As many of you may know, a lot of research is done using guppies and their ectoparasite Gyrodactylus turnbulli as a model system for ecological and epidemic dynamics:
Gyrodactylus on a fish. Video credit: Christina Gheorghiu.
The really cool thing about these parasites is that they’re easy to observe and to quantify, so we can collect data on their presence and growth-rates over time without having to sacrifice the host (guppy). Also they reproduce exponentially and are transmitted directly via host-host contact thus causing epidemics in the wild, a characteristic atypical of most macroparasites. These two characteristics combined make G. turnbulli a very convenient model parasite for studying epidemic dynamics, as has been done here, here, and here. Epidemics and high parasite loads (sometimes reaching well over 300 parasites per fish) lead to high levels of guppy mortality, making this system also well suited for the study of host-parasite coevolution.
Body condition (a metric of individual weight to length ratio) is used as a common proxy for health or well being. While research on how an individual’s food intake can affect their overall body condition and disease resistance is abundant, the question of how these relationships might translate to the population scale remained unanswered. Thus we decided to investigate how food availability in the environment and host body condition relative to others in the population affected the overall outcomes of an epidemic, using the guppy-Gyrodactylus system as a model. To answer this question, we set up laboratory populations of guppies and subjected them to different levels of food availability and measured their relative body conditions (i.e. how much better or worse-off were they relative to all other experimental fish). Then we introduced parasites to the populations by infecting one randomly chosen fish from each group and monitored the epidemics over time. Pretty simple and it gives a huge load of information.
If you want to read more about our methods, measurements and analyses, you can do so here. Below we provide an overview of our most interesting results.
The first cool thing that we found is that in general, host relative body condition was positively associated with parasitism. This would in principle mean that the healthier a fish is, the more parasites it will have; nonetheless it can also mean that the “fatter” a fish is for its length, the more likely it is to have parasites, and when it is infected, the more likely it is to have more parasites. Specifically, we found that the incidence of parasitism (the number of fish that became infected over the course of our experiment) was greater in populations with a higher average condition index. This is pretty counter-intuitive, and it was therefore assumed that those with a high relative condition index would be more resistant to disease.
Another interesting finding was that the relative condition of the fish we used to introduce the disease to the population (the “source” fish) mattered a lot. Almost all of our epidemic variables were significantly affected by this factor. Specifically we found that the peak burden of parasites in the populations was significantly positively impacted by the relative condition index of the “source” fish. Moreover, we found that parasites were more aggregated (crowded on one particular fish) when the condition of the source fish was high, and that this crowding usually occurred on high condition source fish, particularly when the average condition of the population was low. This is exciting because these results indicate that the way in which a disease is introduced to a population, or rather, the characteristics of the host through which it is introduced, can have significant impacts on the course of the epidemic (both the burden and distribution of parasites) in the population as a whole. To our knowledge, this is a novel result that could inspire further investigation.
So why would higher condition fish have more parasites? One theory is that larger fish are so because they invest more energy into growth rather than into defense against disease, making them more susceptible to parasite infection. It is also possible that larger fish simply make better hosts because they provide more resources, allowing the parasites to rapidly grow and reproduce while making them less likely to transfer to a host of lower quality. This idea seems to fit our result that parasites were strongly aggregated on hosts of high condition.
But what about food availability? While our results did not indicate any significant impacts of food availability alone, we did find that the interaction of food availability with the condition of the source fish negatively impacted our epidemic parameters. What this means is that the positive relationship we found between peak parasite burden/aggregation and source condition was dampened (the slope of the regression is not as steep) as food availability increased. We think this means that in populations with high food availability, fish may be able to consume more resources and dedicate that energy towards resistance, rather than fat storage or growth, thus decreasing their “quality” as a host causing parasites to grow at a slower rate and forcing them to disperse throughout the population in search of a better host.
Overall, our results present some new and interesting ideas which we hope to follow-up on in future investigations.