Direct and indirect transmission of four Salmonella enterica serotypes in pigs

Abstract

Background: Feed-borne spread of Salmonella spp. to pigs has been documented several times in recent years in Sweden. Experiences from the field suggest that feed-associated serotypes might be less transmittable and subsequently easier to eradicate from pig herds than other serotypes more commonly associated to pigs. Four Salmonella serotypes were selected for experimental studies in pigs in order to study transmissibility and compare possible differences between feed-associated (S Cubana and S Yoruba) and pig-associated serotypes (S Derby and S Typhimurium).

Methods: Direct contact transmission was studied in four groups of pigs formed by six 10-week-old salmonella negative pigs commingled with two fatteners excreting one of the four salmonella serotypes. Indirect transmission was studied by putting six 10-week-old salmonella negative pigs in each of four salmonella contaminated rooms. Each room had previously housed a group of pigs, excreting one of the four selected serotypes.All pigs were monitored for two weeks with respect to the faecal excretion of salmonella and the presence of serum antibodies. At the end of the trial, eight samples from inner tissues and organs were collected from each pig at necropsy.

Results: In the four direct transmission groups, one pig shed Salmonella (Cubana) at one occasion. At necropsy, S Typhimurium was isolated from one pig.In the indirect transmission groups, two pigs in the Yoruba room and one pig in each of the other rooms were excreting detectable levels of Salmonella once during the study period of two weeks. At necropsy, S Derby was isolated from one of six pigs in the Derby room and S Typhimurium was isolated from four of the six pigs in the Typhimurium room.No significant serological response could be detected in any of the 48 pigs.

Conclusions: These results show that all four selected serotypes were able to be transmitted in at least one of these field-like trials, but the transmission rate was low in all groups and no obvious differences between feed-associated and pig-associated serotypes in the transmission to naïve pigs and their subsequent faecal shedding were revealed. However, the post mortem results indicated a higher detection of S Typhimurium in the ileocecal lymph nodes of pigs introduced into a contaminated environment in comparison with the other three serotypes.

Figure 1

The individual faecal shed of Salmonella in pigs following mixing of two seeder pigs with six naive pigs (DT-study). The two seeder pigs in each group were inoculated with 0.65 × 10⁹ cfu of one of four different serovars of Salmonella eight weeks earlier. Demonstration of Salmonella is illustrated by white, whereas grey illustrates that Salmonella was not detected. In the right column the results from analysis of Salmonella in tissue samples taken post mortem are shown. Within brackets the tissue from which Salmonella was isolated are abbreviated as follows: cecum content = cec, colon lymph node = col, colon tissue = cot, tonsil = ton.

Figure 2

The individual faecal shed of Salmonella in six pigs after their introduction into Salmonella contaminated pens (IT-study). The pens had harboured Salmonella shedding pigs, individually inoculated with 0.65 × 10⁹ cfu of Salmonella eight weeks earlier. The inoculated pigs were removed and replaced with the naive, 10 weeks old pigs. Demonstration of Salmonella is illustrated by white, whereas grey illustrates that Salmonella was not detected. In the right column the results from analysis of Salmonella in tissue samples taken after euthanisation are shown. Within brackets the tissue from which Salmonella were isolated are abbreviated as follows: cecum content = cec, colon tissue = cot, ileocecal lymph node = iln.