Age-structured next generation matrix and R₀ calculation for Mycobacterium avium subsp. paratuberculosis (MAP)

Area of Science:

• Veterinary Epidemiology
• Infectious Disease Modeling
• Animal Genetics

Background:

• Paratuberculosis (Johne's disease), caused by Mycobacterium avium subspecies paratuberculosis (MAP), presents significant eradication challenges in dairy herds.
• Key challenges include age-specific susceptibility, lifelong infection, variable shedding, low diagnostic sensitivity, long latency, and environmental persistence.

Purpose of the Study:

• To quantify MAP transmission dynamics in a closed, year-round calving, free stall dairy herd.
• To evaluate the potential of genetic selection and herd management interventions for reducing the basic reproduction number (R₀).

Main Methods:

• Development of an age-structured next-generation matrix (NGM) based on a compartmental model.
• The NGM incorporates dam-to-calf and environmental transmission pathways.
• Host demography and age-specific exposure are reflected through discrete one-week age groups and locations (calving pen, calf pen, adult pen).

Main Results:

• The baseline NGM yielded a basic reproduction number (R₀) of 2.90.
• The majority of new infections occurred in calves (91.1%), followed by newborns (5.01%) and adults (3.91%).
• Reducing initial susceptibility or increasing the age-specific susceptibility decrease rate were identified as key genetic targets to lower R₀ below one.

Conclusions:

• The study provides a framework for quantifying farm-level MAP transmission.
• Identifies critical host traits and herd management interventions for targeted control strategies.
• Highlights the importance of age-specific dynamics in MAP transmission and control.