AIM: In rural Gambia, birth season predicts infection-related adult mortality. Seasonal factors in early life may programme the immune system, possibly via an effect on thymic development. This study tested whether thymus size in rural Gambian infants is affected by birth season, defined as January-June (harvest, few infections) versus July-December (hungry, high infectious load). METHODS: Thymic volume (thymic index) was assessed sonographically at 1, 8, 24 and 52 wk in 138 singleton infants born over 14 consecutive months. Growth and morbidity were regularly assessed. RESULTS: Mean thymic index increased from 19.9 at 1 wk to 30.9 at 8 wk and 35.7 at 24 wk, then fell to 32.0 at 52 wk. Thymic index was associated with each preceding thymic index up to 24 wk. This tracking persisted after correcting for infant weight and month of measurement (p < 0.01). Thymic index at 1 wk was marginally but non-significantly lower in hungry season births (p = 0.06). Thymic index was lower in hungry season measurements; this effect persisted after adjusting for current weight and infection markers, and was strongest at 8 wk (p = 0.001). CONCLUSION: These results describe a pattern of thymic growth followed by diminution in size. An infant's thymic index tracks despite changes in body weight and season. Thymic index was lower in the hungry season. This appears not to be explained by infection or nutritional status. A possible influence of trophic factors in breast milk merits investigation.
