Even though FG is still not concerned by this FpHPR2 gene deletion [10], the phenomenon of immigration from neighboring countries and over might introduce species carrying the genetic mutation. Our study aimed to assess the accuracy of SD Malaria Ag P.f/Pan® test, which is the RDT currently used in remote healthcare centers of FG, in the rapid diagnosis of malaria. The SD Malaria Ag P.f/Pan® test was sufficiently accurate for diagnosing malaria in suspected patients and the routine monitoring and the detection of passive cases in malaria low transmission areas (Orientation note about malaria diagnosis in the context of low transmission rate, September 2014, WHO). Diagnosing malaria is an emergency to early introduce a curative treatment and prevent an adverse evolution that, in some cases, could lead to death. Microscopic blood smear examination is still the gold standard, but high-quality RDTs can be used, at first, due to their efficacy, large availability, and cheap cost (WHO. Quality assurance for microscopy: malaria microscopy quality assurance manual. Genève, 2009). In FG remote areas, the only way to quickly diagnose malaria disease remains RDTs. The choice of the SD Malaria Ag P.f/Pan® in FG was based on the comparison of performance provided by the WHO reports but not on entomological studies made in FG. (https://www.who.int/diagnostics_laboratory/evaluations/pq-list/malaria/public_report/en/).
In FG, the implementation of a malaria control strategy allowed to reduce the number of recorded malaria cases by 82% during the last ten years, with 3344 cases in 2009 and 212 cases in 2019 (https://www.guyane.ars.sante.fr). In 2011, among 1209 cases reported P falciparum and P vivax represented respectively 31% and 68.5% [10]. In 2019, among the 212 recorded malaria cases, only 6% were P falciparum (13 cases). Forty-two cases were hospitalized, and only 2 cases developed a complicated clinical picture (one with P falciparum and one with P vivax) (https://www.guyane.ars.sante.fr/paludisme-1). In our study, the average share of confirmed (BFT test) P falciparum decreased significantly with 13.3% of positive tests (minus 17.7%), while confirmed P vivax cases raised with 85.1% of positive tests (plus 16.6%) (https://www.guyane.ars.sante.fr/system/files/2017-06/palu%20plan%202015-2018.pdf).
Our study recorded all RDT's performed and compared them to BFT regardless of whether it was a first-line diagnostic test or a monitoring test during the curative treatment. The global sensitivity of SD Malaria Ag P.f/Pan® test was, on average, 96% (88.2 – 1) for P falciparum and 93% (90.6 – 94.2) for P vivax. The sensitivity can be affected by a low rate of parasitemia in the bloodstream [1]. The global specificity was high, with a rate of 99.8% (99.5 – 1) for all species included. In our database, among 12880 concomitant RDT and BFT tests, 10873 tests (84.4%) represented first-line diagnosis tests, and 2007 RDTs were performed during the follow-up period. The latter affected results of the VVP, which was on average 97% (94.5 – 98.9) but significantly different for P falciparum and P vivax with respectively 88% (80.4 – 1) and 98% (94.7 – 99.1).
Most of RDTs performed during the follow-up period were done during the two first years, which can explain the annual variation of the PPV. Furthermore, there is a significant variation of the global PPV week after week during the first 28 days of our study. It is also explained by the number of false positives related to the persistence of the protein-encoding for Plasmodium in the blood. In our study, we have not collected clinical information. Consequently, we were not able to determine if there were an immunological factor or infectious agents to explain the calculated PPV [12,13]. Thus, a positive result of the RDT must be confirmed by a BFT. On the ground, there is a real risk of not making the correct diagnosis by placing too much importance on the rapid test result in case of positivity.
The negative predictive value was, on average, 99.7% (99.3 – 99.5) without a significant difference between P falciparum and P vivax. A low density of parasitemia can affect the accuracy of the NPV [14]. It was not the case in our study where we did not find false-negative results. We can deduce that the negativity of SD Malaria Ag P.f/Pan®, used as a first-line diagnosis test, allowed to reasonably eliminate malaria diagnosis even though perform BFT remains the gold standard.
In our study, 2007 concomitant RDT/BFT tests were carried during the follow-up period of patients during treatment. Our results have shown that the RDT result was still in the majority positive up to 28 days, even though parasites were no longer detectable with BFT. The literature emphasizes a remaining RDT positivity up to 63 days after treatment. Previous findings mention that PfHRP2 RDTs are still positive after treatment for longer than combination or pLDH RDTs [15]. This is explained by a slower degradation of PfHRP2 compared to pLDH after parasite elimination [15,16]. Indeed, PfHRP2 antigen, which is progressively eliminated from the bloodstream, can be responsible for false positives [17,18]. In opposition, pLDH is quickly eliminated after one week from the bloodstream [19]. Thus, the initial parasite density influence false positives result and PfHRP2 persistence [16].
Furthermore, patients benefiting from ACT treatment have a higher probability of remaining positive to RDTs than those who received non-ACT drugs [16]. In practice, the persistence of positivity of RDT after treatment makes it obsolete for monitoring. Thus, the SD Malaria Ag P.f/Pan® test should not be used to assess the efficiency of the treatment set up in these conditions.
During the last two years, the majority of malaria cases in FG were autochthonous (83% first-quarter 2019, 78% first-quarter 2020) while the remaining part was from Brazil (range 9 to 14 %), Suriname (range 2 to 5 %), or Africa (range 2 to 7%) [8]. Illegal gold miners (garimpeiros) inside the rainforest and native populations at border areas between Suriname and Brazil are at risk of malaria resurgence [7,20,21]. The epidemic of malaria experienced a pic in 2017 at the Amazonian border between FG and Brazil. Multifactorial causes were pointed out, such as migration matters from Brazil and Venezuela, local politics, logistics issues, and others [22]. Most cases occurred in forest areas except Saint Georges d'Oyapock at Brazil's border (Amapà region). https://www.guyane.ars.sante.fr/paludisme-1. In FG, the malaria incidence is low, so that all patients with positive RDT tests are considered as recently infected. However, in 2015, Orpal-1, a study carried out on a population of 421 gold miners, has shown that the wide majority were Brazilian citizens (93.8%). This study has shown that the prevalence of carriage, determined by PCR, was 22.3% (95% CI: 18.3 – 26.3) to 84% asymptomatic. Species identified were P falciparum and P vivax, 47.9% and 37.2%, respectively with 10.6% co-infections. Thus, there is a real risk of periodic reintroduction of the disease in FG [23]. It is estimated that illegal gold miners are around 10.000 people (www. guyane.gouv.fr). Orpal-1 results with only 4.2% of the group, targeted as accountable for malaria cases spreading in FG, might be non-representative of the reality. Further investigations are required to clarify this key point.
By the way, in the Brazilian Amazon basin, the lack of PfHRP2 protein is very variable. The isolates collected in Acre state had the highest percentage of deletions with 31.6% (95% CI: 21.6-43.1), while in Rondonia state, the prevalence of deletion was only 3.3% (95% CI: 0.4-11.5) and absent in Para state [24] but 100% of deletion for Pf HRP3 [25]. In 2013, within the annual meeting of AMI/RAVDERA, in Lima, Peru, CDC reported in the Macapa region 9% of PfHRP2 deletion (only 13 samples were screened) and 36% of PfHRP3 deletion. More globally, during the same meeting, CDC mentioned PfHPR2 deletion in 14% of cases in Brazil, 14.1% in Suriname, 33.3% in Peru, 7.5% in Colombia, and 4% in Bolivia [26]. Moreover, there is no accurate data about imported malaria cases except the distribution shares that may depend on immigration waves (www.guyane.ars.sante.fr). Besides, we do not know the immigration journey of patients and if they are coming from endemic countries with a significant rate of PfHPR2 deletion. In theory, it represents a risk of false-negative tests that should be better estimated by further studies [27].