I just read a neat 2013 Ecology Letters paper. Here’s the question: how does host density affect disease transmission?
Civitello et al. (2013) studied a simple aquatic disease system. Daphnia dentifera consume fungal spores (Metschnikowia bicuspidate) as they forage on algae, the fungus reproduces in the Daphnia host, and when the host dies, more spores are released into the environment.
Typically, we might predict that as the density of Daphnia increases, we might see increased disease transmission. There are more hosts eating more spores and thus more spores are being produced, so an increase in transmission seems to make sense. However, Civitello et al. (2013) proposed two ways that increased host density might decrease transmission. First, increasing the number of foraging Daphnia should increase the number of spores being taken out of the environment. So, perhaps higher Daphnia densities actually deplete spore densities, making susceptible hosts less likely to consume spores. Second, Daphnia are known for their intraspecific interference competition. The more Daphnia you have, the lower their foraging rates, and perhaps the lower the encounter rates between susceptible hosts and spores.
To figure out how Daphnia density affects fungus transmission, Civitello et al. (2012) did four things. First, they built submodels of disease transmission that just looked at Daphnia’s consumption of spores. Then they did an experiment where they varied Daphnia and spore densities. They used that infection and spore consumption data to parameterize their submodels, and to see which submodel performed the best. Next, they stuck their submodels into a full epidemiological model of the system and determined how Ro and infection prevalence changed with host density. Finally, they compared the results of the overall model (with each submodel) to field data.
Here’s what they found. In the experiment, transmission increased with spore density, but decreased with host density. The submodels that included interference competition among the Daphnia best described those results. When they put that submodel into the full epidemiological model, they found something surprising: the model predicted that infection prevalence and Ro should have unimodal relationships with host density. Guess what? That’s exactly what they saw in the field! Infection doesn’t occur until you have some minimum density, it increases to some optimum, and then declines until the prevalence is 0 at host densities where interference competition reduces foraging on spores below levels that can maintain the parasite population. NEAT!
Do you remember my recent posts about how the vital rates of malaria and mosquitoes (and Ro) have unimodal relationships with temperature? Now here’s a paper saying that Ro has a unimodal relationship with host density, instead of a positive, linear relationship. Are you sensing a trend? It looks like transmission rates don’t just increase linearly to infinity.
Here’s my favorite line of the paper: “The linear interference model statistically crushed the standard and phenomenological alternatives.” I hope to be cool enough someday to say that my hypothesis CRUSHED the status quo. I’ve added that goal to my bucket list.
What do you think? Were you surprised that competition can regulate parasite transmission?
Civitello, D.J., S. Pearsall, M.A. Duffy, and S.R. Hall. 2013. Parasite consumption and host interference can inhibit disease spread in dense populations. Ecology Letters.