Infectious agents and occurrence of shigellosis

The infectious agents causing shigellosis are different species of Shigella bacteria. Although these bacteria may cause mild cases of acute watery diarrhoea, dysentery is the real threat in shigellosis. The bacteria infect and destroy cells lining the patient's large intestine (colon), causing ulcers and bleeding, which results in the characteristic appearance of blood and mucus in the stool.

Although Shigella infection can occur at any age, it is rare in infants less than six months of age and most common in children aged two to three years. This age-distribution is unlike amoebic dysentery, which is rare in children less than five years of age. Two-thirds of the cases of shigellosis and most of the deaths are in children below ten years, and (like all diarrhoeal diseases) the effects are most severe in malnourished children.

Shigella bacteria can be easily transmitted from person to person and rapidly cause epidemics, particularly under conditions of overcrowding, where personal hygiene is poor, such as in prisons, institutions for children, and refugee camps. Small doses of the infectious agent – as few as ten organisms – are enough to transmit the infection, which means it can be transmitted easily to close contacts. Another reason for the rapid spread is that after recovery, infected individuals can transmit the bacteria in their faeces for up to four weeks after the illness. By contrast, epidemics of amoebic dysentery rarely occur. Therefore, if an epidemic of dysentery occurs in your community, you should suspect the most likely cause is bacillary dysentery due to Shigella bacteria.