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An epidemiological investigation of porcine-origin feed ingredients and the occurrence of porcine epidemic diarrhea on Midwestern United States pork farms
Porcine epidemic diarrhea (PEDv) is a disease of pigs caused by a virus in the family Coronaviridae. PEDv was confirmed for the first time in the U.S. on May 16, 2013. An investigation of the Canadian outbreak by the Canadian government identified an empirical association between feeding porcine blood plasma (subsequently found to be PCR positive for PEDv genetic material) and occurrence of the disease. To investigate the association between exposure to porcine-origin feed ingredients and the occurrence of PEDv, an epidemiological investigation of Midwestern U.S. pork farms was undertaken in May 2014. Feed delivery data related to pig farming clients confirmed as being infected with PEDv (case farms) were obtained from a commercial feed manufacturer’s information system. All deliveries of feed in the two-weeks prior to each PEDv outbreak that contained either spray dried plasma protein (SDPP), granulated red-blood cells (SDBC), choice white grease (CWG), or hydrolyzed porcine proteins (HPEP) were identified. The batching number for each feed delivery was then ‘back-traced’ through the feed manufacturer’s information system to identify the lot-level detail for each of the ingredients listed above that were included in the deliveries. This list of ‘risk-associated lot numbers’ was then ‘forward-traced’ to identify the complete list of clients to whom feed deliveries were made that included these same ingredient lot numbers but were not infected with PEDv (control farms). The risk of PEDv that was associated with exposure to specific lots of porcine-origin feed ingredients was then determined by estimating the odd’s ratio (OR) for each ingredient (at both the ingredient-level and the lot-level). Forty-three cases and 418 controls were included in the study with the most important limiting factor being the availability of ingredient data at the lot-level. Two sources of SDPP (five and six lots, respectively), and one source each of SDBC (three lots), CWG (ten lots), and HPEP (one lot) were included in the analysis of risk. No ingredients were positively associated (OR > 1 and p < 0.05) with PEDv case farms. Significant differences in risk were identified amongst lot numbers of several ingredients and in these instances; Mantel-Haenszel adjustments to the OR were made. Amongst all ingredients, only a single-lot of CWG was significantly positively associated with PEDv cases. Inclusion of commercially manufactured porcine-origin feed ingredients in swine feed does not appear to be strongly associated with the occurrence of PEDv in pigs consuming that feed.
Keywords: porcine-origin feed ingredients, porcine epidemic diarrhea, epidemiology