Malaria epidemiology in low-endemicity areas of the northern coast of Ecuador: high prevalence of asymptomatic infections

Malaria is endemic in the coastal and Amazon regions of Ecuador in areas below 1500 m of altitude, however, the number of malaria cases has decreased >99% since 2001 and only 558 cases were reported in 2015 (Fig. 1); [1]. During the last 10 years, malaria control efforts have been highly effective and according to the Pan American Health Organization (PAHO), the country is currently in malaria pre-elimination phase [2, 3], closer to elimination than its neighbours Colombia and Peru. According to the World Health Organization, Ecuador is among the 21 countries that are in track for malaria elimination in 2020 [4]. Ecuador is focusing in fast detection of acute malaria clinical cases and treatment with artemether–lumefantrine (Plasmodium falciparum) and chloroquine + primaquine (Plasmodium vivax). Malaria surveillance activities are focused in the Northwest coast and in the Amazon regions, the current endemic areas [2].

Presently transmission is low and unstable with periodic outbreaks of both P. falciparum and P. vivax, still occurring on both sides of the Andes [1, 5]. The northwest coast of Ecuador, particularly Esmeraldas province, which limits on the North with the Nariño department of Colombia has historically been an area of P. falciparum transmission [5]. Although this region has experienced a substantial decrease in the number of reported malaria cases, it still maintains transmission and in the 2012–2013 period, it reported an outbreak of 150 P. falciparum cases in Esmeraldas city [5, 6]. In contrast, during the same period, San Lorenzo which historically had reported the highest P. falciparum annual parasite incidence (API) reported only ~20 cases [6]. In 2014 only 20 malaria cases were reported in Esmeraldas province but >100 cases were reported in both 2015 and 2016 [1].

Despite the low transmission, several challenges remain for Ecuador to progress towards the elimination goal. Major challenges are the accurate estimation of malaria prevalence, particularly in view of the number of asymptomatic cases potentially harbouring low levels of parasitaemia not detectable by microscopic examination and the instability of malaria control activities. As in many other regions, microscopic examination of thick blood smears is the gold standard for malaria diagnosis due to its good performance to confirm symptomatic infections. However, many infections have been shown to be submicroscopic and asymptomatic [7], leading to an underestimation of the number of infected individuals in endemic communities. These latter cases remain in the communities as parasite reservoirs for mosquito infection and require active case search as well as the use of more sensitive diagnostic methods. Although asymptomatic infections are considered to be the result of incremental immunity due to frequent exposure to parasite infections, and to reduce the disease burden, they contribute to maintaining malaria transmission [7‐10]. Although a growing number of studies have estimated the prevalence of asymptomatic cases in diverse areas of Latin America such as Brazil, Colombia and Peru, as well as in regions in other continents with greater transmission intensities such as the Solomon Islands and Papua New Guinea (PNG) [7, 11‐13], the proportion of asymptomatic malaria carriers in Ecuador has not been reported, leading to an underestimation of the real malaria incidence. This knowledge gap as well as others concerning the biology of vectors have to be addressed in malaria elimination programmes. Although Anopheles albimanus is considered the main vector species in northern Ecuador and southwest Colombia [14], the knowledge on its bionomy and potential vectors is limited [15, 16]. These knowledge gaps must be addressed for malaria elimination programmes.

The aim of this study was to better understand the epidemiological situation in the northern region of Ecuador, in view of the current malaria elimination plans. Four communities of Esmeraldas province were investigated by using sociodemographic and epidemiological surveys.

Ecuador has experienced ~99% reduction in malaria clinical cases in the last 15 years and is now pointing towards an elimination phase. Despite periodic focal outbreaks, the country has shown a malaria transmission decreasing trend. In 2014, Ecuador reported the lowest malaria incidence of the last decade but in 2016 the reported incidence (917 cases), almost doubled that of the previous year. The study indicated the presence of a significant number of asymptomatic malaria parasite carriers in the study sites, with parasitaemia detectable only by qPCR which is probably an indication of an official under reporting of cases.

The periodic outbreaks, particularly the most recent one, appear to depend on several features. First, as most countries do, the National Malaria Control Programme (NMCP) of Ecuador uses microscopy and more recently RDT for malaria diagnosis. Although these two methods have great value for passive case detection of symptomatic cases, they show very limited sensitivity to detect asymptomatic cases which frequently display submicroscopic parasitaemia levels. Second, the intense population mobility from and to Colombia, may contribute to frequent reintroductions of malaria parasites. These and other issues may slow down the elimination process.

Despite the fact that moving from malaria control to elimination, may appear as simple as intensifying control activities, elimination requires a significant sustained effort with critical strategic programme changes to ensure the goal of reaching zero cases.

Because the NMCP does not usually seek identification of asymptomatic infections, they frequently remain undiagnosed, and even if they are searched, thick smears have limited sensitivity. Furthermore, there is no policy about treatment of these infections, and therefore asymptomatic carriers may be contributing to maintaining the parasite transmission to Anopheles as suggested by previous studies [7, 9, 26]. In areas with low transmission levels, asymptomatics could be critical reservoirs in the maintenance of transmission, thus hampering elimination. The reported 7.5% prevalence by PCR in the study areas was higher than that reported for the northern coast of Peru (1.2%) [20], but comparable to the prevalence recently found in the Colombian border (9.5%) [7] and the Peruvian Amazon (10.9%) [27], suggesting that even though the number of reported cases in Ecuador is lower than on the Colombian border, the transmission level may be similar. This finding is of great importance because, officially, Colombia is considered to be one of the countries with greater transmission in the Latin America region, while Ecuador is among those in pre-elimination phase. However, these results are similar to those reported previously from the Colombian coast, where PCR detected 26 times more parasites than microscopy when reactive case detection was used [7]. These results underscore the shortcomings of microscopy as a diagnosis technique and indicate that active surveillance in the area by the NMCP may not be enough to attain elimination. Moreover, all reported cases in this study were submicroscopic despite parasitaemia varying between 1 and 800 parasites/μl by PCR. It is likely that the amount of submicroscopic infections could be even higher because the PCR used here has a detection limit of ~2 parasites/μl [7]. In this study, a real time PCR was used due to the convenience of the DNA transportation in filter paper. More sensitive methods such as RNA reverse transcriptase could have been used but the inability to preserve RNA was a limitation in the study setting.

On the other hand, the presence of asymptomatic cases indirectly indicates that despite the low transmission intensity the community has developed substantial levels of clinical immunity, even though overall, less than one-third of the population displayed positive titers to both parasite species. Even in El Pedregal where the highest reactivity was recorded, less than half of the study population was positive. The seroreactivity found was in general low, but the frequency of seropositive individuals appeared to correlate with the malaria prevalence, it was higher in El Pedregal which displayed greater number of submicroscopic infections. While this study used a basic serological approach, more information could have been obtained from an analysis of age dependent antibody response and IFAT in all the studied population.

It is surprising that, although Esmeraldas province is the only area of Ecuador still regularly reporting P. falciparum cases, in this study, P. vivax was by far the most prevalent parasite (94% of infections against only 6% caused by P. falciparum). In this province only four cases of P. vivax were reported in 2013 [6]. This finding might be explained by the high prevalence of asymptomatic P. vivax infections only detectable by q PCR, and is in contrast with a greater prevalence of P. falciparum reported in southern Colombia during the same period [7, 28]. A possible explanation for this could be related to the fact that a large part of the population may express reduced Duffy antigens on the surface of their RBCs as a result of heterozygous genotypes as has been shown in other populations that allow P. vivax infections at low parasitaemia and develop immunity against this parasite [29], consistent with the frequency of antibodies to P. vivax in El Pedregal and La Boca.

The high prevalence of P. vivax in an area where it is rarely reported brings new challenges from the treatment point of view. Ecuador uses a radical cure treatment that includes a 3-day chloroquine course and a 7 day primaquine treatment. The need for primaquine treatment for P. vivax, in an area not previously reported, will have important consequences in a possible elimination setting, considering that the status of glucose-6-phosphate dehydrogenase (G6PD) deficiencies is uncertain. Even though P. vivax treatment has been performed without G6PD deficiency testing in Ecuador, models predict that the north of the country, an area where P. vivax is rarely reported has a higher prevalence of G6PD deficiency than other areas where vivax infections are more common [30]. As elimination approaches, the treatment of asymptomatic P. vivax patients will need to go hand in hand with the determination of the G6PD status for each patient.

The communities of El Pedregal, Ricaurte and La Boca with a greater historical exposure to malaria presented better knowledge about malaria but comparatively lower than in the adjacent coast of Colombia [31]. In contrast, in El Guadual, a community located towards the mountain side there was a lack of knowledge about malaria and no prevention practices were identified, this may correspond to recent introduction of malaria at these higher altitudes. Indeed, antibodies in this community indicated a lower previous exposure to the parasite than in the lowland study sites.

Although most people had a basic knowledge about malaria, practices to prevent malaria transmission were not very common. However, geo-referencing of malaria cases allowed mapping specific disease foci which facilitate targeting by control and elimination activities including education and community participation.