Converging seasonal prevalence dynamics in experimental epidemics

Area of Science:

• Ecology
• Epidemiology
• Parasitology

Background:

• Seasonal patterns in infectious disease prevalence are common but poorly understood.
• Empirical studies on disease seasonality mechanisms are scarce.
• An aquatic host-parasite system (Daphnia magna and Hamiltosporidium tvärminnensis) exhibits natural seasonality.

Purpose of the Study:

• To experimentally investigate how initial parasite prevalence affects long-term seasonal disease dynamics.
• To determine the influence of starting conditions on the establishment and regularity of seasonal prevalence patterns.
• To test the long-term effects of different initial prevalence levels (5%, 50%, 100%) over four years.

Main Methods:

• Outdoor experimental populations of Daphnia magna were established with varying initial Hamiltosporidium tvärminnensis prevalence (5%, 50%, 100%).
• Prevalence dynamics were monitored over a four-year period.
• Seasonal changes and their amplitude were analyzed across different initial prevalence treatments.

Main Results:

• All treatments showed prevalence changes, but the 100% initial prevalence group exhibited significantly different seasonal patterns compared to 5% and 50%.
• Populations with 5% and 50% initial prevalence displayed strong, regular seasonality within the first year.
• A high initial prevalence (100%) resulted in low-amplitude seasonal changes throughout the experiment, indicating a prolonged effect of starting conditions.

Conclusions:

• The time required for disease prevalence to exhibit regular seasonal changes is strongly dependent on the initial infection prevalence.
• In short-lived host populations like Daphnia magna in rock pools, initial conditions may persist, preventing the full expression of typical seasonal dynamics.