Prevalence of Malaria Parasite Infections among U.S.-Bound Congolese Refugees with and without Splenomegaly

During 2013–2019, more than 50,000 Congolese refugees living in east Africa resettled to the United States. During 2014–2015, 14% of Congolese refugees resettling to the United States from the Kyangwali Refugee Settlement in the Kikuube district of Uganda were noted to have splenomegaly. ¹ A subsequent investigation of splenomegaly in Congolese refugees resettled to the United States highlighted malaria infection as a relatively common condition, although the definitive etiology of splenomegaly in this population was not determined. ² These observations led to enhanced diagnostics for malaria infection testing for U.S.-bound Congolese refugees from Kikuube district, Uganda, to better manage malaria by detecting additional cases and by providing species-specific treatment. We determined the prevalence of plasmodial infection among 803 U.S.-bound Congolese refugees evaluated under this enhanced diagnostics program.

As part of the U.S. Refugee Resettlement Program, all U.S.-bound refugees originating in sub-Saharan Africa receive presumptive predeparture treatment with artemether/lumefantrine (AL), unless contraindicated, as recommended by the U.S. CDC to prevent symptomatic Plasmodium falciparum malaria disease during travel or following arrival. Treatment is administered in a standard 3-day course under direct observation and should be completed no sooner than 5 days before departure.

In 2015, in response to the observed high prevalence of splenomegaly in U.S.-bound Congolese refugees, the CDC added diagnosis and management recommendations, stating that all refugees with clinical splenomegaly (enlargement of the spleen) identified at their initial medical examination (occurring 3–6 months before departure to the United States) should receive a malaria rapid diagnostic test (RDT; SD Bioline Malaria Ag P.f/Pan test, Abbott Diagnostics Korea Inc., Gyeonggido, Republic of Korea) and, if positive, receive antimalarial treatment with AL. In addition, the CDC recommended that all refugees with splenomegaly receive primaquine after arrival in the United States, if glucose-6-phosphate dehydrogenase activity is normal, to presumptively eliminate Plasmodium vivax and Plasmodium ovale hypnozoites.

Despite these recommendations, many refugees had persistent splenomegaly months to years after arrival in the United States. ² In addition, there were increasing reports of breakthrough malaria, particularly Plasmodium malariae despite predeparture treatment with AL, ² which was of concern because of increasing reports of AL treatment failure for P. malariae infections. ³ Because P. malariae has been well associated with splenomegaly, ^4–6 the reports of P. malariae in Congolese refugees raised additional concern that the documented persistent splenomegaly could be related to inadequate malaria presumptive treatment, especially if baseline prevalence of P. malariae infection was higher than expected.

In response to these concerns, and given the continuing high burden of malaria in U.S.-bound Congolese refugees from Uganda, the CDC expanded its guidance for malaria testing and treatment in 2018 to include the RDT for P. falciparum for all Congolese refugees (with or without splenomegaly) plus PCR. Refugees with a positive RDT and those diagnosed with splenomegaly (with or without a positive RDT) were treated with AL at the initial examination (approximately 6 months before departure). ⁷ In addition, when PCR results became available (at a later date), refugees with positive results who had not previously received appropriate treatment based on species, or had a newly identified infection, were offered treatment. ⁸ It should be noted that in addition to enhanced diagnostics and treatment at initial medical evaluation, all refugees without a contraindication (including those who received previous treatment based on a positive RDT, positive PCR, or diagnosis of splenomegaly) received AL treatment completed no sooner than 5 days before departure for the United States, consistent with previous guidance. ⁹ An outline of this guidance is provided in Table 1.

The program was instituted to enhance the diagnosis and management of malaria in U.S.-bound Congolese refugees. The goal of this evaluation is to retrospectively report prevalence of malaria based on enhanced testing and to correlate the malaria findings with detection of splenomegaly. The proposed evaluation was reviewed by a CDC human subject advisor and determined to be a non-research evaluation of program activities.

Malaria RDT results were interpreted by International Organization for Migration laboratory personnel. DNA was extracted from blood dried on Whatman 3-MM filter paper with Chelex (Cetiva, Marlborough, MA), ⁷ with nested PCR amplification of species-specific sequences of 18S subunit ribosomal RNA genes and discrimination of species by electrophoresis of amplicons, as described previously. ⁸ Clinical data, including the presence of splenomegaly and RDT results, were collected retrospectively from medical records.

During February–March 2018, 803 refugees had a physical examination including an abdominal examination and tested for malaria parasites by RDT and PCR. Demographic characteristics were compared between individuals with and without detectable malaria infections using chi-squared tests. Adjusted odds ratios and 95% CIs were calculated using multivariable logistic regression models. All models were adjusted for age, gender, family size, and birth country.

This cohort consisted of an approximately equal number of women (51%) and men (49%), with age largely less than 55 (median = 14, interquartile range = 6–28) years; 41% were born in Uganda to Congolese parents, and 59% were born in the Democratic Republic of the Congo (DRC). A total of 187 (23%) refugees were positive by either RDT or PCR, 78 (10%) were positive by malaria RDT only, 35 (4%) were positive by malaria PCR only, and 74 (9%) were positive by both RDT and PCR. In the 109 infections identified by PCR, 87 (80%) had monoinfections with P. falciparum, eight (7.3%) with P. ovale, seven (6.4%) with P. vivax, one (0.9%) with P. malariae, and six (5.5%) mixed infections (Table 2).

Most (75%) malaria infections detected by PCR were among children aged 14 years and younger. The highest prevalence of infection was in those aged 6–14 years (24%) and the lowest in those aged 25–54 years (5%). In the adjusted regression analysis (Table 3), odds of malaria infection detected by PCR were more than four times higher among children aged 6–14 years than adults aged 25–54 years (OR = 4.6, 95% CI = 1.3–15.6) and almost seven times higher among adults aged 55 years or older than that in the same reference group (OR = 6.6, 95% CI = 2.1–20.5).

Splenomegaly was observed in 92 refugees (11%), with the highest prevalence among those aged 6–14 years (19%), similar to previous reports in this population. ¹ Of the 92 refugees with splenomegaly, malaria parasite infections were detected by RDT in 27 (29%) and by PCR in 18 (20%). Adjusting for age, gender, family size, and birth country, odds of splenomegaly were higher among those with malaria infection detected by PCR than those without (OR = 1.5, 95% CI = 0.8–2.8) and among those who were RDT positive as compared with those RDT negative (OR = 1.8, 95% CI = 1.0–3.0), but the association was significant only for the RDT comparison (Table 3). Of note, among those positive by RDT, an unusually high proportion (51%) were negative by PCR.

This analysis has limitations. This was a retrospective analysis of data collected during routine care and not designed as a prospective study. Malaria prevalence may vary as transmission intensity changes over time, and this cross-sectional investigation offers only a snapshot of the evaluation period. Furthermore, the high rate of PCR negativity among those who tested positive by RDT was unexpected because PCR offers a more sensitive test for parasitemia. Expert microscopy was not included in the enhanced program evaluation. In the operational setting, either RDT or PCR may have been inaccurate because of undetermined factors. Potential contributors to discrepancies between RDT and PCR results include persistence of circulating histidine-rich protein beyond that of plasmodial DNA in those recently treated for malaria (including self-treatment in advance of their medical examination for resettlement), errors in reading of RDTs in the field setting, false-negative PCR assays due to polymerase inhibitors in samples, and false-positive PCR assays due to contamination from other samples. ^9,10

In conclusion, malaria was prevalent in U.S.-bound Congolese refugees living in Kikuube district, Uganda, in February–March 2018. Splenomegaly was prevalent in this population at rates similar to those found in 2014. ¹ Efforts to delineate the optimal treatment regimen for malaria in the population before migration to the United States would be beneficial, particularly as 18% of PCR-positive samples had P. ovale or P. vivax. Traditionally, presumptive treatment has not focused on relapsing (P. vivax and P. ovale) forms of malaria because U.S.-bound refugees have mostly not originated in areas with high prevalence of these parasites. In light of the prevalence of P. ovale and P. vivax that is higher than that expected in Congolese refugees, presumptive treatment for relapsing malaria in some refugee populations should be reconsidered. There was no clear direct association between malaria parasitemia and splenomegaly, indicating that active malaria parasitemia is not necessary for splenomegaly or that splenomegaly persists over time despite parasite clearance. Nonetheless, the background high prevalence of malaria infection suggests a likely role for malaria infection in the etiology of splenomegaly, as previously suggested. ^11,12 Despite anecdotal reports of high prevalence of P. malariae infection in Congolese refugees, < 1% of detected species were P. malariae in this program. The small number of P. malariae cases identified suggests this species is not a driver of splenomegaly. Etiologies other than malaria that could account for or contribute to splenomegaly should be further explored.