PKD-Workshop: Questions concerning the Proliferative Kidney Disease

1.Environmental factors

  • What are the effects of the PKD regarding the (known) rise of temperature (within the last 10-20 years)?
  • How is the meaning of other factors, for example light, pollution, etc. for the outbreak of the disease?
  • Is there any knowledge of outbreaks of PKD in lakes?

2.Fish as a host

  • Are the fish obligatory hosts or are they so called dead-end hosts; or, the other way around: Can the life-cycle of the parasite be completed independently of the fish?
  • What is the critical temperature of water and its necessary duration that leads to an outbreak of the disease within brown trout?
  • Rainbow trouts (O. tshawytscha, O. kisutch) are assumed to be the most sensitive species, brown trouts and species of Salvelinus are considered to be less sensitive (de Kinkelin & Gay, in press). What are these statements based on? Is it only due to the fact that rainbow trout are in general held at higher temperature than brown trout? Own results showed a higher prevalence of PKD in brown trout (35%) than in rainbow trout (26%). On the other side, rainbow trout showed in general better condition indices than brown trout, a fact that might indicate a higher susceptibility of brown trout to the disease.
  • Are there any other fish species known to be infected (e.g. cyprinids)?

3.Susceptibility

  • Are there any known differences regarding susceptibility between farmed fish and wild fish?
  • Are there any genetic differences regarding susceptibility between different strains of the same species?

4.Transmission

  • Are the parasites to be secreted completely from the fish (brown trout) or are they to survive within the fish in order to strike anew in the following year?
  • Does the developmental stages, which are deliberated from the fish, infect bryozoans?
  • Is there really no evidence that the parasite can be transmitted from fish to fish?

5.Epidemiology

  • What is known about the mortality rates of brown trout? In open waters?
  • What are the effects of the disease on populations in open waters in general?
  • Is the brown trout more sensitive to PKD than the rainbow trout?
  • What is the earliest possible time of the year to release fry into river stretches with PKD-occurrence in order to induce a resistance?
  • Can PKD-infected fry transmit the disease to PKD-free open waters, assumed that bryozoans (known as hosts) are present?
  • Can PKD potentially be found in any open waters where bryozoans live? Is the proof only a question of space of time?

6.Resistance and immunity

  • What is the mechanistic base for the resistance? There are hints on cell-mediated immunity (Hedrick et al., 1993).
  • What are the conditions to establish an efficient resistance (temperature, density of the parasite, etc.)?
  • How long does it take to establish an efficient resistance?
  • Can resistant fish release infective spores?
  • Does the acquired resistance remain for the rest of the life?
  • How long can the sporogene stages be proved in the fish after recovery, this obviously differs between fish species?
  • Is it possible that the disease can outbreak anew as a result from the development of new (extra)sporogene stages, which - perhaps - was not yet detected due to the restricted capabilities of diagnostic tools before the development of PCR and immuno-histochemistry?
  • Can the parasite survive in the fish until a reinfection takes place?
  • Is the decline in mortality in 0+ fish seen from September onwards, due to developing immunity per se; is the recovery mainly associated with a reduced propagation of the parasite due to the falling water temperatures in autumn or is it a combination of both of these? (Ferguson, 1981).

7.Diagnosis

  • Is a reliable proof of the existence of the parasite by molecular methods (PCR) possible throughout the year? Even if the density of the parasite is very low? Are there any differences between different species of fish?
  • What about the certainty of a negative result?

8.Bryozoans as hosts

  • How does the parasite hibernate? Only in reduced, living colonies of bryozoans? What is known about the conditions of hibernation (temperature, etc.)?
  • There are different types of spores in bryozoans and in fish – does this fact point to different types of parasites?
  • Fredericella sultana is the only known species of bryozoan that does hibernate as a living colony (Gay et al., 2001) – is therefore an infection in winter only possible if this species is present in the waters?
  • Is there an increased spreading of the bryozoans reported, which is caused by eutrophication or reoligotrophication and rise of the water temperature?
  • Was there a spread of new (exotic) species of bryozoans in Europe during the past 10-20 years, and if so: are these bryozoans more vulnerable to the parasite?
  1. Stocking measures with brown trout in Swiss rivers
  • We gave the following recommendations:
  • stocking of PKD-free river stretches only with fry free of PKD
  • stocking of PKD-positive waters with fry from PKD-positive hatcheries (we assume this can reduce the mortality rate due to a possibly acquired resistance in the hatchery)
  • stocking of PKD-free waters with fry from PKD-positive hatcheries should be omitted in any case
  • stocking of rivers in autumn (hypothesis: fish infected but no mortality, next summer: acquired immunity)

Is this, to the best of your knowledge, a reasonable set of recommendations?

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