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      Inter-epidemic Acquisition of Rift Valley Fever Virus in Humans in Tanzania

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          Abstract

          Background

          In East Africa, epidemics of Rift Valley fever (RVF) occur in cycles of 5–15 years following unusually high rainfall. RVF transmission during inter-epidemic periods (IEP) generally passes undetected in absence of surveillance in mammalian hosts and vectors. We studied IEP transmission of RVF and evaluated the demographic, behavioural, occupational and spatial determinants of past RVF infection.

          Methodology

          Between March and August 2012 we collected blood samples, and administered a risk factor questionnaire among 606 inhabitants of 6 villages in the seasonally inundated Kilombero Valley, Tanzania. ELISA tests were used to detect RVFV IgM and IgG antibodies in serum samples. Risk factors were examined by mixed effects logistic regression.

          Findings

          RVF virus IgM antibodies, indicating recent RVFV acquisition, were detected in 16 participants, representing 2.6% overall and in 22.5% of inhibition ELISA positives (n = 71). Four of 16 (25.0%) IgM positives and 11/71 (15.5%) of individuals with inhibition ELISA sero-positivity reported they had had no previous contact with host animals. Sero-positivity on inhibition ELISA was 11.7% (95% CI 9.2–14.5) and risk was elevated with age (odds ratio (OR) 1.03 per year; 95% CI 1.01–1.04), among milkers (OR 2.19; 95% CI 1.23–3.91), and individuals eating raw meat (OR 4.17; 95% CI 1.18–14.66). Households keeping livestock had a higher probability of having members with evidence of past infection (OR = 3.04, 95% CI = 1.42–6.48) than those that do not keep livestock.

          Conclusion

          There is inter-epidemic acquisition of RVFV in Kilombero Valley inhabitants. In the wake of declining malaria incidence, these findings underscore the need for clinicians to consider RVF in the differential diagnosis for febrile illnesses. Several types of direct contact with livestock are important risk factors for past infection with RVFV in this study’s population. However, at least part of RVFV transmission appears to have occurred through bites of infected mosquitoes.

          Author Summary

          Rift Valley fever (RVF) is a disease of animals and people that is caused by the RVF virus. During epidemics, humans get RVF through direct contact with animals or through mosquito bites. In East Africa, epidemics occur every 5–15 years following unusually high rainfall. In between epidemics, the transmission of RVF might occur at low level. In an epidemic-free period, we measured whether people in the Kilombero Valley in Tanzania had evidence of past and recent RVF infection in their blood sample, and studied risk factors. Three per cent of people had been infected recently, and 12% had evidence of past infection, with increased risk with age, among milkers and among people eating raw meat. Some people with past or recent infection reported they had not had contact with animals. Households keeping livestock had more members with evidence of past infection. The findings show that people get infected with RVF in between epidemics, and that various types of contact with livestock are important risk factors. There is also evidence that some people get infected with RVFV by mosquitoes in the epidemic free period. Clinicians in the Kilombero Valley should consider RVF in the differential diagnosis of patients with fever.

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          Most cited references21

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          Rift Valley fever epidemic in Saudi Arabia: epidemiological, clinical, and laboratory characteristics.

          This cohort descriptive study summarizes the epidemiological, clinical, and laboratory characteristics of the Rift Valley fever (RVF) epidemic that occurred in Saudi Arabia from 26 August 2000 through 22 September 2001. A total of 886 cases were reported. Of 834 reported cases for which laboratory results were available, 81.9% were laboratory confirmed, of which 51.1% were positive for only RVF immunoglobulin M, 35.7% were positive for only RVF antigen, and 13.2% were positive for both. The mean age (+/- standard deviation) was 46.9+/-19.4 years, and the ratio of male to female patients was 4:1. Clinical and laboratory features included fever (92.6% of patients), nausea (59.4%), vomiting (52.6%), abdominal pain (38.0%), diarrhea (22.1%), jaundice (18.1%), neurological manifestations (17.1%), hemorrhagic manifestations (7.1%), vision loss or scotomas (1.5%), elevated liver enzyme levels (98%), elevated lactate dehydrogenase level (60.2%), thrombocytopenia (38.4%), leukopenia (39.7%), renal impairment or failure (27.8%), elevated creatine kinase level (27.3%), and severe anemia (15.1%). The mortality rate was 13.9%. Bleeding, neurological manifestations, and jaundice were independently associated with a high mortality rate. Patients with leukopenia had significantly a lower mortality rate than did those with a normal or high leukocyte count (2.3% vs. 27.9%; odds ratio, 0.09; 95% confidence interval, 0.01-0.63).
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            An Outbreak of Rift Valley Fever in Northeastern Kenya, 1997-98

            In December 1997, 170 hemorrhagic fever-associated deaths were reported in Carissa District, Kenya. Laboratory testing identified evidence of acute Rift Valley fever virus (RVFV). Of the 171 persons enrolled in a cross-sectional study, 31(18%) were anti-RVFV immunoglobulin (Ig) M positive. An age-adjusted IgM antibody prevalence of 14% was estimated for the district. We estimate approximately 27,500 infections occurred in Garissa District, making this the largest recorded outbreak of RVFV in East Africa. In multivariate analysis, contact with sheep body fluids and sheltering livestock in one’s home were significantly associated with infection. Direct contact with animals, particularly contact with sheep body fluids, was the most important modifiable risk factor for RVFV infection. Public education during epizootics may reduce human illness and deaths associated with future outbreaks.
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              Interepidemic Rift Valley Fever Virus Seropositivity, Northeastern Kenya

              Rift Valley fever (RVF) is a mosquito-borne zoonosis that is expanding its range in Africa and the Middle East. Economic effects can be catastrophic for meat and dairy producers, e.g., high illness and mortality rates among affected livestock herds ( 1 , 2 ) prompting World Organization for Animal Health–mandated international embargoes of livestock exports. These epidemics are even more devastating for pastoral nomads and local herders; many adult animals can die, affecting the next crop of newborns and the survival of locals who are economically and physically dependent on milk and meat during the epidemic. During large RVF outbreaks, extensive numbers of human infections occur as well, leading to substantial healthcare challenges in resource-limited settings. RVF symptoms in persons are typically fever, myalgia, and malaise; in a noteworthy minority of cases retinitis, encephalitis, hemorrhagic fever, and death occur. Overall mortality rate is ≈1% ( 3 , 4 ). RVF is caused by the phlebovirus, Rift Valley fever virus (RVFV), which was originally isolated in Kenya and is endemic to other countries of East Africa, South Africa, and the Senegal River valley ( 3 , 5 – 7 ). The virus, introduced repeatedly into Egypt since the 1970s, and most recently into the Arabian peninsula (Yemen and Saudi Arabia) in 2000 ( 8 – 10 ), is embedded in ecosystems by vertical transmission in certain floodwater Aedes mosquito species ( 1 ). Consequently, RVF outbreaks are strongly linked to excessive rainfall and local flooding. The most recent Kenyan Rift Valley fever outbreak occurred during El Niño rains from November 2006 through April 2007 ( 11 , 12 ). The largest RVF outbreak in Kenya took place in an El Niño–related flooding period in 1997–1998 ( 13 ). Even within different climate zones, RVFV transmission may vary considerably as a function of fine-scale differences in local environment. Evidence of prior RVFV infection can be tested by ELISA for anti-RVFV immunoglobulin (Ig) G ( 14 , 15 ). Earlier studies have shown that RVFV seroprevalence in Kenyan populations has been as high as 32% in high-risk areas during epidemics ( 13 ). During interepidemic periods, observed community RVFV seroprevalence rates have ranged from 1% to 19% in different settings within Kenya ( 16 ). Because RVF outbreaks typically occur in remote locations under extreme weather conditions, relatively little is known about the underlying health status of at-risk communities. Likewise, debate continues regarding the likely dominant mode of animal-to-human transmission during combined epizootics and epidemics. RVFV reemergence, caused by floodwater mosquitoes, is followed by widespread amplification in high-risk animal populations and progressively greater prevalence among animals. When epizootic conditions are right, additional mosquito species will feed on viremic animals and subsequently transmit RVFV to humans, creating a potential epidemic. Humans can also become infected through exposure to infectious animal tissues or bodily fluids such as abortus, birthing fluids, milk, or blood. Among pastoral nomads and other herders in the semiarid regions of Africa, family members could be differentially exposed depending on traditional gender-specific duties, thereby altering the risk-modifying effects of age or gender. Specific types of animal exposure that are the most risky, and important nonanimal exposures have not yet been elucidated. Knowing which forms of exposure provide the greatest RVFV transmission risk may be useful for endemic or epidemic public health education and for targeting interventions (such as animal vaccination) that can decrease infection or illness during an epidemic. The goals of this study were to 1) determine the baseline human population health status in an area that has suffered repeated RVF outbreaks; 2) identify which animal and nonanimal exposures are associated with RVFV seropositivity; 3) evaluate whether seropositivity, exposures, and risks differ among town and village settings in a high-risk region of northeastern Kenya; and 4) assess whether interepidemic human RVFV transmission occurs. Materials and Methods Location Our study was a location-stratified household-based cluster sampling of human populations residing in 2 areas near Masalani Town, Ijara District, situated in a semiarid region of Northeastern Province, Kenya. The study was performed in March and April 2006, ≈8.5 years after the previous RVF outbreak of 1997–1998, and well before the floods during the fall of 2006 that were associated with the most recent RVF epizootic/epidemic. On the basis of our study objectives, the balanced sampling frame for selection of the planned 250 participants was divided between a rural village, Gumarey (centered at 1° 40′12′′S, 40°10′48′′E), and a town, Sogan-Godud (centered at 1°41′24′′S, 40°10′12′′E). Both are sublocations defined within the Kenya Census and are located within 500 m of each other and within 10 km of the Tana River, which is prone to flooding during periods of excessive rainfall. Flatness of the local terrain, combined with poor drainage, makes the area a prime environment for RVFV transmission during floods, as evidenced by ongoing RVF outbreaks. Gumarey has a largely seminomadic pastoralist population, and local homes are traditional grass huts. Sogan-Godud is a larger town with more permanent tin-roofed dwellings and stores (Figure 1). Figure 1 Photographs depicting differences between sublocations in northeastern Kenya. Sogan-Godud (A) has more permanent dwellings and stores with tin-roofed buildings. Gumarey (B) has more semipermanent traditional dwellings and animal grazing areas. Population Study recruitment was begun after consultation and approval by local administrators and religious leaders. After an initial demographic census was conducted to determine the current local population and its distribution, 270 survey participants were selected by randomized cluster sampling of households in the 2 designated subsections of Masalani town. Children 7 years of age provided individual assent. The study sample comprised a locally representative ethnic mix of >99% Somali or Bantu and 5 years of age and 1 mL from children mean + 2 standard deviations for control serum and absolute value >0.2 were deemed positive. Each sample was run in duplicate, and OD values were averaged. Any OD discrepancy between duplicate tests was resolved by repeat testing. Pooled RVFV-positive serum samples were used as the positive plate controls, and pooled RVFV-negative North American serum samples were used as the negative plate controls. Serologic screening was performed at the Division of Vector-Borne Diseases in Nairobi and confirmed at Case Western Reserve University; correlation of results was excellent. Confirmatory plaque reduction neutralization test (PRNT) was performed at University of Texas Medical Branch at Galveston to assess the risk of false-positive results secondary to ELISA cross-reactivity with related viruses. Confirmatory testing using PRNT was performed on all positive samples (n = 33) and an age- and location-matched set of negative samples (n = 33) ( 17 ). All ELISA-positive samples had PRNT titers >80; most had titers of 320. All but 1 ELISA-negative sample had titers 15 years of age were more at risk, p = 0.0001), gender (male participants were more at risk, p = 0.011), location (those from Gumarey were more at risk, p = 0.001), home flooding (p = 0.024); contact with a dead human body (p = 0.0001); contact with cattle (p = 0.012); and involvement in sheltering (p = 0.003), butchering (p = 0.0001), skinning (p = 0.0001), cooking (p = 0.005), milking (p = 0.0001), birthing livestock (p = 0.0001), or disposing of an aborted animal fetus (p = 0.0001). Other reported exposures varied significantly between the 2 sublocation groups. Those from Sogan-Godud were more likely to have used mosquito nets (odds ratio [OR] 5.2, p = 0.0001) and mosquito coils (OR 8.2, p = 0.0001) to reduce insect exposure. Those from Gumarey were more likely to have had goat contact (OR 2.6, p = 0.046), had cattle contact (OR 4.7, p = 0.0001), consumed raw milk (OR 4.1, p = 0.0001), sheltered livestock (OR 2.6, p 3 times the risk of women participants (20% vs. 9%; adjusted OR 2.78, 95% CI 1.18– 6.58 for male participants vs. female participants), but this difference did not remain significant within sublocation analysis. After age, gender, and location were controlled for, those who had disposed of an aborted animal fetus, were 3 times more likely to be seropositive (72.7% vs. 35.7%, adjusted OR 2.78, 95% CI 1.03–7.52). Age and location, but not gender, were associated with disposal of an aborted animal fetus, such that those who were older or who were from Gumarey were more likely to dispose of an abortus (Technical Appendix 2). Subgroup analysis by village showed significant predictors of RVFV seropositivity in Gumarey to be an ill family member, disposal of an aborted fetus, and gender (Technical Appendix 2). Displacement by flood was also associated with RVFV seropositivity in Gumarey but could not be included in the model because every seropositive participant was displaced by floods and this factor was overdetermined. Male participants were >3 times as likely to be seropositive compared with female participants: (adjusted OR 3.45, 95% CI 1.17–10.19). Disposal of an aborted animal fetus (adjusted OR 15.12, 95% CI 4.445–51.35) and presence of an ill family member (adjusted OR 18, 95% CI 1.35–246.97) were also associated with RVFV seropositivity. In Sogan-Godud, the logistic model to predict seropositivity included age, such that the odds of seropositivity increased 5% for every 1-year increase in age (adjusted OR 1.05, 95% CI 1.019–1.091) (Technical Appendix 2). Children 15 years of age. The adjusted OR for seropositivity (calculated from the overall logistic model) was 1.05; 95% CI 1.03–1.07 per year of age (Figure 4). This difference persisted at the sublocation level with those children in Sogan-Godud still with significantly lower risk than adults. Figure 4 Rift Valley fever virus immunoglobulin G seropositivity by decade of age and village of residence; Gumarey had a higher rate than Sogan-Godud in almost all age groups. Symptom History and Physical Examination Findings Symptoms and signs reported on the survey questionnaire included fever, malaise, myalgia, chills, backache, eye pain, headache, rash, red eyes, photophobia, poor appetite, flushing, nausea, vomiting, meningismus, poor vision, epistaxis, hematemesis, hematochezia, bruising, confusion, vertigo, stupor, and coma. Of these, a past history of myalgias (OR 6.03, p = 0.0001), backache (OR 3.86, p = 0.003), eye pain (OR 2.28, p = 0.034), red eyes (OR 2.75, p = 0.008), meningismus (OR 2.97, p = 0.004), poor vision (OR 2.74, p = 0.008), and coma (OR 14.55, p = 0.005) were statistically associated with RVFV seropositivity in the study population (Appendix Table 4). Upon physical examination, no nonocular finding was specifically associated with RVFV seropositivity. Ophthalmologic Findings Of the 18 identified cases of substantial retinal disease in the survey population, 7/18 (38.9%) were seropositive compared with 11/18 (61.1%) who were seronegative (p = 0.003, χ2 8.75). All participants with eye disease were >21 years of age, and all seropositive participants with eye disease were >50 years of age. The OR of late eye disease associated with RVFV exposure (seropositivity) was 4.99 (p = 0.003) (Appendix Table 5). Measured visual acuity ranged from 6/5 to 6/60 (equivalent to 20/17–20/200) in the seronegative group and 6/5–6/36 (20/17–20/120) in the seropositive group, although both groups included those with extremely poor vision who could decipher only large objects (measured by finger counting) or who could not perceive light. Visual acuity differed statistically among groups and was more likely to be worse in the RVFV–seropositive group (visual impairment defined as >20/80: 12% of seronegative vs. 25% of seropositive participants; p = 0.047, χ2 3.94). Among the 18 participants with retinal disease, 14 (78%) had visual impairment, and among the 7 seropositive participants with retinal disease, 5 (71%) had visual impairment. No distinctive lesion was associated with RVFV seropositivity, though the eye diseases differed among the groups. Seropositive participants with eye disease had of optic atrophy (3), retinal hemorrhage (2), and retinal scarring (3). One person had retinal hemorrhage and scarring. By contrast, seronegative participants with eye disease had uveitis (1), vasculitis (1), maculopathy (3), peripapillitis (1), retinal scarring (1), optic scarring (2), retinal atrophy (1), and retinal degeneration (1). Discussion This study highlights the variability in RVFV seroprevalence in high-risk settings. In northeastern Kenya, older age, rural village location, male gender, disposal of an aborted fetus, and eye disease were associated with RVFV seropositivity. RVFV seropositivity was relatively high in our sample population in Masalani town, Kenya, particularly in the village area (Gumarey), where seropositivity rates were nearly 4 times higher than in the town area (Sogan-Godud); these areas were separated by only 500 m. Clues to the reasons for this discrepancy in seroprevalence were identified in our study. Those from Gumarey were more likely to have mosquito and animal exposures than those from Sogan-Godud. These risk factors, coupled with the most important predictors of rural seropositivity, male gender, and disposal of an aborted animal fetus, yield evidence for disparate risks for RVFV infection in different communities. As identified in our prior work, RVFV seroprevalence can vary significantly across Kenya ( 16 ). Our current study shows that large seroprevalence discrepancies can also occur over short distances. Spatial risk assessments of RVF in animals in Senegal have been predicted by using measurements of seasonal rainfall, land surface temperature, distance to perennial water bodies, and time of year ( 18 ). Designing such risk maps with human risk factor data may enable improved surveillance systems and better prediction of the spatial distribution of RVFV. This information, gathered with satellite imagery ( 19 ) and large-scale cluster analysis ( 20 ), can be used not only to predict large outbreaks but also to identify local hot spots of RVFV transmission to optimize RVF control in resource-limited settings. For each year of life, the odds of being RVFV seropositive increased by 5%. Male participants were nearly 3 times more likely to be seropositive than female participants, a risk that was noted in the 1997 RVF outbreak investigation ( 13 ). The difference in seropositivity among genders is not explained on the basis of reported animal or nonanimal exposures, which were comparable and not statistically different between genders. The increased seropositivity among male participants may have a biological basis, given that outcome of infection and resultant immune response to other viruses have been linked to gender differences ( 21 ). Disposal of an aborted animal fetus was associated with nearly 3 times increased odds of RVFV seropositivity. This finding may indicate the importance of RVFV transmission by aerosolization of blood and amniotic fluid during animal birthing. It is unknown whether aerosol or vector-borne transmission is the dominant form of transmission during interepidemic or epidemic periods. Our analysis indicates that disposal of an aborted animal fetus was a common associated risk factor at both the composite and sublocation level. Planned repeat sampling of our cohort since the most recent outbreak of 2006–2007 may enable the determination of the primary mode of epidemic transmission. We found evidence of interepidemic human transmission of RVFV, which has not been previously shown. Our validation of seropositive young children, born after the documented outbreak in 1997–1998, indicate that low-level interepidemic transmission to humans is continuing in the Masalani area and likely in other areas of Kenya ( 16 ). The natural reservoir for RVFV and the mechanism by which humans become infected during interepidemic periods are unknown. Wild animals have been shown to be infected with RVFV, but further studies must determine whether these animals play a role in RVFV maintenance between outbreaks ( 22 ). We demonstrated statistically significant differences between the seropositivity rates of those with and without eye disease. Those with chronic retinal disease were 5 times more likely to be RVFV seropositive. We did observe a difference in visual acuity between RVFV seropositive and seronegative persons in our sample tested 8 years after the 1997–1998 outbreak, and perhaps greater changes may have been present during acute RVF disease. Although there were no ocular findings that were pathognomonic for prior RVFV infection, the detected retinal disease supports evidence from previous studies on the oculopathogenesis of RVFV ( 23 ). No specific nonocular examination finding was associated with RVFV seropositivity, but several reported symptoms were statistically more common among those who were RVFV seropositive. Most of these symptoms were severe neurologic manifestations of disease, such as neck stiffness, confusion, and coma. RVFV can cause encephalitis ( 1 ), and this type of inflammation may explain the higher prevalence of these reported symptoms among seropositive participants. Myalgia and backache may be present in most of the nonsevere RVF cases and are not specific to RVFV infection. Poor vision, which was noted to be more common among RVFV seropositive participants in our sample, may be an indicator for RVF retinitis, a common sequela of RVFV infection ( 23 , 24 ). RVFV IgG ELISA and PRNT antibodies are believed to last decades after infection and therefore provide a reliable index of prior RVFV exposure. In contrast, though less well studied, it appears that IgM is lost in 50% of patients after 45 days and is absent in 100% by 4 months after infection ( 25 ). We did not perform IgM testing in our study, although it might have yielded useful additional information about acute RVFV infection. We also recognize that seropositive results may be false positive due to cross-immunoreactivity with viruses in the same family, although discrepancies between the neutralization test and the ELISA were only 4.9% in this population. The use of confirmatory PRNT testing of ELISA-positive samples can greatly improve viral specificity ( 26 ). Our study was limited by its cross-sectional design; therefore, we are unable to conclude whether the identified risk factors specifically caused RVFV exposure. The validity of the associations in this study relies on accurate recall of exposures by the study participants. Although we asked about timing of symptoms and exposures, language differences during questioning limited our accurate collection of these data. Our study may have limited generalizability; we tested risk factors from a small population in Masalani, and risks may vary in other parts of Kenya or in other countries. Data on animal exposures were collected in a binary fashion, so no information about magnitude or duration of contact is known, which may have an effect on risk estimations. We also had no quantitative exposure data for the RVFV vectors in our study area. This study highlights the large-scale variability in exposure and RVFV seropositivity among Kenyan villages and emphasizes the effect of age, gender, location, and animal husbandry in RVFV transmission. This information is useful for local public health agencies so that they can target protective interventions according to risk factors in different populations. Further studies are needed to examine the epidemiologic, biological, and genetic basis for the increased risk among persons of male gender and to quantify the potential public health impact of modifying the rural environment. RVFV transmission is known to be ongoing in livestock in areas where RVFV is endemic during interepidemic periods; we have shown that this extends to humans, confirming past observations ( 27 ). Ongoing efforts to predict hot spots of infection on both small and large scales is useful only when at-risk communities are able to use the information to target mosquito or vaccine control efforts and prevent outbreaks. As RVF expands its geographic range and becomes recognized as a disease of global importance for human and animal health, more research is needed to define the most accessible modes of transmission control. Supplementary Material Technical Appendix 1 Study participants received the following structured interview regarding housing, animal exposure, motor function, visual function, and recent or remote Rift Valley fever-related symptoms. Technical Appendix 2 Binary Logistic Regression Analysis to Predict Rift Valley Fever Virus seropositivity Appendix Table 1 Associations of potential predictors with Rift Valley fever virus seropositivity* Appendix Table 2 Association between (village) location and potential model predictors for Rift Valley Fever virus seropositivity* Appendix Table 3 Testing of association between predictors of Rift Valley fever seropositivity* Appendix Table 4 Association of signs and symptoms with Rift Valley fever virus seropositivity* Appendix Table 5 Testing of association of eye disease with Rift Valley fever virus seropositivity
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS Negl Trop Dis
                PLoS Negl Trop Dis
                plos
                plosntds
                PLoS Neglected Tropical Diseases
                Public Library of Science (San Francisco, CA USA )
                1935-2727
                1935-2735
                27 February 2015
                February 2015
                : 9
                : 2
                : e0003536
                Affiliations
                [1 ]Ifakara Health Institute, Ifakara, Tanzania
                [2 ]Department of Biomedical Sciences, Institute of Tropical Medicine, Antwerp, Belgium
                [3 ]Department of Infectious and Parasitic Diseases, Faculty of Veterinary Medicine, University of Liege, Liege, Belgium
                The Kenya Medical Research Institute, KENYA
                Author notes

                The authors have declared that no competing interests exist.

                Conceived and designed the experiments: RDS DB EG. Performed the experiments: RDS MA EG. Analyzed the data: RDS ENA ET DB. Wrote the paper: RDS ENA ET DB EG.

                Article
                PNTD-D-14-01396
                10.1371/journal.pntd.0003536
                4344197
                25723502
                19fad59e-3665-4811-87fb-1b3c99b867e7
                Copyright @ 2015

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

                History
                : 13 August 2014
                : 13 January 2015
                Page count
                Figures: 1, Tables: 3, Pages: 11
                Funding
                This work was funded through a scholarship awarded to RDS by Belgian Directorate General for Development Cooperation (DGDC), through Institute of Tropical Medicine, Antwerp. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
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                All relevant data are within the paper.

                Infectious disease & Microbiology
                Infectious disease & Microbiology

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