Rare Diseases Epidemiology: Update and Overview

Eva Bermejo-Sánchez, Manuel Posada de la Paz

This work summarizes the main aspects to be considered around birth defects (or congenital anomalies) clusters. Most birth defects (BD), considered individually, fall into the definition of rare diseases (RD), according to their low frequency. Likewise, many RD are congenital, because their manifestations are present at birth or can be even evident before the delivery. It has been estimated that overall 7.9 million children are born each year with serious BD of genetic or partially genetic origin, and additional hundreds of thousands more are born with serious BD of post-conception origin.A "birth defect cluster" can be defined as an unusual aggregation of cases (grouped in place and time) that is suspected to be greater than expected, even though the expected number may not be known. These clusters are incidents or occurrences that let us turn the challenge of identifying the causal agent(s) involved in the origin of such clusters, into an opportunity to exert primary prevention, and thus achieve the ultimate goal of enabling infants being born healthy. Therefore, any program or system involved in BD surveillance and research should devote part of its activities to detect and investigate clusters, to ensure that such opportunity for primary prevention will be conveniently leveraged. Regardless the type of cluster, there are several phases that must be undertaken sequentially for proper control and the maximum benefit for the population: cluster detection, evaluation and investigation, management, adoption of preventive measures, and communication of the results to the public or target population.