Page 919 - The Toxicology of Fishes

The Effects of Polycyclic Aromatic Hydrocarbons in Fish from Puget Sound, Washington 899

To determine if salmon from a contaminated environment are also more susceptible to an infectious
disease, we collected juvenile fall-run Chinook salmon from an urban estuary and from a nonurban estuary
and the respective releasing hatcheries upstream from these estuaries, and we exposed them in the
laboratory to the marine bacterial pathogen Listonella anguillarum. We found that juvenile Chinook salmon
from the contaminated estuary were more susceptible to L. anguillarum-induced mortality than were fish
from the corresponding hatchery upstream from the estuary, which were not exposed to contaminants. In
contrast, juvenile fall Chinook salmon from a nonurban estuary showed no increase in susceptibility to
L. anguillarum-induced mortality compared to fish from the corresponding hatchery (Figure 22.14). These
disease challenge studies indicated that juvenile Chinook salmon with contaminant-associated immuno-
dysfunction were also more susceptible to one of their natural pathogens (Arkoosh et al., 1998). Follow-
up laboratory exposure studies with sediment extracts and contaminant model mixtures determined that
contaminants, apart from other estuarine variables specifically associated with the Duwamish and Hylebos
Waterways, could independently suppress immune function and increase disease susceptibility in juvenile
Chinook salmon (Arkoosh et al., 1994, 2001). Because these fish were exposed to mixtures of contami-
nants, there was some uncertainty regarding the relative contributions of PAHs and other chemicals present
in sediment (e.g., PCBs) to the observed reductions in disease resistance. In a recent disease challenge
study with L. anguillarum, Palm et al. (2003) found little indication of reduced disease resistance in
juvenile Chinook salmon exposed to dietary PAHs at environmentally relevant concentrations; however,
these studies were conducted in freshwater, in spite of the fact that L. anguillarum is a saltwater pathogen,
so the results may not reflect the virulence of L. anguillarum in the natural environment.
To better characterize the effects of PAHs on immunocompetence, we exposed juvenile rainbow trout
(Oncorhynchus mykiss) to PAHs in their diet at environmentally relevant concentrations, similar to those
found in stomach contents of Chinook salmon (Oncorhynchus tshawytscha) from contaminated estuaries
in Puget Sound (Arkoosh et al., 1998), and examined changes in disease resistance and expression of
immune-regulating genes (Bravo, 2005). Like salmon in earlier experiments, these fish exhibited higher
susceptibility to the pathogen, Aeromonas salmonicida, than fish fed the control diet. When we profiled
gene expression in head kidney using microarrays, we found that over 20 immunologically relevant
genes were differentially expressed after pathogen challenge. Transcripts from five immune genes—inter-
leukin 8 (IL-8), transport associated protein 1 (TAP1), NF-κB essential modulator (NEMO), recombi-
nation activating gene 2 (RAG 2), and a major histocompatibility complex II (MHC II) gene—were also
measured by RT–PCR at the time points examined by microarray. These genes participate in innate and
adaptive immunity and some are key regulators of immune response such as NF-κB modulator (Mann
et al., 2001). Moreover, they have been previously been described as important components of resistance
of salmon to A. salmonicida (Gerwick et al., 2002; Vanya et al., 2005). All selected genes were
significantly down regulated by 2 days post-challenge, suggesting that PAH exposure decreases tran-
scription of genes involved in the immune response.
Evidence also suggests altered immune function in English sole exposed to PAHs in the field, PAH-
contaminated sediment, or PAH-contaminated sediment extracts from Eagle Harbor (Arkoosh et al.,
1996; Clemons et al., 2000) (Figure 22.15 and Figure 22.16). Eagle Harbor, the site of a former creosote
plant, is located at Bainbridge Island in Puget Sound and has sediments characterized by high levels of
PAHs. In sole exposed to PAHs, the leukoproliferative (mitogenic) response and macrophage production
of cytotoxic reactive oxygen intermediates were augmented. In other species, increased production of
reactive oxygen species by macrophages has been associated with an increase in peroxidative damage
of kidney and gill tissues (Bravo, 2005; Fatima et al., 2000), but the effects of augmented reactive oxygen
intermediate production and leukoproliferative response on English sole’s immune function are unknown.
Studies are currently underway to better establish the linkage between these changes and disease
susceptibility in English sole.

Growth
Studies over several years with juvenile Chinook salmon from the Duwamish and Hylebos Waterways
suggest that exposure to PAHs may suppress growth in this species (Casillas et al., 1995, 1998a). Growth
was monitored in juvenile salmon collected from these sites and held in the laboratory for 90 days, and
it was found that growth rates for the fish from urban estuaries were lower than those for fish from the