A polymerase chain reaction (PCR) assay was adapted and optimised for specific detection
of Lawsonia intracellularis genomic DNA segment in swine faeces. Lawsonia
intracellularis is the aetiological agent of porcine proliferative enteropathy (PPE) and the
PCR represents the first diagnostic test suitable for ante-mortem use in affected swine.
Various methods designed to extract bacterial DNA from faeces were evaluated to establish
a convenient and optimum protocol. The PCR was utilised in pig challenge studies to
investigate the excretion patterns of L. intracellularis in weaner pigs orally inoculated with
pure cultures of L. intracellularis. This challenge work demonstrated that the PCR was a
suitable tool for detection of infection, and indicated that individual animals could excrete
L. intracellularis organisms for periods of up to ten weeks post-challenge. Such an
excretion period has major implications for the transmission of organisms in the field. For
example, if infected growers are still shedding L. intracellularis organisms upon entry to
the breeding population, then this is a possible route for the transmission of disease to
younger, susceptible pigs.
younger, susceptible pigs.
A more extensive, two-part investigation of the epidemiological aspects of PPE in the field
followed. The investigation comprised a farm sampling study and a questionnaire postal
survey. In the farm sampling study, faeces samples were collected serially over a ten month
period from breeding gilts and their litters. Samples were subjected to PCR for the
detection of infection, allowing estimation of within-herd prevalence, as well as
determination of possible transmission patterns. The assay successfully detected the
presence of L. intracellularis in the weaners and/or growers of three of the five farms
selected for this study. The within-herd prevalence for these age-groups ranged from 10 to
30%. The PCR also confirmed infection in several of the adult breeding boars and gilts.
The relative expense of the PCR assay dictates that its practical application, at least for the
purposes of research, must be targeted. Thus, the sampling study was coupled with a postal
questionnaire survey. Mailing questionnaires to almost 600 commercial production units
achieved a 56% response rate. This provided a sufficiently large number of herds to allow
statistical analysis of possible risk factors involved in the epidemiology of PPE. This survey
indicated that the 1993 to 1995 period-prevalence of PPE in the UK was 31%. Based on the
number of sows, herd size was an important risk factor, even when herds with under 50
sows were excluded (p<0.005). There was an important link between the occurrence of PE
and nucleus herds, with five out of six nucleus herds in the study having had PE diagnosed
in the previous three years. This link was strengthened in that herds obtaining replacement
breeding boars from nucleus stock were at an increased risk of PE (p<0.05). Factors which
could affect the exposure of animals to the faecally-contaminated environment were also
significant. Surprisingly, slatted or meshed floors were linked to PE, especially in the
younger age-groups (p<0.05). Batch movement of pigs on a house basis was significantly
protective against PE (p<0.05), but batch movement on a pen basis only was neither a
protective nor a risk factor.
Epidemiology of PPE is complex, but the PCR has proved to be a valuable tool, capable of
screening for the presence of L. intracellulars in field conditions. Additionally, its use has
permitted some initial conclusions to be drawn regarding the excretion and transmission
patterns of this unique organism.