Analytical Epidemiological Studies of Spotty Liver Disease in Australian Cage-free Laying Hens

Abstract

Spotty liver disease (SLD) is an infectious disease mainly reported in free-range laying hens, causing significant mortality and production losses. Since the isolation and identification of Campylobacter hepaticus and C. bilis, the causative agents of Spotty liver disease (SLD), significant advancement has been made in characterising the pathogen and understanding the transmission and spread of the disease. Yet, an in-depth understanding of SLD epidemiology is still lacking. An opinion survery (Chapter 2) was first used to scope the industry experience and hypotheses on SLD risk factors. This was followed by the preliminary cross-sectional study (Chapter 3) in cage-free laying flocks (n=24) across different states of Australia. It was found that the presence of a scratch area inside the poultry house is a risk factor for SLD (P = 0.003). Notably, despite the absence of a scratch area, almost half of the laying flocks in fully slatted houses (barn and free-range) in the preliminary study developed clinical SLD. This highlighted that other putative risk factors could precipitate SLD occurrence. The second survey searched for further risk factors that may exist in houses with a floor fully covered by slats (n=49) (Chapter 4). The study indicated that layer houses with naturally ventilated environmental control systems are a key determinant of the risk for SLD (P = 0.002). Further analysis showed that a higher nest density (birds per m2 nest space) has a stronger risk of causing SLD (P = 0.015). A third field study was conducted to study putative risk factors for SLD in flocks (n=48) from sheds with a scratch area (Chapters 5 and 6). The absence of cool cells, i.e. less efficient shed cooling, was thought to be a statistically important risk factor for SLD (P = 0.028). To the author’s knowledge, this thesis is the first to report on risk factors, more importantly key determinants, of SLD based on a series of descriptive and analytical epidemiological studies.