Challenging the host-parasite binary

parasite kaledascope

In a critique of conventional approaches to considering host-parasite relationships in the context of disease management and ecology, Horwitz and Wilcox (2005) describe the host-parasite classification as “necessarily arbitrary”. The basis of this is that parasites and hosts exist in, and belong to, complex adaptive systems. The co-evolution of parasites and their hosts has been well documented and is reflected in sentiments that parasites and hosts are “intimate enemies”. However, this is typically viewed as an adversarial “arms race” as opposed to an opportunity for mutual growth. Management of parasitic infections is therefore conventionally an attempt to prevail over, rather than coexist with, parasites, which has its flaws.

Horwitz and Wilcox (2005) discuss the complexity and dynamism of parasite-host interactions, including the long and short-term benefits of being parasitised as reasons why current management approaches are unsustainable. They point out how our overuse of antibiotics in treating parasite-borne diseases, such as Lyme disease, has the incidental effect of driving selection of drug-resistant parasites. They also highlight how this overuse of antibiotics hinders the development of immunological responses in our population. They suggest that, in turn, our current practices may have sabotaged the human species on an evolutionary scale.

In light of this, I believe we need to consider whether our public health approaches to parasite control should also take into account intergenerational consequences and to whom we, as scientists, owe a duty of care. This is an ethical dilemma that is increasingly relevant as we continue to change ecosystems with climate and habitat destruction. When enacting public health policy, how do we balance the immediate disease burden and the flow-on effects to the human species?

While I agree with the overall sentiment that our approaches are short-sighted, we should not consider endangering the immediate population for a speculative evolutionary advantage. Attempting to adjust the host-parasite relationship is in and of itself a manipulation to intentionally skew evolutionary outcomes. In addition, from an ethical standpoint, focussing on the long-term fitness of the human species at the expense of the existing population is irresponsible. Public confidence in science and health policy would be undermined if we prioritised maintaining continuous host-parasite interactions above optimal treatment of parasitic diseases. I certainly don’t believe that the potential that mass-vaccination programs may place a selective pressure on parasites to develop mechanisms for dispersal to the non-vaccinated justifies abandonment of such programs.

I agree wholeheartedly that our approaches to parasites must be more “rounded”. Taking a multi-faceted view is the only way we can manage disease risk. However, I think that placing too great an emphasis on evolutionary repercussions of our actions is dangerous. Instead, we should prioritise the adoption of more integrated, approaches to preventing infection or re-infection. This would not only protect people at risk of exposure, but also restrict the movement of parasites, without withholding them from their natural, zoonotic ecosystems.

In summary, Horowitz and Wilcox highlight the unsustainability of our current practices in parasite management as well as the consequent complex moral quandary that arises in consideration of alternative approaches.

Tunan (Nicole) YuChristina Spry

 - Tunan (Nicole) Yu (ANU Undergraduate student) in collaboration with Dr. Christina Spry

References

Horwitz, P., Wilcox, B.A. (2005) Parasites, ecosystems and sustainability: an ecological and complex systems perspective. International Journal for Parasitology, 35: 725 - 732  https://doi.org/10.1016/j.ijpara.2005.03.002
 

Read more about Parasites: The Good, The Bad, and The Ugly.