Evaluation of a real-time quantitative PCR to measure the wild Plasmodium falciparum infectivity rate in salivary glands of Anopheles gambiae
© Marie et al.; licensee BioMed Central Ltd. 2013
Received: 18 March 2013
Accepted: 4 June 2013
Published: 2 July 2013
Evaluation of malaria sporozoite rates in the salivary glands of Anopheles gambiae is essential for estimating the number of infective mosquitoes, and consequently, the entomological inoculation rate (EIR). EIR is a key indicator for evaluating the risk of malaria transmission. Although the enzyme-linked immunosorbent assay specific for detecting the circumsporozoite protein (CSP-ELISA) is routinely used in the field, it presents several limitations. A multiplex PCR can also be used to detect the four species of Plasmodium in salivary glands. The aim of this study was to evaluate the efficacy of a real-time quantitative PCR in detecting and quantifying wild Plasmodium falciparum in the salivary glands of An. gambiae.
Anopheles gambiae (n=364) were experimentally infected with blood from P. falciparum gametocyte carriers, and P. falciparum in the sporozoite stage were detected in salivary glands by using a real-time quantitative PCR (qPCR) assay. The sensitivity and specificity of this qPCR were compared with the multiplex PCR applied from the Padley method. CSP-ELISA was also performed on carcasses of the same mosquitoes.
The prevalence of P. falciparum and the intensity of infection were evaluated using qPCR. This method had a limit of detection of six sporozoites per μL based on standard curves. The number of P. falciparum genomes in the salivary gland samples reached 9,262 parasites/μL (mean: 254.5; 95% CI: 163.5-345.6). The qPCR showed a similar sensitivity (100%) and a high specificity (60%) compared to the multiplex PCR. The agreement between the two methods was “substantial” (κ = 0.63, P <0.05). The number of P. falciparum-positive mosquitoes evaluated with the qPCR (76%), multiplex PCR (59%), and CSP-ELISA (83%) was significantly different (P <0.005).
The qPCR assay can be used to detect P. falciparum in salivary glands of An. gambiae. The qPCR is highly sensitive and is more specific than multiplex PCR, allowing an accurate measure of infective An. gambiae. The results also showed that the CSP-ELISA overestimates the sporozoite rate, detecting sporozoites in the haemolymph in addition to the salivary glands.
In malaria endemic countries, Plasmodium falciparum is transmitted to the human host by the bite from a female Anopheles mosquito. Anopheles gambiae sensu stricto (s.s.) is the most widespread malaria vector throughout the afrotropical belt. In the context of malaria eradication, it is essential for malaria-surveillance programmes to estimate accurately the risk of malaria transmission. Currently, the main indicator of Plasmodium transmission is the measure of the entomological inoculation rate (EIR) , which is the number of infective mosquito bites per human per night. In field settings, the EIR is commonly estimated by using captured adult mosquitoes. Evaluation of infection prevalence in salivary glands can be measured by counting sporozoites by microscopy  or by using the enzyme-linked immunosorbent assay on the head-thorax of the mosquito to detect the surface circumsporozoite protein (CSP-ELISA) . Both methods are known to be labour intensive and it has been shown that CSP-ELISA overestimates the real infection rate by detecting the CSP from the oocysts bursting, two to three days before the sporozoites actually reach the salivary glands [2, 4].
Research efforts in recent decades have led to the development of molecular biology tools for detecting Plasmodium falciparum in human blood  and in mosquito samples . Among these, a multiplex PCR was developed by Padley et al to detect the four major species of Plasmodium (P. falciparum, Plasmodium malariae, Plasmodium ovale, and Plasmodium vivax) in human blood samples  and was applied to detect them in Anophele s mosquitoes. Multiplex PCR is based on the detection of a Small SubUnit of ribosomal RNA (SSU rRNA) of each Plasmodium species but it requires a significant amount of parasite DNA, which is not easily achieved with small tissues like a single pair of salivary glands. Specific and sensitive methods such as quantitative PCR (qPCR) have also been developed to measure the prevalence and intensity of infection in human blood samples [8, 9]. In mosquito samples, quantification of P. falciparum oocysts in Anopheles stephensi and in wild An. gambiae s.s. has also been achieved through real-time PCR. The latter study evaluated the difference in susceptibility of malaria infection (oocyst stage) between the M and S molecular form of An. gambiae s.s. in Cameroon. In addition, Vernick et al estimated the infection prevalence of P. falciparum (parasite culture) in An. gambiae (insectary-reared mosquitoes) by reverse transcriptase PCR using specific sequences of the Small SubUnit of ribosomal RNA (SSU rRNA) of the sporogonic stages. Recently, a duplex real-time PCR was developed for the detection of the four Plasmodium species in field mosquitoes from Benin based on species-specific primers and probes for the gene encoding the small subunit (18S) of Plasmodium rRNA . However, in this study, the use of the head-thorax of mosquitoes leads to an inaccurate estimation of the EIR, which should be based only on the sporozoites present in salivary glands.
Therefore, it is important to develop sensitive and rapid diagnostic tools for detecting Plasmodium in salivary glands of the Anopheles vectors, as this will reveal the true proportion of infective mosquitoes and, consequently, only those that can transmit malaria parasites. The aim of the present study was to evaluate the sensitivity and the specificity of a quantitative PCR method in the detection of wild P. falciparum sporozoites in An. gambiae salivary glands. First, the qPCR assay based on the mitochondrial cytochrome c oxydase subunit 1 (COX-1) gene described by Boissiere et al  was tested on infected salivary glands to detect and quantify P. falciparum. A comparison of the qPCR method with a multiplex PCR based on the Padley method was also made to identify the most sensitive method. Finally, a comparison of the infectivity rates obtained with these two techniques with those obtained with the CSP-ELISA was performed on the carcasses of mosquitoes without salivary glands. CSP-ELISA was considered the current reference method used in the field. In this paper, experiments were conducted in semi-field conditions. Anopheles gambiae mosquitoes were fed on blood from asymptomatic children containing high similar gametocyte densities (from 52.7 to 60.6 gametocytes/μL). In natural settings, mosquito infectivity rate depends on several factors such as gametocyte density, sex ratio and multiclonality of parasites [14–17]. In consequence, this original approach allowed to mimic field conditions, and thereby to evaluate the potential application of this qPCR in field settings. Data showed that qPCR is highly sensitive but more specific than the multiplex PCR. Moreover, this study confirmed that the CPS-ELISA overestimates the infectivity rate by detecting the circulating sporozoites in addition to those present in salivary glands.
All procedures involving human subjects used in this study were approved by the Cameroonian National Ethical Committee (statement 099/CNE/SE/09). Children identified as gametocyte carriers were enrolled as volunteers after their parents or legal guardians have signed an informed consent form.
The Kisumu strain of An. gambiae was provided by the Laboratoire de Lutte contre les Insectes Nuisibles, Institut de Recherche pour le Développement, France. The colony was established and maintained at the insectary in OCEAC (Yaoundé, Cameroon) for the experimental infections. Adult mosquitoes were maintained in standard insectary conditions (27±2°C, 85±5% RH, and 12 h light/dark) and provided with 6% sterile sucrose solution.
Experimental infections and salivary gland dissection
Female mosquitoes were fed on P. falciparum gametocyte carriers. Infectious feeding was performed as previously described [18, 19]. Females, three to five days old, were starved for 24 h and allowed to feed on human blood containing P. falciparum gametocytes for 35 min. Unfed and partially fed mosquitoes were removed by aspiration and discarded. Fully engorged females were kept in the insectary until dissections 14 days after the infectious blood meal. Mosquitoes were cold-anaesthetized and salivary glands were dissected in 10 μL of buffer containing 7 M urea, 2 M thiourea, and 4% CHAPS (GE, Healthcare). Samples were kept frozen individually at −20°C until processing.
After the dissection of salivary glands, the carcass-thorax-head were tested by ELISA for the presence of P. falciparum CSP as described by Burkot and modified by Wirtz et al. The monoclonal antibody and positive controls were provided by the Centers for Disease Control and Prevention (CDC, Atlanta, GA, USA). Mosquitoes were considered positive when the optical density (OD) was higher than the mean plus three standard deviations of the negative controls (OD=0.059).
DNA extraction from the salivary glands was performed using DNAzol® (Molecular Research Center, Inc, Cincinnati, OH, USA) according to the manufacturer’s instructions. Extracted DNAs were eluted in a final volume of 20 μL water and were stored at –20°C. DNA extraction was checked for the presence of mosquito DNA by specific PCR for An. gambiae species .
Identification of Plasmodium falciparum by multiplex PCR
The infection status of P. falciparum was determined by multiplex PCR as previously described  based on the detection of a Small SubUnit of ribosomal RNA of each Plasmodium species with five primers: universal reverse Plasmodium primer 5′-GTATCTGATCGTCTTCACTCCC-3’, P. malariae forward 5′-CGTTAAGAATAAACGCCAAGCG-3′, P. falciparum forward 5′- ACAGACGGGTAGTCATGATTGAG-3′, P. ovale forward 5′-CTGTTCTTTGCATTCCTTATGC-3′, and P. vivax forward 5′-CGGCTTGGAAGTCCTTGT-3′. PCR was performed on 5 μL of eluted DNA with the Taq Hot Start Master mix (Qiagen, Valencia, CA, USA) following the manufacturer’s instructions. PCR amplification was carried out under the following conditions: an initial incubation cycle to activate the enzyme for 45 sec at 95°C followed by 43 cycles of amplification involving 45 sec at 95°C, 90 sec at 60°C and a final extension of 5 min at 72°C.
Quantitative real-time PCR
qPCR was performed on 1 μL of eluted DNA with the EvaGreen® dye (5X HOT Pol EvaGreen® qPCR Mix Plus (ROX), Euromedex, Souffelweyersheim, France) in the 7300 Real-Time PCR System (Applied Biosystems, Foster City, CA, USA). Specific primers used for the qPCR were 5′-TTACATCAGGAATGTTATTGC-3′ and 5′-ATATTGGATCTCCTGCAAAT-3′ [9, 22]. They amplified a 120-bp sequence of the P. falciparum cytochrome c oxidase subunit 1 (Cox1) mitochondrial gene. The reaction mixture was prepared following the procedure of Boissière et al. Absolute qPCR was performed following the amplification program of an initial melting cycle for 15 min at 95°C followed by 40 amplification cycles at 95°C for 15 sec and 58°C for 30 sec. The melting temperature was determined using a dissociation curve. Curves were generated after amplification: at 95°C for 15 sec (DNA denaturation), at 60°C for 30 sec (double stranded DNA), and at 95°C for 15 sec (single stranded DNA). Fluorescence was monitored allowing the identification of the specific melting point. As described by Boissière et al, standard curves using 3D7 strain DNA were generated from serial dilution methods and resulting in a quantification range of 6 to 60,000 genomes/μL. These standards were used to determine the concentration of sporozoites in the salivary glands of An. gambiae.
Statistical analyses were performed using the statistical software R , and all differences were considered significant at P values of <0.05. The means of the amplification efficiencies between the standard samples and the salivary gland samples were compared using the Mann-Whitney-Wilcoxon test. Cohen’s kappa co-efficient (κ) was calculated to measure the agreement between the qPCR and multiplex PCR. Methods were compared using the McNemar test.
Comparison of qPCR and multiplex PCR techniques for detection of Plasmodium falciparum sporozoites in salivary glands of Anopheles gambiae
Salivary glands samples
Some multiplex qPCR assays have also used the EvaGreen® dye, as it was done in the present study [29, 30]. Therefore, it seems possible to adapt the present qPCR method so as to carry out multiplex qPCR detection of the four species of Plasmodium. Evagreen® dye is a DNA-binding dye with many features that make it superior to the SYBR® Green I for qPCR [29, 31]. Furthermore, this dye is compatible with all common real-time PCR cyclers  and is currently about half the price (€0.16 per reaction) of the SYBR® Green (€0.53 per reaction) commonly used. The duplex qPCR performed by Sandeu et al used the Taqman technique (€1.12 per reaction). In conclusion, the qPCR developed here is cost-effective and therefore suitable for large field studies. It is also cheaper than the multiplex PCR (€1.60 per reaction).
Comparison of CSP-ELISA with qPCR and multiplex PCR for detection of Plasmodium falciparum sporozoites
Estimation of malaria transmission requires sensitive and specific tools for the evaluation of infective mosquitoes, i e, detection of sporozoites in Anopheles salivary glands. This study showed that real-time quantitative PCR can be used to detect and quantify sporozoites of wild P. falciparum in the salivary glands of An. gambiae. This qPCR can be performed on small samples such as the DNA of P. falciparum sporozoites extracted from a single pair of salivary glands of An. gambiae with a sensitivity of six genomes/μL. In the present study, the real-time quantitative PCR was compared for the first time with multiplex PCR and CSP-ELISA methods.
qPCR is highly sensitive but more specific than multiplex PCR. Moreover, qPCR with EvaGreen® dye is reliable, reproducible, and cost-effective. This method is feasible for evaluating the P. falciparum infection rate in the salivary glands and it can lead to an accurate estimation of the risk of transmission in field settings, which were overestimated by CSP-ELISA. Improving the estimation of the EIR with this method could have significant implications on vector control strategies and on the evaluation of their effectiveness.
We thank Antoine Berry at Service de Parasitologie-Mycologie, Centre Hospitalier Universitaire de Toulouse, Hôpital Rangueil, (Toulouse, France) for the parasite culture and M.N. Lacroix at Laboratoire de lutte contre les insectes nuisibles (Montpellier, France). We are grateful to volunteers from Mfou primary schools and their parents or guardians for participating in this study, to the medical team from the Mfou hospital for assistance in the field, and to the technical staff of the OCEAC for P. falciparum infections. We are particularly grateful to Etienne Onana from the OCEAC for mosquito rearing and salivary glands dissections.
This work was supported by funds from the Institut de Recherche pour le Développement (IRD) and the European Community’s Seventh Framework Programme (FP7) (grant agreement N° 242095, EviMalaR). AM was supported by an EviMalaR scholarship. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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