猪群规模对猪群健康的影响（三）

猪群规模影响疾病产生的生物学原因

Biological reasons for an effect of herd size on disease occurrence<<

猪群规模增大，引入和传播病原的风险随之增加，以下会对其生物学原因进行论述。笔者注意到，通过管理手段（例如，生物安全措施、全进全出式生产及按日龄分群生产），也许能够降低大规模猪群所引发的风险，从而降低病原进入猪群或在猪群内部传播的风险。但是，美国一些小规模猪群所有者的管理方式风险较高，如与邻居交换公猪、参加集市和牲畜展览，以及在单一用途上更频繁地使用卡车，这样就削弱了小规模猪群的养殖优势。

In this section, we describe plausible biological reasons for increased risk of introduction and spread of infectious agents with increasing herd size. We note that large herds might be able to mitigate many of these increased risks by the use of management procedures (e.g. biosecurity practices, all-in, all-out production and age-segregated production) which decrease the risk of introduction of pathogens into herds or their within-herd transmission. In addition, owners of smaller herds in theUSAfrequently use riskier management practices, including swapping boars with neighbors, increased likelihood of attending fairs and livestock shows, and more frequent use of trucks for a single purpose, which counteract the benefits of small herd size.

猪群外部引入病原风险更大

Greater risk of introducing infectious agents form outside the herd<<

猪群规模增大，可能会增加猪群与外部潜在感染源直接和间接接触。如种猪与生长猪（20公斤左右）的引入、饲料和屠宰猪的运输以及人员出入。因此，若不针对上述事项采取防护措施，通常会增加引入病原的风险。以下，将更为详细地论述管理因素与猪群规模的相互关系（见“猪群规模相关管理因素的影响”）。

The number of direct and indirect contacts that a swine herd has with potential outside sources of infection may increase as the herds expand in size. These contacts include introduction of breeding and feeder pigs, transportation of feed and of slaughter animals and visitor. Consequently, unless protective management practices are used as outlined above, the risk of introduction of pathogens will also usually increase. We describe the interrelationships between management factors and herd size in more detail below (see Influence of management-related factors that are associated with herd size).

引入病原携带猪

Introduction of infectious agents with carrier pigs<<

为证实猪群规模效应，从外部购入后备母猪替换猪群中一定比例的母猪。假定后备母猪数量为n，源猪群中病原感染率为p，Marchevsky等（1989）认为由引种导致至少感染一头母猪的风险（P）服从二项分布。假设p = 0.01且在引种前未进行病原体检测，那么，一个500头母猪的猪群每年替换掉20%的母猪，其引入病原的风险(P)为0.64，同等替换率条件下，一个100头母猪的猪群，其风险(P)为0.18。新场启动时，从同一源猪群引种，同样会发生与猪群规模相关的类似现象。从理论上讲，大规模猪群所有者如果想要降低风险，可以从未感染猪群购买猪，或者只引入检测阴性的猪。

To demonstrate the effects of herd size, consider herds that replace a fixed proportion of the female breeding herd with gilts purchased from outside herds. Assuming a binomial distribution, Marchevsky et al. (1989) showed that the risk (P) of introducing at least 1 infected female is where n is the number of gilts introduced and p is the prevalence of the infectious agent in the source population. For example, assuming that p = 0.01 and no testing for the agent was done prior to the introduction, then a herd of 500 sows replacing 20% of females per annum would have a risk (P) of 0.64 of introducing an infectious agent compared with a risk of 0.18 for a herd of 100 sows with the same replacement rate. A similar herd size-related phenomenon occurs when herds are initially established from the same source without mitigations. One possible strategy for owners of larger herds to mitigate this theoretically increased risk is to purchase pigs from non-infected herds or to only introduce test-negative pigs into their herds.

P=1-(1- p)n (1)

Thorburn等人(1991)建议对公式1作出修正，使其包含未替换的样本，且可对公式进一步修正，即在感染率不同的猪群中对不同数量的猪进行抽样。以下不再赘述此类修正。另外，为与下一部分的模型进行直接比较，假设病原携带猪引入受感染猪群后，病原就会在该猪群内传播。

Thorburn et al. (1991) suggested a modification of equation 1 to include sampling without replacement, and the equation could be further modified to include sampling of variable numbers of pigs from herds with different prevalences of infection. For simplicity, we do not present these modifications here. Also, to allow direct comparison with the model in the following section, we assume that, once an infectious agent is introduced into a herd with an infected pig, it is transmitted to other pigs in the same herd.

空气传播引入病原

Introduction of infectious agents transmitted by airborne routes<<

实验数据表明，即使不引种或者限制与其他猪群接触，病毒性疾病通过空气传播引入病原同样会增加大型猪群的感染风险。如猪呼吸道冠状病毒（PRCV）和猪伪狂犬病毒（PRV）等传染病（据报道在丹麦发生过空气传播导致的传染病）在大规模猪群的感染风险明显增加（Henningsen等，1988； Christensen等，1990； Flori等，1995）。此外，这些研究发现，猪群规模的自然对数变换能够最优表述猪群规模对PRCV阳性或PRV阳性的单一影响。

Even for herds not introducing pigs and with limited contacts with other herds, there is empirical evidence that large swine herds are at increased risk of airborne introduction of viral diseases. For example, for infections such as PRCV and PRV (for which airborne transmission in Denmark has been reported), an increased risk of infection was evident in large herds (Henningsen et al., 1988; Christensen et al., 1990; Flori et al., 1995). In addition, the authors of these three studies found that a natural logarithmic transformation of herd size was the best formulation of the independent effect of herd size on the odds of positive PRCV or PRV status.