Populations of Salmonella typhimurium in internal organs of experimentally infected carrier swine

Abstract

Summary Experiments were conducted to determine comparative populations of Salmonella typhimurium in the most commonly infected body organs of long-term carrier swine. Naturally farrowed Salmonella-free pigs (n = 58) were orally exposed to S typhimurium when they were 47 days old. Necropsy of 3 to 5 randomly selected pigs was conducted at 3, 7, 10, 14, and 17 days and at 3, 4, 5, 6, 8, 12, 16, 20, 24, and 28 weeks after exposure. Mean populations (log10/g) of S typhimurium in palatine tonsils, ileum, cecum (wall and contents), ascending colon (wall and contents), and mandibular and ileocolic lymph nodes were estimated at each necropsy, using a most-probable-number method of bacteriologic examination. Populations of organisms in cecum and colon were similar to each other throughout the duration of the study. Mean populations (log10/g) associated with cecal and colonic walls decreased from 6.1 and 6.6, respectively, during the first postexposure (pe) week to ≤ 1.67 from pe weeks 4 to 28. Populations (log10/g) associated with cecal and colonic contents decreased from 5.6 and 5.5, respectively, at pe day 3 to 2.5 and 2.7, respectively, at pe week 4, and remained ≤ 2.8 until week 28. Populations (log10/g) associated with intestinal walls and contents were closely correlated during the study. Population (log10/g) in the ileum was ≥ 5.3 from pe days 3 to 17, then varied between 5.4 and -0.4 up to pe week 28. Population (log10/g) of S typhimurium in the tonsils varied from 6.4 to 5.5 up to pe week 5, then decreased to a range of 5.3 to 4.0 until pe week 24, and further decreased to 1.1 at pe week 28. Population (log10/g) in mandibular lymph nodes decreased from 3.5 at pe day 3 to 0.8 at week 8, then ≤ 1.0 up to week 28. In ileocolic lymph nodes, population decreased from 4.4 during the first pe week to ‒0.2 at 8 weeks, then ≤ 1.5 up to week 28. Fecal samples obtained weekly or biweekly throughout the study were 50 to 100% culture-positive (av, 85.4%). Results indicate that the long-term carrier state of S typhimurium in most infected body organs of swine exists mainly at a low, but fairly stable population.