Salmonellosis in wild birds and its relationship with the infection in finishing pigs

The potential relationship between Salmonella infection in wild birds and pigs was investigated. Feces from pigs, wild birds, and bird droppings or other environmental samples from 25 finishing farms were cultured for Salmonella isolation. In 17 (68%) farms Salmonella was isolated. Out of 57 Salmonella isolates found, 32 (56.1%) were Typhimurium. In 6 (24%) farms the same Salmonella serotype was isolated from samples from different origins and similar AR and PFGE patterns were found, which would support the existence of a transmission cycle of Salmonella infection between birds and pigs in this area. Preventing bird access to farm premises is highly recommended. Introduction Pork meat is considered an important source of Salmonella infection for humans in Europe (EFSA, 2008) and the reduction of the prevalence of Salmonella serovars with public health significance in pig herds is considered a major objective in the continent (Regulation (EC) No 2160/2003). The control of pig salmonellosis is difficult. Salmonella spp. are ubiquitous, survive outside the host for months and are able to infect a large variety of wildlife (Murray, 2000). Besides, the Salmonella serovars commonly observed in Europe rarely produce disease, hampering the identification of infected herds. The success of Salmonella control programs relies thus on a good knowledge on the epidemiology of this infection. Factors such as feeding, management, farm design, etc., play a significant role in the spread and maintenance of this infection among pigs (Funk et al, 2004). The role that wildlife may have on the epidemiology of this infection has been suggested many times, but it has been less investigated. With this study we try to gain further insight on the epidemiology of pig salmonellosis in an area of high prevalence in Spain through the search for phenotypic and genotypic similarities among Salmonella isolates from pigs and wild birds that may suggest inter species transmission. Material and methods Mist netting was used to humanely trap birds around pig farms. Birds were identified and kept in sterilized dark cages in groups of less than 5 animals of the same species until they defecate. Feces were collected through sterile swabs and birds released after measured and tagged by a licensed bander. Pools of 5 pig feces (from a minimum of 7 pens) and other environmental samples likely contaminated by bird feces were also collected from fattening units. All samples were cultured by triplicate (ISO 6579:2002) and Salmonella isolates serotyped at the National Centre for Animal Salmonellosis (Madrid, Spain) following the White-Kauffmann-Le Minor scheme (Grimont and Weill, 2007). Salmonella isolates were tested against a panel of 10 antimicrobials (i.e. nalidixic acid, ciprofloxacin, cefotaxime, ampicillin, chloramphenicol, streptomycin, gentamicin, sulphisoxazole, trimethoprim, and tetracycline) using the KirbyBauer disk diffusion method (Murray et al, 2003) and following the antimicrobial concentrations recommended by the European Committee on Antimicrobial Susceptibility Testing (Anonymous, 2007) and the Clinical and Laboratory Standards Institute (CLSI) (Anonymous, 2005). Salmonella strains were classified as resistant (R), intermediate (I) or susceptible (S), according to the CLSI guidelines. Genotyping was carried out by Pulsed-Field Gel Electrophoresis (PFGE) after digestion with XbaI following the CDC standardized laboratory protocol (CDC, 2004). Salmonella Braenderup H9812 was used as molecular size marker (Hunter et al., 2005). Samples from pigs and wild birds were collected from 25 pig farms. An average of 9.4 (SD=5.6) fecal samples (individual or pools) from wild birds and of 8.3 (SD=3.2) pools of fresh pig feces were collected per farm. In addition, in 23 of these farms environmental samples (i.e. bird feces and samples from aisles, windows or underneath the feed silos) were also collected (an average of 2.7 per farm; SD=1.8).