對(duì)于即將設(shè)立大規(guī)模生豬養(yǎng)殖體系的決策者們不得不考慮的一個(gè)問題：?jiǎn)螆?chǎng)的規(guī)模設(shè)置為多大合適？2400頭生產(chǎn)母豬/4800頭還是20000頭生產(chǎn)母豬？有傾向于小而美的--雞蛋放在多個(gè)筐子里的；也有大干猛上、氣吞河山的--不玩則已，動(dòng)則萬(wàn)頭母豬的。最終單場(chǎng)或者單點(diǎn)群體規(guī)模的大小由各種因素綜合妥協(xié)而定，個(gè)中因素不乏：拿地成本、糞污集中處理--環(huán)保成本、運(yùn)營(yíng)管理成本分?jǐn)偂⒅苓呹P(guān)系維護(hù)成本、管理&生產(chǎn)效率等等，其中一個(gè)因素：豬群健康狀況的維持以及疫病發(fā)生時(shí)的穩(wěn)定乃至凈化的成本，無(wú)論是非瘟語(yǔ)境下抑或是常見的仔豬流行腹瀉或者藍(lán)耳病或者口蹄疫的感染壓力下，健康是效益藍(lán)圖眾多個(gè)零前面的那個(gè)數(shù)字，若此數(shù)字為0則所有的效益藍(lán)圖都將化為難途。豬群規(guī)模大小對(duì)豬群健康的影響到底幾何？

本連載將從生物學(xué)原理及獸醫(yī)流行病學(xué)層面，結(jié)合管理及環(huán)境相關(guān)因素，針對(duì)豬群規(guī)模的度量、豬群規(guī)模對(duì)疾病的影響等，通過(guò)統(tǒng)計(jì)學(xué)模型，更科學(xué)直觀地剖析豬群規(guī)模與豬群健康的關(guān)系。此研究得到了丹麥農(nóng)業(yè)部（KVL-VEL-4）以及美國(guó)農(nóng)業(yè)部的動(dòng)植物檢疫局、獸醫(yī)處（合作協(xié)議第12-34-99-0020-CA號(hào)）以及加利福尼亞戴維斯分校食用動(dòng)物健康中心的資助。他山美玉，更可攻玉，請(qǐng)參考。

豬群規(guī)模對(duì)豬群健康的影響（一）

Empirical and theoretical evidence for herd size as a risk factor for swine diseases – Part 1

摘要Abstract<<

豬病研究中，豬群規(guī)模通常是影響豬群健康的風(fēng)險(xiǎn)因子，但現(xiàn)有流行病學(xué)研究，很少在生物學(xué)原理方面對(duì)其進(jìn)行充分論述（無(wú)論是正相關(guān)或負(fù)相關(guān)）。從生物學(xué)角度講，豬群規(guī)模與疾病正相關(guān)主要是因?yàn)椋?dāng)豬群規(guī)模較大時(shí)，從其他種群引入病原的風(fēng)險(xiǎn)較高，病原在豬群內(nèi)及豬群間的傳播風(fēng)險(xiǎn)較高，另外，豬群規(guī)模效應(yīng)與管理效果和環(huán)境因素相關(guān)。然而，相比小規(guī)模豬群所有者，大規(guī)模豬群所有者可能會(huì)更多通過(guò)管理和圈養(yǎng)方式降低風(fēng)險(xiǎn)。在評(píng)估管理因素、豬群規(guī)模與疾病間的關(guān)系時(shí)，通過(guò)胸膜炎，肺炎和偽狂犬病的研究來(lái)闡述所涉及的流行病學(xué)問題。今后研究中，建議（i）以最能真實(shí)反映群體風(fēng)險(xiǎn)狀態(tài)的方式來(lái)確定豬群規(guī)模；（ii）在評(píng)估管理方面的風(fēng)險(xiǎn)因素研究時(shí)應(yīng)考慮豬群規(guī)模；（iii）以種群為基礎(chǔ)，研究管理因素自身以及管理因素、豬群規(guī)模、豬群密度、豬只密度之間的相互關(guān)系；（iv）對(duì)畜群規(guī)模可能造成的任何影響，作出生物學(xué)上的合理假設(shè)；（v）描述源種群和研究樣本中的豬群規(guī)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.

引言Introduction<<

豬群中，管理與環(huán)境因素對(duì)病原體的引入和存在起到?jīng)Q定性作用，對(duì)豬群多項(xiàng)生產(chǎn)力指標(biāo)也有影響。自二十世紀(jì)七十年代早期進(jìn)行的研究（B?ckstr?m，1973；Lindqvist，1974；Aalund等人，1976）認(rèn)為在影響豬健康的各項(xiàng)因素中，豬群規(guī)模與疾病關(guān)系密切。然而，在已有研究中，關(guān)于豬群規(guī)模效應(yīng)（無(wú)論是正相關(guān)或負(fù)相關(guān)）的生物學(xué)解釋并不常見或未見詳細(xì)說(shuō)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 (B?ckstr?m, 1973; Lindqvist, 1974; Aalundet al., 1976). However, the biological basis for the herd-size effect (whether positive or negative) is not frequently known or specified in published studies.

大多數(shù)流行病學(xué)研究顯示，豬群規(guī)?？梢詻Q定與之相關(guān)的管理變量，例如，全進(jìn)全出、多點(diǎn)式生產(chǎn)、生物安全、防疫接種，以及雇傭員工（美國(guó)農(nóng)業(yè)部：動(dòng)植物衛(wèi)生檢疫局：獸醫(yī)處，1995年）。盡管流行病學(xué)家認(rèn)為豬群規(guī)模與疾病之間存在的關(guān)聯(lián)頗具研究?jī)r(jià)值，且為今后研究提供了方向，但這一研究與養(yǎng)豬生產(chǎn)者之間沒有直接的關(guān)聯(lián)。例如，擁有大規(guī)模豬群的生產(chǎn)者通常能實(shí)現(xiàn)與其對(duì)應(yīng)的經(jīng)濟(jì)效應(yīng)，因此不太愿意減小豬群規(guī)模，因?yàn)檫@樣通常會(huì)增加生產(chǎn)成本，降低每頭豬的銷售毛利。此外，就生產(chǎn)設(shè)施而言，大型豬群的資本投資較大，這也使生產(chǎn)者不愿減小豬群規(guī)模。不過(guò)，在某些國(guó)家，動(dòng)物福利、環(huán)境和政治因素可能會(huì)限制豬群的最大規(guī)模。同樣，由于大規(guī)模豬群為市場(chǎng)提供了最大數(shù)量的生豬，隨著公眾對(duì)豬肉相關(guān)食源性疾病的關(guān)心，更多人也開始關(guān)注豬群規(guī)模對(duì)病原傳播的影響。

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.

豬群規(guī)模與流行病學(xué)關(guān)系的研究已逐漸展開，因?yàn)橐?guī)模大小可根據(jù)現(xiàn)有數(shù)據(jù)庫(kù)進(jìn)行估算，或通過(guò)問卷調(diào)查確定。但是，在許多研究中，群體風(fēng)險(xiǎn)狀態(tài)、與豬群規(guī)模相關(guān)的管理和環(huán)境因素，或者其他一些不確定因素，如區(qū)域內(nèi)豬群和豬只密度，這些是否為引起豬群規(guī)模效應(yīng)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.

筆者認(rèn)為，未來(lái)研究豬流行病學(xué)的目標(biāo)之一是能夠更好地闡述豬群規(guī)模效應(yīng)，以便可以將增加豬群規(guī)模所致的潛在不利（有益）影響最小化（最大化）。重點(diǎn)在于區(qū)分真實(shí)效應(yīng)和虛假效應(yīng)，假設(shè)效應(yīng)歸因于未經(jīng)改進(jìn)的管理或環(huán)境因素，那么改變豬群規(guī)模則毫無(wú)益處。同樣，查明豬群規(guī)模效應(yīng)的原因，可以給養(yǎng)殖戶和決策者提供更具體的合理防控建議。

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.

本文回顧了關(guān)于豬群規(guī)模及豬病的流行病學(xué)研究，敘述了豬群規(guī)模的度量方法，用生物學(xué)相關(guān)理論解釋了豬群規(guī)模可能對(duì)疾病產(chǎn)生的影響，并根據(jù)經(jīng)驗(yàn)示例豬群規(guī)模與疾病風(fēng)險(xiǎn)之間關(guān)系。文中大多數(shù)例子是關(guān)于胸膜炎、肺炎和偽狂犬病等，這些疾病在豬群規(guī)模效應(yīng)研究方面常被提及，且存在正相關(guān)關(guān)系。文章最后一部分，對(duì)未來(lái)豬流行病學(xué)研究提出建議，涉及評(píng)估豬群規(guī)模效應(yīng)以及管理和環(huán)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.

關(guān)于“豬群”的定義多種多樣，一般指身體有接觸或接近，具有共同的生產(chǎn)要素（如，飼料來(lái)源相同、體系內(nèi)基因及種豬來(lái)源相同）。一些定義更適用于直接接觸的疾?。ㄈ纾臧_），而非間接接觸傳播的疾?。ㄈ纾i胸膜肺炎放線桿菌）。本文將豬群廣義上劃分為單體建筑內(nèi)飼養(yǎng)、物理隔離的多個(gè)建筑內(nèi)飼養(yǎng)，單點(diǎn)式或多點(diǎn)式散養(yǎng)的群體。同一建筑物或地點(diǎn)的豬，其日齡及生產(chǎn)用途都有可能不同。這與先前研究中的定義無(wú)太大差別、

（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).

豬群規(guī)?，F(xiàn)有研究及度量

Published studies and measures of herd size<<

現(xiàn)有豬病研究將豬群規(guī)模本身，或連同管理及環(huán)境因素一并視為風(fēng)險(xiǎn)因子。本文的參考文獻(xiàn)選自在2001年前（包括2001年）見刊的英文文章。文獻(xiàn)查找方式為搜索以下關(guān)鍵詞：“豬群規(guī)?！?，“豬群規(guī)模與患病風(fēng)險(xiǎn)”，“風(fēng)險(xiǎn)因子”，“豬群管理”，“管理因素”，“單位飼養(yǎng)密度”以及“豬群密度”。通過(guò)這些文獻(xiàn)，研究如何度量豬群規(guī)模，并根據(jù)豬的身體系統(tǒng)進(jìn)行分類研究，進(jìn)而判斷較大豬群規(guī)模與風(fēng)險(xiǎn)之間的關(guān)系為“正相關(guān)”，“負(fù)相關(guān)”或“不相關(guān)”（表1）。但是，一些文章沒有采用統(tǒng)計(jì)分析，沒有提出如何對(duì)豬群規(guī)模進(jìn)行分類，或沒有統(tǒng)一使用多變量分析，因此，這些分類略顯主觀?；谶@些限制，本文主要采用定性研究。筆者曾考慮使用meta分析（薈萃分析）進(jìn)行對(duì)比，并將結(jié)果匯總為單一的效應(yīng)指標(biāo)，但由于缺乏可比較性，所以無(wú)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.

現(xiàn)有文獻(xiàn)表明，豬群規(guī)模與疾病之間的關(guān)系并非一成不變（表1）。不過(guò)，可以看出，包括偽狂犬病在內(nèi)的呼吸系統(tǒng)疾病，豬群規(guī)模與患病概率通常正相關(guān)。但是，無(wú)法確定不贊成發(fā)表負(fù)面結(jié)果的偏見是否會(huì)影響這一結(jié)論。但是，有證據(jù)表明，較大豬群規(guī)?？深A(yù)防某些疾病，如弓形蟲病和乳斑肝。此保護(hù)作用可能是由于大規(guī)模豬群更傾向于采用隔離圈養(yǎng)系統(tǒng)，從而減少了與寄生蟲的接觸。

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.

假設(shè)這些研究人員想要通過(guò)使用一種理想的方式，在豬群處于一定規(guī)模時(shí)，量化其感染或患病的風(fēng)險(xiǎn)，但現(xiàn)有文章表明，豬群規(guī)模的度量方法多種多樣。針對(duì)母豬的疾病，通常根據(jù)母豬（后備母豬和母豬）的數(shù)量來(lái)確定豬群規(guī)模。需要注意的是，母豬數(shù)量可以是當(dāng)前存欄量或是已分娩母豬的數(shù)量，且關(guān)于“母豬”的定義也有多種。豬場(chǎng)內(nèi)母豬群規(guī)模大小的調(diào)節(jié)方式，包括剔除未達(dá)到育齡的種豬（盡管已經(jīng)標(biāo)記為后備母豬）或剔除豬場(chǎng)中的淘汰母豬。涉及育肥豬的疾病，豬群規(guī)模的度量指標(biāo)更多變（如，每年屠宰的豬的數(shù)量，處于斷奶至出欄日齡之間的數(shù)量，豬群內(nèi)所有豬的數(shù)量，以及每棟豬舍內(nèi)豬的數(shù)量）。針對(duì)通過(guò)空氣傳播的偽狂犬病病毒（PRV）（Christensen等，1990）和豬呼吸道冠狀偽狂犬病毒(PRV)（Henningsen等，1988；Flori等，1995），在丹麥?zhǔn)褂谩盁崃慨a(chǎn)生單位”（HPU）的量值表示豬群規(guī)模。HPU（20℃，1 HPU = 1000瓦）是指不同日齡的豬只在不同溫度下的熱損失（因通風(fēng)要求）估算值（Strom，1978年）：1頭母豬和其仔豬= 0.4 HPU；1頭非哺乳期母豬、公豬或后備母豬= 0.2 HPU；1頭保育豬/育肥豬= 17 HPU。此法適用于分娩至育肥階段體重、大小、日齡不同的豬群，有利于將種豬群和育肥豬群進(jìn)行規(guī)模對(duì)比。尚未衡量此法在單一標(biāo)準(zhǔn)下針對(duì)不同類型豬群的相對(duì)優(yōu)勢(shì)，但研究集約化飼養(yǎng)模式下氣溶膠傳播的豬呼吸道疾病，此類方法更加適用，因?yàn)檫@種情況下各個(gè)年齡段豬的患病風(fēng)險(xiǎn)都是恒定的。進(jìn)行被動(dòng)和主動(dòng)免疫之前，大量病毒和細(xì)菌可能造成疾病首次流行，此時(shí)可假設(shè)患病風(fēng)險(xiǎn)與日齡無(wú)關(guān)。相反，如果傳播主要通過(guò)于鼻與鼻接觸，則該方法更適合于大欄內(nèi)育肥豬，而非限位欄內(nèi)的種豬。對(duì)于后者，采用HPU度量豬群規(guī)模已不適用。

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) (Christensenet al., 1990) and porcine respiratory coronavirus pseudorabies virus (PRV) (Henningsenet al., 1988; Floriet 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.

編輯：姚紅

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