The common denominator of rare diseases is the infrequency of their occurrence in the human population. Often debilitating lifelong disease or disorder condition with a prevalence of 1 or less, per 1000 population is defined by the World Health Organization (WHO) as a rare disease. Yet, a few developed and developing countries have their own definition to suit their requirements. This has resulted in heterogeneity in rare disease definition and therefore consensus among nations does not exist. Nevertheless it is generally accepted that a disease having fewer than 100 patients per 100,000 population is described as rare disease and fewer than 2 patients per 100,000 is described as ultra rare disease. Ultra rare diseases are rarest of rare diseases. Rare and ultra rare diseases are sometimes referred to as orphan and ultra orphan diseases, respectively.
Table 1. Rare diseases and Disorders (RDD) population are statistically derived from published data of respective national population census of 2011 or later. These derived data are based on the lower limit of global prevalence estimate. Table 2A & B. Similarly estimated data for states and territories of India.
Of the several thousand, currently pegged around 7000 reported rare diseases or disorders, except for a handful, most do not have epidemiological data available. Each one of these uncommon diseases widely varies in its prevalence. Considering the sheer number of identified rare and ultra rare diseases and their varying prevalence, it becomes nearly impossible to ascertain the total number of rare (and ultra rare) disease patients in the world. However, on the basis of available prevalence data on few select rare diseases, there is a consensus of sorts that exists among the researchers about the number of total global rare disease afflicted population. Gross estimate of the number of rare or ultra rare disease patients is 6 - 8% of global population and is widely accepted by the researchers. This consensus, fortunately, allows us to fathom the situation at hand and also helps provide geographic regional estimates. Here an attempt is made to derive ‘probable’ numbers for rare and ultra rare disease patients in various South Asian countries based entirely on officially reported current population of respective countries (Table 1). Similarly, rare disease patient numbers for individual states and territories in India is derived as well (Table 2A & B).
A head count for every individual rare disease may not be possible. The caveat is that the real number of rare disease affected population in the world cannot be ascertained unless the prevalence of every individual rare disease is established by systematic studies. There may be many unreported rare diseases, ultra rare diseases waiting to be described in developing countries. South Asian countries where consanguinity is prevalent may harbor unique geography-specific rare diseases and might add another dimension to the scenario. The estimated 6 - 8% population encompasses not only patients with 'diseases' but also patients with 'disorders'. A person with polydactyly (a condition of having additional digits beyond normal) is not considered to be a patient in this respect, but it is a deviation from normal yet non-debilitating and therefore it is a disorder. Thus the numbers in the Tables 1 & 2 incorporate both rare disease and disorder patients. PDF of this document Download.
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