AbstractHerd size is frequently studied as a risk factor for swine diseases, yet the biological rationale for a reported association with herd size (whether positive or negative) is rarely adequately discussed in published epidemiological studies. Biologically plausible reasons for a positive association between herd size and disease include a greater risk of introduction of pathogens from outside the herd, greater risk of transmission of pathogens within and among herds when the herd is large, and effects of management and environmental factors that are related to herd size. However, compared with owners of small herds, owners of large herds might more frequently adopt management and housing practices that mitigate this theoretically increased risk. We used studies of pleuritis, pneumonia and pseudorabies to describe the epidemiological issues involved in evaluations of the relationship between management factors, herd size and disease. In future studies, we recommend that (i) herd size be measured in a way that best characterizes the true population at risk; (ii) studies that evaluate management-related risk factors should account for herd size wherever possible; (iii) population-based studies of the interrelationships among management factors and between management factors, herd size, herd density and pig density be done; (iv) likely biological reasons for any herd-size effect be postulated; and (v) the distribution of herd sizes in the source population and the study sample be described.
摘要 猪群规模作为猪病的一个风险因素经常被研究,但在已发表的流行病学研究中,很少充分讨论所报告的猪群规模与疾病相关的
生物学原理(无论是正相关还是负相关)。从
生物学角度看,猪群规模与疾病呈正相关的合理原因包括:从猪群外部引入病原体的风险更大;猪群规模大时,病原体在猪群内部和猪群之间传播的风险更大;以及与猪群规模相关的管理和环境因素的影响。然而,与小规模牛群的所有者相比,大规模牛群的所有者可能会更频繁地采取管理和饲养措施来降低理论上增加的风险。我们通过对胸膜炎、肺炎和伪狂犬病的研究,描述了在评估管理因素、牛群规模和疾病之间的关系时所涉及的流行病学问题。在今后的研究中,我们建议:(i) 对牛群规模的测量应能最好地描述真正的风险种群;(ii) 评估与管理相关的风险因素的研究应尽可能考虑牛群规模;(iii) 对管理因素之间以及管理因素、牛群规模、牛群密度和猪密度之间的相互关系进行基于种群的研究;(iv) 假设任何牛群规模效应可能的
生物学原因;(v) 描述源种群和研究样本中牛群规模的分布。