Intrafamilial person-to-person spread of bacillary dysentery due to Shigella dysenteriae in southwestern Saudi Arabia

Objective: To identify the factors that influence transmission of bacillary dysentry (BD) within families during a propagated outbreak of bacillary dysentery, Design: A retrospective cohort study. Setting: Eighteen neighbouring villages in rural Gizan, southwestern Saudi Arabia. Subjects: Two hundred and thirty three cases of ED were identified among seventy nine families. Results: Secondary cases of ED occurred in 57 of 79 families with a primary case of ED. The secondary attack rate per cent (AR%) within families ranged between 7.7% and 80%, Age of primary cases did not correlate with degree of secondary AR% in exposed families (p>0.04; p>0.05); however, within households, the age of the first secondary cases (median = two years) was usually less than the age of the primary case (median = six years). Children under five years of age constituted 43% of secondary cases, The median interval between successive cases within a house ranged from three and seven days. Two hundred and twenty eases (94.4%) gave history of close contact within another case of ED. Cases of ED were exposed to close relatives with ED (79.1%), neighbours (11.4%), and friends (9.5%), Risk factors influencing the spread of ED within families included two rooms or fewer per house (OR = 4.3, 9.5% CI 1.3-14.3), family size of five or more (p = 0.012, two-failed Fisher's exact test), and presence of more than two persons per room (OR = 11.2, 95% CI 3.1-42.4), Conclusion: Person-to-person secondary transmission can amplify the spread of bacillary dysentery within households and neighbouring villages. Crowding was a risk factor that amplified transmission of ED within families.