Proliferative kidney disease in juvenile salmonids in UK rivers
S W Feist, R Gardiner, E Peeler and M Longshaw
CEFAS Weymouth Laboratory, Barrack Road, The Nothe, Weymouth, Dorset, DT4 8UB, UK
Background
Early surveys of wild fish stocks were primarily aimed at determining the distribution and host
range of PKD. Many fish species were obtained, generally by electro-fishing and seine netting of
rivers in England and Wales (Seagrave et al., 1981). Some work was also undertaken in a Scottish
trout stream (Wootten & McVicar, 1982). It rapidly became clear that salmonids and pike were the
main species affected by PKD. Later studies confirmed the findings of the earlier work, but also
included examinations of juvenile fish (Bucke et al., 1991; Feist & Bucke, unpublished). More
recently, a survey conducted with the Fish Health Inspectorate at CEFAS Weymouth has provided
new data on the effect of the disease on juvenile salmonids which is presented here.
Susceptibility of wild fish
The range of susceptible fish species has generally been accepted for several years and includes
most European and North American salmonid species. In addition, grayling (Thymallus thymallus)
and pike (Esox lucius) are also known to be susceptible; the former, both from natural and
experimental exposure routes (Feist & Bucke, 1993). However, brook trout (Salvelinus fontinalis)
do not appear to show clinical signs of the disease although the parasite is able to reach the renal
tubules. Most recently, Arctic char (S. alpinus) have been shown to be highly susceptible to the
infection, which results in large numbers of later stages of the parasite developing in the renal
tubules with relatively mild host reaction (Kent et al. 2000). Curiously, there has been a single
report of a ‘PKX-like’ organism reported from carp gills (Voronin & Chernysheva, 1993). This
may indicate a new host species for the parasite, but more likely an infection with a closely related
species. This clearly needs further investigations to clarify the nature of this organism.
Actual evidence of a serious impact of PKD on wild salmonid populations is difficult to obtain. It is
known from the surveys that wild fish, in particular brown trout, show severe clinical signs of PKD
but perhaps not surprisingly, obvious mortalities that could be attributed to PKD have not been
reported. Increased predation of affected fish and efficient scavenging of mortalities as well as
possible low levels of disease are all possible explanatory factors. It is also known that juvenile fish
are most susceptible to the disease and that survivors are relatively immune. However, it should be
remembered that juvenile fish tend also to be more susceptible to other pathogens that might be
present in the environment.
Survey of salmonids in UK rivers
Recent findings from our survey of wild salmonids from 26 sites on 23 river systems showed that
brown trout and Atlantic salmon were the most susceptible species. Up to 75% of juvenile (0+)
brown trout were found with severe clinical PKD at particular sites and up to 33% of salmon had
mild disease signs, again at particular sites only. However, fish from several sites sampled did not
show evidence of disease. The clustering of disease by river strongly suggests that there are
environmental factors affecting the prevalence of PKD. In both species, co-infections with an
unidentified Chloromyxum species were a common finding. The significance of this infection is
still unknown. However, histopathological changes appeared to be limited to the renal tubules and
PKD-Workshop, July 2 and 3, 2001, EAWAG Kastanienbaum, Switzerland
Project “fischnetz”, www.fischnetz.ch
an inflammatory response to the parasite was not observed. On sampling the fish, it was noted that
brown trout with PKD were clearly stressed and were invariably the first to die after capture.
It is clear that there is a great deal that we do not understand on the impact of PKD on wild fish
stocks and in particular on the various factors affecting infection rates in the wild. For any real
assessment of long-term impact there is a need for long-term prospective observational studies to
gather data consistently and on a regular basis. Without this it would be almost impossible to detect
changes in the population structure attributable to disease and PKD in particular.
A
multidisciplinary approach, including fisheries biologists and stock assessment scientists, is required
to assess the factors affecting the temporal changes in fish population levels and structure. The
timing of release of the infectious agent in different geographical areas, the differences in
susceptibility between fish species and between fish populations are likely to be important. In
addition there is increasing evidence that adverse water quality exacerbates disease prevalence and
recent findings have shown that the prevalence and severity of PKD increased with decreasing water
quality (Schmidt et al. 1999). Other environmental factors may also directly affect the population
level and possibly indirectly through increasing the prevalence and severity of PKD and the
prevalence of other diseases. It is therefore crucial that studies of PKD in wild fish also assess
important environmental factors and the prevalence of other diseases. Multivariate regression
analysis, using data from a range of sites and rivers, is required in order to assess the impact of
PKD, adverse environmental factors, and the synergistic effect of disease and environmental factors,
on population levels. The analysis of a study that collects data from a number of rivers and from a
number of sites of the same river must also take into account the clustered nature of the data. One
potential factor affecting the level of PKD in wild fish is the prevalence of PKD in farmed fish on
the same river system. Therefore, any study of PKD in wild fish should also investigate the disease
status of farmed fish utilising the same river system.
Bryozoans have been shown to harbour stages of the causative agent of PKD (Tetracapsula
bryosalmonae) which is the infective stage to fish, and there are very many factors affecting the
bryozoan hosts which will influence their distribution and the prevalence of T. bryosalmonae. It is
quite possible that the prevalence of PKD is primarily determined by the factors affecting the
distribution of T. bryosalmonae. This opens up a whole new field of investigation and there will
certainly be many interesting discoveries in the coming years, some of which may indicate realistic
possibilities for control methods based on the management of bryozoan populations.
Although there has been some work on performance indicators in diseased fish, this is an area of
particular importance when attempting to assess the potential impact on wild fish. A variety of
physiological parameters could be measured, for example swimming performance and respiratory
function.
References
Bucke, D., Feist, S. W. & Clifton-Hadley, R. S., (1991) The occurrence of proliferative kidney
disease (PKD) in cultured and wild fish: further investigations. J. Fish Dis., 14, 583-588
Feist, S. W. & Bucke, D. (1993) Proliferative kidney disease in wild salmonids. Fisheries
Research, 14, 51-58.
Kent, M. L., Khattra, J., Hedrick, R. P. & Devlin, R. H. (2000) Tetracapsula renicola n. sp.
(Myxozoa: Saccosporidae); the PKX myxozoan - the cause of proliferative kidney disease of
salmonid fishes. J. Parasitol., 86(1), 103-111.
PKD-Workshop, July 2 and 3, 2001, EAWAG Kastanienbaum, Switzerland
Project “fischnetz”, www.fischnetz.ch
Schmidt, H., Bernet, D., Wahli, T & Burkhardt-Holm, P. (1999) Influence of water quality on fish
health - comparison between river water and treated sewage effluents. Abstract O-041, European
Association of Fish Pathologists IXth International Conference, Rhodes, Greece. 19-24th
September, 1999.
Seagrave, C. P., Bucke, D., Hudson, E. B. & McGregor, D. (1981) A survey of the prevalence and
distribution of proliferative kidney disease (PKD) in England and Wales. J. Fish Dis., 4, 437-439.
Voronin, V. N. & Chernysheva, N. B. (1993) An intracellular gill parasite as the possible agent of
mortality during swim-bladder inflammation in common carp, Cyprinus carpio L. J. Fish Dis., 16,
609-611.
Wootten, R & McVicar, A. H. (1982) Some preliminary observations on proliferative kidney
disease
PKD-Workshop, July 2 and 3, 2001, EAWAG Kastanienbaum, Switzerland
Project “fischnetz”, www.fischnetz.ch