Erysipelothrix rhusiopathiae infections

M-L PENRITH AND B T SPENCER

GENERAL INTRODUCTION: REGULAR, NON-SPORING, GRAM-POSITIVE RODS

Erysipelas in pigs

Synonyms: Diamond skin disease, swine erysipelas, vleksiekte (Afrik.), rouget (Fr.), mal rubra (Port.)

Introduction

Erysipelas is a peracute, acute, subacute or chronic infectious disease of pigs caused by Erysipelothrix rhusiopathiae. The peracute or acute forms manifest as an often fatal septicaemia. The common names ‘diamond skin disease’ or ‘vleksiekte’ (Afrik.) are derived from the subacute form, which is characterized by large, roughly diamond-shaped, slightly raised, well-demarcated, purplish-red patches on the skin. Pigs suffering from the chronic form of the disease are usually unthrifty and may have vegetative valvular endocarditis and/or chronic polyarthritis.

A bacillus that was probably E. rhusiopathiae was isolated by Koch in 1878 from the blood of mice suffering from septicaemia.39 The organism was associated with the disease known as ‘rouget’ in pigs in 1882/83, which was accurately described for the first time by Löffler in 1886.³⁹

Outbreaks of erysipelas occur only sporadically in South Africa, probably because most of the breeding stock are vaccinated regularly.

In pigs, erysipelas can result in reduced production due to mortality, unthriftiness and increased condemnations at slaughter.

Aetiology

Erysipelothrix rhusiopathiae is usually regarded as the only species in the genus, although, based on differing N-acetyl-beta- glucosaminidase activity, a new species, E. tonsillae, has been proposed.^{28, 31} The taxonomic position of the genus in relation to other bacterial genera has not yet been resolved.¹⁴

The organisms are facultatively anaerobic, non-motile, non-sporulating, usually slender, straight or slightly curved rods 0,2 to 0,4 μm in width and 0,5 to 2,5 μm in length. They may, however, be filamentous and 4 to 60 μm or more in length.^{14, 39} They are Gram-positive, stain well with the various aniline dyes,³⁹ ferment glucose and lactose weakly, and produce hydrogen sulphide in triple sugar iron agar.^{14, 23} They grow readily on most of the standard laboratory media, but cultivation is enhanced by the addition of glucose, and, to a lesser extent, blood and serum to the medium in a slightly alkaline pH. Most strains grow better if isolated primarily in a reduced oxygen environment containing 5 to 10 per cent carbon dioxide.³⁹ Smooth (S-form) colonies are round, convex, slimy, translucent, about 0,3 to 1,5 mm in diameter, and have entire or undulating edges. These colonies are formed within 24 to 48 hours, and are surrounded by a narrow rim of incomplete haemolysis (α-haemolysin) when grown on a blood agar medium.^{14, 32} Rough (R-form) colonies are generally opaque, flatter and larger than S-form colonies,^{14, 23} and have irregular edges, and a surface which appears matt. Serovars 1 and 2 generally produce S-form colonies on blood agar, while less virulent serovars produce R-form colonies. However, morphological distinction between the two forms is not always accurate and intermediate forms exist; S-form colonies may change to intermediate or R-form colonies, and vice versa.^{14, 39}

Of the 25 serovars that have been identified, serovars 1 and 2 are those most frequently isolated from cases of erysipelas; the others are relatively rare.^{7, 12, 14, 23, 39}

Erysipelothrix rhusiopathiae is remarkably resistant for a non-sporulating organism. It retains its infectivity in putrefied meat for more than four months, in pickled and salted bacon for several weeks, in well-smoked hams for more than three months, in drinking water for five days, and in sewage for up to 35 days.^{11, 14, 38} The survival of the organism is favoured by environmental factors such as low temperature, alkaline pH and abundant organic matter.¹⁴ Survival in soil under any conditions does not exceed 35 days.³⁹

There is a high correlation between pigs and mice of susceptibility to artificially induced infection with highly or less virulent strains, while mixed results are obtained in infections with strains of intermediate virulence.⁵ White mice are highly susceptible to infection with E. rhusiopathiae, and for this reason they are used in a test known as the mouse protection test, which is used for the identification of new isolates of E. rhusiopathiae. Almost all strains of E. rhusiopathiae tested by the tube or slide technique are coagulase-positive. This technique is useful for differentiating between E. rhusiopathiae, Listeria spp. and Corynebacterium spp. ³³

Epidemiology

Erysipelothrix rhusiopathiae is a cosmopolitan pathogen. Erysipelas can occur in pigs kept under a variety of conditions and, unlike most of the currently important diseases of pigs, is not associated with intensification of production. Sporadic outbreaks in pigs occur periodically in South Africa, ⁹ and the disease was recently reported in Kenya.³⁶ Other susceptible species are humans, cattle, sheep, horses, white mice, pigeons, turkeys and several other species of birds,^{11, 14, 34} but the disease is probably only important in pigs, sheep and turkeys.³⁹

Pigs of all ages may contract the disease. It has been suggested that pigs aged between two months and one year, and pregnant sows, are most susceptible,^{15, 20} while, according to Wood,³⁹ pigs younger than three months and older than three years are least susceptible. However, recent reports of field...