Infection of pigs following exposure to contaminated pen floors

Abstract

The objective of these studies was to determine if pigs could acquire infection when exposed to different levels of environmental contamination with Salmonella Typhimurium. In experi- ment 1, pigs were euthanised after 2, 3 and 24 hours of exposure to a highly contaminated envi- ronment (105 organisms/100cm2). Tonsils, caecal contents, ileocaecal junction, lung, liver, spleen and ileocaecal lymph nodes were evaluated. Salmonellae were detected in at least one sample from each pig after exposure for 2 or more hours. Salmonellae were recovered most frequently from tonsils (100% positive), segments of the ileocaecal junction (94.4% positive) and caecal con- tents (89% positive). In experiment 2, pigs were exposed to a less contaminated environment (102 organisms/100cm2) for time periods of 1, 3, 6 and 24 hours. Tonsils, caecal contents and mesen- teric lymph nodes were collected for Salmonella spp. isolation and examined. Salmonellae were not detected in any samples from pigs euthanised after exposure for 1 hour. Salmonella spp. were recovered from the caecal contents of all pigs after exposure for 3, 6 and 24 hours. Introduction It is recommended that pigs be rested for a minimum of 2 hours prior to slaughter, to recover from the stress of transport and to improve meat quality. However, recent research has shown that pigs reared in an environment free of Salmonella spp. or on farms with low levels of infection may acquire infection during transport to the abattoir or while held in lairage (Hurd et al., 2001a), (Hurd et al., 2002), (Morgan et al., 1987). In the lairage up to 90% of environmental sam- ples collected may test positive for Salmonella spp., indicating that the lairage may be an impor- tant potential source of infection for incoming animals (Swanenburg et al., 2001). Experimental infection with Salmonella spp. may be carried out by inoculation per os, intranasal instillation or intraperitoneal injection. However, in order to simulate a natural mode of transmission in the stud- ies described here, animals experimentally infected by intranasal instillation were used to seed the environment with salmonellae. The use of infected animals, which were shedding salmonel- lae, also ensured that challenged pigs were exposed to Salmonella spp., which had been pas- saged in vivo. Recent data obtained using a simulated lairage model have shown that pigs can become rapidly infected with Salmonella spp. when exposed to an environment contaminated with these organisms (Hurd et al., 2001b). In contrast with these findings, in Denmark Boes et al. (2001) found relatively low levels of cross contamination among pigs from Salmonella spp. nega- tive herds held in lairage for two to four hours in pens contaminated with infected faeces. It appears that the level of environmental contamination present in the lairage and the length of time for which pigs may be exposed to this contamination are important factors in determining the level of cross contamination which can occur in pigs prior to slaughter. The primary objective of this study was to determine if short-term exposure to an environment contaminated with Salmonella spp. such as occurs in the lairage is sufficient to produce infection in pigs. Therefore in order to ascertain the minimum exposure time necessary to produce infection, pigs were exposed for one, two, three, six or twenty-four hour periods. The effect of two different levels of contamination was also investigated.