Shigella

(endorsed 1996)

Guideline

Escherichia coli (or alternatively thermotolerant coliforms) are used to indicate the presence of pathogenic Shigella spp. If explicitly sought, pathogenic Shigella spp. should not be detected. If detected, advice should be sought from the relevant health authority or drinking water regulator.

General description

Bacteria of the genus Shigella cause bacillary dysentery. Although shigella infection is not often waterborne, major outbreaks resulting from waterborne transmission have been described. The isolation of Shigella spp. from drinking water indicates recent human faecal contamination, but this occurs only rarely. This possibly indicates the limitations of the method rather than absence of the organisms, as there is no useful enrichment or selective medium for isolation of these bacteria. Techniques used have been designed for isolation of Salmonella spp. and are not optimal for Shigella spp.

Australian significance

No conclusive evidence for the transmission of shigellosis through water supplies in Australia has been reported. The incidence of infection by Shigella in Australia is low except in central Australia, and among travellers returning from abroad.

Treatment of drinking water

Standard disinfection procedures eliminate Shigella spp. from water, provided that turbidity is low.

Method of identification and detection

Shigella spp. are Gram-negative, nonsporing, nonmotile rods, growing both aerobically and anaerobically. Metabolism is both respiratory and fermentative; acid, but usually not gas, is produced from glucose but lactose is seldom fermented. Catalase is usually produced, except by Shigella dysenteriae type 1, while oxidase is produced by one serotype only. Nitrates are reduced to nitrites (APHA method 9260 E 1992).

Shigella spp. are serotyped on the basis of their somatic O antigens. Both group and type antigens are distinguished, group antigenic determinants being common to a number of related types. Serological typing is adequate for all species except S. sonnei.

Health considerations

Shigella spp. have a low infective dose and are highly pathogenic for humans. Characteristic bloody diarrhoea results from the invasion of the colonic mucosa by the bacterium; the process is probably highly species-specific. Shigella spp. have no natural hosts other than the higher primates, and effectively, humans are the only source of infection in the community. Among the enteric bacterial pathogens, Shigellae seem to be the best adapted to cause human disease. Transmission occurs directly between susceptible individuals, and the infectious dose is lower than for other bacteria.

Derivation of guideline

The isolation of Shigella spp. from drinking water indicates recent human faecal contamination and, in view of the extreme virulence of the organisms, is of crucial public health significance.

The effect on the community of noncompliance with the guideline will depend on the Shigella strain involved, the numbers, and the susceptibility of the population. Cases of shigellosis will almost certainly result.

Reference

APHA Method 9260E, (1992). Detection of pathogenic bacteria: Shigella. Standard Methods for the Examination of Water and Wastewater, 18th edition. American Public Health Association, Washington DC.