For travellers,
P. falciparum is the main target of malarial prevention because this species is widely endemic, very common and clinically severe [
1]. Although very rarely life-threatening, relapsing malaria species,
P. vivax and
P. ovale, are also of concern for travellers in endemic areas. In contrast to
P. vivax, which is widespread in Asia and Central and Southern America,
P. ovale is only endemic in some African countries [
5]. Two subspecies of
P. ovale have been identified in West Africa [
6].
Plasmodium ovale is responsible for rare, travel-acquired infection [
7]. However, the French Army experienced a significant increase in the incidence rate of
P. ovale malaria attacks among French soldiers returning from the Ivory Coast between 2002 and 2007, but not among soldiers in the field, while the incidence of
Pl. falciparum regularly decreased over the same period [
3,
4]. These changes could reflect epidemiological changes in the local transmission of malaria and/or a lower effectiveness of doxycycline chemoprophylaxis on
P. ovale than on
P. falciparum. Indeed,
P. ovale was very uncommon while using the association chloroquine-proguanil in the French Army before 2002. Thus, preventing
P. ovale infection with doxycycline seems challenging because of the lack of action on hypnozoites. A similar failure has also been recently reported with atovaquone plus proguanil chemoprophylaxis [
8].
Diagnosing
P. ovale infection in infected patients is a challenge in the field or on return to non-endemic countries [
9]. Symptoms can be due to a primary infection or, later to relapses that occur in one patient in ten because of delayed parasitaemia from intrahepatic dormant forms [
1]. Shortening the time-lapse before effective treatment of this relapsing malaria is required to reduce morbidity, limit sick-leave and, diminish the risk of spleen rupture [
10]. This experience illustrates a few traps that physicians should be aware of when attempting a diagnosis. First, all clinical features of
P. ovale attacks are not specific. Nevertheless, a tertian fever-evocative although non-specific - was present in more than half the patients, possibly due to the time lapse to the delayed diagnosis, which had allowed synchronization of the replication cycle. Moreover, digestive or respiratory symptoms, which were present in half cases, can wrongly suggest a viral infection. However, any fever, associated or not, after return from an endemic area, should be considered to be due to malaria and be followed up by immediate thick and thin blood smears. As a result of health education before and during the mission, all the patients promptly consulted a general practitioner and underwent a search for malaria in a nearby laboratory. In contrast to
Plasmodium falciparum and
Plasmodium vivax malaria, the haemogram appeared not to be very helpful for
Plasmodium ovale because anaemia is frequently missing and thrombocytopaenia mild or absent [
7], especially within the first days of the attack. Biological markers for haemolysis were also missing in five cases out of six. Most of all, the sensitivity of routine microscopic searches for
P. ovale in imported cases is very low [
11], as shown in this series. Although more pyrogenic than
P. falciparum[
12], the level of parasitaemia for
P. ovale is far lower at the same time after disease onset [
7]. Unfortunately, all available rapid antigenic tests currently lack sensitivity to
P. ovale, while they are highly sensitive to
P. falciparum[
13‐
15]. They generally fail to detect
P. ovale infection when thick films and blood smears are negative. This could be due to the very low level of circulating antigen or to inadequate antigens used for these tests. This could explain some acute unexplained fevers in the French Army in the field in Africa [
3]. When the diagnosis of imported
P. ovale malaria is suspected, routine microscopic searches with thick and thin blood smears should be repeated, up to three times and in an expert laboratory, if possible [
9]. Considering the high sensitivity and specificity of molecular detection of
P. ovale using PCR, this tool marks real progress in confirming the diagnosis, although it is still not routinely available [
5,
7,
16]. It can be used as a second-line diagnosis tool to identify infra-microscopic parasitaemia, especially for unexplained relapsing fever in travellers.