Swine dysentery is a mucohemorrhagic diarrheal disease of pigs that affects the large intestine.
Etiology and Pathogenesis
The essential causal agent is Brachyspira hyodysenteriae, an anaerobic spirochete that produces a hemolysin, although other organisms may contribute to the severity of lesions. B hyodysenteriae produces strong β hemolysis on blood agar under anaerobic incubation conditions. Other strongly β-hemolytic Brachyspira have been described that produce lesions of swine dysentery when inoculated into pigs, namely B suanatina, some strains of B intermedia, Brachyspira sp SASK 30446, and B hampsonii. The Brachyspira proliferate in the large intestine and causes degeneration and inflammation of the superficial mucosa, hypersecretion of mucus by mucosal epithelium, and multifocal bleeding points on the mucosal surface. The organism does not penetrate beyond the intestinal mucosa. Decreased ability of the mucosa to reabsorb endogenous secretions from the unaffected small intestine results in diarrhea.
The first signs are partial anorexia, passage of soft feces, and possibly fever. The course is variable. Some pigs die peracutely. More commonly, a mucoid diarrhea with flecks of blood and mucus develops and progresses to a watery mucohemorrhagic diarrhea. After several days, the feces are brown and contain flecks of fibrin and debris. Diarrheic pigs are dehydrated, profoundly weak, gaunt, and emaciated.
The diffuse lesions are confined to the cecum, spiral colon, and rectum. The affected mucosa is covered with a layer of transparent or gray mucus, often with suspended flecks of blood in early stages, with a mixture of blood, fibrin, and necrotic debris in more advanced cases, and a yellow, necrotic debris late in the course.
Clinical signs and necropsy findings are usually sufficient for a presumptive diagnosis. Confirmation is based on demonstration of typical histologic lesions in the large intestine and isolation of strongly β-hemolytic Brachyspira by anaerobic culture. Concurrent diseases are not uncommon. Differential diagnoses include intestinal spirochetosis, proliferative enteritis, salmonellosis, and heavy whipworm infections.
Treatment and Control
Therapeutic use of antibacterials is effective if started early. Water medication is preferred at first. Because drug-resistant strains are prevalent, it is essential to choose a drug to which the organism is sensitive. Bacitracin, carbadox, lincomycin, tylosin, tiamulin, and virginiamycin are commonly used. The disease may be eradicated from infected premises without total depopulation by a persistent and carefully planned program that includes treatment of carrier pigs with bactericidal drugs and thorough cleaning and disinfection of vacated facilities. Mice are an important reservoir of infection for B hyodysenteriae, and any eradication attempt must include elimination/reduction of the mouse population on the farm. In addition, B hyodysenteriae will survive >60 days in pig waste at refrigerator temperatures.
Last full review/revision September 2013 by D. L. Hank Harris, DVM, PhD