Malaria parasite prevalence in asymptomatic migrants residing in Sweden
Authors: Wångdahl Andreas , Bogale Rebecca Tafesse , Eliasson Isabelle , Broumou Ioanna , Faroogh Fariba , Lind Filip , Vashchuk Ganna , Hildell Adina , Franson Suzanne , Hallberg Emil , Grip Isabelle , Nordling Irene , Gervin Angelica , Kaitoly Shelan , Tekleab Berhane , Wyss Katja , Mendez Ana Requena , Hertting Olof , Färnert Anna 
Published: September 2022

Background

Asymptomatic infections with malaria parasites are common in populations in endemic areas. These infections may persist in migrants after arrival in a non-endemic area. Screening to find and clear these infections is generally not implemented in non-endemic countries, despite a potential negative health impact. We performed a study to evaluate the Plasmodium parasite prevalence in migrants living in Sweden.

Methods

Adults and children born in Sub-Saharan Africa (SSA) were invited in the study between April 2019 and June 2022 at 10 different sites. Rapid diagnostic tests (RDT) and real-time PCR were used to detect malaria parasites. Prevalence and test sensitivity were calculated with 95% confidence intervals (CI). Univariate and multivariable logistic regression were used to evaluate associations with PCR positivity.

Findings

In total, 789 individuals were screened for Plasmodium spp., of which 71 (9·0%) were detected by PCR and 18 (2·3%) were also RDT positive. A high prevalence was detected in migrants with Uganda as the country of last residence, 53/187 (28·3%), particularly in children, 29/81 (35·8%). Among the PCR positive, 47/71 (66·2%) belonged to families with at least one other member testing positive (OR 43·4 (95% CI 19·0-98·9), and the time lived in Sweden ranged between 6-386 days.

Interpretation

A high malaria parasite prevalence was found in migrants from SSA, particularly in children. Awareness of asymptomatic malaria infection is needed and screening for malaria in migrants arriving from high endemic countries should be considered.

Funding

Swedish Research Council, Stockholm County Council and Centre for Clinical Research, Västmanland, Sweden.

Research in context

Evidence before this study

Asymptomatic malaria infections are common in endemic areas, and migrants may still be infected when arriving in a non-endemic country. Previous studies have shown a parasite prevalence between 3-31.3% in migrants arriving in non-endemic countries, largely depending on patient origins and the diagnostic method used. No European country recommends screening for malaria, whereas in Australia screening of high risk groups is recommended, and in the US, presumptive antimalarial treatment is given. Robust data are few to establish which migrants are at highest risk of infection and who should be screened. In addition, the duration of asymptomatic plasmodium infections has not been assessed thoroughly, and available data are based on case-series which may represent extreme examples of long duration.

Added value of this study

This study is one of the largest cross-sectional studies that evaluate the prevalence of malaria in migrants living in a non-endemic country. The study was primarily conducted in a primary health care level, resembling a potential way to screen for malaria and to avoid the overestimation of the prevalence which is usually perceived in hospital-based studies. Apart from describing a parasite prevalence of 9% in asymptomatic migrants, using real-time PCR, we also identified country of last residence as one of the key indicators to identify the risk of carrying malaria parasites, with a parasite prevalence of 53/187 (28.3%) in individuals where Uganda was the country of last residence. Other risk factors for PCR-positivity were evaluated and children were at particular risk of PCR positivity clustering of asymptomatic malaria infections in families are described. Duration of infection could also be estimated due to the lack of re-exposure for malaria in Sweden.

Implications of all the available evidence

The results presented in this study summarise the best available epidemiological information for the prevalence of malaria according to PCR and RDT techniques among a large sample of migrants living in a non-endemic setting and can be used to inform screening decisions, suggesting that screening for malaria is motivated in migrants from Sub-Saharan Africa after arrival in a non-endemic country.