制粒對豬流行性腹瀉病毒感染的飼料的影響

畜牧人小李

　　制粒對豬流行性腹瀉病毒感染的飼料的影響
　　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

　　豬流行性腹瀉病毒（PEDV）是一種對熱敏感的病毒，也給美國養豬業帶來滅頂之災。由于PEDV病毒具有熱敏感性，我們推測用蒸汽機和造粒機模仿傳統商業化的熱處理過程或許會減輕PEDV的傳染性。制粒作為一種常見的飼料加工過程，涉及到蒸汽力和切應力的使用，這都會導致被處理飼料溫度的升高。本試驗設計了兩種熱處理方法通過定量反轉錄PCR和生物測定分析來探究制粒機采用不同的處理時間和溫度是否會影響PEDV的數量和傳染性。

　　在試驗一中按3×3×2析因設計設置了三種制粒溫度（68.3、79.4和90.6℃）、三種處理時間（45、90和180 s）和兩種劑量的病毒接種量（低劑量：1×102組織培養感染劑量50（也就是50%細胞產生細胞病理效應的使用劑量）/g；或高劑量：1×104組織培養感染劑量50/g）。未接種PEDV和已接種且未經制粒處理作為對照。低劑量PEDV接種組的飼料與高劑量接種組飼料相比多出6.8±1.8循環閾值（Ct）的PEDV（P < 0.05）。先不考慮時間和溫度，與接種了PEDV但飼料未處理的試驗組相比，制粒過程降低了可檢測到的PEDV病毒RNA的數量。無論劑量高低，飼喂感染了PEDV且未經過處理的飼料，豬只第2-7天（試驗第2天至試驗結束）的糞便中PEDV是陽性的。然而，只要經過上文中的9種時間×溫度處理模式中的任何一種處理過后，豬糞樣品或盲腸內容物中就檢測不出PEDV的RNA。

　　根據試驗一的結果，我們設計了第二個試驗來探究較低的處理溫度對PEDV數量和感染性的影響。在試驗二中，接種了PEDV的飼料在以下5種溫度（37.8、46.1、54.4、62.8和71.1℃）處理30s。這5種越來越高的處理溫度導致飼料的平均Ct值分別為32.5、34.6、37.0、36.5和36.7。所有樣品含有可檢測出的PEDV的RNA。然而，通過生物測定技術發現只有來自處理溫度為37.8和46.1℃的豬只身上檢測出感染性。

　　試驗二的結果表明制粒溫度在54.4℃以上時可有效降低豬飼料中PEDV的數量和感染性。然而由于它只是特定時間點的緩解步驟，所以仍需更深入的研究來阻止制粒后的污染。


　　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.

來源：豬營養國際論壇CSIS   翻譯：李光燃