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The objective of this study was to further define subclinical ileitis by measuring the impact of varying doses of Lawsonia intracellularis on clinical signs, L intracellularis shedding and seropositivity, performance, and gross and histopathological intestinal changes in weaned pigs.
A total of 144 two-week-old pigs originating from one farrowing unit were weighed and randomly assigned to 24 pens of six pigs each at the beginning of the study (Day -14). Pigs were acclimated for a period of 14 days (Days -14 to 0), challenged with an L intracellularis inoculum on Day 0, and observed for a period of 21 to 22 days, at which time each pig was euthanized and necropsied. Blood and fecal samples were collected from two randomly selected pigs in each pen on Day -14, Day 14, and at the end of the study (Day 21 or 22). Serum samples were analyzed for antibody to L intracellularis by IPMA and fecal swabs were tested for L intracellularis by PCR. There were six treatment groups with four pens of six pigs per treatment. Each study group was treated on Day 0 with a different dose of L intracellularis inoculum: Treatment A (nonchallenged, control) and Treatments B, C, D, E, and F which were challenged with serial 10-fold dilutions resulting in doses of approximately 104 (F) to 108 (B) organisms per pig. Clinical scores for attitude, abdominal appearance, fecal consistency, and fecal blood were recorded for each pig three times per week, from Day 7 to Day 21. All pigs were individually weighed on Days -14, 0, 7, 14, and 21 or 22. Feed intake was calculated on each weigh day.
By Day 14, approximately 15% of pigs in Treatments B and C were PCR-positive, while the other treatment groups remained PCR-negative. On Days 21 and 22, fecal samples from 12.5% to 83.6% of the challenged groups were PCR-positive, and 12.5% to 98.0% of challenged pigs were seropositive for L intracellularis antibodies. More pigs in Treatments C and D were PCR-positive for L intracellularis than in Treatment A (nonchallenged controls), and more pigs in Treatments B, C, and D were seropositive than in either Treatment E or the nonchallenged controls. No animals in the nonchallenged control group became positive either by fecal PCR or by IPMA. Clinical scores for abdominal appearance, pig attitude, and fecal blood were predominantly normal for all treatments across all observation periods. There was no significant difference in mean fecal consistency score between treatment groups until Day 14. Between sampling days 14 and 18, there was a numerical dose response between the number of organisms administered and the mean fecal consistency score for the pens. Fecal consistency scores were significantly higher than those of the nonchallenged controls in Treatments B and C on Day 14, in all challenged groups on Day 16, and in Treatments B, C, and D on Day 18. Growth rate was numerically lower in challenged groups than in controls Days 0 to 6. Average daily gain was significantly lower in Treatment B than in the nonchallenged controls for Days 7 to 13. On Day 14, pigs in Treatments B and E weighed significantly less (by approximately 2 kg) than did the nonchallenged controls. Average daily gain was significantly lower than controls in all challenged groups for Days 14 to 21 and 22 and Days 0 to 21 and 22, paralleling the lower body weights observed during these periods. Average daily feed intake was also significantly lower in all challenged groups than in the nonchallenged controls for Days 7 to 13, Days 14 to 21 and 22, and Days 0 to 21 and 22. There was a significantly poorer feed conversion ratio over the entire post-challenge period (Days 0 to 21 and 22) with increasing inoculum dose. Generally, performance was poorer (Days 0 to 21 and 22) for all performance measures with increasing inoculum doses. Mortality was distributed across all treatment groups, with no significant difference between treatments. Between 8% (Treatment F) and 33% (Treatment D) of the pigs in challenged pens had gross lesions consistent with L intracellularis infection at necropsy on Days 21 and 22, and no lesions were observed in the nonchallenged controls. More positive animals were identified by H&E scores (31% to 61%), WS scores (33% to 54%), and IHC scores (62% to 78%) than by gross pathology (8% to 33%). The most sensitive indicator was IHC, with 62% to 78% of the challenged pigs having scores that were significantly different from those of the controls. The proportions of pigs in which the ileum was histologically positive for ileitis were significantly greater in Treatments C and D than in the nonchallenged control group. When all four post mortem diagnostic measures of ileitis were considered, the proportion of pigs positive was highest in Treatment D when compared to the nonchallenged controls.
The present study demonstrates both induction of subclinical ileitis after inoculation with a mucosal homogenate containing L intracellularis and a dose response to the number of organisms in the inoculum, creating a spectrum of illness from subclinical disease at the lowest inoculum dosage to clinical disease at higher doses and even death of some infected pigs. The consistency of poor performance in the challenged groups made performance parameters the most sensitive indicators to identify the disease process experienced by these pigs. The impact of the subclinical form of ileitis on growth was the most remarkable observation of this study. Even at the lowest inoculum dose (Treatment F), average daily gain during the trial period was 37% lower than that of the nonchallenged pigs, and feed conversion was 27% higher. There was no clinical difference in mean fecal consistency scores between Treatments E and F (the two lowest doses of inoculum administered) and the nonchallenged control groups except for one day (Day 16). Fecal shedding of L intracellularis was detected in some pigs, many of these subclinically infected. Although no clinical signs of disease were observed, histopathological evidence of L intracellularis infection was found. This may be extremely important for the economics of swine farming. Not only do subclinically infected pigs perform suboptimally, but their feces may infect other pigs in the herd.
The results confirm that subclinical infection can have a significant impact on growth performance, and that subclinically infected pigs can serve as a source of infection for other animals in the herd. Moreover, a significantly smaller proportion of these subclinically affected animals are positive by routine diagnostic testing methods such as seroconversion and fecal PCR.
Paradis MA, Gebhart CJ, Toole D, et al. Subclinical ileitis: Diagnostic and performance parameters in a multi-dose mucosal homogenate challenge model. J Swine Health Prod. 2012;20(3):137–141.