Parasites are aggregated, but how are contaminants distributed among the hosts? And why does it matter?
A recent paper by André Morrill, Jenn Provencher and myself recently asked and partially answered these questions. We reviewed the literature over 23 years to show that interest in the combined effects of contaminants and parasites on host health was increasing exponentially.
In actual fact, the studies were of three main types: one type was the extent to which parasites bio-magnify the contaminants of their hosts. This effect of such parasites on contaminant burdens in hosts may well have host health effects. Much to build on there! Why parasites (typically intestinal helminthes are studied) even do such a thing is fascinating?
The second type of study was comparative in nature: for example, to what extent do hosts from contaminated habitats harbor more parasites than host from less contaminated or (more) pristine habitats. Many researchers were interested in this type of question because of the potential dual impacts of both environmental degradation and parasites on their hosts. But many of those same studies show negative correlations between contaminants and parasites. This is perhaps because the contaminants are detrimental to infective stages of the parasites or detrimental to the intermediate hosts.
The third type of study concerned to what extent there were correlations between actual contaminant concentrations in host tissue and the number of parasites that the host was harboring. If parasites are aggregated in the host and contaminant distributions follow a lognormal distribution (as they often do) then the neutral co-distribution of these two factors follows a hollow curve (see the paper for theoretical null expectations). Of course, there could be a positive relationship if contaminants reduced effective immunity against the parasites in question. And there are some studies that show that increased contamination leads to increased parasitism as expected if contaminants interfere with host immunity. But such linkages are still far from being widely tested.
What does this mean for studies on the dual impacts of parasites and contaminants on host? If there is no underlying relation between these two factors, one has to be concerned about how to even study the potential dual impacts. Most of the hosts would have low numbers of parasites and low contaminant burdens (if one factor does not affect the other) and relatively few hosts would have average numbers of parasites combined with average levels of contaminants (just by chance). What happens to many host individuals is probably more important to the population than what happens to few host individuals. So it makes some sense at least to compare hosts with combined levels of contaminants that are still low with those hosts having low levels of only one of the two factors, in terms of host reproductive output and survival. The fact that such hollow curves are being reported both in studies of parasite co-infections and parasite contaminant relations should help garner more interest in how to test impacts of multiple assaults.