Disclosures: The authors report no relevant financial disclosures.
May 26, 2021
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Researchers identify ‘hidden’ life cycle of malaria parasites in human spleen

Disclosures: The authors report no relevant financial disclosures.
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Researchers said they have identified a new “hidden” life cycle of malaria parasites in the human spleen — a finding that could have implications for malaria elimination programs, one of them said.

Findings from a study of 15 largely untreated and asymptomatic adults in a malaria-endemic region of Indonesia were published today in The New England Journal of Medicine. Lab testing confirmed that nine of the participants were infected with Plasmodium falciparum and six with Plasmodium vivax.

Malaria parasite infographic
Source: Kho S, et al. N Engl J Med. 2021;doi:10.1056/NEJMc2023884.

For the study, the researchers collected spleen tissue and peripheral blood from each participant. All participants had had a splenectomy.

The researchers discovered a large biomass of asexual-stage parasites in the patients’ spleens, with nonphagocytosed red cells that had parasite densities hundreds to thousands of times higher than those observed in circulating blood (P = .02 for P. falciparum; P = .03 for P. vivax).

Among the total asexual stage P. vivax parasites identified, 98.7% (95% CI, 89.4-99.8) were found in the spleen, they reported. Additionally, they estimated that the asexual-stage P. falciparum splenic biomass “was at least two times as large as that in the circulation.”

Nicholas Anstey

“From a public health perspective, some people with large numbers of viable parasites hiding in the spleen did not have parasites detectable in the blood, even by PCR. This hidden reservoir of infection is likely another factor limiting the success of malaria elimination programs relying on mass testing of blood and only treating those with detectable infection,” Nicholas Anstey, MBBS, FRACP, PhD, an infectious diseases physician at the Menzies School of Health Research in Darwin, Australia, told Healio.

“Chronic asymptomatic malaria should be considered as predominantly an infection of the spleen, with just a small component of parasites circulating in peripheral blood,” Anstey said.

He said they were surprised to discover much higher densities of P. vivax parasites in the spleen rather than in the peripheral blood.

“Our study was in chronic asymptomatically infected residents of a malaria-endemic area rather than in patients with acute symptomatic malaria, because trauma-related splenectomy is rare during acute untreated malaria,” Anstey said. “We think there will also be a hidden life cycle of malaria parasites in the spleen in patients with acute symptomatic malaria, but in patients with acute malaria, we don’t yet know what proportion of the total body biomass of malaria parasites are hiding in the spleen and what proportion are circulating in the blood.”