19       Erysipelothrix rhusiopathiae

 

Erysipelothrix rhusiopathiae is a non-motile, Gram-positive, facultative anaerobe. It is catalase-negative, oxidase-negative, resistant to high salt concentrations and grows in the temperature range of 5–42°C and in the pH range of 6.7–9.2. It can survive for several weeks in the environment but does not replicate outside the animal host. Isolates from animals with acute infections form smooth colonies, while isolates from chronically infected animals form rough colonies. Smears from smooth colonies yield slender rods whereas rough colonies are usually composed of short filaments which decolorize readily. The bacterium grows on nutrient agar but growth is improved in media containing blood or serum.

Erysipelothrix rhusiopathiae causes erysipelas in pigs and turkeys worldwide. Sheep and other domestic animals are occasionally infected. Up to 50% of healthy pigs may harbour E. rhusiopathiae in their tonsils. Carrier pigs may excrete the organism in faeces and in oronasal secretions.

Infection is usually acquired by ingestion of material contaminated with pig faeces. Entry may occur through the tonsils, skin or mucous membranes. Virulence factors include a capsule which protects the organism against phagocytosis and several exoenzymes. The enzyme neuraminidase assists in adherence to, and penetration of, endothelial cells and hyaluronidase facilitates dissemination into host tissues. Swine erysipelas can occur in four forms. The septicaemic and cutaneous (‘diamond’) forms are acute, while arthritis and vegetative endocarditis are chronic forms of the disease. Chronic arthritis has the most significant negative impact on productivity. Septicaemia occurs after an incubation period of about three days. Some pigs may be found dead; others are febrile and depressed. Mortality may be high in some outbreaks and pregnant sows may abort. In the diamond-skin form, systemic signs are less severe. Pigs are febrile and cutaneous lesions progress from small, light pink or purple, raised areas to more extensive diamond-shaped erythematous plaques. Arthritis, which is commonly encountered in older pigs, can present as stiffness, lameness or reluctance to bear weight on affected limbs. Joint lesions can progress to erosion of articular cartilage with eventual fibrosis and ankylosis. In vegetative endocarditis, the least common form, wart-like thrombotic masses are present usually on the mitral valves. Many affected animals are asymptomatic but some may develop congestive heart failure or die suddenly if stressed by physical exertion or by pregnancy. Clinical presentation and the type and location of lesions may suggest swine erysipelas. Diamond-shaped lesions are pathognomonic. Erysipelothrix rhusiopathiae can be isolated on blood agar and identified using biochemical tests. However, PCR-based procedures, including a combination of enrichment broth cultivation and PCR, have been described for detection of the pathogen and for identification. Typing procedures include serotyping and, more recently, molecular techniques such as pulsed-field gel electrophoresis and nucleotide sequencing of the spaA gene. The latter encodes the spaA surface protein, a major protective antigen. Live and attenuated vaccines are available for the prevention of erysipelas in pigs, although some workers suggest that the use of live vaccines may be associated with chronic arthritic forms of the disease.

Turkey erysipelas affects birds of all ages. Toms may excrete the organisms in their semen and turkey hens may die suddenly within five days of artificial insemination.

Non-suppurative polyarthritis of lambs may result from entry of E. rhusiopathiae through the navel or, more commonly, through docking or castration wounds. Post-dipping lameness, which affects older lambs and adult sheep, is due to cellulitis and laminitis associated with contamination of the dipping fluid by the pathogen.