Tuesday, June 24, 2014

Definition of Rare Disease

In June, 2014, my book, entitled Rare Diseases and Orphan Drugs: Keys to Understanding and Treating the Common Diseases was published by Elsevier. The book builds the argument that our best chance of curing the common diseases will come from studying and curing the rare diseases.

Short excerpt from the Introduction chapter:
“The beginnings and endings of all human undertakings are untidy.”
—John Galsworthy

In the U.S., Public Law 107-280, the Rare Diseases Act of 2002 states: “Rare diseases and disorders are those which affect small patient populations, typically populations smaller than 200,000 individuals in the United States” [1]. Since the population of the U.S. is about 314 million (in 2013), this comes to about one case for every 1570 persons. This is not too far from the definition recommended by the European Commission on Public Health; fewer than one in 2000 people. It is important to have numeric criteria for the rare diseases, because special laws exist in the U.S. and in Europe to stimulate research and drug development for diseases that meet the criteria for being “rare” (see Section 14.2). Unfortunately, it is very difficult to know, with any certainty, the specific prevalence or incidence of any of the rare diseases (see Glossary items, Prevalence, Incidence). A certain percentage of the cases will go unreported, or undiagnosed, or misdiagnosed. Though it is impossible to obtain accurate and up-to-date prevalence data on every rare disease, in the U.S. the National Institutes of Health has estimated that rare diseases affect, in aggregate, 25–30 million Americans [2].

There seems to be a growing consensus that there are about 7000 rare diseases [3]. Depending on how you choose to count diseases, this may be a gross underestimate. There are several thousand inherited conditions with a Mendelian inheritance pattern [4]. To these, we must add the different types of cancer. Every cancer, other than the top five or ten most common cancers, occurs with an incidence much less than 200,000 and would qualify as a rare disease. There are more than 3000 named types of cancer, and many of these cancers have well-defined subtypes, with their own morphologic, clinical, or genetic characteristics. Including defined subtypes, there are well over 6000 rare types of cancer [5–8]. Regarding the rare infectious diseases, well over 1400 different infectious organisms have been reported in the literature [9]. A single infectious organism may manifest as several different named conditions, each with its own distinctive clinical features. For example, leishmaniasis, an infectious disease that is common in Africa but rare in Europe, may present in one of four different forms (cutaneous, visceral, diffuse cutaneous, and mucocutaneous). When we add in the many rare nutritional, toxic, and degenerative diseases that occur in humans, the consensus estimate of the number of rare diseases seems woefully inadequate. Nonetheless, the low-ball “7000” number tells us that there are many rare diseases; way too many for any individual to fully comprehend.

Later chapters explain that the rare diseases, as a group, share a common set of biological properties that distinguish them from the common diseases.

I urge you to read more about this book. There's a good preview of the book at the Google Books site. If you like the book, please request your librarian to purchase a copy of this book for your library or reading room.

- Jules J. Berman, Ph.D., M.D.

tags: rare diseases, orphan drugs, orphan diseases, common diseases, definition of rare diseases, rare disease organizations, organizations for rare diseases, rare disease research