The highest burden of sickle cell disease (SCD) is in sub-Saharan Africa where over 300,000 children are born each year with SCD, and related childhood mortality rates remain high (1, 2). Malaria and SCD have a similar geographic distribution across endemic regions in sub-Saharan Africa and Asia, where malaria is often a major cause of infection-related hospitalization in SCD (3–5). In people with SCD, malaria can worsen baseline anaemia, trigger acute complications of SCD, and increase the likelihood of gram-negative bacteraemia—all of which potentially increase morbidity and risk of death (6, 7).

Hydroxyurea (HU) was the first drug approved as a disease modifying agent in sickle cell anaemia and is recommended for use, irrespective of phenotypic severity. HU has multiple benefits in SCD including increases in absolute haemoglobin (Hb), mean corpuscular volume (MCV) and foetal haemoglobin (HbF) levels, reductions in white blood cell (WBC), neutrophil, platelet, and reticulocyte counts, and increased nitric oxide availability. At the cellular level, these effects inhibit sickle polymerization, reduce inflammation, and improve blood rheology (8, 9).

There was earlier conflicting evidence about the benefits or safety of HU on the predisposition to and clinical course of malaria; however, the REACH trial showed a lower incidence of malaria in children on HU at maximum tolerated dose (10). HU-associated reductions in the absolute neutrophil count (ANC) below 3.0 × 10⁹/L significantly decreased malaria incidence within the study population. The NOHARM study also demonstrated that HU was safe in very young children (ages 1.00–3.99 years), with no difference in malaria incidence or severity between children on HU or placebo. NOHARM study participants received malaria chemoprophylaxis, and there were 12 episodes of clinical malaria over a 1 year follow-up period (11).

Our group has previously shown high (>95%) days 28 and 42 cure rates in SCD patients with acute malaria, using recommended antimalarials such as artemisinin-based combination therapies (ACT). These studies were conducted prior to the introduction of HU as a disease modifying agent in Ghana, where 1 in 50 newborns is affected with SCD (12, 13). It is unknown which changes occur in haematological or biochemical parameters of children on HU therapy who are concomitantly treated with ACT. There are no published reports on the interplay, if any, between HU and ACT during the acute phase of malaria, when significant changes in blood cell parameters may occur, or any influence of HU on parasite clearance from peripheral blood. One of HU’s therapeutic effects in SCD is through transient reduction of intracellular HbS concentration within erythrocyte precursors in favour of HbF, while haemoglobinopathies have been demonstrated to influence the activity of artemisinin based antimalarials by altering their accumulation and binding to target molecules within parasitized erythrocytes (14). Additionally, while artemisinin derivatives are well tolerated and typically do not have major haematological side effects, there are reports of delayed haemolytic anaemia and dose-dependent decreases in neutrophil counts (15, 16). These effects could potentially exacerbate HU and malaria-associated alterations in blood cell parameters. Furthermore, a retrospective study from Portugal in returned adult travellers with non-severe falciparum malaria, found that there were statistically significant elevations in liver enzymes, specifically alanine aminotransferase (ALT) and aspartate aminotransferase (AST), in those who were treated with a standard 3 days course of artemether-lumefantrine (AL) compared to a historical cohort of patients treated with a quinine-doxycycline regimen. These liver enzyme elevations were clinically asymptomatic, did not require treatment interruption and resolved without any specific interventions (17). HU has been implicated in both worsening of hepatic dysfunction in patients with SCD and improvements in sickle cell hepatopathy (18, 19).

Studies that evaluate changes in clinical and laboratory parameters in SCD patients on HU during acute malaria would provide information to help guide optimal malaria treatment policy, especially since HU use in Africa has become more widespread and is considered standard of care in SCD. Therefore, our main study objectives were to: (i) evaluate differences, if any, in selected haematological and clinical chemistry parameters between children with SCD on HU therapy treated with artemether-lumefantrine (AL) for uncomplicated malaria, compared to those not on HU therapy, (ii) determine parasite clearance and treatment efficacy and (iii) frequency of occurrence of adverse events (AEs) in both groups during a 28 days follow up period.

The median (range) duration of HU treatment (for those on HU) was 28 (3–60) months and the median (range) HU dose was 25 (18–35) mg/kg/day.

Malaria and SCD both remain diseases of public health significance. Deployment of HU as disease modifying therapy for SCD in malaria endemic countries has the potential to improve SCD care; however, concomitant antimalarial drug administration within the context of HU therapy may require further safety data and or assurance as well as demonstration of efficacy. This study provides preliminary data on systematic changes in haematological and clinical chemistry parameters and preliminary efficacy data among SCD patients with uncomplicated malaria on HU therapy treated with AL. The study also provides data on the dynamics of these changes, from acute malaria illness through recovery, between antimalarial treated HU+ and no-HU recipients. Additionally, the baseline data of the malaria treated SCD HU+ and no-HU participants have been compared with the corresponding steady state SCD HU+ or no-HU group. The findings thus provide data on malaria- or SCD-distinctive changes during the acute and immediate post-recovery periods.

Our results showed lower Hb levels in participants with acute malaria compared to those in steady state, although this was only significant in the HU+ group. While RET was similar in those on HU (RDT-positive vs. steady state), it was significantly higher in the RDT-positive no-HU group compared to steady state no-HU. SCD causes alterations in blood cell indices characterized by a reduction in Hb and increase in RET, while malaria also causes accelerated breakdown of both parasitized and uninfected red cells, potentially worsening the effect on Hb and RET (21, 22). Although HU is known to cause reticulocytopenia, the expected increase in RET in the context of acute malaria haemolysis was not initially seen in the RDT-positive HU+ group, despite having significantly lower Hb than those in steady state. Over the follow up period, the RET was similar in both RDT-positive HU+ and no-HU groups until day 28, when the HU+ group had significantly higher RET in keeping with greater bone marrow stimulation in this sub-group as their Hb levels were also higher by day 28 (23). Further evaluation with a larger group of participants is needed to verify these findings.

WBC, ANC, and platelets are elevated in SCD steady state, due to underlying chronic inflammation and increased expression of endothelial adhesion molecules. The myelosuppressive effect of HU usually counteracts these changes (8, 24). We however found significantly higher ANC levels in those with acute malaria vs. steady state, irrespective of HU status. In the REACH study, HU-induced myelosuppression with ANC <3.0, was protective against malaria through incompletely defined mechanisms that may include decreased inflammation (10). In the current study, WBC was also increased in both HU+ and no-HU malaria groups compared to those in steady state, although this was not statistically significant for the HU+ group. Surprisingly, platelet counts were comparable across all groups at baseline. In malaria pathogenesis, platelets play both beneficial and deleterious roles, with malaria being a known cause of thrombocytopenia. Thrombocytopenia is also a marker of disease severity in P. falciparum and P. vivax infections (25, 26). It is not clear why our acute malaria cohorts did not show this expected platelet drop, but the lack of differences compared to those in steady state could be because they had uncomplicated infection. Interestingly, comparing only the HU+ malaria and no-HU malaria groups, we found marked differences between groups in the platelet counts over the 28 days follow up period, although the patterns of change were similar, with both groups showing relatively lower platelet counts on day 3, a peak on day 14 and values close to baseline on day 28.

Although the HU+ malaria group had a higher mean parasite density than the no-HU malaria group, this was not statistically significant. SCD does not share the protective effect of HbS against severe P. falciparum malaria seen in sickle cell trait, yet parasite burden is usually lower, and SCD individuals develop clinical malaria at lower parasitaemia than non-SCD. HbS-containing red cells may present a hostile environment for malaria parasite development as infected sickled red cells are preferentially removed from circulation by splenic macrophages. HU reduces HbS concentration and increases HbF levels in red cells, and improvements in deformability and cellular hydration, leading to red cells less prone to premature haemolysis. Increased HbF may also be associated with restriction of parasite growth in vitro (27), and HU may act synergistically with antimalarials to prevent parasite growth (28). In vitro studies show that ribonucleotide reductase inhibitors interfere with malaria parasite development at the schizont stage through inhibition of DNA synthesis, while HU may increase expression of endothelial intracellular adhesion molecule 1, facilitating parasite adhesion to the endothelium (29).

The finding of higher mean parasitaemia at enrolment in the HU+ malaria group could have caused a significant drop in haemoglobin at the start of infection and may account for the (unexpectedly) similar haemoglobin levels at malaria diagnosis in the HU+ and no-HU groups. It may also explain the higher prevalence of anaemia-associated clinical signs at presentation in the HU+ group. The increased Hb by days 7 and 28, in both groups, are in keeping with complete recovery from the acute malaria infection.

There were comparable ANC between the HU+ and no-HU malaria groups at baseline and significantly higher ANC in the HU+ group on days 3 and 14 but a reverse trend by day 28. However, the days 3 through 14 ANC among the HU+ group was skewed by the single post-splenectomy participant who had persistent parasitaemia through day 14. Neutrophils play a crucial role in the body’s defense against pathogens and are implicated in the phagocytosis of malaria parasite-infested RBC and free merozoites. Neutrophils also produce reactive oxygen species that inhibit parasite growth (30). While these changes may be incorrectly interpreted as evidence of underlying bacterial infection, the degree of neutrophilia is linked to an increased likelihood of developing severe malaria due to the formation of neutrophil extracellular traps and transient immunosuppression (30), and both leukocytosis and neutrophilia represent adverse prognostic factors for severe SCD-related complications such as stroke. The finding of persistent parasitaemia-associated neutrophilia signals and confirms the adverse effects of malaria among SCD patients and the need for prompt malaria treatment in this patient population.

Lymphocyte counts on days 3 and 28 were comparable between the HU+ and no-HU groups. However, the corresponding counts were markedly different between days 7 and 14. While the implication of these dynamics are unknown, T cell lymphocytes play a role in priming phagocytic cells to capture or kill malaria parasites as well as helping B lymphocytes produce functional anti-parasitic antibodies (31). Lymphocytes are also important in SCD pathophysiology as invariant natural killer T cells have been implicated as a critical immune profile that potentiates SCD and may act as a possible therapeutic target. The implication of the comparatively higher lymphocyte counts in the HU+ group in the immediate post-treatment phase are unclear, even as HU is known to have antiproliferative effects on T lymphocytes in vitro (32) and specifically targets quiescent lymphocytes in vivo (33).

Despite lack of any significant differences in biochemical parameters on Day 0 between both malaria groups, the temporal patterns showed marked variations in AST and ALT between days 0 and 28. The nadir of ALT and AST occurred in the HU+ group on day 3, with no change in BIL on day 3—a finding that was not seen in the no-HU group. The latter group had an increase in the AST and ALT values on day 3, consistent with previous reports of self-limiting, disease and/or drug-related liver injury (17, 34). Although HU can cause acute elevations in liver enzymes, it is well tolerated in children with SCD with no short- or long-term hepatotoxicity. In a single treatment centre in the United Kingdom, significant reductions in the ALT and AST were seen in children with sickle cell anaemia who had good adherence to HU, signifying a beneficial effect on hepatocytes (35).

Being the most prescribed ACT in sub-Saharan Africa (36), several systematic studies have shown an overall high (>95%) AL treatment success in sub-Saharan Africa (37, 38), with comparatively high efficacy rates reported among SCD patients (12, 39). However, a lower AL therapeutic efficacy has been reported among young underweight children from Africa (40), while AL treatment has also been associated with a higher treatment failure risk, or reduced lumefantrine absorption in underweight children (41, 42). Among the general population of children in Burkina Faso and Uganda, there have also been recent concerns of AL treatment failure (43, 44). Furthermore, AL treatment has been associated with delayed parasite clearance among SCD patients and SCD-associated pathophysiological changes such as impairment of hepatorenal function, albuminuria (45), malnutrition (46), or alterations in blood cell indices, may act in concert, with implications for overall treatment outcome in SCD patients. In this study, however, absent parasitaemia in all study participants by day 3 demonstrates rapid parasite clearance and supports the continued high AL efficacy in our study population. The recurrence of parasitaemia in a splenectomized patient, is consistent with evidence of prolonged parasite clearance in individuals with impaired splenic function and with the findings from a recent report from Sudan, of parasite recrudescence after antimalarial treatment in a splenectomized SCD patient on HU (47).

Children with SCD treated with AL for acute uncomplicated malaria had rapid parasite clearance and high day 28 cure rates irrespective of whether they were on prior HU therapy. However, distinct pattern of changes in haematological and blood chemistry parameters between HU+ and no-HU groups in this pilot study, including rapid decline in ALT and AST among the HU+, in contrast with elevated ALT and BIL levels in the no-HU group during the acute illness phase, suggest that further focused studies are needed to evaluate the significance of these observations in the specific context of long term safety of HU during acute malaria and its treatment. Furthermore, our finding of recurrent parasitaemia in a single splenectomized patient, highlights the need for studies to refine the role of the spleen in SCD patients with acute malaria and to advocate for antimalarial preventive interventions for high-risk SCD patients in endemic areas.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by Institutional Review Board, Korle Bu Teaching Hospital; and the Ghana Health Service Ethics Review Committee. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation in this study was provided by the participants’ legal guardians/next of kin.

CS: Conceptualization, Data curation, Investigation, Methodology, Supervision, Validation, Writing – original draft, Writing – review & editing. SKA: Conceptualization, Data curation, Investigation, Methodology, Supervision, Validation, Writing – original draft, Writing – review & editing. AA: Data curation, Investigation, Methodology, Supervision, Validation, Writing – original draft, Writing – review & editing. MN: Conceptualization, Investigation, Methodology, Supervision, Validation, Writing – review & editing. YB: Conceptualization, Investigation, Methodology, Supervision, Validation, Writing – review & editing. NS: Conceptualization, Investigation, Methodology, Supervision, Validation, Writing – review & editing. SYA: Investigation, Methodology, Supervision, Validation, Writing – review & editing. AS: Data curation, Formal analysis, Investigation, Methodology, Validation, Writing – review & editing. LH: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Validation, Writing – review & editing. MO: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Writing – review & editing. GA: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing.