GENERAL INTRODUCTION: SPIROCHAETES

Introduction

Swine dysentery (SD) in its typical acute disease manifestation is characterized by dysentery and the development of acute diphtheritic to necrotic enteritis with haemorrhage of the caecum and colon in weaned, growing, finishing and adult pigs. It results in temporary loss of condition, chronic wasting, or death. It is an important enteric disease in many countries.

The disease was first described in 1921 by Whiting and his co-workers in the USA who were able to reproduce it in transmission experiments using the gut contents of affected animals, thus proving that it was transmissible.⁴⁸ It was recorded in many countries of the world but the aetiological agent was unknown until 1971 when Taylor and Alexander described the isolation of a spirochaete and the reproduction of the disease by feeding pure cultures of it to pigs.⁴⁵ Shortly after that, Harris and his co-workers in the USA confirmed their findings and named the organism Treponema hyodysenteriae.^{14, 16} Both the organism and the disease have been extensively studied since then. This knowledge has been applied to methods used to diagnose, treat and control the disease, but has not yet resulted in a universally applicable vaccine.

Aetiology

The aetiological agents of SD are Brachyspira hyodysenteriae (formerly Serpulina or Treponema hyodysenteriae),³⁵ the more recently described  B. hampsonii⁷from North America, and B. suanatina in Northern Europe.³⁹ These species are included in a group of large oxygen-tolerant anaerobic spirochaetes found primarily in the large intestines of pigs and other mammals and birds. Brachyspira hyodysenteriae is 6 to l0 µm in length, flexible and active when viewed by phase contrast microscopy, and stains readily with aniline dyes such as carbol fuchsin. The cell contains a protoplasmic cylinder 350 nm in diameter with pointed ends and 7 to 14 fibrils inserting at each end and is surrounded by an envelope. Brachyspira hampsonii and B. suanatina are somewhat shorter in length and have 10-14 (B. hampsonii) and 7-8 (B. suanatina) flagella per cell end, respectively.^{33, 34}

The organisms can be grown on blood agar in atmospheres containing 5 per cent carbon dioxide and 95 per cent hydrogen. They form colonies 1 mm in diameter surrounded by β-haemolysis after 48 hours’ incubation.

Brachyspira hyodysenteriae is strongly β-haemolytic, indole positive, β-galactosidase negative, hippurate negative, and is antigenically distinct from other Brachyspira spp. With the exception of B. hampsonii being indole negative,³³ these features are shared with B. hampsonii and B. suanatina. It is sensitive to drying⁸ and acid conditions (pH <6,0), and is readily destroyed by heat but can survive in organic matter for days or weeks. Its specific antigens include 16 and 36 kDa proteins, 10 kDa envelope protein and a 46 kDa periplasmic flagellar protein.¹⁹

Historically, serotyping, multilocus enzyme electrophoresis (MLEE), 16S rRNA sequences, DNA:DNA hybridization, whole cell DNA probes, restriction enzyme analysis (REA), pulsed field gel electrophoresis, and recently, random amplification of polymorphic DNA (RAPD), matrix-assisted laser desorption ionization time-of-light spectrometry (MALDI-TOF), multi-locus sequence typing (MLST), multiple-locus variable-number tandem-repeat analysis (MLVA), and whole genome sequencing have all been used to identify/classify B. hyodysenteriae, but not B. hampsonii and B. suanatina.⁵¹

Epidemiology

Descriptions of the epidemiology and pathogenesis in this chapter are based on studies of B. hyodysenteriaeand not on B. hampsonii and B. suanatina. Swine dysentery occurs worldwide, is common in many European countries, and has recently been described as a re-emerging disease in for example the USA. However, official national incidence figures are scarce. Increasing numbers of herds are being founded using SD-free breeding stock housed in isolation. Dysentery-free pigs are being used in feeding enterprises, and separate site weaning and rearing operations are all reducing the prevalence of the disease in many countries.

Swine dysentery is transmitted to healthy, susceptible pigs by the ingestion of the faeces of affected or carrier animals or of material contaminated with their faeces.

Faecal shedding of the organism occurs during the incubation period of the disease (in the field this varies between 7 and 60 days but is usually 4 to 14 days in experimentally infected animals), its clinical course, and for up to 90 days following clinical recovery. Farms on which an outbreak of the disease has occurred remain infected unless depopulation and disinfection or a whole herd treatment have been carried out.⁴⁹  At least two electrophoretic types (ETs) or restriction endonuclease analysis (REA) types have been identified on individual farms, and B. hyodysenteriae appears to be a recombinant species with an epidemic population structure in which a few ETs have spread widely.⁴⁷

Pigs of all ages are affected although the peak...