Can Scavengers Actually Reduce Disease Transmission?

Many organisms are vulnerable to a wide array of diseases and parasites throughout the course of their lives, but could scavengers help reduce that vulnerability? (Image Credit: The High Fin Sperm Whale, CC BY-SA 4.0)

Do scavengers prevent or promote disease transmission? The
effect of invertebrate scavenging on Ranavirus transmission (2019) Le Sage et al., Functional Ecology, https://doi.org/10.1111/1365-2435.13335

The Crux

As intimate as the host-parasite relationship is, it is important to keep in mind that it is embedded within a complex web of other interactions within the local ecological community. To add to this complexity, all of these interactions can feed back on and effect the host-parasite relationship. One ubiquitous part of all communities is the scavenger, an organism that feeds on dead and decomposing organisms. The authors of this paper wanted to investigate how scavengers affect disease transmission in local communities.

This question in interesting because it can easily go either way, depending on the community in question. Scavengers could lower disease transmission by eating infected organisms, thus removing contagious elements from the environment. However, scavengers could also increase transmission by promoting the spread of contagious elements in the community via their own waste after they consume infected tissues.

What They Did

The authors ran three experiments to assess the ability of a scavenger to impact disease dynamics. The first involved giving a salamander carcass to one of four different scavenger groups at three different densities, and allowing the scavengers to feed for one day. This allowed them to test for differences in scavenger type, as well as scavenger density. The second experiment involved exposing healthy salamander larvae to infected salamander corpses, which had either been fed upon by a scavenger or was blocked by a mesh wall. Each of these treatments also had a control, which consisted of healthy larvae exposed to non-scavenged, non-blocked infected carcasses.

The third experiment assessed how many carcasses an individual scavenger could effectively consume. A single scavenger was placed into an aquarium, after which either one, two, eight, or ten salamander carcasses were added. The scavengers were given two days to feed, after which the authors measured the amount consumed.

Did You Know: Opportunistic Scavengers

The scavengers used in this study do not solely consume animal carcasses, they also act as apex predators in the ponds that they call home. Dragonfly larvae and large beetle larvae compete with one another for both food and space, and will not hesitate to eat their competitors OR members of their own species.

What They Found

The two largest and most voracious scavengers (beetle and dragonfly larvae) ate the most of the salamander carcass, consuming 84% and 63% respectively. The two smaller scavenger groups (backswimmers and gammarids) ate significantly less, consuming 18% and 12% respectively.

68% of the healthy salamander larvae that were given free access to the infected salamander carcass became infected, while only 23% of the larvae exposed to a scavenged-carcass became infected. Interestingly, this was only slightly higher than the percentage of larvae infected when the carcass was behind a screen (16%).

For the scavenging efficiency experiment, individual beetles could only consume ~1-2 carcasses per day, so treatments with carcass abundances above 4 were saturated, meaning that the scavengers were full and couldn’t eat any more.

Dytiscid beetles like this are one of the many predators that act as scavengers in small ponds. (Image Credit: B Mlry, CC BY-NC 2.0)

Problems

The specific disease used in this experiment (Ranavirus) appears to be transmitted best when a healthy individual eats an infected individual. While this does happen in nature, another common vector for disease transmission involves the spread of infective stages throughout the water column, which then infect healthy organisms through touch alone. It would have been interesting to see the authors investigate other diseases/parasites that are transmitted via other routes.

So What?

Previous work and theory on the potential of scavengers to affect disease transmission has focused on just that, the potential of a scavenger to have an effect. This is due to the observational nature of many previous studies, as opposed to the experimental and manipulative approach the authors used in this paper.

The authors showed that scavengers reduce disease transmission in local communities when they consume infected carcasses. This opens the door for further studies in a variety of other systems, exploring how other disease, hosts, and scavengers interact and affect disease dynamics. Despite the variation that almost certainly exists in the natural world, this study has shown that yet another part of a host’s local community can interact with and affects its relationship with parasites and disease.

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