The “Unhealthy Herd Hypothesis”: Understanding When Predation Increases Parasitism

The field of disease ecology has mainly focused on host-parasite interactions as an isolated relationship, but ongoing research continues to find that predators play an important role in these interactions. In the past, predation was thought to reduce disease in populations of prey, known as the “healthy herds hypothesis”. Now, researchers are learning that predators can increase disease prevalence in their prey in a variety of ways. 

The “healthy herds hypothesis” speculates that predators can significantly decrease the presence of parasites in their prey by consuming infected individuals. Further research on this topic has revealed that the impact of predators on disease prevalence in their prey varies greatly, and often predators increase parasitism in their prey. This review focuses on mechanisms by which predation can increase disease in prey populations. 

This review by CEID’s Robert Richards and a team of experts on disease ecology examines the mechanisms by which predators influence parasitism and provides recommendations for future research to learn more about factors that promote predators to spread disease. 

Firstly, “sloppy predators” can increase parasitism by consuming their prey in a manner that increases transmission of parasites to other prey. Typically, predation methods that spread the tissue, internal organs, or blood of  prey contributes to disease spread given that the prey behaviors expose them to these infection sources (ex: filter feeding or scavenging). 

Another mechanism of predator-spreading is “partial predation”, where the predator causes a wound where parasites can directly enter into the prey’s body. Furthermore, the prey’s immune system must dedicate energy to repairing the wound, taking focus away from parasite defense functions. Predators can also be vectors for disease spread by becoming contaminated with the parasite after preying on an infected individual. When the predator preys on another individual and wounds it, the infection may spread.

Predators can also spread disease by ingesting parasites, since more resilient parasites can pass through the digestive tract and be excreted. Factors such as the type of feces excreted, prey avoidance/preference to consume excrement, and the parasite dose needed to infect prey can impact this type of predator spreading. 

Selective predation is another mechanism of predator-spreading. Predators may avoid infected prey, leading to increased parasitism in the prey population. Certain predators preferentially prey on specific size or age groups of prey, which may be more or less likely to carry infections.

In response to predation pressure, prey often alter their behavior, physiology, and immunity. Future studies should focus on the relationship between predation pressure and these changes in prey, as well as examining multiple mechanisms of predator transmission in a predator-prey system.

By: Brenna Daly