In a provocative paper analyzing the intricacies of this complex issue, Hall et al. (2006) begin their analysis of the subject by asking "Will an increasingly warmer world necessarily become a sicker world?" They posed this question because, in their words, "increased temperatures can accelerate the fitness of parasites, reduce recruitment bottlenecks for parasites during winter, and weaken hosts," while further noting that "warmer temperatures may allow vectors of parasites to expand their range," which would enable them to "introduce diseases to novel habitats," which is something climate alarmists frequently claim about mosquitoes and malaria. However, as they continue, "these doom-and-gloom scenarios do not necessarily apply to all taxa or all situations," and they note that "warming does not necessarily increase the fitness of all parasites."

Enlarging upon these latter points, the four biologists and their statistician co-author write that the "virulence of parasites may not change, may decrease, or may respond unimodally to increasing temperatures (Stacey et al., 2003; Thomas and Blanford, 2003)," and in this regard they further note that "vital rates increase with temperature until some optimum is reached," and that "once temperature exceeds this optimum, vital rates decline gradually with increasing temperature for some taxa, but rapidly for others," such that "in some host-parasite systems, a parasite's optimum occurs at cooler temperatures than the optimum of its host," citing the work of Carruthers et al. (1992), Blanford and Thomas (1999) and Blanford et al. (2003) on fungus-grasshopper associations in substantiation of this scenario. In such cases, as they describe it, "a host can use warmer temperatures to help defeat its parasites through behavioral modification of its thermal environment."

However, the situation sometimes can be even more complex than this; for Hall et al. write that "warmer temperatures can also lead to shifts in temperature optima (Huey and Hertz, 1984; Huey and Kingsolver, 1989, 1993)," and that "the exact evolutionary trajectory of host-parasite systems in a warmer world may depend sensitively upon underlying genetic correlation structures and interactions between host genotypes, parasite genotypes, and the environment (Blanford et al., 2003; Thomas and Blanford, 2003; Stacey et al., 2003; Mitchell et al., 2004)." Consequently, they conclude that "longer-term response of the physiology of host-parasite systems to global warming becomes difficult to predict."

But these considerations are not the end of the story either; for the researchers note that "other species can profoundly shape the outcome of parasitism in host populations," and that "predators provide an important example" because, as they elucidate, predators "can actually inhibit epidemics by selectively culling sick hosts and/or by maintaining host densities below levels required for parasites to persist (Hudson et al., 1992; Packer et al., 2003, Lafferty, 2004; Ostfeld and Holt, 2004; Duffey et al., 2005; Hall et al., 2005)." When all is said and done, therefore, Hall et al. conclude that "global warming does not necessarily mean that disease prevalence will increase in all systems."

Three years later, and writing in the journal Trends in Parasitology, Morgan and Wall (2009) echoed many of the salient points raised in the Hall et al. paper while further elucidating the complex nature of this topic. With respect to the relationship between temperature and parasitic development, the two authors note that "just as development rates of many parasites of veterinary importance increase with temperature, so [too] do their mortality rates [increase]." And they reiterate the notion that "temperature will also affect mortality indirectly through the action of predators, parasitoids, pathogens and competitors, whose development and abundance are also potentially temperature sensitive," so that, in the end, "the net effect of climate change could be complex and far from easily predicted."

In perusing the subject in greater detail, as they elucidate some of the many complexities involved, the two UK researchers indicate that "several biological mechanisms (including increased parasite mortality and more rapid acquisition of immunity), in tandem with changes in husbandry practices (including reproduction, housing, nutrition, breed selection, grazing patterns and other management interventions), might act to mitigate increased parasite development rates, preventing dramatic rises in overall levels of diseases." However, because "optimum mitigation strategies will be highly system specific and depend on detailed understanding of interactions between climate, parasite abundance, host availability and the cues for and economics of farmer intervention," as they characterize the situation, they conclude "there is a need for research that considers likely effects of climate change and mitigation strategies in terms of the whole host-parasite system, including anthropogenic responses, and not just in terms of parasite population dynamics." One year later, two such papers were published, both of which attempted to account for the response of human behavior on parasite abundance and disease incidence.

Focusing on cutaneous myiasis (blowfly strike) in sheep, Wall and Ellse (2011) employed a stochastic simulation model "to examine the changes in the seasonal incidence of ovine cutaneous myiasis on farms in the United Kingdom and the likely effects of changes in husbandry and control strategies" in the face of projected changes in climate. And in doing so, the authors made a special point of noting that "the ability of this model to successfully account for observed patterns of strike has been confirmed previously by comparison of predicted with observed strike incidence patterns observed on 370 farms in England and Wales," citing the work of Wall et al. (2002).

According to the two UK researchers, "the simulations show that the range of elevated temperatures predicted by current climate change scenarios result in an elongated blowfly season with earlier spring emergence and a higher cumulative incidence of strike," and that "overall, higher temperatures increased strike incidence disproportionately in ewes in early summer, but had relatively less direct effect on the pattern of lamb strike incidence," noting that "a 3°C increase in average temperature approximately doubles the cumulative incidence of strike in lambs but results in four times more strikes in ewes." However, as expressed in the concluding words of the researchers' abstract, "the simulations suggest that integrated changes in husbandry practices are likely to be able to manage expected increases in strike, given the range of climate changes currently predicted." And as they elucidate in the body of their text, "modest changes in husbandry practices should be able to manage expected increases in strike, under the range of climate changes currently predicted," demonstrating that "consideration of the likely impact of climate change must take into account animal management practices as well as parasite biology (Morgan and Wall, 2009)."

Working with two additional co-authors on the same host and parasite, Wall et al. (2011) came to a similar conclusion, reporting that "the models suggest that simple changes in some husbandry practices, such as shearing or trap use, could have an important effect in reducing early season ewe strike incidences," and that "practical measures exist which, with modest changes in husbandry practices, should be able to manage expected increases in strike." The key message of this latter work, however, was the authors conclusion that "simple extrapolations of the known effects of temperature on ectoparasite development," in an attempt to "predict changes in disease incidence in a warmer climate," is simply "too simplistic." Quite to the contrary, they write that "attempts to predict the likely impact of climate change on disease incidence must take into account changes in farmer behavior and animal management practices as well as parasite biology."

In one final study, Bentley and Burgner (2011) examined the host/parasite relationship between juvenile sockeye salmon (Oncorhynchus nerka) and the tapeworm Triaenophorus crassus in an Alaskan watershed that had experienced a 1.9°C increase in summer water temperature over the prior 46 years. At the onset of conducting their experiment, Bentley and Burgner hypothesized that the warming of the region "would have resulted in a corresponding increase in fish metabolism, and thus potential consumption rates, that would increase infestation rates of the tapeworm Triaenophorus crassus." To test their hypothesis, they compared infestation rate data for T. crassus collected between 1948 and 1960 with similar data obtained in 2008 and 2009 from the Wood River system of Bristol Bay, Alaska.

In the words of the two U.S. researchers from the University of Washington's School of Aquatic and Fishery Sciences, the following results were observed: (1) "comparing the average summer air temperature to the parasite prevalence of juvenile sockeye salmon, we found no significant relationship over the fifteen years of collected data," (2,3) "evaluating the influence of average summer air temperature on the parasite infestation rates of juvenile sockeye salmon, we again found no significant relationship for either parasite abundance or parasite intensity," (4) "when we compared the 13 years of historic parasite prevalence to equivalent data collected in 2008 and 2009, we did not find a statistically significant positive long-term trend in the data," (5) "the parasite abundance of examined sockeye salmon smolts also did not exhibit a statistically significant long-term trend using the eight years of historic data and the two years of contemporary data," and, finally, (6) "evaluating the relationship between time and parasite intensity produced similar results as the other five comparisons, with there not being a statistically significant positive relationship."

In light of the above findings, Bentley and Burgner write, in the concluding sentence of their paper, that their data demonstrate that "the complex effects of warming have not summed to generate a measurable change in the infestation rates of juvenile sockeye salmon in the Wood River system." Given the many factors involved in host/parasite inter-relationships, for example, together with their great complexities, it is quite possible that global warming will never significantly impact parasite infestation rates in the animals they attack. Therefore, global warming alarmists would do well to temper their over-the-top rhetoric on this projected future tragedy that they claim will occur because of CO2-induced global warming, since ever more data continue to indicate that this issue will likely turn out to be a non-problem ... or a problem that is easily solved by a modicum of human intervention.

References
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