The relationship between infestation level and the welfare of cage-free laying hens experimentally infested with northern fowl mites

Northern fowl mites are the most economically important and damaging ectoparasite in the U.S. egg industry. The heterogeneous nature of mite infestation poses a unique welfare challenge; some hens become severely infested and others are more resistant to infestation. This study sought to characterize the relationship between mite infestation level and laying hen welfare parameters. Purebred white laying hens (replicate 1: n = 393 hens; replicate 2: n = 600 hens) were randomly allocated to cage-free floor pens (n = 40 hens/pen; 5 pens/room) in a research facility at 17 wk. Each hen was infested with 50 northern fowl mites at 24 wk. At 12, 16, 20, 24, 26, 30, and 40-41 wk, welfare assessments were conducted and each hen was assessed for the following parameters: body weight; condition of the beak, footpads, feathers (neck, back, left wing, right wing, tail, and belly); toe damage; keel bone damage (deviations and tip fractures); and mite-associated measures (mite score, skin inflammation and area of inflammation, and skin lesions). After peak infestation (30 wk), hens were sorted into infestation level groups based on their mite score using PROC HPCLUS (SAS 9.4). Body weight, inflammation area, and feather count were analyzed with PROC GLIMMIX. All other traits were analyzed with PROC LOGISTIC and odds ratios were calculated. Footpad condition, feather condition, and keel bone damage worsened with age. Toe damage improved with age. Hens that would develop high infestation levels had worse wing and tail feather condition but better belly feather condition prior to infestation. Mite population peaked 6 weeks post-infestation and declined over the remainer of the study. Higher infestation levels were associated with worse skin lesion and skin inflammation scores and tended to be influenced by the number of feathers on the vent immediately prior to mite infestation. No differences in body weight were observed despite mite infestation. Northern fowl mite infestation level exhibits differential effects on laying hen welfare, particularly feather condition, and results in reduced welfare due to lesions and inflammation.