Epidemiology - Shigella

Epidemiology

Shigellosis occurs worldwide. Estimated 150 million cases occur annually worldwide. The incidence of shigellosis in developing countries is nearly 20 times more than in developed countries.

·           It is estimated that 30% of these infections are caused by S. dysenteriae.

·           S. flexneri is the most common cause of shigellosis in devel-oping countries.

·           S. sonnei is the most common cause in the industrial world.

◗ Geographical distribution

Shigellosis is worldwide in distribution but shows a lot of variations epidemiologically between the nature and extent of the infection in the industrially advanced and in the devel-oping and poor countries. Endemic shigellosis is found in all age groups and is caused by all species in developing countries where environmental sanitation is poor. In India, S. flexneri is the major species and constitutes 50–85% of all Shigella iso-lates followed by S. dysenteriae (8–25%), S. sonnei (2–24%), and S. boydii (0–8%).

◗ Habitat

Shigella species are strict human pathogens. They are found inthe large intestine of infected human hosts. They are not found in any other animal hosts.

◗ Reservoir, source, and transmission of infection

Infected patients or, less often, carriers are reservoirs of infec-tions for shigellosis. Chronic carriage is rare, because the bacilli are not excreted in feces within a few weeks, except in some malnourished children or in patients with AIDS. Shigellosis is transmitted by:

·           Fecal–oral route by hand-to-mouth infection through con-taminated fingers. Because as few as 10–200 bacilli can cause disease, shigellosis spreads rapidly in areas where sanitary standards and the level of personal hygiene are low.

·           Contaminated food and water: Food and water contaminatedwith human feces containing Shigella spp. is the main source of infection.

·           Fomites such as door handles, water taps, lavatory seats.

·            Flies, which may transmit the infection as mechanical vectors.

·           Sexually among young male homosexuals due to oro-analcontact.

Shigellosis is primarily a disease of children. Nearly, 70% of all infections occur in children younger than 15 years. Patients at highest risk for disease are malnourished children, young children in daycare centers, nurseries, and custodial institu-tions; siblings and parents of these children. Shigella infection in malnourished children often causes a vicious cycle of further impaired nutrition, recurrent infection, and further growth retardation. Endemic disease in adults is common in house-hold contacts of infected children and in male homosexuals.

◗ Typing of Shigella

For epidemiological purposes, Shigella species have been classi-fied into many colicin types, depending upon the biochemical characteristics of the microorganisms.