The feces may be red, resembling tomato ketchup, or may have a black, tarry appearance. Death usually results from dehydration caused by severe diarrhea. In herds where the disease has become well-established, the only signs affected pigs may exhibit are lower weight gain and reduced feed efficiency. These carrier-shedder pigs are the major source of the disease on infected farms, and also transmit it to noninfected farms. Shedding of S. hyodysenteriae in the feces of birds, dogs and mice makes these species potential vectors of swine dysentery among herds.
Several factors determine the extent and severity of swine dysentery in a swine herd.
The dose of S. hyodysenteriae to which pigs are exposed depends on the degree of environmental contamination, which in turn is directly related to swine population density, level of hygiene, segregation of sick and poor-doing animals, and effectiveness of the medication strategy. Very little is known about the relative virulence of individual strains of S. hyodysenteriae . Environmental stressors decrease the resistance of pigs to diseases, causing recurrence of diarrhea in carrier pigs, and poor response to treatment in sick animals. A key element to rational treatment and effective control of swine dysentery is correct diagnosis at the onset of an outbreak of the disease. Diagnosis of swine dysentery in chronically affected herds can be difficult and may require several submissions to a diagnostic laboratory before final confirmation is possible.