The life cycles of all of the five species of Plasmodium naturally infecting humans are complex, but P. vivax (and P. ovale) has a single additional stage that hugely magnifies its epidemiological and clinical complexity. This parasite places dormant forms in the liver that cause multiple clinical attacks (relapses) over about 2 years.

I am guilty of naming (41 years ago!) the plasmodial stage to which you refer, Alan, viz. the hypnozoite. Leaving aside the situation as regards P. ovale, it would seem that P. vivax recurrences can also (in addition to hypnozoites) have extravascular merozoites as their origin. It is a long and complicated story. The article for which the website is given in the question contains links to about 13 recent publications on the subject. This stuff isn't in textbooks (yet).

The forms of human malaria caused by other Plasmodium species cause significant morbidity but are rarely life-threatening. Cases of severe P. vivax malaria have recently been reported among populations living in (sub)tropical countries or areas at risk. P. vivax and P. ovale can remain dormant in the liver. Relapses caused by these persistent liver forms (“hypnozoites”) may appear months, and rarely several years, after exposure. Relapses are not prevented by current chemoprophylactic regimens, with the exception of primaquine. Latent blood infection with P. malariae may be present for many years, but it is very rarely life-threatening.

According to WHO, if the Parasitemia recurs within 16 days of treatment has been administered, a relapse is unlikely; but after this time, a relapse can not be distinguished from a recrudescence and that an infection to P. vivax that recurs

within 28 days. Whatever its origin, it must be resistant to chloroquine, if the treatment was properly administered. That is, it is necessary to define if they are in endemic areas or if the parasite is resistant; after making sure you have given the right treatment.

World Health Organization,WHO. WHO guidelines for the treatment of malaria. WHO/HTM/MAL/2006.1108. Geneva: WHO Press; 2006.

If the website given in the original question is accessed (the article gives links to about 13 recent publications that are relevant), it will be seen that the whole story is changing. In the light of current (2019) knowledge, furthermore, the time to recurrence (ref. the WHO criteria) is no longer a valid criterion as to whether a relapse or a recrudescence is involved. Understanding of the significance of the response or non-response to treatment has also changed, especially since 2018. There are now more questions than answers.

following

According to WHO, if the Parasitemia recurs within 16 days of treatment has been administered, a relapse is unlikely; but after this time, a relapse can not be distinguished from a recrudescence and that an infection to P. vivax that recurs

The WHO view regarding time-associated relapse/recrudescence is no longer valid. See:

https://malariaworld.org/blog/significance-time-clinical-recurrence-vivax-malaria

Thanks, Miles.

A pleasure, Alan.

Thanks Dr Miles. Your experience in this field is very important to us.

Thank you, José.

Please also have a look at the following references.

Preprint What is the explanation for Plasmodium vivax malarial recurr...

Article The effect of chloroquine dose and primaquine on Plasmodium ...

Article Evaluation of Plasmodium vivax malaria recurrence in Brazil

Article Single-Dose Tafenoquine to Prevent Relapse of Plasmodium vivax Malaria

Thanks, Arvind. Curiously, the preprint is not on the subject that the title thereof says it is on, i.e. the content is incompatible with the first part of the title. That aside, the first author of the second publication presented on the subject at two conferences last year that I attended (and I cited a related article of his in a recent paper).

Yes, the blood-and-liver concept is the conventional one, Sadanand. As I indicated in conjunction with the original rhetorical question, understanding of the situation is in the process of changing. See the following and click on the links (words in blue) in the article:

http://theconversation.com/why-does-malaria-recur-how-pieces-of-the-puzzle-are-slowly-being-filled-in-108833