制粒对猪流行性腹泻病毒感染的饲料的影响

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　　制粒对猪流行性腹泻病毒感染的饲料的影响
　　R. A. Cochrane, L. L. Schumacher, S. S.Dritz, J. C. Woodworth, A. R. Huss, C. R. Stark, J. M. DeRouchey, M. D. Tokach,R. D. Goodband, J. Bia§, Q. Chen, J. Zhang, P. C. Gauger, R. J. Derscheid, D.R. Magstadt, R. G. Main and C. K. Jones

　　猪流行性腹泻病毒（PEDV）是一种对热敏感的病毒，也给美国养猪业带来灭顶之灾。由于PEDV病毒具有热敏感性，我们推测用蒸汽机和造粒机模仿传统商业化的热处理过程或许会减轻PEDV的传染性。制粒作为一种常见的饲料加工过程，涉及到蒸汽力和切应力的使用，这都会导致被处理饲料温度的升高。本试验设计了两种热处理方法通过定量反转录PCR和生物测定分析来探究制粒机采用不同的处理时间和温度是否会影响PEDV的数量和传染性。

　　在试验一中按3×3×2析因设计设置了三种制粒温度（68.3、79.4和90.6℃）、三种处理时间（45、90和180 s）和两种剂量的病毒接种量（低剂量：1×102组织培养感染剂量50（也就是50%细胞产生细胞病理效应的使用剂量）/g；或高剂量：1×104组织培养感染剂量50/g）。未接种PEDV和已接种且未经制粒处理作为对照。低剂量PEDV接种组的饲料与高剂量接种组饲料相比多出6.8±1.8循环阈值（Ct）的PEDV（P < 0.05）。先不考虑时间和温度，与接种了PEDV但饲料未处理的试验组相比，制粒过程降低了可检测到的PEDV病毒RNA的数量。无论剂量高低，饲喂感染了PEDV且未经过处理的饲料，猪只第2-7天（试验第2天至试验结束）的粪便中PEDV是阳性的。然而，只要经过上文中的9种时间×温度处理模式中的任何一种处理过后，猪粪样品或盲肠内容物中就检测不出PEDV的RNA。

　　根据试验一的结果，我们设计了第二个试验来探究较低的处理温度对PEDV数量和感染性的影响。在试验二中，接种了PEDV的饲料在以下5种温度（37.8、46.1、54.4、62.8和71.1℃）处理30s。这5种越来越高的处理温度导致饲料的平均Ct值分别为32.5、34.6、37.0、36.5和36.7。所有样品含有可检测出的PEDV的RNA。然而，通过生物测定技术发现只有来自处理温度为37.8和46.1℃的猪只身上检测出感染性。

　　试验二的结果表明制粒温度在54.4℃以上时可有效降低猪饲料中PEDV的数量和感染性。然而由于它只是特定时间点的缓解步骤，所以仍需更深入的研究来阻止制粒后的污染。


　　Effect of pelleting on survival of porcine  epidemic diarrhea virus–contaminated feed
　　R. A. Cochrane, L. L. Schumacher, S. S.Dritz, J. C. Woodworth, A. R. Huss, C. R. Stark, J. M. DeRouchey, M. D. Tokach,R. D. Goodband, J. Bia§, Q. Chen, J. Zhang, P. C. Gauger, R. J. Derscheid, D.R. Magstadt, R. G. Main and C. K. Jones

　　Porcine epidemic diarrhea virus (PEDV) is a heat-sensitive virus that has devastated the U.S. swine industry. Because of its heat sensitivity, we hypothesized that a steam conditioner and pellet mill mimicking traditional commercial thermal processing may mitigate PEDV infectivity. Pelleting, a common feed processing method, includes the use of steam and shear forces, resulting in increased temperature of the processed feed. Two thermal processing experiments were designed to determine if different pellet mill conditioner retention times and temperatures would impact PEDV quantity and infectivity by analysis of quantitative reverse transcription PCR and bioassay. In Exp. 1, a 3 × 3 × 2 factorial design was used with 3 pelleting temperatures (68.3, 79.4, and 90.6°C), 3 conditioning times (45, 90, or 180 s), and 2 doses of viral inoculation (low, 1 × 102 tissue culture infectious dose 50 (the concentration used to see cytopathic effect in 50% of the cells)/g, or high, 1 × 104 tissue culture infectious dose 50/g). Noninoculated and PEDV-inoculated unprocessed mash were used as controls. The low-dose PEDV–infected mash had 6.8 ± 1.8 cycle threshold (Ct) greater (P < 0.05) PEDV than the high-dose mash. Regardless of time or temperature, pelleting reduced (P < 0.05) the quantity of detectable viral PEDV RNA compared with the PEDV-inoculated unprocessed mash. Fecal swabs from pigs inoculated with the PEDV-positive unprocessed mash, regardless of dose, were clinically PEDV positive from 2 to 7 d (end of the trial) after inoculation. However, if either PEDV dose of inoculated feed was pelleted at any of the 9 tested conditioning time × temperature combinations, no PEDV RNA was detected in fecal swabs orcecum content. Based on Exp. 1 results, a second experiment was developed to determine the impact of lower processing temperatures on PEDV quantity and infectivity. In Exp. 2, PEDV-inoculated feed was pelleted at 1 of 5 conditioning temperatures (37.8, 46.1, 54.4, 62.8, and 71.1°C) for 30 s. The 5 increasing processing temperatures led to feed with respective mean Ct values of 32.5, 34.6, 37.0, 36.5, and 36.7, respectively. All samples had detectable PEDV RNA. However, infectivity was detected by bioassay only in pigs from the 37.8 and 46.1°C conditioning temperatures. Experiment 2 results suggest conditioning and pelleting temperatures above 54.4°C could be effective inreducing the quantity and infectivity of PEDV in swine feed. However, additional research is needed to prevent subsequent recontamination after pelleting as it is a point-in-time mitigation step.

来源：猪营养国际论坛CSIS   翻译：李光燃