Observations on the distribution of monophasic Salmonella Typhimurium on pig farms in Great Britain

Ten pig herds were visited and intensively sampled to determine the within group prevalence, distribution of contamination and numbers of Salmonella organisms excreted by infected pigs. The distribution of infection was highly variable but on all farms with breeding pigs the breeding herd was involved, even though the occurrence of the organism was greater in growing and fattening pigs. Infection was less common in farrowing areas. Involvement of wild birds and contamination of soil on outdoor units was frequently found. Numbers of organisms excreted were typically low but levels of up to 106 cfu/g were found in a small number of samples. The role of breeding pigs was clearly illustrated by the finding of five different serovars, including monophasic S.Typhimurium, in a single batch of replacement gilts delivered to one outdoor breeding farm. Introduction Monophasic variants of Salmonella Typhimurium (mST) have emerged in pigs and other species in many countries during the last twenty years, but the emergence of phage type DT193 variants with resistance to ampicillin, streptomycin, sulphonamide and tetracycline in most European countries has been particularly dramatic, and has resulted in a substantial number of human cases (EFSA, 2010). The reason for the emergence and rapid spread of mST in UK pigs since 1996 is unknown but one hypothesis could be increased involvement of breeding pigs or higher numbers of organisms shed in faeces, leading to more rapid spread of infection. This study was therefore begun in order to investigate qualitatively and quantitatively the distribution, and subsequent persistence, of infection on a series of different types of pig farms. Materials and Methods Ten commercial pig herds (3 outdoor breeding, 3 indoor breeder finisher, 1 outdoor grower, 4 outdoor finisher, 2 indoor finishers) in which monophasic S.Typhimurium S.4(5)12:i:or S.4,12:i:had been isolated or suspected were visited by the authors. A combination of naturally pooled faeces (taken with a large gauze swab), individual faeces (60 per epidemiological group or if less than 60 pigs in the group a number of faeces equivalent to the number of pigs was taken) and wildlife and environmental samples were taken and returned to the laboratory on the day of collection. Culture of the swab samples was begun on the day of collection but individual samples were held at 4oC until the next day when 2 g was aliquoted and tested. The remainder of the individual samples was retained until a Salmonella result was obtained then a maximum of 40 samples from each farm as semiquantified by a dilution-enrichment technique as described by Wales et al (Wales et al., 2006). Salmonella culture was carried out using a modification of ISO6579 : Annex D, using Rambach agar as the single isolation medium. Salmonella isolates were serotyped and a selection were also phage typed. Results A mixture of Salmonella serovars was found in pig areas on all farms except E and F, where only monophasic S.Typhimurium (mST) was present. Only the qualitative results from breeding farms are shown in table one because of space limitations but other data will be presented at the conference. In larger farms both regular mST and ‘Copenhagen’ variants were sometimes found and phage types DT193 and DT120 could be present concurrently. Monophasic and regular S.Typhimurium was more likely to be found in weaned and fattening pigs rather than the breeding herd but in most cases a low to moderate prevalence was also found in breeding pigs, particularly gilts. Involvement of wild birds, particularly on outdoor units, and rodents was identified. Pooled water, transport vehicles and various environmental samples were