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 BD were identified among seventy nine families.

RESULTS

Secondary cases of BD occurred in 57 of 79 families with a primary case of BD. 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 cases (94.4%) gave history of close contact within another case of BD. Cases of BD were exposed to close relatives with BD (79.1%), neighbours (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, 9.5% CI 1.3-14.3), family size of five or more (p = 0.012, two-tailed 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 BD within families.