Shigellosis, also called bacillary dysentery, is caused by four species; Shigella dysenteriae, Shigella flexneri, Shigella boydii and Shigella sonnei.
Bacillary dysentery is primarily a human disease often acquired by drinking water contaminated with human faeces or by eating food washed with contaminated water. Man is the only significant reservoir of Shigella infection. In the UK most cases are associated with foreign travel, however, there are occasional reports of UK-acquired cases associated with sexual transmission, predominantly among men who have sex with men.
Infection can result following ingestion of as few as 10 organisms. The incubation period is between 12 and 96 hours. Illness is characterised by diarrhoea, sometimes with blood and mucus and is common amongst young children although infection can occur in all ages after travel to areas where hygiene is poor. Invasive disease is rare but extra intestinal complications such as Haemolytic Uraemic Syndrome can occur. Cases maintain a low level of infectivity for as long as the organism is excreted in the stool. Shigella species may survive for up to 20 days in favourable environmental conditions and this may lead to transmission through contact with contaminated fomites.
In July 2011, an increase in UK acquired cases of Shigella flexneri predominantly among men who have sex with men (MSM), aged between 30-50 years, some of whom were HIV positive, was identified in the Greater Manchester area. In London, an increase in Shigella flexneri was noted during 2010/11, some of which were seen in MSM. The profile of the patients presenting with Shigella flexneri is similar to that associated with the ongoing outbreak of Lymphogranuloma venereum infection. Sexual transmission is likely to be fuelled by the low infectious dose, immunodeficiency due to HIV infection and serosorting (sex between partners with the same HIV status). Travel may also play a role in introducing Shigella species to at risk populations.
health protection teams and health professionals who work in sexual health have been asked to be alert for cases of Shigella flexneri acquired in the UK among MSM. A stool culture should be obtained from patients presenting with acute diarrhoea and microbiologists should refer Shigella isolates to the Reference Laboratory, Colindale according to their usual protocol. To help interrupt onward transmission, patients with laboratory confirmed infection should be treated with ciprofloxacin, subject to antimicrobial sensitivity. The emergence of ciprofloxacin resistance should be monitored closely and, if necessary, antimicrobial treatment switched to ensure treatment remains effective.
Further details of the outbreak investigation and recommended measures for infection control are available in the Health Protection Report.