Epidemiology of imported malaria give support to the hypothesis of ‘long-term’ semi-immunity to malaria in sub-Saharan African migrants living in France
Introduction
France is the industrialised country most affected by imported malaria, accounting for 50% of all imported cases in Europe. The incidence of imported malaria in France has ranged from 4000 to 7000 cases per year over the past 10 years [1], [2], [3], [4]. Although the annual incidence appears to be decreasing, national epidemiological data in France [1], like those from our cohort in Bordeaux [5], demonstrate a paradoxical increase in the proportion of malaria patients of sub-Saharan Africa origin living in France and visiting family or relatives in their country of origin (VFR). It is widely accepted that a prolonged duration of stay in a stable transmission zone for Plasmodium falciparum malaria [6], [7] is associated with a certain level of protection against malaria, due to repeated exposure to the parasite, expressed as a reduced risk of severe malaria [8], [9], [10], [11], [12]. This state of protection, termed ‘semi-immunity’, is generally considered to be labile, partial and quick to disappear within months when ‘semi-immune’ individuals no longer live in an endemic area and are not regularly exposed to infecting bites of the Anopheles mosquito. However, the notion that semi-immunity to malaria is always transient may not be correct [10], [11], [13], [14], [15] and studies of imported malaria in Europe indicate that these cases are generally less severe when they occur in VFR [1], [2], [3], [4], [16], [17], [18], [19], [20], [21], [22], [23], [24]. These observations suggest that semi-immunity to malaria may persist in VFR, particularly since this group is less likely to use malarial prophylaxis than travellers of European origin (TEO) [4], [18], [19], [20], [21], [22], [23]. In this context, the milder severity of P. falciparum malaria in VFR could signify ‘long-term’ semi-immunity to malaria acquired following a history, albeit remote, of long-term residence in a stable transmission zone in sub-Saharan Africa. We therefore consider as strong the hypothesis of ‘long-term’ semi-immunity to malaria awaited to persist over a 12 months period following departure from an endemic area. From an epidemiological point of view, we hypothesize that VFR can be considered to be ‘semi-immune’ to P. falciparum malaria if the clinical and biological presentation of P. falciparum malaria in VFR is less severe than that in TEO.
Our aim was to investigate the factors associated with imported malaria that would favour the hypothesis of a ‘long-term’ semi-immunity to malaria in VFR by comparing the epidemiological, clinical and biological characteristics of imported P. falciparum malaria in VFR and TEO presenting at the University Hospital Center (CHU) of Bordeaux, France.
Section snippets
Materials and methods
A retrospective, descriptive study was carried out using the medical files of adult patients treated for malaria in consultation or hospitalization at the CHU of Bordeaux, between 1st January 2000 and 31st December 2007. Inclusion criteria were a positive diagnosis of P. falciparum malaria, age ≥15-years and main residence in France. The patients were divided into two groups: group of travellers of sub-Saharan Africa origin and visiting family or relatives in their country of origin (VFR group)
Results
This retrospective study identified 526 adult cases of imported malaria treated between 2000 and 2007 in the CHU of Bordeaux. These 526 cases have already been described elsewhere [5]. Among these patients, the place of residence and country of birth were identified in 474 cases. Forty-nine patients were excluded as their main place of residence was sub-Saharan Africa (25 French expatriates and 24 newly arrived sub-Saharan African migrants). No case among descendants of African migrants were
Discussion
Our aim was to investigate the factors associated with imported malaria that would favour the hypothesis of a ‘long-term’ semi-immunity to malaria among VFR. Our method was to compare the epidemiological, clinical and biological characteristics of imported P. falciparum malaria in a cohort of VFR and TEO recruited at the CHU of Bordeaux, France.
In our study, VFR were less often affected by severe malaria than TEO. From univariate analysis, this finding was evidenced despite the VFR poorer
Authors' contributions
TP conceived and designed the study and has made substantial contributions for acquisition analysis and interpretation of data, drafting and revising the manuscript. AD has made substantial contributions for acquisition, management and analysis of data. MM and MCR have been involved in revising the manuscript critically. DM participated as guarantor in the design and coordination of the study, and contributed to draft the manuscript, revising it critically for important intellectual content.
Conflict of interest
None.
Acknowledgements
We thank K. Ezzedine who provided a proofreading assistance for free.
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