Herdlevel

Herdlevel is a term used in veterinary epidemiology and livestock management to describe the health status or risk of infection at the level of an entire herd, rather than at individual animals. It is used to summarize whether a group of animals is free of a disease, exposed, or infected, and to guide interventions such as vaccination, quarantine, or movement controls. Common herdlevel measures include herd-level prevalence or seroprevalence, defined as the fraction of herds in a population that have at least one positive animal, or that meet a defined disease status.

In practice, herdlevel assessment often relies on sampling a subset of animals within each herd and applying decision rules to determine the herd’s status. Data can come from serology, molecular tests like PCR, or observable clinical signs. Two-stage sampling designs are common: selecting herds first, then sampling within herds. Estimates must account for test sensitivity and specificity at the herd level, as well as within-herd variation.

Herdlevel metrics are used to allocate resources, target vaccination campaigns, implement control measures, and monitor progress toward disease elimination. They enable policymakers and producers to evaluate risk and prioritize actions across a population of herds rather than focusing solely on individual animals.

Limitations include potential misclassification due to imperfect tests, sampling bias, and ecological fallacy when inferring individual risk from herdlevel results. The concept intersects with herd immunity discussions, where the proportion of immune animals at the population level influences transmission dynamics across herds.