studying the impact of such parasite export on the nature of local parasite populations.

Microhabitat use, not temperature, regulates intensity of Gyrodactylus cichlidarum long-term infection on farmed tilapia--are parasites evading competition or immunity?

Gyrodactylids (Monogenea) are ectoparasites of fish, some of which negatively affect commercially valuable fishes. Temperature strongly regulates population dynamics of these viviparous flatworms in farmed and wild fish populations, with most gyrodactylid species showing positive temperature-abundance associations. In agreement with epidemiological theory, numerous laboratory studies demonstrate that these parasites cannot persist in confined fish populations without periodic introduction of susceptible hosts. Extinction of gyrodactylid populations is due to host immunity, which develops in several fish species. In this one-year study, we followed populations of the recognized pathogen Gyrodactylus cichlidarum infecting four genetic groups of confined tilapia (wild type Nile tilapia Oreochromis niloticus niloticus, red O. n. niloticus, Mozambique tilapia O. mossambicus and a red synthetic population called Pargo-UNAM) kept under farming conditions and subject to natural environmental fluctuations. Based on the antecedents given, we postulated the following three hypotheses: (1) parasite abundance will be regulated by water temperature; (2) parasites will induce host mortality, particularly during periods of rapid infrapopulation growth; and (3) gyrodactylid populations will eventually become extinct on confined fish hosts. We disproved the three hypotheses: (1) parasite numbers fluctuated independently of temperature but were associated to changes in microhabitat use; (2) although gyrodactylid populations exhibited considerable growth, no evidence was found of negative effects on the hosts; and (3) infections persisted for one year on confined fish. Microhabitat use changed over time, with most worms apparently migrating anteriorly from the caudal fin and ending on the pectoral fins. Gyrodactylid populations followed similar trajectories in all fish, aggregating and dispersing repeatedly. Several instances were found where increased parasite dispersion coincided with increased intensity of infection; as well as the opposite, where increased aggregation coincided with parasite population declines. Three alternative explanations could account for these observations: that parasites (1) experience differential mortality on different anatomical regions of the fish; (2) migrate to avoid intraspecific competition; and (3) migrate to escape localized immune responses induced by infection. Our data do not allow us to demonstrate which of these alternatives is correct, so we discuss the merits of each. We provide circumstantial evidence in support of the third explanation, because as shown in other fish host-gyrodactylid interactions where immune responses have been characterized, in this study worms progressively moved away from fins with high mucus cell density to those with low density - what would be anticipated if immune defenses occur and reach the fish surface through mucus.