University of Helsinki, Helsinki 2006
Epidemiological aspects and improved differential diagnostics of porcine Brachyspira pilosicoli
Doctoral dissertation, April 2006.
Brachyspira pilosicoli causes porcine intestinal spirochaetosis, which is manifested by a mild, persistent diarrhoea among weaned pigs at the age of 7-14 weeks. The growth of diseased pigs is retarded and their feed conversion is lowered, resulting in diminished production. These studies were designed to investigate the molecular epidemiology of B. pilosicoli in Finnish sow herds and to improve laboratory diagnostics for B. pilosicoli. Infectivity of the rare hippurate-negative biotype of B. pilosicoli was examined by an infection trial and eradication of B. pilosicoli from a sow herd was demonstrated.
A high genetic diversity was observed among 131 B. pilosicoli strains obtained from 49 sow herds located in the two major pork production areas of Finland. A high discriminatory power of pulsed-field gel electrophoresis (PFGE) was established with either SmaI or MluI used as a restriction enzyme. Common genotypes between the herds were rare, and no clustering of the genotypes according to the two geographical areas was observed. A single genotype could persist in a herd for several years; however, genetic recombination among B. pilosicoli strains might occurr. The epidemic nature of B. pilosicoli infection in Finnish pig farms was shown, and the role of migrating vectors or fomites for horizontal transmission was assessed as minor.
The high discriminatory power of PFGE was further exploited to investigate hippurate-negative phenotypes of B. pilosicoli. No relationship between hippurate-negativity and genotypes of B. pilosicoli was detected. This finding was substantiated by comparative analyses of 16S rDNA nucleotide sequences of hippurate-negative and -positive isolates; different nucleotide positions of the strains were not predictive of the hippurate hydrolysis reaction. B. pilosicoli isolates from the same herd could possess an identical genome despite having a different capacity for hydrolysing hippurate. The congruent phylogeny of hippurate-negative and -positive B. pilosicoli was further ascertained by an ultrastructural study; the two biotypes shared all of the features unique to the species B. pilosicoli. In conclusion, the expression of hippurate hydrolysis can vary within a single B. pilosicoli clone, and thus, the hippurate test can not be used alone for differentiation of porcine B. pilosicoli.
An occasional hippurate-negativity can disturb the phenotype-based differential diagnostics of B. pilosicoli. A B. pilosicoli-specific D-ribose test was therefore established to strengthen the diagnostics protocol. Sixty unrelated B. pilosicoli strains and 35 strains of other porcine Brachyspira species were studied for D-ribose utilization by an indirect method based on recording the pH reduction of a broth culture in the presence of D-ribose. All B. pilosicoli strains, regardless of the hippurate reaction, could utilise D-ribose, whereas the strains of the other Brachyspira species were D-ribose-negative. These results enabled the construction of an amended classification scheme for phenotypic differentiation of porcine B. pilosicoli.
Experimental pigs were inoculated either with a hippurate-negative B. pilosicoli strain or with a B. pilosicoli type strain. Somewhat unexpectedly, the pigs in both trial groups remained healthy. Only the hippurate-negative B. pilosicoli strain could be reisolated from two of eight infected pigs. An explanation for the silent infection - or lack of infection - may lie in the challenge procedure, strain attenuation, environmental conditions, absence of concomitant enteropathogens or the feeding regimen. The pathogenicity of hippurate-negative B. pilosicoli should be further studied in modified conditions for trial pigs.
Eradication of a chronic B. pilosicoli infection without total depopulation was attempted in a 60-sow farrowing herd. The principles for eradication of B. hyodysenteriae, the cause of swine dysentery, were applied. Special attention was paid to sanitary measures, relocation of animals according to age groups to nearby shelters, husbandry principles and adequate medication. The eradication was successful; the diarrhoea of the young growers disappeared, and B. pilosicoli was not detected in any sample during the 4.5-year follow-up.
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© University of Helsinki 2006
Last updated 06.04.2006