The hypnozoite concept, with particular reference to malaria

Miles B. Markus

In 1978, the nature of the hypnozoite was discussed in an article that appeared in a relatively obscure journal, which is also where the term was adopted for Plasmodium (a little-known fact). As a result, that commentary on the use of the word "hypnozoite" has been almost completely overlooked. Although the publication is now more than three decades old, the analysis remains valid today. It is explained in the present paper that like "merozoite" and "sporozoite", the name "hypnozoite" is applicable not only to a latent stage in the life cycle of Plasmodium but to some apparently dormant forms of other kinds of apicomplexan parasites as well. Merozoites of different genera of parasitic protozoa are not necessarily the same biologically and/or otherwise. Similarly, although the hypnozoite concept relates primarily to pre-merozoite stages, some atypical post-divisional apicomplexan forms might also be hypnozoites. Examples are likewise given of latent organisms that, in contrast, are clearly not hypnozoites, such as dormant merozoites in malaria infections. Lastly, the plasmodial hypnozoite is placed in context in relation to the relatively unfamiliar (nomenclaturally) malarial bradysporozoite, chronozoite, dormozoite, merophore, merosome and x body. This paper is based on a presentation by the author, as a Life Member of the American Society of Tropical Medicine and Hygiene, to its 59th Annual Meeting in Atlanta, Georgia, USA, 3-7 November 2010.