Observations on the epidemiology of sickle cell disease.

The four common genotypes of sickle cell disease in Jamaica are homozygous sickle cell (SS) disease, sickle cell-haemoglobin C (SC) disease, sickle cell-B+ thalassemia, and sickle cell-BO thalassaemia with respective incidence at birth of 3.2, 2.0. 0.34, and 0.16 per 1000 live births. Haematological indices, clinical features, and over-all prognosis vary between these genotypes and also between patients within individual genotypes. Although symptomatic selection has tended to emphasize more severely affected patients, this wide variation of clinical and haematological severity is especially apparent in SS disease. Factors contributing to this variability in SS disease include the persistence of foetal haemoglobin, the association with alpha thalassemia, and the interaction with environmental factors of which socioeconomic status is the most obvious. Further elucidation of factors determining the severity of SS disease will increase understanding of the pathogenetic mechanisms in the disease and may also identify new possibilities for therapeutic intervention.