In many places, diarrhoeal diseases are endemic with seasonal peaks. However, when serious outbreaks of acute diarrhoeal disease occur, the common cause is either:

- Shigella dysenteriae type 1 (Sd1), which causes bacillary dysentery, or

- Vibrio cholerae serogroup 01 or 0139, which causes cholera.

Shigella dysenteriae type 1 (Sd1)

This is the most virulent of the four serogroups of shigellae, and is often resistant to most of the common antimicrobials. The illness caused by Sd1 often includes abdominal cramps, fever, and rectal pain. Less frequent complications with Sd1 include sepsis, seizures, renal failure, and haemolytic/uraemic syndrome. The organism is highly infectious, and readily transmitted by direct person-to-person contact as well as by food and water.

Shigella dysentery type 1 always should be considered as a possible cause of the outbreak when there is an unusual increase in the weekly number of cases of bloody diarrhoea or deaths from bloody diarrhoea.

Vibrio cholerae 01 and 0139

Cholera has spread widely since 1961 and now affects at least 98 countries.

Most people infected have no symptoms or only mild diarrhoea. However, those with a severe case of the disease can die within hours of onset from fluid/electrolyte loss through profuse diarrhoea and, to a lesser extent, vomiting. Although high death rates can occur when treatment is unavailable, case fatality can be reduced to below 1% with proper facilities and care. The organism is spread almost exclusively by ingestion of food or water contaminated directly or indirectly by faeces or vomit from infected individuals.

A cholera outbreak should be suspected if either or both of the following occur:

- a patient older than five years develops severe dehydration or dies from acute watery diarrhoea;

- there is a sudden increase in the daily number of patients with acute watery diarrhoea, especially patients who pass the “rice water” stools typical of cholera.