This issue of Emerging Infectious Diseases contains 2 independent reports of Rickettsia sibirica infections in Spain and Portugal. The authors identify the agent both as a subspecies (1) and as a strain (2). This inconsistency reflects a lack of consensus regarding the use of subspecies designations for Rickettsia taxa, so appropriate designation of these pathogens as strains or subspecies remains problematic.

Tick-borne typhus of northern Asia (or North Asian spotted fever) was first discovered in the 1930s, and its etiologic agent, R. sibirica, was formally described in 1949. In 1993, an isolate designated “mongolitimonae” was first discovered in the People’s Republic of China (3), considered a new species (4), and finally described as a new genotype of R. sibirica (5). A proposal to create 2 subspecies, R. sibirica sibirica and R. sibirica mongolitimonae, was recently published (6). A new prokaryote name must be both effectively and validly published. To become effectively published, a name must meet certain rules, as defined by the International Code of Nomenclature of Bacteria (7). To become validly published, the name must then appear on a Validation List published in the International Journal of Systematic and Evolutionary Microbiology (8). This provides an orderly system for bacterial names to be properly introduced and published in the scientific literature. Valid and nonvalid names are listed on regularly updated websites (9,10). Names introduced without being validated have no standing in bacterial nomenclature. That is currently the status of these proposed R. sibirica subspecies. Therefore, the use of strain designations is still appropriate.

If, and when, the subspecies names are validated, they are likely to be adopted and routinely used in the literature. Rickettsial taxonomy continues to evolve, and future changes should be determined by critical scientific judgments and general consensus within the scientific community.

Dr Massung is chief of the Rickettsial Zoonoses Branch, Division of Viral and Rickettsial Diseases, National Center for Zoonotic, Vector-borne and Enteric Diseases, Centers for Disease Control and Prevention. His research interests include laboratory and epidemiologic investigations targeting the detection, prevention, and control of rickettsial and Bartonella spp. diseases and Q fever.

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