Assistant Professor of Biology Cynthia Downs is the lead author of a recently published paper in Trends in Parasitology titled “Scaling of Host Competence.” Downs and co-investigators developed a framework for investigating how the body size of the parasites’ hosts affected how effectively the hosts spread the disease-causing parasite to other hosts. They demonstrated this model using three human-relevant parasites – lyme disease, rabies, and West Nile virus.
Downs has been interested in how traits scale (change) with body size since the first paper she wrote during her Ph.D. program. Currently, her lab is exploring how body size shapes the architecture of the immune system; she currently has a National Science Foundation grant to study this idea.
“Large hosts are not small hosts writ large when it comes to disease risk,” said Downs. “That is, large hosts are disproportionally more or less better at amplifying a parasite then would be expected if the large hosts were just a scaled up version of the small host. If you put the individual in a community made of populations of different host species, the scaling of population density helps reduce the effectiveness of large hosts of amplifying a parasite. The framework we developed, can be used to develop testable predictions for host-parasite systems, and those predictions can be used to informed models for disease spread.
“Most emerging disease in humans are zoonotics that originated in animal hosts; Lyme disease is an example. Animal hosts can act as a reservoir for these parasites, so understanding how the composition of communities affects the spread of disease-causing parasites is important for understanding disease risk in humans.
“This research provides a framework for asking questions about the importance of specific species in the spread of parasites. For example, I think that it's interesting that our framework predicted that population density might mitigate the predicted high competence of large host species for some parasites. By extension, our framework predicts that the naturally low density of large species acts as a defense against diseases-causing parasites and that increasing the density of populations of large species could increase the probability of disease outbreaks.
“The importance of density in the spread of parasites is well known, but our model suggests that host body size might be important when predicting how changes in population density affect the spread of disease. A change in the density of a large host might affect parasite dynamics in a community differently than a change in the population density of a small host.”