Empirical and theoretical evidence for herd size as a risk factor for swine diseases – Part 1
猪病研究中，猪群规模通常是影响猪群健康的风险因子，但现有流行病学研究，很少在生物学原理方面对其进行充分论述（无论是正相关或负相关）。从生物学角度讲，猪群规模与疾病正相关主要是因为，当猪群规模较大时，从其他种群引入病原的风险较高，病原在猪群内及猪群间的传播风险较高，另外，猪群规模效应与管理效果和环境因素相关。然而，相比小规模猪群所有者，大规模猪群所有者可能会更多通过管理和圈养方式降低风险。在评估管理因素、猪群规模与疾病间的关系时，通过胸膜炎，肺炎和伪狂犬病的研究来阐述所涉及的流行病学问题。今后研究中，建议（i）以最能真实反映群体风险状态的方式来确定猪群规模；（ii）在评估管理方面的风险因素研究时应考虑猪群规模；（iii）以种群为基础，研究管理因素自身以及管理因素、猪群规模、猪群密度、猪只密度之间的相互关系；（iv）对畜群规模可能造成的任何影响，作出生物学上的合理假设；（v）描述源种群和研究样本中的猪群规模分布。Herd 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.
猪群中，管理与环境因素对病原体的引入和存在起到决定性作用，对猪群多项生产力指标也有影响。自二十世纪七十年代早期进行的研究（Bäckström，1973；Lindqvist，1974；Aalund等人，1976）认为在影响猪健康的各项因素中，猪群规模与疾病关系密切。然而，在已有研究中，关于猪群规模效应（无论是正相关或负相关）的生物学解释并不常见或未见详细说明。Management and environmental factors are considered important determinants of the risk of the introduction and maintenance of an infectious agent in swine herds and also affect many measures of herd productivity. Among factors influencing pig health, herd size is considered to be an important correlate of disease and has been studied since the early 1970s (Bäckström, 1973; Lindqvist, 1974; Aalund et al., 1976). However, the biological basis for the herd-size effect (whether positive or negative) is not frequently known or specified in published studies.大多数流行病学研究显示，猪群规模可以决定与之相关的管理变量，例如，全进全出、多点式生产、生物安全、防疫接种，以及雇佣员工（美国农业部：动植物卫生检疫局：兽医处，1995年）。尽管流行病学家认为猪群规模与疾病之间存在的关联颇具研究价值，且为今后研究提供了方向，但这一研究与养猪生产者之间没有直接的关联。例如，拥有大规模猪群的生产者通常能实现与其对应的经济效应，因此不太愿意减小猪群规模，因为这样通常会增加生产成本，降低每头猪的销售毛利。此外，就生产设施而言，大型猪群的资本投资较大，这也使生产者不愿减小猪群规模。不过，在某些国家，动物福利、环境和政治因素可能会限制猪群的最大规模。同样，由于大规模猪群为市场提供了最大数量的生猪，随着公众对猪肉相关食源性疾病的关心，更多人也开始关注猪群规模对病原传播的影响。 In most epidemiological studies, some measure of herd size can be determined and this is often used as a surrogate for management variables that are herd size-related, such as all-in, all-out and multisite production, biosecurity and vaccination practices, and the use of hired employees (United States Department of Agriculture: Animal and Plant Health Inspection Service: Veterinary Services, 1995). Although the demonstration of an association between herd size and disease might be considered interesting by epidemiologists and provide direction for future studies, such a finding has little immediate relevance to swine producers. For example, producers with large herds often realize economies of scale and are therefore unlikely to reduce herd size voluntarily because such a change would usually result in increased production costs and a lower gross margin per pig sold. In addition, large herds have greater capital investment in facilities that might also preclude them from decreasing herd size voluntarily. In some countries, however, animal welfare, environmental and political considerations may ultimately result in limits on the maximal size of swine herds. Also, public-health concerns about foodborne disease associated with pig meat will increase interest in the effects of herd size on pathogen prevalence because larger herds contribute the greatest number of slaughter pigs to the market.猪群规模与流行病学关系的研究已逐渐展开，因为规模大小可根据现有数据库进行估算，或通过问卷调查确定。但是，在许多研究中，群体风险状态、与猪群规模相关的管理和环境因素，或者其他一些不确定因素，如区域内猪群和猪只密度，这些是否为引起猪群规模效应的首要因素，仍然未知。 Interest in studying herd size associations in epidemiological studies has probably developed partly because estimates of size are frequently available in existing databases or can be obtained readily by questionnaire. In many studies, however, an unanswered question is whether the primary factor that produces the herd-size effect is the size of the population at risk, management and environmental factors that are related to herd size, or some other unmeasured factor, such as herd and pig density in an area.笔者认为，未来研究猪流行病学的目标之一是能够更好地阐述猪群规模效应，以便可以将增加猪群规模所致的潜在不利（有益）影响最小化（最大化）。重点在于区分真实效应和虚假效应，假设效应归因于未经改进的管理或环境因素，那么改变猪群规模则毫无益处。同样，查明猪群规模效应的原因，可以给养殖户和决策者提供更具体的合理防控建议。 We believe that one goal of future epidemiological studies of swine disease is to better characterize the nature of the effect, if any, of herd size, so that potential adverse (beneficial) effects of increases in herd size can be minimized (maximized). Accordingly, it is important to differentiate real from spurious herd-size effects because a change in herd size would have no benefit if the effect was attributable to management or environmental factors that were not modified. Also, determination of the reason for any herd-size effect would allow more specific advice to be given to farmers and decision-makers on appropriate preventive measures.本文回顾了关于猪群规模及猪病的流行病学研究，叙述了猪群规模的度量方法，用生物学相关理论解释了猪群规模可能对疾病产生的影响，并根据经验示例猪群规模与疾病风险之间关系。文中大多数例子是关于胸膜炎、肺炎和伪狂犬病等，这些疾病在猪群规模效应研究方面常被提及，且存在正相关关系。文章最后一部分，对未来猪流行病学研究提出建议，涉及评估猪群规模效应以及管理和环境因素。 In this paper, we review epidemiological studies of herd size and swine diseases, describe measures of herd size, present biological explanations for possible effects of herd size on disease occurrence, and give examples of the relationships between herd size and disease risk based on our experience. Most of our examples are studies of pleuritis, pneumonia and pseudorabies because these are the diseases that have been studied most frequently with regard to herd-size effects and positive associations have often been found. In the final section, we make recommendations for future epidemiological studies of swine diseases that involve the evaluation of the effects of herd size and management and environmental factors.关于“猪群”的定义多种多样，一般指身体有接触或接近，具有共同的生产要素（如，饲料来源相同、体系内基因及种猪来源相同）。一些定义更适用于直接接触的疾病（如，疥癣），而非间接接触传播的疾病（如，猪胸膜肺炎放线杆菌）。本文将猪群广义上划分为单体建筑内饲养、物理隔离的多个建筑内饲养，单点式或多点式散养的群体。同一建筑物或地点的猪，其日龄及生产用途都有可能不同。这与先前研究中的定义无太大差别、（Christensen和Gardner，2000）。 Many definitions of ‘herd’ are possible, reflecting physical contact, physical proximity or shared inputs (e. g common feed source, genetics and the source of pigs in a production pyramid). Some definitions are more appropriate to diseases transmitted by direct contact (e.g. mange) than diseases transmitted by indirect contact (e.g. Actinobacillus pleuropneumoniae). For this paper, we broadly define a herd to be a cluster or aggregate of pigs in a single building or multiple physically separated buildings or outdoor lots located on a single site or on multiple sites. Pigs in the same building or site can be of different age and production classes. This is similar to the definition used in a prior study by one of us (Christensen and Gardner, 2000).
Published studies and measures of herd size<<
现有猪病研究将猪群规模本身，或连同管理及环境因素一并视为风险因子。本文的参考文献选自在2001年前（包括2001年）见刊的英文文章。文献查找方式为搜索以下关键词：“猪群规模”，“猪群规模与患病风险”， “风险因子”，“猪群管理”，“管理因素”，“单位饲养密度”以及“猪群密度”。通过这些文献，研究如何度量猪群规模，并根据猪的身体系统进行分类研究，进而判断较大猪群规模与风险之间的关系为“正相关”，“负相关”或“不相关”（表1）。但是，一些文章没有采用统计分析，没有提出如何对猪群规模进行分类，或没有统一使用多变量分析，因此，这些分类略显主观。基于这些限制，本文主要采用定性研究。笔者曾考虑使用Meta分析（荟萃分析）进行对比，并将结果汇总为单一的效应指标，但由于缺乏可比较性，所以无法采用此方法。We reviewed published studies of swine diseases in which herd size was evaluated as a risk factor either individually or in addition to other management and environmental factors. Our literature review was restricted to manuscripts published in English up to and including the year 2001. The search was done using the keywords ‘herd size’, ‘herd size and disease risk’, 'risk factor’, 'pig management’, 'management factor’, 'stocking density' and ‘herd density’. We read the papers to determine how herd size was measured and then categorized studies according to the body system studied and evaluated associations with larger herd size as being ‘positive’, ‘negative’ or ‘none' (Table 1). These classifications were often subjective because in some papers there were no statistical analyses or description of how herd size was classified, and multivariable analyses were not used consistently. Because of these limitations, most of our evaluations were qualitative. We considered the use of meta-analysis for a more formal comparison and summary of the results into a single effect measure, but the lack of comparability of studies precluded this approach.现有文献表明，猪群规模与疾病之间的关系并非一成不变（表1）。不过，可以看出，包括伪狂犬病在内的呼吸系统疾病，猪群规模与患病概率通常正相关。但是，无法确定不赞成发表负面结果的偏见是否会影响这一结论。但是，有证据表明，较大猪群规模可预防某些疾病，如弓形虫病和乳斑肝。此保护作用可能是由于大规模猪群更倾向于采用隔离圈养系统，从而减少了与寄生虫的接触。 Our evaluation of the published papers indicated that the relationship between herd size and disease varied with the disease studied (Table 1); however, it is interesting to us that for respiratory diseases, including pseudorabies, the relationship between large herd size and disease was typically positive. However, we are unable to determine whether bias against publication of negative results might have influenced this conclusion. In contrast, we found evidence that large herd size was ‘protective' for some diseases, such as toxoplasmosis and liver milk spots. Presumably, this protective effect was in part attributable to more frequent use of confinement systems in large herds, which reduced exposure to the parasites.假设这些研究人员想要通过使用一种理想的方式，在猪群处于一定规模时，量化其感染或患病的风险，但现有文章表明，猪群规模的度量方法多种多样。针对母猪的疾病，通常根据母猪（后备母猪和母猪）的数量来确定猪群规模。需要注意的是，母猪数量可以是当前存栏量或是已分娩母猪的数量，且关于“母猪”的定义也有多种。猪场内母猪群规模大小的调节方式，包括剔除未达到育龄的种猪（尽管已经标记为后备母猪）或剔除猪场中的淘汰母猪。涉及育肥猪的疾病，猪群规模的度量指标更多变（如，每年屠宰的猪的数量，处于断奶至出栏日龄之间的数量，猪群内所有猪的数量，以及每栋猪舍内猪的数量）。针对通过空气传播的伪狂犬病病毒（PRV）（Christensen等，1990）和猪呼吸道冠状伪狂犬病毒(PRV)（Henningsen等，1988； Flori等，1995），在丹麦使用“热量产生单位”（HPU）的量值表示猪群规模。HPU（20℃，1 HPU = 1000瓦）是指不同日龄的猪只在不同温度下的热损失（因通风要求）估算值（Strom，1978年）：1头母猪和其仔猪= 0.4 HPU；1头非哺乳期母猪、公猪或后备母猪= 0.2 HPU；1头保育猪/育肥猪= 17 HPU。此法适用于分娩至育肥阶段体重、大小、日龄不同的猪群，有利于将种猪群和育肥猪群进行规模对比。尚未衡量此法在单一标准下针对不同类型猪群的相对优势，但研究集约化饲养模式下气溶胶传播的猪呼吸道疾病，此类方法更加适用，因为这种情况下各个年龄段猪的患病风险都是恒定的。进行被动和主动免疫之前，大量病毒和细菌可能造成疾病首次流行，此时可假设患病风险与日龄无关。相反，如果传播主要通过于鼻与鼻接触，则该方法更适合于大栏内育肥猪，而非限位栏内的种猪。对于后者，采用HPU度量猪群规模已不适用。 In the reviewed papers, herd size was measured in many different ways, although we assumed that one of the underlying goals of the authors was to use the chosen herd size measure to characterize the true population at risk for the infection or disease of interest. For diseases of sows, herd size was usually given as the number of breeding females (gilts and sows). It is important to note that the number of females may be based on the current inventory or the number of sows farrowed, and many definitions of a sow are possible. Female herd size estimates may be adjusted by excluding those not of breeding age (though already marked as replacement gilts) or cull sows that are present on a farm. For diseases in finishing pigs, the choice of a herd size measure tended to be more variable (and included the number of pigs slaughtered per annum, the number of pigs in the herd between weaning and market age, the total number of pigs in the herd, and the number of pig places per barn or building compartment). Another herd size measure used in Danish studies of possible airborne transmission of pseudorabies virus (PRV) (Christensen et al., 1990) and porcine respiratory coronavirus pseudorabies virus (PRV) (Henningsen et al., 1988; Flori et al., 1995) was the number of ‘heat-producing units’ (HPU). The HPU (1 HPU = 1000 watts at 20℃) is an estimate of the heat loss (and hence the ventilation requirement) at different temperatures for different age classes of pigs (Strom, 1978): 1 sow and her litter = 0.4 HPU; 1 non-Iactating sow, boar or gilt = 0.2 HPU; 1 grower/finisher = 17 HPU. This measure accounts for differences in size and weight of different age classes of pigs in farrow-to-finish herds and facilitates the comparison of sizes of breeding and finishing herds. The relative advantage of this method for accounting for pigs of different types in a single measure has not been evaluated, but such an approach seems intuitively appealing for studies of respiratory diseases transmitted by aerosols in intensively housed pigs where risk is constant across all ages. An assumption of age-independent risk might be realistic for first epidemics of many viral and bacterial respiratory agents before the risk starts to be influenced by passive and active immunity. In contrast, where transmission occurs primarily by nose-to-nose contact it would be more appropriate to give greater weight to finishing pigs in pens rather than breeding animals in crates. In the latter case, the use of HPU as a herd size measure would be inappropriate.