A shared pathogen reservoir can tip widespread infection into mass mortality

Within wood frog (Rana sylvatica) breeding ponds, within-season infection prevalence and environmental viral concentration jointly predict die-off timing, revealing a sharp transition to mass mortality.

• Why it matters: Mass mortality is often abrupt and hard to forecast from host traits or environmental conditions alone, limiting understanding of how low-intensity infections escalate.
• What they did: A field study of ranavirus epizootics compared die-off versus non-die-off pond-years, then a compartmental model linked waterborne viral concentration to dose-dependent progression from low- to high-intensity shedding states.
• The result: The model reproduces a two-phase trajectory and a sharp mortality threshold driven mainly by the high-to-low shedding ratio, host density, and pond volume, implying environmental feedback can trigger the shift in homogeneous hosts.

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