Food animals carry human pathogens in their gastro-intestinal (GI) tract and on their hides without displaying any physical or external symptoms (Hancock, Rice, Thomas, Dargatz, & Besser, 1997; Letellier, Messier, & Quessy, 1999; Vali et al., 2007). The hides or fleece of food animals are often soiled with feces, blood, dirt, and other foreign materials and are recognized as major sources of contamination in slaughter plants (Barkocy-Gallagher et al., 2003; Bell, 1997; Sheridan, 1998). Foodborne pathogens present on these materials may contaminate the sterile underlying muscle and fat tissue during slaughter through direct transfer or aerosols (Sofos, 2004b). Contamination is first introduced during the first knife incision for hide removal. Thus, this step should be considered as an important contamination transfer point during slaughter (Gill, Bryant, & Landers, 2003; Nou et al., 2003). In order to reduce pathogen levels transferred to carcasses and to the plant environment during hide removal, slaughter plants may implement various animal hide washing or cleaning strategies (Sofos, 2005) which may be applied before or after stunning (Fig. 6.1).
Clipping or hair trimming (Baird, Lucia, Acuff, Harris, & Savell, 2006; Small, Wells-Burr, & Buncic, 2005), spray washing with warm, cold, or ozo-nated water (Bosilevac, Shackelford, Brichta, & Koohmaraie, 2005), or antimicrobial solutions (Baird et al., 2006; Biss & Hathaway, 1995; Small et al., 2005), as well as bathing animals in a trough (Biss & Hathaway, 1995), may be used to reduce the levels of external hide contamination. Power-hosing animals for up to 10 min (Biss & Hathaway, 1995) with cold potable water (10-18°C) in cases of excessive soiling may significantly reduce levels of pathogenic bacteria (Byrne, Bolton, Sheridan, McDowell, & Blair, 2000). Spray washing with cold water seems to be especially effective in cleaning of small animals such as sheep or goats. Pre-stunning washing of sheep is routinely used in New Zealand regardless of contamination level, but in some instances it is applied to animals with extensive soiling of the pelt only (Biss & Hathaway, 1995, 1996; Byrne, Dunne, Lyng, & Bolton, 2007). United States federal regulations require that animals are not wet or at least not dripping at the time of slaughter (Reed, 1996). Therefore, washing should be performed with ample time to allow animals to dry off, or excessive water may be removed with blowing air or vacuum. Animal cleaning and washing, however, may cause stress when applied to live animals, which may be considered inhumane, and have undesirable effects on meat quality.
Live animals may be segregated and slaughtered in groups, based on degree of soiling. Processing speed of heavily soiled animals may be reduced, additional workers may be placed on the slaughter line, or other hygiene measures to reduce hide-to-carcass contamination may be employed (Biss & Hathaway, 1995; Byrne et al., 2007). This approach has been implemented in some
European countries for cattle and sheep. For example, the Irish Department of Agriculture and Food requires a five-scale classification of cattle intended for slaughter. Based on their cleanliness and degree of dampness, they use colored tags to keep track of this classification during slaughter (Anonymous, 1997). However, a study conducted at a Finnish beef slaughter plant indicated that carcasses from excessively filthy animals, even when harvested with additional precautions, experienced higher microbial contamination levels compared to control carcasses fabricated from relatively clean animals (Ridell & Korkeala, 1993). Similarly, an Australian survey indicated that slaughter plants which are frequently processing soiled animals, on average, have a higher prevalence of E. coli on fabricated beef carcasses (Kiermeier et al., 2006). Perhaps excessively soiled animals need to be completely restricted from entering a slaughter facility (Ridell & Korkeala, 1993), or slaughtered at the end of the shift, but this approach may not be feasible due to facilities or economical limitations. In Australia, overnight misting of cattle in a holding pen with water containing detergent results in cleaned animal hides by the time of slaughter.
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