What are the three ways to measure prevalence?

• A. Across regions, across species, within herd
• B. Point, period, lifetime prevalence
• C. Within-herd, between-herd, animal level
• D. Individual, population, community prevalence

Answer: (C)

Prevalence can be described at different levels of organization, which is why this trio fits best. Within-herd prevalence is the proportion of animals in a single herd that are diseased or seropositive, and it’s useful for understanding how widespread the infection is inside that specific herd. Between-herd prevalence (herd-level prevalence) looks across multiple herds and is typically expressed as the proportion of herds that have at least one case, or, alternatively, as the average within-herd prevalence across herds; this tells you how widespread the disease is across the population of herds. Animal-level prevalence combines all data to give the proportion of infected animals across the entire population, regardless of herd boundaries. Together, these three perspectives capture the distribution of disease from the fine-scale unit (within a herd) to the broader grouping (between herds) and the overall population level (animal-level). The other options describe prevalence in time (point, period, lifetime) or in broader contextual terms, not the common hierarchical ways of measuring prevalence across units and animals.