Research note: Spatial and temporal distribution of poultry red mite infestations in non-caged barn and free-range laying hen systems

Poultry red mite (PRM; Dermanyssus gallinae) is a widespread ectoparasite of laying hens, causing major health, welfare, and economic losses. Its nocturnal feeding and concealment in housing structures make infestations difficult to detect and control. PRM control requires insight into spatial and temporal infestation dynamics, which in non-caged laying hen systems may be influenced by housing characteristics, including structural complexity and outdoor access. This study investigated variation in PRM infestations within poultry houses, between housing systems, and over time. Two aviary-housed laying hen flocks on the same farm, one flock housed indoors and one flock with free-range outdoor access, were monitored longitudinally for 9 months. PRM infestation was assessed every 2 weeks using AviVet® traps placed at multiple locations and quantified by trap weight. In the outdoor house, PRM were first detected at sidewall locations adjacent to the outdoor range and spread throughout the house within 4-6 weeks. After Exzolt® (fluralaner) treatment, the house remained PRM-negative for 3 months. Reinfestation occurred at the same sidewalls, where the highest PRM levels were observed, with trap weights ranging from 0.2 to 473.4 mg per trap. Lower levels (0.1–60.9 mg per trap) were found at the rear, central, and front areas. The indoor house maintained low PRM levels (0.1–16.4 mg per trap). To examine variation across housing systems, a cross-sectional study was conducted in 6 laying hen houses on 4 farms: 1 indoor house without covered outdoor range, 3 with a covered outdoor range, and 2 organic free-range houses. Three of the 6 houses were PRM-positive (mean trap weights per house: 1922.1, 24.4, and 51.8 mg). PRM distribution varied across flocks, with no significant differences between structural locations. Overall, PRM infestations in non-caged systems showed substantial spatial and temporal variation within and between houses. These findings highlight the need for structured, multi-location, and repeated PRM monitoring to detect early infestation hotspots and enable timely interventions to support layer health and welfare and reduce economic losses.