Epidemiological risk factors and path models for Spotty Liver Disease in Australian cage-free flocks incorporating a scratch area

Spotty Liver Disease (SLD) is a major disease component in free-range and barn lay egg production systems. SLD, due to Campylobacter hepaticus or C. bilis, may cause mortality and egg production losses. A survey of 48 cage-free layer flocks under 40 weeks of age was conducted in Australian cage-free flocks that have a scratch area for dust bathing within the house, searching for key determinants of increased risk of the occurrence of SLD. A 'key determinant' is a factor that is amenable to management which has an ability to affect the outcome of a disease. To address this objective, a comprehensive questionnaire covering many aspects of farm design and management was administered. Independent variables were assessed across occurrence or non-occurrence of SLD. Continuous variables were initially screened by Student's t-test and categorical variables were examined using contingency tables and Fisher's exact (2-tailed) test. Variables displaying an association of P < 0.20 with SLD occurrence were selected for further evaluation. The selected variables were examined for collinearity and confounding and refined to a smaller number of candidate factors. The reduced number of variables were further assessed using multiple logistic regression, searching for closer statistical associations (P < 0.05). In cage-free layer houses where a scratch area is included within the house, the risk of an outbreak of SLD is increased by the absence of an evaporative cooling system (cool cell) as provided in tunnel ventilation despite the compromise of having open pop holes. In houses that did utilise tunnel ventilation, a lower number of birds placed per square metre of available nest space may decrease the risk of SLD (the upper 95 % confidence limit of nest density for non-case flocks was 102 birds/ m²). Several path models for SLD were developed as a base for continued assessment.