For simplicity, we often assume that all hosts have an equal probability of becoming infected by and transmitting parasites and pathogens. But of course, we know that it isn’t how real systems work. For instance, in real systems, hosts vary in their propensity to become infected by pathogens, and that variation is one probable cause of the parasite ecology “law” that macroparasites are aggregately distributed among hosts. We call hosts that are highly susceptible to a given pathogen “superreceivers,” and hosts that are highly likely to transmit a pathogen are “superspreaders.”
Here’s a question for you to ponder: are superspreaders usually superreceivers and/or are superreceivers usually superspreaders? For instance, sex workers are at high risk for contracting HIV (=superreceivers) because they frequently change sex partners, and they’re also highly likely to spread HIV (=superspreader), if they have it, in comparison to the average person. In that case, the superreceivers are also superspreaders. When that happens, we might predict really explosive epidemics whenever “patient zero” is a superreceiver+superspreader, because R0 will be very, very high.
But consider the Tasmanian devil example that I posted about recently. Tasmanian devils that bite lots of individuals are highly likely to contract Tasmanian devil facial tumor disease; they’re superreceivers. But being bitten by an infected individual doesn’t seem to transmit the infectious cancer to the receiving host, so devils that bite frequently don’t transmit any more frequently than devils that don’t bite frequently. Therefore, the superreceivers in that system aren’t superspreaders.
Now let’s talk about a really cool system that I somehow haven’t blogged about yet. House finches are hosts for an emerging bacterial pathogen (Mycoplasma gallisepticum – Mg) that jumped from poultry into house finches in the 1990s. This pathogen causes conjunctivitis in the house finches – a symptom you don’t often think about in wildlife! In a really neat recent paper, Adelman et al. (2015) showed that birds that spent more time on bird feeders were more likely to become infected by (superreceivers) and transmit (superspreaders) Mg. This is a really cool example of a pathogen that appears to be transmitted by “fomites”: inanimate objects that the pathogen can survive on when off the host.
We probably don’t have enough examples in the literature to determine whether superspreaders are usually superreceivers or to look for generalities in systems where this occurs. But we’re accumulating more examples all the time! Stay tuned.
…if academics were at higher risk of developing conjunctivitis when they sought out free food, I’d have some very squinty-eyed colleagues.
Adelman, J.S., S.C. Moyers, D.R. Farine, and D.M. Hawley. 2015. Feeder use predicts both acquisition and transmission of a contagious pathogen in a North American songbird. Proc Biol Sci. 282(1815): 20151429.