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HOCK burn, the name given to dermatitis of the plantar surface of the hocks of broiler chickens, is visible as black or brown skin lesions ( Greene and others 1985 , Martland 1985 ). These lesions are characterised histologically by areas of epidermal necrosis surrounded by epidermal hyperplasia, with congestion of capillaries, leucocytosis and diffuse heterophil infiltration beneath ( Greene and others 1985 , Martland 1985 ). Hock burn is part of the syndrome of contact dermatitis, which also includes footpad dermatitis and breast blisters ( Greene and others 1985 , Martland 1985 , McIlroy and others 1987 , Haslam and others 2007 ). Lesions are distributed most frequently on the footpad, followed by the hock and the breast ( Greene and others 1985 ).
Contact dermatitis has been identified as a visible indicator of broiler welfare ( Bessei 2006 , Manning and others 2007 , Allain and others 2009 ). In Europe, legislation has now linked allowable stocking densities to the prevalence of these lesions ( EU 2007 ). As a consequence, contact dermatitis affects the wellbeing of birds as well as the economic situation of broiler farms. Lesions have been associated with failure of birds to meet their target weight ( Martland 1985 ). Birds with hock lesions are processed into lower-value products, which affects income in this high-volume, low-margin farming system.
Risk factors for hock burn and pododermatitis have been identified by observational and experimental studies ( Haslam and others 2007 , Buijs and others 2009 ). These include the sex of the bird, rearing system, litter quality, leg weakness, age and weight at slaughter ( McIlroy and others 1987 , Sørensen and others 2000 , Dawkins and others 2004 , Pagazaurtundua and Warriss 2006 , Haslam and others 2007 , Buijs and others 2009 ). However, few studies to date have taken the hierarchical structure of the broiler industry into account during data analysis. Birds are reared in sequential crops (six to seven per year) within a single broiler house; many houses may be located on a farm; and many farms may be owned or managed by the same company. In addition, each flock is delivered to the abattoir in several loads over a number of days.
These 'clustered' data break...