Inter- and intrafamilial distribution of giardia lamblia infection

Cluster randomization trials in tropical medicine: a case study

Field trials in tropical medicine are often designed so that intact social units (e.g., families, schools, communities) rather than independent individuals are randomized to an intervention group. Reasons are diverse, but include administrative convenience, a desire to reduce the effect of treatment contamination, and the need to avoid ethical issues that might otherwise arise. Dependencies among cluster members typical of such designs must be considered when determining sample size and analysing the resulting data. Failure to do so can result in false conclusions that the treatment is effective. The purpose of this paper is to compare different methods which can be used to construct tests of the effect of treatment when outcomes are binary (e.g., infected/uninfected). The discussion will be illustrated using data from a trial which randomly assigned families to either a control group or a screening and treatment programme for imported intestinal parasites.

Clustering of Ascaris lumbricoides and Trichuris trichiura infections within households

A survey of 428 households in a shanty town in Coatzacoalcos, Mexico, revealed high prevalences of Ascaris lumbricoides and Trichuris trichiura. The data were analysed separately for A. lumbricoides and T. trichiura in order to investigate the spatial distribution of heavy infections through the town. Within each age class, those individuals with egg counts in the upper 20% of the range for that age were classified as "heavily infected". When the data were stratified by household size, it was found that the distribution of "heavily-infected" individuals was not random in the community. "Heavily-infected" individuals were found together in households; fewer household units had a single heavily-infected individual than would be expected by chance. Such a pattern could result either from genetic similarities among family members influencing their ability to mount an effective immunological response to infection, or focal transmission in the vicinity of the home, or both. This result may have important implications for the development of community control programmes.