Disease's hidden death toll: Using parasite aggregation patterns to quantify landscape-level host mortality in a...

Area of Science:

• Ecology
• Parasitology
• Wildlife disease

Background:

• Infectious diseases are a major cause of mortality in humans and livestock, and likely even more so in wildlife populations.
• Estimating disease-induced mortality in wildlife, particularly for endemic infections, is challenging due to the difficulty of direct observation.
• Quantifying the population- and landscape-level effects of endemic pathogens on host mortality is a critical but rarely answered question.

Purpose of the Study:

• To indirectly estimate the population- and landscape-level mortality induced by an endemic parasite, *Ribeiroia ondatrae*, in amphibian populations.
• To develop and apply a novel mathematical and statistical framework for inferring host mortality from parasite abundance data.
• To provide a rare glimpse into the magnitude of effects endemic parasites can have on wildlife populations.

Main Methods:

• Combined laboratory experiments, extensive field data, and novel mathematical models to indirectly estimate mortality.
• Developed a flexible Bayesian statistical model that uses patterns of parasite abundance aggregation to infer host mortality.
• Relaxed restrictive assumptions on mortality timing and sampling, and accommodated data from both lab and field settings.

Main Results:

• Trematode infection was associated with an average of 13% to 40% population-level mortality across 301 amphibian populations.
• For three of four amphibian species, models predicted some populations experienced over 90% mortality due to infection.
• At the landscape scale, a small proportion of sites (<20%) contributed to a large majority (>80%) of amphibian mortality.

Conclusions:

• Endemic parasites can cause substantial population-level mortality in wildlife, with significant landscape-level consequences.
• The developed theoretical framework for indirectly inferring parasite-induced mortality can be applied to other host-parasite systems.
• This study highlights the hidden death toll of pathogens on wildlife populations and the importance of understanding these impacts.