33-18: Shigellosis

Shigella dysentery is a common disease, often self-limited and mild but occasionally serious. S sonnei is the leading cause in the United States, followed by S flexneri. S dysenteriae causes the most serious form of the illness. Shigellae are invasive organisms. The infective dose is low at 10²–10³ organisms. There has been a rise in strains resistant to multiple antibiotics.

A. Symptoms and Signs

The illness usually starts abruptly, with diarrhea, lower abdominal cramps, and tenesmus. The diarrheal stool often is mixed with blood and mucus. Systemic symptoms are fever, chills, anorexia and malaise, and headache. The abdomen is tender. Sigmoidoscopic examination reveals an inflamed, engorged mucosa with punctate and sometimes large areas of ulceration.

B. Laboratory Findings

The stool shows many leukocytes and red cells. Stool culture is positive for shigellae in most cases, but blood cultures grow the organism in less than 5% of cases.

Bacillary dysentery must be distinguished from salmonella enterocolitis and from disease due to enterotoxigenic Escherichia coli, Campylobacter, and Yersinia enterocolitica. Amebic dysentery may be similar clinically and is diagnosed by finding amoebas in the fresh stool specimen. Ulcerative colitis is also an important cause of bloody diarrhea.

Temporary disaccharidase deficiency may follow the diarrhea. Reactive arthritis is an uncommon complication, usually occurring in HLA-B27 individuals infected by Shigella. Hemolytic-uremic syndrome occurs rarely.

Treatment of dehydration and hypotension is lifesaving in severe cases. Recommended empiric antimicrobial therapy is either a fluoroquinolone (ciprofloxacin, 750 mg orally twice daily for 7–10 days, or levofloxacin, 500 mg orally once daily for 3 days) or ceftriaxone, 1 g intravenously once daily for 5 days. If the isolate is susceptible, trimethoprim-sulfamethoxazole, 160/80 mg orally twice daily for 5 days, or azithromycin, 500 mg orally once daily for 3 days, is also effective. High rates of resistance to amoxicillin make it a less effective treatment option.