NEW YORK (Reuters Health) - Beyond blood, bone marrow is a major reservoir of the malaria parasite Plasmodium vivax, according to an international group of researchers.
As Dr. Matthias Marti told Reuters Health by email, "Detection of a significant P. vivax parasite reservoir in bone marrow suggests that we are underestimating parasite burden in infected people based on blood smears only."
In a paper online May 8 in mBio, Dr. Marti of the University of Glasgow, U.K., and colleagues note, "Mature P. vivax asexual and transmissive gametocyte stages occur in the blood circulation, and it is often assumed that accumulation/sequestration in tissues is not an important phase in their development."
They add that although Plasmodium falciparum malaria is more widespread, "Plasmodium vivax malaria continues to cause major public health burdens worldwide." Also, "Observations of moderate numbers of mature as well as immature P. vivax stages in the circulation have probably contributed to general misconceptions that P. vivax malaria is benign."
To gain more information on transmission, the team used specific antibodies and quantitative histological analysis to examine P. vivax stage distributions in human blood samples and major tissues of monkey models.
Comparison of transcriptomes of P. vivax and P. falciparum blood-stage parasites showed "similar dynamics of asexual and sexual gene expression, even though the development time for P. vivax gametocytes is much shorter than that for P. falciparum gametocytes."
Also, they say, "Our results indicate relatively lower counts of mature asexual and immature gametocytes in the circulating blood of P. vivax patients."
"We suspected there was a parasite reservoir somewhere in the body," Dr. Marti pointed out in a statement, and in fact, in the animal samples, parasites in the bone marrow and liver accounted for about 30% of the total parasite burden. P. vivax gametocytes mature rapidly in the bone marrow and can be transmitted to mosquitoes before the onset of illness.
"This," Dr. Marti told Reuters Health, "has implications for diagnosis and treatment, but also for the estimation of the human infectious reservoir and ultimately for our ongoing efforts to eliminate this parasite."
Plasmodium expert Dr. Fabio T. M. Costa of the University of Campinas, Brazil, said the paper has elegantly demonstrated "Plasmodium vivax sequestration, and most important, where this parasite accumulates."
"I believe this article closes one cycle of questions regarding P. vivax," Dr. Costa, who was not involved in the study, told Reuters Health by email." Nevertheless, he added, "how this sequestration leads to severe complications in vivax malaria will be a new chapter in this intriguing story!"
Materials provided by Medscape.