Swine erysipelas (SE) or its equivalent in other languages _Schweinerotlauf, vlekziekte, rouget du porc, mal rossino, entrace eresipelatoso, rozyca, and erisipela del cerdo_is a disease caused by the bacterium Erysipelothrix rhusiopathiae (Sneath et al. 1986) and manifested by acute or subacute septicemia and chronic proliferative lesions. The disease is worldwide in distribution and is of economic importance throughout Europe, Asia, and the Australian and American continents.
The identification of SE as a disease entity began in 1878 when Koch isolated from an experimental mouse an organism that he called "the bacillus of mouse septicemia. " In 1882-83 Pasteur and Thuillier briefly described the organism isolated from pigs with rouget. In 1886 Löffler published the first accurate description of the causative agent of Schweinerotlauf and described the infection in swine.
In the United States the recorded history of SE began when Smith (1885) isolated the causative organism from a pig. The disease was not considered important, however, until serious outbreaks were reported in South Dakota in 1928; by 1959 acute SE had been reported in 44 states. Since that time the prevalence of SE apparently has decreased overall (Wood 1984). However, the disease is still considered to be of economic importance, especially in the chronic form, and outbreaks of acute SE continue to occur sporadically in endemic areas.
E. rhusiopathiae occurs in most parts of the world, and SE occurs in most areas where domestic swine are produced. The organism also causes polyarthritis of sheep and lambs and serious death losses in turkeys. It has been isolated from body organs of many species of wild and domestic mammals and birds as well as reptiles, amphibians, and the surface slime of fish.
In humans E. rhusiopathiae causes erysipeloid, a local skin lesion that occurs chiefly as an occupational disease of persons engaged in handling and processing meat, poultry, and fish as well as of rendering-plant workers, veterinarians, game handlers, leather workers, laboratory workers, and the like. The organism occasionally is isolated from cases of endocarditis in humans and rarely causes acute septicemic disease.
ETIOLOGY
E. rhusiopathiae, the causative agent of SE, is a gram-positive bacillus with a marked tendency to form elongated filaments.
Physicochemical Characteristics
MORPHOLOGY AND STAINING. The morphology of E. rhusiopathiae is variable. In smears or cultures made directly from tissues in cases of acute infection, the organism appears as slender, straight or slightly curved rods, 0.2-0.4 by 0.8-2.5 µm, occurring singly or in short chains (Fig. 31.1). An occasional coccoid or clubbed form may be seen. Palisades and angular formations ("snapping division") are common. The organism is nonmotile, non-spore-forming, and non-acid-fast. It stains readily with ordinary dyes and is gram-positive but is easily decolorized. After several subcultures on an artificial medium, filamentous forms of the organism begin to appear. These forms may predominate in old cultures or in chronic lesions. Filamentous forms are somewhat thickened, are greatly elongated (4-60 µm), and may form a mass resembling mycelia, especially in a liquid medium (Fig. 31.1). The filamentous forms sometimes have a beaded appearance when Gram's stain is used.
GROWTH CHARACTERISTICS. Growth of E. rhusiopathiae at 37°C in a nutrient broth appears at 24 hours as a faint turbidity with no odor or pellicle. Shaking typically reveals a momentary appearance of rolling clouds. Slight sedimentation will be seen after 36-48 hours of incubation. Growth is much heavier in broth enriched with serum. In gelatin stabs incubated at 22ûC for 4-8 days, growth of the organism radiates out from the stab in all directions, resembling a test-tube brush.
Colonies of E. rhusiopathiae on agar media at 48 hours are tiny (less than 1 mm), are transparent, and vary from smooth to rough, depending on cellular morphology (Fig. 31.1). Colonies of most strains have entire edges, but some strains form colonies that are slightly larger and have somewhat undulate edges. Granulelike structures usually appear under a colony just below the surface of the agar. Dissociation from smooth to rough form may occur during the development of a colony, producing a sector (Fig. 31.1); the morphology of cells from these intermediate forms will include a variety of shapes from short, curved rods to short filaments.
Most strains of E. rhusiopathiae produce a narrow zone of partial hemolysis on blood agar, usually with a greenish color. Rough colonies do not induce hemolysis.