Biology Reference
In-Depth Information
The consequences and the virulence of epidemics on populations are multifactorial, and
so is the etiology of cancer (Thompson and Hall 1993). One of these factors is undoubtedly
the fitness of the immune system, which in turn, as discussed above, can be affected by
many factors, including exposure to anthropogenic compounds. The difficulty of linking
exposure to potentially immunotoxic chemicals with a higher occurrence of a disease or an
increase of clinical signs lies in the plasticity of each of the organizational levels of the sys-
tem (i.e., molecular, cellular, tissue, organ, individual, and population). Fortunately, every
single perturbation at the molecular or cellular level will not translate into clinical signs
or pathology. However, this plasticity is limited and can also be influenced by different
factors. Therefore, for a specific group within a population, an individual can be basically
categorized as “resistant,” “normal,” or “sensitive” to a particular pathogen (Figure 6.1).
Morbilliviruses are known to be highly virulent and to cause serious mortality events in
a sero-negative population of mammals (Barrett 1999). However, exposure to immunosup-
pressive compounds could contribute to the general stress affecting the immune system,
amplifying the mass mortalities associated with epizootic episodes or tumor occurrence.
This has been partly supported by several studies reporting higher concentrations of PTSs
in animals that have died from morbillivirus infections, as compared with free-ranging
animals or animals that died from physical trauma (Kuehl et al. 1991; Hall et al. 1992;
Aguilar and Borrell 1994; Kajiwara et al. 2008b).
Moreover, serological evidence has established that harbor and gray seals from the less
contaminated Canadian East coast have been infected with a virus that was similar or
identical to PDV before 1988 with no signs of unusual mortalities (Henderson et al. 1992;
Ross et al. 1992). Differences in species sensitivity to immunotoxic compounds, and par-
ticularly to PCBs, could also explain the higher sensitivity of harbor seals as compared to
gray seals (Hammond et al. 2005).
In conclusion, in marine mammals and humans, the establishment of a relationship
linking exposure to PTSs, the impacts on the immune system, and the host's reduced resis-
tance to infection and cancer is complicated by numerous confounding factors. However,
there is a very convincing body of evidence indicating that environmental pollution can
significantly contribute to stress on the immune response resulting in major consequences
on the health of marine mammal populations.
Sensitive
Normal
Resistant
Strongly affected
Moderately
Mildly or unaffected
Resistance to a pathogen
FIGURE 6.1
Resistance to a pathogen. Shift from normal resistance to a pathogen in a population after exposure to immu-
notoxic stress. The exposed population (heavy line) will be at risk of developing clinical signs and disease
compared to the unexposed population (fine line).
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