“Cannibalism and infectious disease: friends or foes?”

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Benjamin G. Van Allen, Forrest P. Dillemuth, Andrew J. Flick, Matthew J. Faldyn, David R. Clark, Volker H. W. Rudolf, and Bret D. Elderd

Cannibalism spreads disease? Not so fast! It can be a little more complicated...

Two fall armyworm (Spodoptera frugiperda) caterpillars of the same age (within 24 hours). The caterpillar on the left was infected with the baculovirus Autographica californica multinucleate nucleopolyhedrovirus (AcMNPV) shortly after hatching and remained at that stage while the uninfected larger caterpillar continued to grow and is in the 4th instar in this photo. The smaller, infected individual is a very inviting target for cannibalism by its much larger conspecific.
(Figure 4 from Van Allen et al., © 2017 The University of Chicago Press)

To a cannibal, a potential meal (a.k.a. the victim) may represent either a perfectly balanced protein snack or a breeding ground of virulent pathogens and parasites. The risk of disease transferred from victim to cannibal may weigh heavily against its nutritional advantages. Thus, potential disease transmission is no small part of why cannibalism is often considered a dangerous pursuit, at least from the cannibal’s perspective. This common line of thinking suggests that parasites may benefit more from cannibalism than the cannibal, even though cannibalism is ubiquitous in nature. This may be true for us humans, but for most species, cannibalism can actually stop or slow the spread of disease. Ben Van Allen and colleagues in the Elderd lab at LSU and Rudolf lab at Rice University combined their expertise in disease ecology and cannibalism theory to show that, as cannibalism increases in a population, it becomes harder and harder for parasites to invade and disease outbreaks to occur. Even if a parasite perfectly survives the transfer from victim to cannibal, as host population death rates increase due to cannibalism, the environment becomes worse for parasite transmission. Essentially, cannibalism can eliminate infected individuals and their disease from the population before epidemic thresholds are reached and outbreaks start. While there are parasites that may benefit from cannibalism indirectly by exploiting loopholes in the host life cycle, this work suggests that for some populations, cannibalism may be just what the doctor ordered. Read the Article