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Revista da Sociedade Brasileira de Medicina Tropical

Print version ISSN 0037-8682On-line version ISSN 1678-9849

Rev. Soc. Bras. Med. Trop. vol.52  Uberaba  2019  Epub Mar 28, 2019

http://dx.doi.org/10.1590/0037-8682-0537-2018 

Major Article

Asymptomatic Plasmodium infection in a residual malaria transmission area in the Atlantic Forest region: Implications for elimination

Renata Bortolasse Miguel1  2 

Hermano Gomes Albuquerque1  2 

Maria Carmen Arroyo Sanchez3 

José Rodrigues Coura1  2 

Simone da Silva Santos1 

Sidnei da Silva4 

Carlos José de Carvalho Moreira1 

Martha Cecilia Suárez-Mutis1  2 
http://orcid.org/0000-0003-2809-6799

1Laboratório de Doenças Parasitárias, Instituto Oswaldo Cruz/Fiocruz, Rio de Janeiro, RJ, Brasil.

2Programa de Pós-graduação em Medicina Tropical, Instituto Oswaldo Cruz/Fiocruz, Rio de Janeiro, RJ, Brasil.

3Laboratório de Soroepidemiologia e Imunobiologia, Instituto de Medicina Tropical de São Paulo, São Paulo, SP, Brasil.

4Laboratório de Parasitologia, Instituto Nacional de Infectologia, Rio de Janeiro, RJ, Brasil.


Abstract

INTRODUCTION:

Elimination of malaria in areas of interrupted transmission warrants careful case assessment to avoid the reintroduction of this disease. Occasional malaria cases are reported among visitors of the Atlantic Forest area of Brazil, while data on residents of this area are scarce.

METHODS:

A sectional study was carried out to examine 324 individuals living in a municipality where autochthonous cases were detected.

RESULTS:

Asymptomatic Plasmodium infections were detected in 2.8% of the individuals by polymerase chain reaction (PCR), with one case of P. falciparum (0.3%), two cases of P. vivax (0.6%), and six cases of P. malariae (1.9%). The thick blood smears were negative in all individuals. Serological tests performed in 314 subjects were reactive in 11.1%, with 3.5% for P. falciparum and 7.7% for P. vivax. A subsample of 42 reactive individuals for any Plasmodium species showed P. malariae in 30.9% of specimens. Individuals who entered the Atlantic Forest region were 2.7 times more likely to exhibit reactive serology for P. vivax compared with individuals who did not enter this region (p<0.05). Children <15 years had a higher chance of reactive serology for P. falciparum and P. vivax than individuals ≥15 years of age (p<0.05). Individuals living in the Paraiso district had a higher chance of reactive serology for P. vivax compared to other districts (p<0.05). No associations were found between sex, past exposure to malaria, or serological response to antibodies of any Plasmodium species.

CONCLUSIONS:

The implications of these results for the elimination of malaria were discussed.

Keywords: Malaria; Plasmodium infection; Extra-Amazonian region; Elimination; Rio de Janeiro State.

INTRODUCTION

Malaria endemic areas are restricted to the Amazonian region of Brazil where Anopheles darlingi Root 1926 is the main vector implicated in transmission1. An average of 1,296 cases are reported annually outside of this region, most of which (89%) have been imported from elsewhere2. While 50% of the malaria cases in the extra-Amazonian region of Brazil were autochthonous in the middle of the 20th century, this proportion has dropped to only 0.05% to date2. Rio de Janeiro state, located in Southeastern Brazil, used to be considered a highly endemic area for malaria with the reported presence of An. darlingi3. Cases of infection by P. falciparum and P. vivax occurred particularly in the lowlands (Baixada Fluminense region) in the proximity of the capital of the state4. The Global Malaria Eradication Campaign supported the implication of control measures against vectors and the discovery of new drugs for infected people, leading to the interruption of transmission by 1968 with An. darlingi now rarely found in this area5. In recent years, a new malaria epidemiological scenario is emerging in Rio de Janeiro, with the presence of cases imported from endemic areas of Brazil and outside the country and occasional autochthonous cases including outbreaks6-9. In 2016, the World Health Organization (WHO) launched the “Global Technical Strategy for Malaria 2016-2030”, with the goal of “preventing a resurgence of malaria in all countries and areas that are malaria-free”10. A recent malaria outbreak caused by P. simium was described in tourists visiting some regions of the dense and protected Atlantic Forest in Rio de Janeiro state9. Malaria cases in this area are detected predominantly among visitors but it is unclear whether the local residents are also affected. We therefore carried out a cross-sectional study to verify the status of Plasmodium infections in residents of the Atlantic Forest area of Rio de Janeiro state. The novel epidemiology of malaria in interrupted transmission areas must be carefully assessed, particularly considering worldwide efforts to eliminate the disease2,10.

METHODS

A cross-sectional study was carried out during the fall of 2011 in Guapimirim, a municipality located 86 km away from Rio de Janeiro, Brazil. Six localities were eligible for participant recruitment because nearby autochthonous cases were detected, the rural areas Garrafão, Orindi, and Paraíso and the peri-urban areas Barreira, Caneca Fina, and Monte Olivete (Figure 1). The required sample size was estimated using an expected prevalence of 4% for Plasmodium infection as a variable with an acceptable error of 2%. The final sample size was calculated as at least 312 individuals. Residents of both sexes older than 5 years of age who gave their informed consent (for adults) or received consent from the legal guardian (for children under 18 years) were randomly recruited. A semi-structure questionnaire on personal data, exposure to malaria, daily habits, knowledge about disease, clinical information, and history of febrile symptoms in the month preceding the study period was administered. Thick and thin blood smears were collected from each participant according to standard protocols of the Brazilian Ministry of Health. Total blood (5 mL) was collected in Vacutainer® tubes (Becton Dickinson New Jersey, USA) containing ethylenediaminetetraacetic acid (EDTA) for DNA extraction for parasitological diagnosis of malaria by polymerase chain reaction (PCR). Ten milliliters of blood were collected without anticoagulant for serological studies.

FIGURE 1: Study locations and prevalence of asymptomatic Plasmodium infections in the municipality of Guapimirim, Rio de Janeiro, Brazil. 

Parasitological diagnosis

Thick blood smears were stained and 100 fields examined at 1,000× magnification11. DNA was extracted for molecular diagnosis from 200 μL of whole blood using a commercial DNA Purification Kit (Illustrablood genomicPrep MiniSpin Kit), according to the manufacturer’s instructions (GE Healthcare, Pittsburgh,USA). PCR was performed following the nested PCR protocol by Snounou et al. with minor modifications12,13. Agarose gel electrophoresis was performed, gels stained with GelRedTMnucleic acid gel stain, Biotium, Fremont, CA, USA and visualized under UV light. A sample was considered positive if a 120, 144, or 205 bp PCR product (P. vivax, P. malariae, and P. falciparum, respectively) was detected. A sensitivity level of 0.001% for PCR detection of parasitemia levels is appropriate for the diagnosis of subpatent infections14. Positive results were re-tested twice to verify the results.

Serological analysis

Serological examinations were performed using enzyme-linked immunosorbent assay (ELISA) with erythrocytic antigens for P. falciparum and P. vivax and immunofluorescence assay (IFA) for P. malariae. IgG antibodies were detected by ELISA using a crude blood stage P. falciparum antigen extracted with Zwittergent® (Calbiochem, Billerica, MA, USA)15 and MSP1-19 recombinant antigen of P. vivax16. Reactions were assessed by measuring the absorbance at 492 nm using Titertek Multiskan MCC/340 (Labsystems Diagnostics Group, Vantaa, Finland). To determine the cut-off for the ELISA using P. falciparum and P. vivax antigens, receiver operating characteristic (ROC) curves were constructed based on the absorbance of positive and negative samples17 (Figure 2). The reactivity index (RI=absorbance/cut-off) was calculated for all samples, and samples with RI ≥1.1 were considered positive. Samples with values between 0.9 and 1.1 (gray zone) were considered inconclusive. IgG antibodies against P. malariae were detected using the IFA protocol described by Ferreira and Sanchez16. As the quantity of P. malariae antigen was low, this serological test was performed only for the PCR-positive samples and all reactive and inconclusive serologies. The experiments were performed at the Laboratory of Seroepidemiology and Immunobiology at the Institute of Tropical Medicine of São Paulo, Brazil.

FIGURE 2: Receiver operating characteristic (ROC) curves to determine enzyme-linked immunosorbent assay (ELISA) performance for anti-PvMSP119 IgG and anti-Pf IgG. Panel A shows the curve obtained for samples fromP. vivax-infected patients (n=41) and healthy subjects (n=37). Panel B shows the curve for samples fromP. falciparum-infected patients (n=41) and healthy subjects (n=37). PvMSP119-and Pf -ELISAs considered a cut-off of 0.100 in the ROC curves. Anti-IgG PvMSP119 for P. vivax was detected in 100.0% (range: 90.5-100.0, CI: 95%) of P. vivax cases with a specificity of 97.6% (range: 87.1-99.9, CI: 95%). Anti-IgG for P. falciparum was detected in 94.4% (range: 81.3-99.3, CI: 95%) with a specificity of 100.0% (range: 91.2-100.0, CI: 95%).  

A malaria case was defined as an individual with any of the typical malaria symptoms (fever, chills, sweating, headache) at the time of the interview and with a positive thick blood smear or PCR result. An asymptomatic Plasmodium carrier was defined as an individual with Plasmodium species detected in the thick blood smear and/or the PCR but without symptoms 30 days before and after sample collection and without using antimalarial drugs.

Ethical considerations

All the procedures followed the ethical standards of the Ethics Committee of Research involving Human Subjects of the Research Institute Evandro Chagas-INI/Fiocruz, Brazil and in accordance with the Helsinki Declaration (Protocol 0229.0.000.009/10 approved).

Statistical analysis

Exploratory analyses were performed using contingency tables and chi-square and Fisher tests to verify possible relationships between the dependent and independent variables. This relationship was modeled using a series of simple binomial generalized linear models (GLM). Only variables with p<0.2 in the bivariate analysis were evaluated in the model. Multiple binomial GLMs were used to model the effects of all independent variables on each dependent variable. Interaction effects were tested in each model formulation, but no significant interaction was found. We employed the Firth’s Bias-Reduced Logistic Model to account for numerical problems and model convergence in the P. falciparum and P. malariae models. Data were analyzed using the freely available EPI2000 statistical program (Centers for Disease Control, Atlanta, Georgia, USA), the R software, and the RStudio software with the Companion to Applied Regression (CAR) package, R Foundation for Statistical Computing, Vienna, Austria.

RESULTS

Demographics aspects

We interviewed a total of 324 individuals [186 women (57.4%) and 138 men (42.6%)], with a mean patient age of 32.6± 9.8 years without differences between women and men (p=0.16). Of these, 215 participants (66.4%) lived in rural areas and 109 (33.6%) in peri-urban areas. Most individuals were born in the state of Rio de Janeiro (260/324, 81.8%). The mean educational duration of the participants was 6.9±3.6 years, and 269 people (85.4%) had attended school and could read and write. The average time of residence in the Guapimirim municipality was 18.6±13.1 years, the time of residence in the respective locality was 17.7±12.8 years, and that in the current home was 9.8±12 years. A total of 36 individuals (36/324, 11.1%) had changed housing in the last 5 years, 20 of which (6.2%) moved within the municipality.

Previous malaria episodes

Of the 324 participants, 316 (97.5%) never had malaria, 4 (1.2%) did not remember, and 4 (1.2%, 2 women and 2 men) had previously suffered a single episode of malaria. One of these women (age: 69 years) had malaria at the age of 27 years while staying in Mozambique and the other (age: 56 years) had malaria at the age of 6 while staying in the municipality of Mage (state of Rio de Janeiro). One of the previously infected men (age: 70 years) had malaria at the age of 6 years but did not remember the place, while the other man (age: 89 years) had malaria due to P. falciparum infection at the age of 59 years while staying in Rondônia (Amazonian region in Brazil). None of these individuals had symptoms associated with malaria in the year preceding the survey and all of them had negative results in the thick blood smear and PCR tests.

Exposure to malaria

Seventy-eight participants (24.1%) had left the Guapimirim municipality in the 6 months prior to the study, most of them during the last 15 days (41 subjects, 12.7%), and 2 residents went to the Amazon region. Only 10.8% of the participants (35/324) used a mosquito net while sleeping, with 7.7% (25/324) using it always. In terms of visiting the forest region for activities such as collecting fruits, walking, working, or leisure, 75.9% (246/324) of the participants said that they did not regularly enter the forest, 3.1% (10/324) reported going occasionally (mountain climbing or walking), 10.2% (33/324) reported going often, 9.9% (32/324) reported going daily because their homes were in the forest, and 3 participants did not answer this question. Most of the participants (73%) had insufficient knowledge of malaria transmission and prevention.

Laboratory tests

None of the subjects had a positive thick blood smear result. Diagnostic PCR was performed with the samples from 320 (98.8%) individuals. Of these, 9 (2.8%) were positive for Plasmodium infections, 1 for P. falciparum (0.3%), 2 for P. vivax (0.6%), and 6 for P. malariae (1.9%). These 9 individuals did not have symptoms suggestive of malaria and all had negative smears. The individual with a positive PCR result for P. falciparum was a man from the Orindi municipality, and those participants with a positive PCR result for P. vivax were 2 women from the Paradise municipality. The 6 participants with positive PCR results for P. malariae, 3 women and 3 men, were residents of the Orindi, Garrafão, Paradise, and Monte Olivetti locations (Table 1). Although the towns of Paradise and Orindi showed a higher frequency of positive samples, there was no statistically significant association.

TABLE 1: Frequency of Plasmodium infections diagnosed by PCR in the residents of the municipality of Guapimirim, Rio de Janeiro, Brazil, according to the different investigated variables.  

P. falciparum P. vivax P. malariae Total
Pos Neg Pos Neg Pos Neg Pos Neg Total % p-value
Age
<15 years 0 80 0 80 1 79 1 79 80 1,3 0.558
≥15 years 1 239 2 238 5 235 8 232 240 3.3
Total 1 319 2 318 6 314 9 311 320 2.8
Gender
M 1 137 0 138 3 135 4 134 138 2.9 0.794
F 0 182 2 180 3 179 5 177 182 2.7
Total 1 319 2 318 6 314 9 311 320 2.8
Localities
Caneca Fina 0 39 0 39 0 39 0 39 39 0 0.08
Garrafão 0 58 0 58 1 57 1 57 58 1.7
Barreira 0 51 0 51 0 51 0 51 51 0
Monte Olivete 0 16 0 16 1 15 1 15 16 6.3
Paraíso 0 54 2 52 1 53 3 51 54 5.6
Orindi 1 101 0 102 3 99 4 98 102 3.9
Total 1 319 2 318 6 314 9 311 320 2.8
Entry to the Atlantic Forest region for
hunting, leisure, or collecting plants
Yes 0 74 0 74 2 72 2 70 72 2.8 0.7005
No 1 244 2 244 4 242 7 241 248 2.8
Total 1 318 2 318 6 314 9 311 320 2.8
Symptoms in the month preceding the study
Yes 0 29 0 29 0 29 0 29 29 0 -
No 1 290 2 289 6 285 9 282 291 3.1
Total 1 319 2 318 6 314 9 311 320 2.8
Previous malaria
Yes 0 4 0 4 0 4 0 4 4 0 -
No 1 315 2 314 6 310 9 307 316 2.8
Total 1 319 2 318 6 314 9 311 320 2.8

Pos: positive; Neg: negative.

Serology

Serological tests were performed in 314 participants and 35 (11.1%) were reactive for any of the three Plasmodium species studied. The P. falciparum IgG antibody test was reactive in 11 participants (3.5%) and inconclusive in 5 (1.6%), while samples from 24 participants (7.7%) were reactive and 4 (1.3%) inconclusive for P. vivax (anti-IgG PvMSP19 antibody). Due to operational constraints, only 42 samples were tested for P. malariae (all positive PCR samples and/or reactive serology for P. vivax or P. falciparum). Thirteen samples were reactive for P. malariae antibodies (30.9%, 13/42) and 11 samples (29.7%) were reactive for more than one Plasmodium species. Table 2 summarizes the presence of Plasmodium species in positive individuals. Table 3 shows the association between positive PCR samples and serological reactivity for Plasmodium species. Cohen’s kappa coefficient (κ) for agreement between molecular and serological results was considered “moderate” for P. malariae (κ=0.54, range: 0.27-0.82, confidence interval [CI]: 95%) and “fair” for all Plasmodium species (κ=0.29, range: 0.11-0.5, CI: 95%).

TABLE 2: Positive serological reactions for IgG anti-Plasmodium species antibodies in the population of the municipality of Guapimirim. 

Parasite Number %
P. vivax (Pv) 14 40.0
P. falciparum (Pf) 3 8.6
P. malariae (Pm)* 7 20.0
Pv+Pf 5 14.3
Pv+Pm* 3 8.6
Pm+Pf* 1 2.8
Pv+Pf+Pm* 2 5.7
Total 35 100.0

*IgG for P. malariae was assessed only for samples positive in the PCR or serological tests (42 samples).

TABLE 3: Relationship between PCR and serology results of individuals in the municipality of Guapimirim, Rio de Janeiro, Brazil. 

PCR IgG anti-PvMSP119 IgG anti-P falciparum IgG anti-P malariae Any Plasmodium species
(n=312) (n=313) (n=42) (n=314)
Positive Negative Total Positive Negative Total Positive Negative Total Positive Negative Total
P. vivax
Positive 1 1 2 1 1 2 1 1 2 1 1 2
Negative 23 287 310 10 301 311 12 28 40 34 278 312
Total 24 288 312 11 302 313 13 29 42 35 279 314
P. falciparum
Positive 0 1 1 0 1 1 0 1 1 0 1 1
Negative 24 287 311 11 301 312 13 28 41 35 278 313
Total 24 288 312 11 302 313 13 29 42 35 279 314
P. malariae
Positive 1 5 6 0 6 6 6 0 6 6 0 6
Negative 23 283 306 11 296 307 7 29 36 29 279 308
Total 24 288 312 11 302 313 13 29 42 35 279 314

Cohen’s kappa index (κ) for associations between molecular and serological results was 0.07 (range: -0.07-0.20, CI: 95%, p>0.05) for P. vivax, -0.07 (range: -0.02-0.01, CI: 95%, p>0.05) for P. falciparum, and 0.542 (range: 0.27-0.82, CI: 95%, p<0.05, “moderate”) for P. malariae. The same calculations were performed for all Plasmodium species and identified a kappa index of 0.29 (range: 0.11-0.46, CI: 95%, p<0.05, “fair”). IgG: Immunoglobulin G: PvMSP1-19 : 19 kDa C-terminal region of the Merozoite Surface Protein 1 de P. vivax.

Table 4 shows variables associated with IgG reactivity. Individuals who entered the Atlantic Forest region (for hunting, leisure, collecting plants, or other activities) had a 2.7 times increased probability of having a reactive serology for P. vivax compared with individuals who did not enter the forest (p<0.05). On the other hand, children <15 years of age had a higher chance of reactive serology for P. falciparum and P. vivax compared with individuals ≥15 years of age (p<0.05). Individuals living in the Paraiso district had a higher chance of reactive serology for P. vivax and any Plasmodium species (but not specifically for P. falciparum and P. malariae) than people living in other districts (p<0.05). There were no associations between sex, symptoms, or past exposure to malaria and serological response to antibodies of any Plasmodium species. Individuals who reported previous episodes of malaria were not reactive to any species of Plasmodium.

TABLE 4: Reactivity associated with IgG anti-PvMSP119, anti-Pf IgG, and anti-Pm IgG in specimens from residents of the Guapimirim municipality, Rio de Janeiro, Brazil. 

P. vivax (n=312) P. falciparum (n=313) P. malariae (n=42) 1 Any species (n=314)
%Pv uOR (CI: 95%) aOR (CI: 95%) %Pf uOR (CI: 95%) aOR (CI: 95%) %Pm uOR (CI: 95%) aOR (CI: 95%) % uOR (CI: 95%) aOR (CI: 95%)
Age
≥15 years 6.3 0.49(0.20-0.18) 0.36(0.14-.96)* 2.1 0.25(0.07-0.8)* 0.03(0.00-0.09)* 29.0 0.71(0.16-3.06) 0.93(0.73-1.16) 9.7 0.57(0.27-1.21) 0.52(0.23-1.19)
<15 years 12 - - 7.9 - - 36.9 - - 15.8 - -
Total 7.7 - - 3.5 - - 30.9 - - 11.1 - -
Sex
Male 7.5 0.96(0.41-.23) - 3.8 1.13(0.34-3.8) - 21.0 0.38(0.09-1.56) - 11.2 1.01(0.49-2.06) -
Female 7.8 - - 3.3 - - 39.1 - - 11.1 - -
Total 7.7 - - 3.5 - - 30.9 - - 11.1 - -
Entry to the forest for hunting, leisure, or collecting plants
Yes 14.3 2.71(1.15-6.4)* 2.73(1.01-7.38)* 4.3 1.31(0.34-5.2) 1.02(0.54-2.20) 44.4 0.98(0.24-4.07) 0.69(0.54-0.86) 17 1.99(0.93-4.23) 2.09(0.87-4.84)
No 5.8 - - 3.3 - - 45.0 - - 9.4 - -
Total 7.7 - - 3.5 - - - - 11.1 - -
Housing location
Barrera 2.0 1 1 0 1 1 33.3 1 1 3.8 1 1
Monte Oliveti 13.3 1.29(0.41-9.1) 12.62(1.06-297) 0 ** - 100.0 ** - 13.3 1.24(0.27-5.74) 5.01(0.55-46.52)
Garrafão 3.7 0.41(0.09-1.8) 2.53(0.23-56.42) 0 ** - 20.0 0.52(0.05-5.20) - 5.5 0.41(0.12-1.38) 1.55(0.24-12.25)
Caneca Fina 15.8 2.67(0.98-7.2) 8.71(1.31-173.74 7.9 2.86(0.72-11.3) - 42.9 1.87(0.35-9.93) - 18.4 2.00(0.81-4.21) 5.17(1.11-37.29)
Orindi 5.9 0.68(0.26-1.7) 3.48(0.55-67.97) 4 1.21(0.35-4.22) - 23.1 0.57(0.127-2.55) - 11.9 1.11(0.53-2.34) 3.69(0.93-24.73)
Paraiso 13.2 2.16(0.85-51) 10(1.62-94.85)* 7.5 2.95(0.83-10.5) - 25.0 0.67(0.147-3.02) - 17 1.83(0.81-4.21) 5.98(1.42- 41.21)*
Total 7.7 - - 3.5 - - 31.0 - - 11.1 - -
Symptoms in the month preceding the study
Yes 3.8 0.46(0.06-.53) - 3.8 1.10(0.13-9.01) 0 ** - 3.8 0.29(0.04-2.27) -
No 8 - - 3.5 - - 30.9 - - 11.8 - -
Total 7.7 - - 3.5 - - 30.9 - - 11.1 - -
Previous malaria
Yes 0 ** - 0 ** - 0 ** - 0 ** -
No 7.8 - - 3.6 - - 30.9 - - 11.3 - -
Total 7.7 - - 3.5 - - 30.9 - - 11.1 - -

*p<0.05, **Undefined. CI: confidence interval. aOR: adjusted odds ratio, uOR: unadjusted odds ratio. 1Only 42 samples.

DISCUSSION

The prevalence of autochthonous malaria is low in Southeastern Brazil and in the Southern states of the Atlantic Forest region18-20. The characteristics of malaria in these areas are different from those observed in the Amazon region18. The extra-Amazonian malaria is often described as asymptomatic, with low parasitemia and predominance of the species P. vivax20-23. The municipality of Guapimirim is very near to the mountain ecosystem of the Atlantic Forest region, and our study was carried out in areas where malaria cases had been reported. Our results showed that the majority of the study population had lived in the municipality for the last 18 years and only 2 participants (0.6%) had a history of travel to an endemic area but without acquiring malaria. Only 1.2% had a single previous malaria episode in the past, but all of them showed negative PCR, thick blood smear, and serology results. In contrast to our expectations only one of these people had contracted malaria in the Amazon region Rondônia.

We observed no human malaria symptomatic cases and all thick and thin blood smears were negative for Plasmodium species. These results are not unexpected because data from the governmental surveillance system have demonstrated that cases occur throughout the year, with an annual average of 4 or fewer cases across the state particularly during the warmer season7,23. Interestingly, malaria outbreaks associated with P. simium occurred in areas close to the municipality of Guapimirim in the summers of 2015 and 20169. In our study, 9 individuals (2.8%) with asymptomatic Plasmodium infection were diagnosed using PCR analysis, 2 of which were positive for P. vivax (0.6%), 1 (0.3%) for P. falciparum, and 6 for P. malariae (1.9%). None of these individuals developed clinical malaria within 30 days of follow-up. Although most of the infected individuals live in the towns of Orindi and Paradise, there were no significant differences between the assessed locations, probably due to the small number of positive samples. It is important to note the presence of asymptomatic infection in individuals with no previous history of malaria, travel to endemic areas, or contact with a previous malaria-infected person. We speculated that these subclinical infections were probably autochthonous cases. Only a few studies on asymptomatic infection by Plasmodium species in the extra-Amazon region have been carried out to date. Curado et al.24 demonstrated the presence of asymptomatic infections with P. falciparum and mixed malaria caused by P. falciparum and P. vivax in the Atlantic Forest region in São Paulo state. Cerutti et al.20 conducted a population-based study in the Espírito Santo state and obtained similar results. No parasites were detected by microscopy in 1,777 blood samples from residents of the area with reported malaria cases, but asymptomatic infections with P. vivax (1.5%), P. malariae (0.9%), and P. falciparum (0.5%) were diagnosed by PCR. De Alencar et al.25 in Espírito Santo state found a prevalence of 3.4% of P. malariae and P. vivax infections. Maselli et al26 revealed a prevalence of P. falciparum (5.14%) and P. vivax (2.26%) infections in healthy blood donors in São Paulo state. Donors with asymptomatic Plasmodium infection could be reservoirs of transfusion-transmitted malaria (TTM). As these individuals have low parasitic infections with negative thick blood smear results, detection can be difficult using routine laboratory tests26. In our study, we identified 2.8% of participants with asymptomatic infections, 6 of which were infected with P. malaria, the most frequent parasite associated with TTM in the Americas. Thus, further studies should be conducted to identify risk factors in these asymptomatic individuals, to establish diagnostic methods, and to prevent TTM.

In contrast to asymptomatic Plasmodium infections that can only be detected by PCR, the presence of anti-Plasmodium antibodies can reveal recent or past infections27. In our study, 11.8% of specimens were reactive to any of the tested antigens, with 3.5% reactive to P. falciparum, 7.7% to P. vivax, and 30.9% of a subsample of 42 individuals reactive to P. malariae. The frequency of reactivity was low in relation to the study of Azevedo (1997, unpublished data) in which 47.8% of samples from the outbreak of 1993 in the Rio Bonito district showed reactivity of IgG antibodies and 17.4% of IgM antibodies (indirect immunofluorescence). These studies were repeated in the area in 1996 with a higher number of subjects and revealed a frequency of 35.4% of IgG antibodies against the asexual blood forms of P. vivax, while no IgM antibodies were found. These findings were corroborated by Mattos et al.6 Cerutti et al.20 performed a cross-sectional study of 65 patients and all specimens showed a positive reaction to all the variants of antibodies against the circumsporozoite protein of P. vivax, with positive reactions to P. vivax classic (VK210) in 25.4%, to VK247 in 6.3%, to P. vivax-like in 10%, and to P. malariae in 15.1% specimens. The same authors assessed healthy individuals that had been in contact with malaria patients and observed a high percentage of antibodies to P. vivax (37.7% IgG and 6.2% IgM) and P. malariae (44.6% IgG and 15.8% IgM) and a prevalence of P. falciparum reactivity of 13.5% for IgM and 13% for IgG antibodies against asexual forms of the parasite. With our results, we cannot conclude that serology can predict infection.

The main finding of our study is the association between entering the forest and positive serology for Plasmodium vivax. The chance to have a positive serology was 2.7 times (range: 1.01-7.38, CI: 95%, p<0.05) higher for people who entered the forest compared with those that did not. These results differ from those obtained by Maselli et al.26 who showed a positive association between residing in the mountain area of the Atlantic Forest region with P. falciparum and P. vivax infections and forest fragmentation. Nonetheless, the study was carried out with samples from blood donors in São Paulo state and the origin of samples could explain the divergences from our results.

The presence of asymptomatic P. falciparum infections detected in our study is a cause of concern, but other authors20,24,26made the same observation in the Espírito Santo and São Paulo states. Recently, Laporta et al.28 showed an unexpectedly high proportion of P. falciparum on anophelines in the São Paulo state. These results are challenging the traditional “bromeliad-malaria” paradigm that proposes a sylvatic cycle in bromeliad areas with an interaction between an An. kerteszia vector (particularly An. cruzii), a non-human primate reservoir (Alouatta species, Brachyteles species, and Cebinae subfamilies), and a P. simium or P. vivax parasite9,30-35. As PCR used in our study cannot differentiate between a P. vivax and P. simium infection, more sophistical analyses are required. Humans could be accidentally infected when they enter the forest for recreation or work by invading the transmission cycle of the parasite in the wild36-40. On the other hand, P. falciparum is a virulent Plasmodium species in the Amazon region and any infection by this parasite should result in malaria disease if the individual is not immune. The fact that individuals do not develop symptoms in the extra-Amazonian region may be due to antigenic variability of P. falciparum, but this question must be further clarified in future studies involving sequencing of parasite DNA29,41. Another important and unexpected finding of our study was the higher chance of obtaining positive serologies for P. falciparum and P. vivax in individuals under 15 years of age compared to older individuals. No previous study has reported a similar trend, and our findings may present a novel epidemiological scenario in this context that needs to be elucidated in future studies.

Enhancing the knowledge on malaria in residents of the extra-Amazonian areas is an important aspect of disease elimination. Although transmission in this area was interrupted 50 years ago5, 73% of actual residents have insufficient knowledge on malaria transmission and prevention. Azevedo et al.6 found a similar percentage in the Nova Friburgo region. It is noteworthy that 79.3% of participants associated the summer with an increase in mosquito density particularly at nightfall, but a large proportion of the population did not use any control measures for avoiding insect bites. Malaria diagnosis is a big challenge in these areas because medical doctors often do not consider the disease when a patient presents with fever22.

According to the WHO, residual malaria occurs sporadically in places with interrupted transmission and remaining determinants of transmission42. These areas require assessment of favorable locations for Anopheles larvae habitats, food sources, and invasion of Anopheles by Plasmodium. All these conditions are present in the Atlantic Forest region, but on a much smaller scale compared to the Amazon region36-40. Albuquerque et al. 43 recently constructed a model for evaluation of territory receptivity in order to strengthen entomological surveillance of Nyssorhynchus mosquitoes for imported malaria cases. The study showed that pluviosity, temperature, geomorphology, and vegetation variables could be used to create a model of surveillance, but a specific model needs to be generated for Kerteszia anophelines responsible for bromeliad malaria in these areas.

Acknowledgments

We offer our deepest thanks to the institutions that provided technical support for our study. We thank Arianni Rondelli Sanchez and Eduardo Milton Ramos-Sanchez for preparing the P. vivax recombinant, Fabiana Maria de Souza Leoratti for preparing the extract, Dr. Silvia Di Santi for blade preparation of P. malariae and Dr. Daniel Câmara for statistical analysis. Special thanks to the research participants in the Guapimirim municipality.

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Financial Support: This research was funded by Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ, financial support process 214879/E_06/2015) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, financial support 001).

Received: December 17, 2018; Accepted: February 19, 2019

Corresponding author: Dra. Martha Cecilia Suárez-Mutis. e-mail: marmutis@ioc.fiocruz.br / marthasuarezmutis@gmail.com

Conflict of Interest: The authors declare that there are no conflicts of interest.

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