Proliferative kidney disease in Switzerland: current state of knowledge

Proliferative kidney disease in Switzerland: current state of

knowledgeT Wahli1, R Knuesel1, D Bernet1, H Segner1, D Pugovkin1, P Burkhardt-Holm2, M Escher3

and H Schmidt-Posthaus1

1 Centre for Fish and Wildlife Health, Institute of Animal Pathology, University of Bern, Switzerland

2 Swiss Federal Institute for Environmental Science and Technology, Duebendorf, Switzerland

3 AQUASANA Ulmiz, Switzerland

Abstract

This study presents an overview of the distributionof proliferative kidney disease (PKD) in wild andfarmed fish in Switzerland based on two sources: (1)the results of routine diagnostic work at the NationalFish Disease Laboratory, and (2) the results of acountry-wide survey for PKD. The first case of PKDin Switzerland was diagnosed in 1979 and since1981, a few cases have been found every year.Affected species were rainbow trout, brown troutand grayling from rivers and fish farms. Most of thediseased fish were found in the lower altitude regionsof the country (Swiss midlands). A seasonal distri-bution of the cases was evident: PKD-positive fishoccurred only from June to November with promi-nent peaks in August and September. Among in-fected fish all length classes were present, but thehighest numbers of affected fish were found in thelength classes <10 cm and 16–20 cm. The eva-luation of the samples from a survey of the occur-rence of PKD in 2000/2001 revealed a muchbroader geographical distribution of the infectionthan expected from routine diagnostic observations.Fish from 56 of 139 rivers analysed by histologicalexamination were positive for parasites. Fish from131 of these sites were examined macroscopically.Of these, trout from 45 sites showed gross alterationsindicative for PKD. Most of the positive fish werefound in the Swiss midlands north of the Alps. Fishfrom six of 65 fish farms investigated were found tobe PKD-positive by histopathological analysis, and

in four of these cases fish could be identified asinfected on the basis of gross examination.

Keywords: fish farms, proliferative kidney disease,rivers, survey, Switzerland.

Introduction

Proliferative kidney disease (PKD) is consideredone of the most serious parasitic diseases ofsalmonids and is responsible for high mortalitiesin fish farms and possibly also in populations ofwild fish (Okamura, Anderson, Longshaw, Feist &Canning 2001). The spread of the causative agentof PKD, Tetracapsula bryosalmonae, is closely linkedto the presence of bryozoans, its invertebrate host.The occurrence of bryozoan genera, of whichcertain species are known as alternate hosts, wasrecently recorded in Swiss river systems (Okamura,unpublished observations). PKD in brown trout hasbeen observed in a number of field studies on troutpopulations in Swiss rivers (Schneeberger 1995;Bernet, Schmidt-Posthaus, Wahli & Burkhardt-Holm 2000; Schmidt-Posthaus, Bernet, Wahli &Burkhardt-Holm 2001).

Proliferative kidney disease may influence popu-lation dynamics and development of populations ofbrown trout, Salmo trutta fario L., in Swiss rivers.Evidence has accumulated during recent years thatangler catches of brown trout in Swiss rivers aredecreasing. In a survey in all Swiss cantons 17 of 26cantons recorded a decrease in brown trout popula-tions, nine cantons reported stable populations andin only one canton were populations increasing.The data were based on the experience of theauthorities, on angling catch statistics and on datagathered by electrofishing surveys. Subsequently,

Journal of Fish Diseases 2002, 25, 491–500

Correspondence Dr Thomas Wahli, Centre for Fish and Wildlife

Health, Institute of Animal Pathology, University of Bern,

Laenggass-Strasse 122, CH-3012 Bern, Switzerland.

(e-mail: [email protected])

491� 2002

Blackwell Science Ltd

a questionnaire was sent to the fisheries authoritiesin all the cantons in order to analyse in more detailthe situation over the last 10 years. The evaluationof the answers indicated decreasing populationsmainly in rivers of the Swiss midlands (Frick,Nowak, Reust & Burkhardt-Holm 1998). A statis-tical analysis performed by the fisheries section ofthe Swiss Agency for Environment, Forest andLandscape (SAEFL), based on the catches of sportfishermen, indicated a decline of more than 40% onaverage from 1986 to 1996 (Friedl 1999). As aconsequence, a nationwide project (�FISCHNETZ�)was set up to coordinate and initiate research on thecauses of declining fish populations and the oftenquoted impaired health status of fish, and toimplement measures to alleviate and improve thesituation (Burkhardt-Holm, Peter & Segner 2002).Twelve hypotheses were raised, one of themaddressing the potential role of infectious diseasesand impaired fish health as causative factors of thedecline in fish numbers.

In order to evaluate the possible role of PKD asan aetiological factor in the decline of brown troutpopulations, detailed information on the spatial andtemporal distribution of this infectious disease, aswell as on the infected fish species and their lengthclasses in Switzerland, was required. Therefore, as afirst step, historical data of the routine diagnosticwork carried out at the National Fish DiseaseLaboratory (NAFUS) at the University of Bernfrom 1979 to 2000 were compiled (Wahli & Escher2000). Then, in a second step, a survey of PKD inall regions in Switzerland was carried out. Thepresent study based on these two data sets providesan overview of the current knowledge of PKDoccurrence in Switzerland.

Materials and methods

Data from routine diagnostic material

Data from diagnostic work at the NationalLaboratory for Fish Diseases are routinely enteredin a database. Fish were diagnosed as PKD positiveon the basis of the macroscopic appearance ofthe kidney typical for PKD and subsequent histo-logical demonstration of Tetracapsula parasites (cf.Schmidt-Posthaus et al. 2001). Fish in which theparasite was found by histology but which werewithout gross changes were categorized as PKD-positive. Each case consisted of one to several fish.The data from 1979 to 2000 were scanned for PKD

cases. Positive cases were evaluated with regard tospecies, size, origin of affected fish and season ofdiagnosis. The frequency of diagnosed PKD caseswas compared with that of other diseases.

PKD survey

A total of 113 river sites and 65 fish farms situatedin all Swiss cantons were included in a surveycarried out from August to October 2000. Fishwere caught from an additional 26 rivers in August2001. The period between August and Octoberwas chosen because outbreaks of PKD are knownto occur most frequently in late summer/earlyautumn (Hedrick, MacConnell & de Kinkelin1993). Fish in rivers were caught by electrofishingwhereas fish in farms were taken by netting.Whenever possible, 20 fish per site were assessed.In some sites, particularly in rivers, the number offish caught was lower. Fish were killed on siteimmediately after capture. Necropsy was per-formed on all fish and after a macroscopicalevaluation the kidney was carefully removed andfixed in buffered formalin (4%). No macroscopicalevaluation was performed on fish from eight sites.In the laboratory, haematoxylin and eosin (H&E)stained sections from all kidneys were preparedaccording to routine procedures. The majority offish analysed were rainbow trout, Oncorhynchusmykiss (Walbaum), (from farm sites) and browntrout (from field and farm sites).

Macroscopically the fish were attributed to oneof three classes: �negative�, �suspicious� and �posi-tive�, according to clinical signs of PKD such asswelling and discolouration of the kidney. Inhistological examinations, fish harbouring T. bryo-salmonae in kidney tissue were classified as positive.A study site was assigned to be �Tetracapsula-positive� when one or more fish provided evidencefor the presence of the parasite. In cases ofconflicting results between macroscopical andmicroscopical examination, final classification wasbased on histological findings.

Results

Evaluation of data from routine diagnosticwork from 1979 to 2000

It has to be emphasized that the diagnostic materialwas not specifically examined for PKD, and therecorded cases were detected by chance. PKD was

Journal of Fish Diseases 2002, 25, 491–500 T Wahli et al. PKD in Switzerland

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diagnosed in 130 of 9000 cases. Affected specieswere rainbow trout (51 of 3128 cases), brown trout(78 of 1752 cases) and grayling, Thymallus thymal-lus (L.), one of 146 cases (Table 1). The disease wasnot found in any other fish species. Tetracapsulabryosalmonae was more frequently found in fishfrom rivers (75 of 662 cases) than from farms (45 of3512 cases from private farms, five of 463 casesfrom government farms) or from hobby ponds andaquaria (five of 389 cases) (Table 1). Most cases ofinfected rainbow trout were found in fish farms andponds (45 of 2930 cases), whereas the majority ofinfected brown trout originated from rivers (68 of551) (Table 1).

With regard to length classes, the highest numberof cases with T. bryosalmonae was in fish of 16–20 cm irrespective of the species (Fig. 1a), followedby the class <10 cm. A considerable number ofcases with infected fish of >25 cm was found inrivers (Fig. 1c). Considering the percentage of caseswith infected fish per length class the highestpercentages of cases with infected brown trout werefound in the 10–15 and 21–25 cm length classes.No clear peak was evident in rainbow trout lengthclasses (Fig. 1b). In rivers the 10–15 cm length classwas most affected (Fig. 1d).

Tetracapsula infections were first recorded inJune and peak values followed in August and

Figure 1 Proliferative kidney disease (PKD) cases according to fish species, origin of fish and fish length: (a) percentages of cases per

length class (in cm) according to species, calculated on all PKD positive cases of the corresponding species, (b) percentages of PKD

positive cases per length class according to species, calculated on all positive cases of the species and length class, (c) percentages of cases

per length class according to origin, calculated on all PKD positive cases from the corresponding origin, (d) percentages of PKD positive

cases per length class according to origin, calculated on all cases of the corresponding origin and length class. Each case consists of one to

several fish.

Table 1 Proliferative kidney disease cases

according to species and origin of fish

(limited to cases with rainbow trout,

brown trout and grayling). Number of

positive cases vs. total cases of correspond-

ing species and origin

Rainbow trout Brown trout Grayling Total

Private fish farm 41/2588 4/846 0/78 45/3512

Government fish farm 0/121 5/309 0/33 5/463

Ponds and aquaria 4/342 1/46 0/1 5/389

Rivers 6/77 68/551 1/34 75/662

Total 51/3128 78/1752 1/146 130/5026


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September. The time course of clinical manifesta-tion of PKD infection was not dependent on theorigin of fish (Fig. 2b), but there was a peak inAugust for brown trout and in September forrainbow trout (Fig. 2a).

No increase in PKD cases over the period ofinvestigation was evident in routine diagnosticmaterial since the 1980s, with the exception of1997. The first positive case was recorded in 1979and from 1981 to 2000 two to nine cases fromdifferent locations were diagnosed every year(Fig. 3). The unusual increase in 1997 wasthe result of a specific field study (Escher 1999),from which fish were included in the routinediagnostic work to determine their health sta-tus. This included an examination for T. bryosal-monae. Most of the T. bryosalmonae-positive casesoccurred in rivers and farms from the Swissmidlands.

A comparison of frequencies of different diseasesrecorded in salmonid species (Table 2) shows PKDto be the most frequently diagnosed disease(11.3%) in rivers, followed by furunculosis

(2.3%), whereas in fish farms other diseases werefound more often.

Results of the survey in 2000/2001

Fish from 131 river sites were assessed macroscop-ically. Fish from 45 of these sites showed kidneyalterations suggesting Tetracapsula infection. In fishfrom 41 of these 45 sites the presence of Tetracap-sula was confirmed histologically while in fish fromfour sites no parasites were detected. Histologicalinvestigation revealed positive fish in five furthersites, although the infected fish showed no clinicalsigns. In addition, parasites were found in fish fromsix of eight sites evaluated only by histology.Overall, parasites were demonstrated in fish from56 of 139 sites (40.3%).

The majority of the positive cases were located inrivers situated in the Swiss midlands north of theAlps. Exceptions were one case in the canton Valais(Rhone-river system), one in the canton Ticino(south of the Alps) and one site at higher altitude(>1000 m) north of the Alps (Fig. 4).

Figure 2 Proliferative kidney disease cases

according to fish species, origin of fish

and season: (a) percentage of positive cases

per month according to species, (b) per-

centage of positive cases per month accord-

ing to origin of fish.


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All affected fish were rainbow or brown trout. Atmost sites 6 40% of the sampled fish were positive(Fig. 5a), whereas in two sites 90–100% of the fishexamined were infected with T. bryosalmonae.

Fish from four of 65 farms showed clinical signstypical for PKD and the presence of Tetracapsula infish from these farms was confirmed by histology.Fish from two further farms proved to be positivefor the parasite histologically, but did not showclinical signs of PKD. Thus, six of 65 farms (9.2%)contained T. bryosalmonae-positive fish. None ofthese sites was a government farm. Only a propor-tion of the fish examined (6 30%) per site wasinfected (Fig. 5b).

Discussion

The evaluation of the diagnostic data allowed theidentification of fish infected with T. bryosalmonaeaccording to species, size, origin and occurrence,with regard to season and year. Rainbow and browntrout were the main species harbouring the parasite.Both species play an important role in Switzerland;rainbow trout is the main species cultured forconsumption whereas brown trout is the majorsalmonid species of most Swiss river habitats. Thisis also reflected by the number of positive andnegative cases per fish species and origin. Whereasthe majority of positive rainbow trout originated

Figure 3 Number of proliferative kidney

disease cases in rivers and fish farms in

Switzerland per year from 1978 to 2000.

One case per positive site included per year.

Table 2 Percentages of different diseases in

cases from routine diagnostic material from

1978 to 2000. Only brown and rainbow

trout included

Total Rivers Fish farms

Proliferative kidney disease (PKD) 2.6 11.3 1.3

Bacterial kidney disease (BKD) 1.6 0.2 2.1

Furunculosis 4.4 2.3 5.1

Infectious haematopoietic necrosis (IHN) 0.4 0.0 0.5

Infectious pancreatic necrosis (IPN) 1.0 0.6 1.2

Viral haemorrhagic septicaemia (VHS) 6.1 1.4 6.9

Figure 4 Distribution of sampling sites

and sites with fish infected with Tetracapsulabryosalmonae in Switzerland. d ¼ sites

without infected fish; m¼ sites with

infected fish.


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from farms, most of the infected brown trout wereobtained from rivers. Stocking of rivers with thenonindigenous rainbow trout has been prohibitedin Switzerland since 1994 [Schweizerischer Bun-desrat (BR) 1993]. However, because of accidentalescapes from farms or earlier deliberate releases,small numbers of rainbow trout can be caught insome rivers. In a few rivers there are reproducingrainbow trout populations originating from earlierstocking measures. The only non-trout speciesfound to be positive for T. bryosalmonae, in onlya single case, was grayling, an important species ofmany Swiss rivers. Arctic charr, Salvelinus alpinus(L.) (n ¼ 295), and northern pike, Esox lucius L.(n ¼ 135), were not found infected although theyare regarded as susceptible (Morris, Adams, Feist,McGeorge & Richards 2000).

The results of routine diagnostic work and of thespecific survey in 2001 indicated a higher pre-valence of the parasite in wild fish populations thanin farmed fish. This may be related to the life cycleof T. bryosalmonae which depends on the presenceof bryozoans (Anderson, Canning & Okamura1999). Environmental conditions in many fishfarms are not suitable for bryozoans, e.g. use of coldwell water in concrete raceways or PVC tanks. Theresulting absence or low number of bryozoans infish farms may explain the low frequency of PKD infarmed fish.

The length distribution of fish infected withT. bryosalmonae may depend on two importantfactors. Smaller fish, corresponding to an age of0+, will have recently had their first contact withthe parasite, to which they are particularly suscep-tible. This would explain the high percentage ofinfected fish of smaller length in relation to the totalnumber of cases. However, less active and thuspotentially infected small fish are less obvious in an

open water course than larger fish with typicalclinical signs and hence the latter will be more oftencollected, possibly resulting in the observed peak inthe larger length class. In many fish farms, rainbowtrout are reared in tanks supplied with ground orwell water in their first year and thus the probabilitythat they come into contact with infectious T. bryo-salmonae stages is low. Subsequently, they are oftentransferred into earth ponds with a higher risk ofinfection and this could explain the length distri-bution of infected rainbow trout.

The presence of infected fish was dependent onthe season, in accordance with findings in othercountries as summarized by Hedrick et al. (1993).The differences between seasonal peak values of thenumber of positive cases of rainbow and browntrout could be as a result of slightly divergenttemperature preferences of the two species. How-ever, no unequivocal data on differences in thetemperature dependency of disease outbreaksbetween brown and rainbow trout are reported(Grande & Andersen 1991).

Fish with PKD are often lethargic and show adarkening of the skin. Both signs can also berecognized in feral fish and might lead to a morefrequent sampling and analysis of such fish. Clinicalsigns of other diseases are often less obvious on feralfish and this could explain why PKD is the mostoften diagnosed disease in feral trout. In contrast, infish farms viral [viral haemorrhagic septicaemia(VHS), infectious haematopoietic necrosis (IHN),infectious pancreatic necrosis (IPN)] and bacterialdiseases [bacterial kidney disease (BKD), furuncu-losis] play a more important role than PKD interms of mortality and frequency of occurrence. Inthe vast majority of cases, analyses of fish fromfarms were not primarily concerned to diagnose thepresence of T. bryosalmonae, but the fish were

Figure 5 Percentage of examined fish

infected per site. River samples: n ¼ 139

of which 56 with infected fish; fish farm

samples: n ¼ 65 of which six with infected

fish.


496� 2002


submitted because of other problems. With manyfarms using spring or well water, temperatures insummer do not often rise to levels provoking aclinical outbreak of PKD. An additional reason forthe relative importance of other diseases in fish isthat other infectious agents are directly transmissi-ble from fish to fish, while for T. bryosalmonae afurther host is needed.

It must be emphasized that diagnostic data areinevitably selective. In the majority of cases, fish aresubmitted because of health problems in farms,ponds or aquaria, whereas infected fish from riversare only sampled when a problem is very promi-nent. Hence, the data do not represent the overallsituation in all rivers and farms. Therefore, in orderto obtain a more realistic idea of the distribution ofPKD in Switzerland, a survey study was performed.The findings of this survey largely confirmed andextended the results of the diagnostic work. Fishmainly from rivers in the Swiss midlands wereinfected by T. bryosalmonae. However, more siteswith positive fish were found than expected fromthe diagnostic work revealing a wider distributionthan previously assumed. The findings concerningfarmed fish demonstrated that PKD is not aneconomically important problem in Swiss troutproduction.

Fish farms in Switzerland do not only producetrout for the retail market, but also for stockingpurposes. Thus stocking with fish from PKD-positive farms could enhance the spread of PKD inwild populations. The fact that none of thegovernment farms producing brown trout forrestocking were infected indicates that these farmsare not a major factor in the distribution of thedisease. However, there is some evidence thatprivate societies of anglers producing their own fishfor restocking could play a role. This is supportedby findings from one canton where infected fishwere found in three sites raising fingerling trout ona private base for restocking. It has also to beemphasized that participation in the survey wasvoluntary for private farms and many small farmsproducing relatively few fish for restocking were notincluded.

Apart from the restocking of rivers with infectedfish other factors could be involved in the spread ofPKD in wild fish populations. Water temperatureplays an important role in the development ofT. bryosalmonae. A temperature of 9 �C is necessaryfor the development of infectious stages of themyxozoan parasite in the bryozoan host (Gay,

Okamura & de Kinkelin 2001). Temperature isalso a critical factor in the outbreak of the disease infish; for rainbow trout, 15 �C was found to be thethreshold temperature (Clifton-Hadley, Bucke &Richards 1987; Gay et al. 2001). A significantrecent increase in the maximum annual watertemperature has been documented for certainstretches of Swiss rivers in which PKD has beenfound. For example, the alpine stretch of the RiverRhine has seen a 2 �C increase in the average watertemperature over the past 20 years (Jakob 1998).The role of global warming in this phenomenon isstill under consideration, although a temperatureincrease caused by heated cooling water fromindustries, sewage effluents and hydro-electric powerstations has also been suggested (Jakob 1998). Thelatter also decrease the velocity of water flow insome stretches of river facilitating the settlement ofbryozoans (Gay et al. 2001). In addition, poorwater quality favours the distribution and prolifer-ation of filter feeding bryozoans (Gay et al. 2001).Between 1960 and 1995 the percentage of Swisshouseholds and industries connected to waste watertreatment facilities rose from 10 to 94%, resultingin a drastic reduction in the emissions of degradableorganic substances into receiving waters. However,approximately 200 000 tonnes of nitrogen com-pounds are released into the environment, which is10 times more than the natural amount (Bundi,Peter, Truffer, Wagner, Mauch & Scheidegger1997). These anthropogenic factors provide favour-able conditions for a spread of bryozoans and thusT. bryosalmonae in Switzerland.

In most sites only a proportion of the fish werefound to be infected with T. bryosalmonae and insome sites fish did not display any clinical signs ofPKD and the presence of the parasites could only bedemonstrated by histology. This finding is inagreement with a study of the health situation offish along a river course, where marked differencesin the degree of pathology caused by PKDwere evident (Schmidt, Bernet, Wahli, Meier &Burkhardt-Holm 1999). In the upper reaches,changes in the kidney were hardly visible and theabundance of parasites was low, but the extent ofchanges clearly increased downstream (Fig. 6).Higher parasite burdens corresponded to more pro-nounced histopathological changes. These findingssuggest that considerable variations in the parasiteburden are common in brown trout populations of ariver. This may be partly explained by differenttemperature regimes and other confounding factors,


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e.g. velocity of water flow, burden of organicdegradable matter along the course of a river.Outbreaks of the disease are only recorded attemperatures above 15 �C (Gay et al. 2001).

In a few cases no parasites were found althoughmacroscopic changes of the kidney were present.This confirms the observation that agents otherthan T. bryosalmonae can cause gross pathologicalsigns comparable to those of PKD (e.g. fungalinfections, Renibacterium salmoninarum) (Hedricket al. 1993). In one of these cases an Ichthyophonusinfection was demonstrated (Schmidt-Posthaus &Wahli 2002). These findings suggest that histolog-ical analyses are indispensable for the correctdiagnosis of PKD. In some cases further techniquessuch as lectin histochemistry (Marin de Mateo,McGeorge, Morris & Kent 1996) and immunohis-tochemistry might prove helpful.

Conclusions

From the evaluation of the two data sets describedhere, it can be concluded that PKD has a widedistribution in Swiss rivers, mainly in rivers of theSwiss midlands. South of the Alps, only a single caseof PKD was found. As archived material prior to1980 is not available, no conclusion on the spreadof the disease over recent decades is possible. Thedata indicate that both brown and rainbow trout aresusceptible to the disease with brown trout the mostaffected species in rivers, whereas in fish farmsmainly rainbow trout are infected.

Government fish farms are a major source of fishfor stocking measures. The data from the routinediagnostic material revealed Tetracapsula-positive

fish in government farms (five of 463 samples),whereas in the survey no positive fish were found inthese farms, although most government farmsparticipated in the survey. This could be a conse-quence of the increasing awareness of this disease.Although it is premature to reach definitiveconclusions, it appears that the introduction ofPKD by stocking fish from government farms is nota factor in further spread of the disease. The impactof private farms is far less clear.

To date no consistent correlation between abun-dance of trout and the presence of PKD in a riverhas been found. The disease was found in riverstretches with high and low abundances of yearlingtrout (A. Peter, personal communication). Thissuggests that other factors besides PKD mightcontribute to the decline of Swiss brown troutyields. This hypothesis is supported by studieswhich have shown that populations of other species,that are not known to be affected by PKD, are alsodecreasing (Gerster 1998). With regard to thepossible impact of PKD on feral fish populations,the disease was recently added to the list ofnotifiable fish diseases in Switzerland (TSV 1995;amendment 3.01).

This study provides for the first time a survey ofthe presence of PKD over the whole territory of anEuropean country. However, the number of sam-ples and sites included do not yet permit anestimate of the overall prevalence. Future studieswill have to include more detailed analyses of theepidemiological situation with particular emphasison the role of PKD in population dynamics. Todemonstrate the relation between the decline oftrout populations and PKD, further research on thisdisease and the causative agent is required includingthe application of new diagnostic tools, e.g. poly-merase chain reaction (PCR), for a more accuratedetection of the parasite outside the period ofclinical disease (Saulnier & de Kinkelin 1997;Kent, Khattra, Hervio & Devlin 1998; Longshaw,Feist, Canning & Okamura 1999).

Particularly important questions concerningPKD in Switzerland, and also in other countriesinclude:

• What is the mortality rate of wild brown troutbecause of PKD and its implication for fishpopulations?

• How can data on PKD gathered from rainbowtrout be extrapolated to brown trout, e.g. criticaltemperature for outbreaks of the disease, with

Figure 6 Degree of changes (i.e. intensity of infection) in fish

sampled at different locations in the River Langeten.


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emphasis on increased water temperatures in thelast 20 years?

• What are the conditions for the development ofresistance in brown trout to PKD under naturalconditions, e.g. how long has the fish to beexposed to infective stages of the parasite todevelop resistance?

• Do fish originating from natural reproductionand trout raised in farms and then stocked torivers differ in their susceptibility for the diseaseand the development of resistance?

Acknowledgements

The authors thank A. Hertig and the cantonalfisheries authorities for help in sampling andmacroscopic evaluation of the river fish. Thanksgoes to all fish farmers who allowed us to samplefish in their facilities. Parts of the study were fundedby grants from the Swiss Agency for Environment,Forest and Landscape (SAEFL) and FISCHNETZ.

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Veterinary Medical Faculty, University of Bern, Bern.


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Schweizerischer Bundesrat (BR) (1993) Verordnung zum Bun-desgesetz uber die Fischerei (VBGF), Bern, SR 923.01. EDMZ,

Switzerland.

Tierseuchenverordnung (TSV) (1995) Amendment 3.2001.

Federal Veterinary Office, Bern, Switzerland.

Wahli T. & Escher M. (2000) Verbreitung der Proliferativen

Nierenkrankheit (PKD) in der Schweiz. Petri Heil 51,

23–25.

Received: 5 September 2001Accepted: 17 June 2002


500� 2002


Documents

Proliferative kidney disease in Switzerland: current state of knowledge