Are you super?

Disease ecology often uses the term, “super-spreader,” which describes an individual responsible for a disproportionate amount of disease transmission. For example, in the diagram below, each circle or triangle represents an infected individual. Each circle infects four more circles, but each triangle infects only one. After only a few rounds of infection, there are far more circles than triangles, and the circles would be called super-spreaders.transmission

However, the spread of infection depends on more than the transmitter. Some individuals may be far more likely to receive an infection than others (these individuals could be called “super-receivers”).

super receivers

As one might guess, if an individual were both a super-spreader and a super-receiver, they would greatly impact disease spread in a population. Whether such individuals exist, however, remains an open question.

Adelman et al. (2015) recently investigated this question in house finches (Haemorhous mexicanus) and their bacterial parasite, Mycoplasma gallisepticum, which causes conjunctivitis. The authors performed two studies. In the first, they monitored the presence of tagged birds at feeders in order assess feeder use, aggressive interactions at feeders, and aspects of sociality (such as local bird group size, number of birds with which a focal bird usually feeds, and a few others). The authors then compared these measures with the presence of infection. Interestingly, the authors found limited connection between a bird’s aggressive or social interactions and whether that bird was infected. But feeder use did strongly associate with infection. Birds that spent more time foraging at feeders were more likely to be infected.

However, because this study did not manipulate infection, it was unclear whether spending more time at feeders led to infection, or whether infection led to increased time at feeders. Consequently, in a second study, Adelman et al. created artificial bird groups in captivity, where each bird had a known foraging propensity. Then, the authors introduced the infection to each group, but to a bird of different foraging propensity each time. In this experiment, the infection spread faster in groups in which the initial infection was on a bird that spent more time at feeders. Although, because groups for which the initial infection was on a “high time forager” also had higher average foraging times than groups for which the initial infection was on a “low time forager,” the result that the parasite spread faster in the “high time forager” groups could be due to the initial individual infected or the high average foraging of the group. Foraging clearly affects transmission of the disease, but it is still unclear whether foraging does so through super-spreaders, super-receivers, or both.

Citations

Adelman, J.S., Moyers, S.C., Farine, D.R. & Hawley, D.M. (2015). Feeder use predicts both acquisition and transmission of a contagious pathogen in a North American songbird. Proceedings. Biological sciences / The Royal Society, 282.