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Intrafamilial Transmission of Bacillary Dysentery Due to Shigella Dysenteriae Type 1 in Southwestern Saudi Arabia.


A propagated outbreak of bacillary dysentery (BD) in Gizan (southwestern Saudi Arabia) affected 18 neighboring villages. Multiple resistant Shigella dysenteriae type 1 was recovered from some cases of BD in the affected villages and from concurrent parallel outbreaks in other parts of Saudi Arabia. This outbreak provided an opportunity to characterize the epidemic features and to identify the factors that influence transmission of BD within families.


A case of BD was defined as any person who was previously well and presented with bloody diarrhea (parasitologic stool examination had excluded amebiasis) between Nov. 20, 1992, and May 13, 1993. BD cases were identified by reviewing diarrheal diseases surveillance reports and checking hospital (including laboratory) logbooks, and primary health care records. Unreported cases were actively found by visiting schools and interviewing cases. We visited houses of all cases of BD in the 18 villages and interviewed all family members. Primary (first case of BD in a household) and secondary cases of BD (cases of BD other than the first one) within the family were identified. Questions included relevant symptoms, date of onset, usual source of food and drinking water, type of feeding for children under 2 years of age and history of contact with other cases of BD.


During the study period, 233 cases of BD were identified in Gizan among 79 families. Outbreaks of BD sequentially spread from one village to another, and cases were identified in a chain of 18 villages mainly in two neighboring administrative sectors (68% of cases). Secondary cases of BD occurred in 57 of 79 families with a primary case of BD. The mean secondary attack rate (AR) within families was 33.6% (range 7.7% to 80%). Age of primary cases showed no relation to degree of secondary AR in exposed families; however, the age of secondary cases was related to the age of the primary case. Children under 5 years of age constituted 43.5% of secondary cases. The median interval between successive cases within a house ranged from 3 to 7 days. Two hundred twenty cases (94.4%) gave history of close contact within another case of bloody diarrhea. Cases of BD were exposed to close relatives with BD (79.1%), neighbors (11.4%) and friends (9.5%). Risk factors influencing the spread of BD within families included two rooms or fewer per house (OR=4.3, 95% CI 1.3-14.3), family size of 5 or more (p=0.012, 2-tailed Fisher's exact test), and presence of more than 2 persons per room (OR=11.2, 95% CI 3.1-42.4)


Person-to-person secondary transmission can amplify the spread of bacillary dysentery within households and neighboring villages. Crowding was an independent risk factor that amplified transmission of BD within families. Mothers and school teachers should be the target of health educational programs intended to interrupt secondary transmission of BD in children. These programs should focus on promoting hygienic personal practices. The epidemiologic and laboratory surveillance systems need to be strengthened to identify and contain recurrences promptly. Doctors need to be informed continuously about resistant strains.