Where the wild things are

Parasitic diseases represent a pressing issue for human health and conservation. While lab studies have provided exquisite detail on host-parasite interactions they often lack whole organism or ecological context. Here Paulina Pontifes, a recent graduate from our Masters programme in Quantitative Methods in Biodiversity, Conservation and Epidemiology introduces us to the emerging field of wild immunology and tells us why it’s time to take immunology where the wild things are.

The wood mouse, Apodemus sylvaticus, is quickly being established as a prime study system for wild immunology. ©Paulina Pontifes

Where the wild things are

Finding food and a place to live are basic requirements for every living organism. Evolution has afforded life a staggering diversity of strategies to fulfil these demands, but one of the most successful is that of the parasites, which find both food and home in the hosts they colonise. Parasitism is such a fruitful strategy that, had Benjamin Franklin been a biologist, he might have included parasites as a third element when he wrote “In this world, nothing can be said to be certain, except death and taxes”.

Parasites thrive at the expense of their host—they divert host resources towards themselves and give little or nothing in return, curtailing host fitness and sometimes causing fatal harm. Under such a one-sided arrangement, it is in the host’s best interest to defend itself. In the struggle against parasitic invasion, the host’s immune system is key in dictating how well parasites establish themselves, survive and reproduce.

Most of our understanding of how the immune system curbs parasitic infections comes from studies on laboratory animals. These animals live in strictly controlled environmental conditions, and share a high degree of genetic relatedness to reduce the variation between individuals. Such precautions ensure that variation doesn’t obscure the molecular underpinnings of the immune system, which immunologists are interested in dissecting. However, in the real world most organisms live in both changing and challenging environments; their immune systems perform under varying environmental conditions, fluctuating resource availability and multiple parasitic challenges. Although helpful for molecular and cellular immunology research aims, laboratory conditions lack ecological realism.

A controlled life for mice in lab based research. Understanding Animal Research [CC BY 2.0], via Wikimedia Commons.

In recent years, an emerging field has begun to buck the laboratory standard in favour of wild animal populations. This field, which goes by the name of wild immunology or eco-immunology, draws from the ecologist’s toolbox to study the immune system at work in the settings under which it evolved, in the presence of diverse parasitic challenges and variable environmental conditions. In stark contrast to molecular immunologist, wild immunologists are interested in how the immune system integrates natural ecological variation.

Although relatively young, the field of wild immunology is growing rapidly and continues to bring new insights  to our understanding of disease dynamics and the immune system. Perhaps one of the most important concepts supported by wild immunology is that the immune response should be considered in the context of the whole organism and its environment.

Fighting off infection is a costly enterprise, but individuals also need to grow and reproduce.  All these needs  draw from a finite energy budget, which in turn depends on the availability of resources and/or energy reserves. While food scarcity is not a condition in lab studies, unless manipulated by researchers, food availability in the wild varies. Under limited resources, not all physiological activities can be equally tended to, and individuals may be faced with resource allocation decisions. For example, if challenged with parasitic infestation during the reproductive season, should organisms invest equally in both defence needs and producing offspring, or prioritise one  over the other? What does this physiological decision depend on? Considering the immune system in context has led to one of the most interesting questions posed in wild immunology research: how is immunity balanced in a variable world?

Here at the Institute, we have begun to explore how the immune system regulates infection under variable resource availability, using the wood mouse as a study system. In a joint effort over the past six years, a collaboration between Dr Simon Babayan’s (IBAHCM) group and that of Dr Amy Pedersen at the University of Edinburgh, has developed an array of molecular and immunological methods to measure the immune response in the wood mouse in the wild. These tools will allow us to disentangle the associations between parasite infection, resource availability and immune response in a wild system.

Getting wild samples can take a bit more time and effort than lab based studies. Here a wild field mouse is being released. Wildlife Wanderer [CC BY 2.0], via Flickr.

This will be a data intensive endeavour, because we intend to keep track of an array of immune genes, ecological and demographic factors and infection status of multiple parasites. However, given the potential role of parasitic infections in shaping population dynamics and their implications for human health, it is crucial to improve our understanding of the factors driving within-host infection outcomes; we must identify immune markers relevant for protective immunity in wild populations and determine the mechanistic links between individual level processes and population dynamics.

By building bridges between ecology and immunology, we are in a better position to explore the complex interactions and processes driving disease transmission, both at the individual and the population level. This is why we must take immunology where the wild things are.

Interested? Find out more about what wild immunologists in the UK are up to at the brilliant site run by the Institute of Evolutionary Biology and the Centre for Immunity Infection and Evolution at the University of Edinburgh, and beyond at the NSF Research Collaborative Network: Refining and Diversifying Ecoimmunology.  

Acknowledgements: A special thanks to Dr Simon Babayan for reviewing a draft of this blog entry and to the Naturally Speaking Team, especially J. Caryl and J. Burgon, for their helpful comments and contributions to this post.  

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