Interim report of the HELCOM CORESET project

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Confounding factors
The multifactorial aetiology of diseases, in this context in particular of externally visible diseases, is generally
accepted. Therefore, externally visible diseases have correctly been placed into the general biological effect
component of the OSPAR CEMP (OSPAR 2010b). Most wild fi sh diseases monitored in past decades are
caused by pathogens (viruses, bacteria). However, other endogenous or exogenous factors may be required
before the disease develops. One of these factors can be environmental pollution, which may either affect
the immune system of the fi sh in a way that increases its susceptibility to disease, or may alter the number
and virulence of pathogens. In addition, contaminants may also cause specifi c and/or non-specifi c changes at
various levels of biological organisation (molecule, sub-cellular units, cells, tissues, organs) leading to disease
without involving pathogens.
The occurrence of signifi cant changes in the prevalence of externally visible fi sh diseases can be considered a
non-specifi c and more general indicator of chronic rather than acute (environmental) stress, and it has been
speculated that they might, therefore, be an integrative indicator of the complex changes typically occurring
under fi eld conditions rather than a specifi c marker of effects of single factors. Because of the multifactorial
causes of externally visible diseases, the identifi cation of single factors responsible for observed changes in
disease prevalence is diffi cult, and scientifi c proof of a link between contaminants and externally visible fi sh
diseases is hard to achieve. Nevertheless, there is a consensus that fi sh disease surveys should continue to
be part of national and international environmental monitoring programmes since they can provide valuable
information on changes in ecosystem health and may act as an “alarm bell”, potentially initiating further
more specifi c studies on cause and effect relationships.
A thorough statistical analysis of ICES data on externally visible diseases (lymphocystis, epidermal hyperplasia/papilloma,
acute/healing skin ulceration) of dab from different North Sea regions, confi rmed the multifactorial
aetiology of the diseases under study since a number of natural and anthropogenic factors (stock
composition, water temperature, salinity, nutrients, contaminants in water, sediments and biota) were found
to be signifi cantly related to the long-term temporal changes in disease prevalence recorded. (Lang and
Wosniok 2000; Wosniok et al. 2000).
The presence of macroscopic liver neoplasms and of certain types of histopathological liver lesions is a
more direct indicator of contaminant effect and has been used for many years in environmental monitoring
programmes around the world. Liver neoplasms (either detected macroscopically or by histopathological
analysis) are likely to be associated to exposure to carcinogenic contaminants, including PAHs, and are therefore
considered appropriate indicators for contaminant-specifi c biological effects monitoring. The study of
liver histopathology (comprises the detection of more lesion categories (non-specifi c, neoplastic and nonneoplastic
toxicopathic lesions), refl ecting responses to a wider range of contaminants (including PAHs) but
also to other environmental stressors and is, therefore, considered an appropriate indicator for both general
and contaminant-specifi c biological effects monitoring.
The liver is the main organ involved in the detoxifi cation of xenobiotics and several categories of hepatocellular
pathology are now regarded as reliable biomarkers of toxic injury and representative of biological endpoints
of contaminant exposure (Myers et al. 1987, 1992, 1998; Stein et al. 1990; Vethaak & Wester 1996;
Stentiford et al. 2003; Feist et al. 2004). The majority of lesions observed in fi eld collected animals have also
been induced experimentally in a variety of fi sh species exposed to carcinogenic compounds, PAHs in particular,
providing strong supporting evidence that wild fi sh exhibiting these lesions could have been exposed
to such environmental contaminants.
Ecological relevance
Fish diseases are considered as ecosystem health indicators, refl ecting ecologically relevant effects of
environmental stressors at the individual and population levels. As such, they differ from other types of
indicators that refl ect changes at lower levels of biological organisation (e. g. molecules, cells) and the
ecological relevance of which is considered as low or unclear (e. g. biomarkers of exposure to contaminants.)
(ICES 2009b)