Efficacy of Praziquantel for Treatment of Plasmodium falciparum Infection in Asymptomatic Gabonese Adults: A Double-Blind Placebo-Controlled Randomized Trial

Abstract

Background

Schistosomiasis and malaria are major poverty-related parasitic infectious diseases. They are often coendemic and affect similar target populations. Mass drug administration and intermittent preventive treatment programs are commonly used for their control. From a programmatic perspective the establishment of preventive treatment programs targeting both diseases simultaneously would be a great advantage. While there is preliminary evidence that certain antimalarial drugs exert a clinically important effect on Schistosomes, little is known on whether praziquantel (PZQ), the only licensed drug to treat schistosomiasis, has activity against Plasmodium parasites. Therefore, the CORMA-MAL study was conducted to evaluate antimalarial activity of PZQ.

Methods

This double-blinded, randomized, placebo-controlled phase 2b trial was conducted in Lambaréné, Gabon and recruited semi-immune adults with an asymptomatic Plasmodium falciparum parasitemia. Participants were randomized (1:1) to receive placebo or 40 mg/kg PZQ once daily for 3 days and followed up for a total of 7 days.

Results

Forty-four participants were recruited. Analytical models indicated a highly significant hourly reduction of microscopically determined parasitemia in the PZQ arm and a nonsignificant decreasing trend in the placebo group (regression slopes of log₁₀-transformed parasitemia: β=−0.006 [P < .001] and β=−0.0013 [P = .3], respectively). However, in total only 9 of 22 (41%) in the PZQ arm compared to 6 of 22 (27%) in the placebo arm reached parasite clearance by day 7.

Conclusions

Findings demonstrate a statistically significant antimalarial activity of PZQ; however, the observed effect is only moderate compared to current first-line antimalarial treatments. Future research should investigate the synergistic potential of combining PZQ and antimalarial drugs in malaria and schistosomiasis control.

Clinical Trials Registration.Pan African Clinical Trials Registry, PACTR202206584817951, 10 June 2022 (https://pactr.samrc.ac.za/TrialDisplay.aspx?TrialID=22586).

Schistosomiasis and malaria are often coendemic. Programmatically, the conception of integrated disease control approaches would be a great advantage. This study revealed a statistically significant but moderate antiplasmodial effect of praziquantel in semi-immune Gabonese adults with P. falciparum infection.

Malaria is caused by Plasmodium parasites transmitted to humans via an infective bite by mosquitos of the genus Anopheles. Despite 2 decades of significant progress in malaria control, it continues to pose a severe global health threat with 263 million cases and 597 000 deaths worldwide in 2023; approximately 95% of global morbidity and mortality occurred in the World Health Organization (WHO) African Region, with children accounting for 76% of all deaths [1]. The second most prevalent parasitic disease in sub-Saharan Africa is urogenital schistosomiasis. It is caused by Schistosoma haematobium and most frequently affects children and adolescents, who constitute the population that is at the same time most affected by malaria [2, 3]. Mass drug administration and preventive treatment programs are commonly deployed for the control of both diseases [4, 5]. The overlap of the most affected patient population and the amenability to preventive treatment programs provides important opportunities for the simultaneous control of malaria and schistosomiasis. A preventive treatment regimen with activity against both pathogens would therefore allow the implementation of integrated control programs against both poverty-related infectious diseases, which may lead to important operational advantages compared to the current vertical disease control approaches.

There is increasing evidence that antimalarial drugs are active against Schistosomes [6–8]. At the same time little is known whether praziquantel (PZQ), the only antischistosomal drug used, is active against Plasmodium parasites [7]. PZQ is the cornerstone of schistosomiasis control programs, with a track record of over 40 years use with good safety and tolerability [2, 5, 9–12]. Preliminary evidence indicates that PZQ may, beyond its primary role in treating schistosomiasis, also exert some activity against Plasmodium parasites. A study conducted by a research group in 1998 in Russia, using an in vivo Plasmodium berghei mouse model, showed that PZQ, dosed as 125 mg/kg body weight, exhibited a relevant antiplasmodial activity during the treatment administration period. This was indicated by the fact that a standardized parasite index markedly decreased while treatment was given; however, it rose again once treatment was stopped. It had an even greater antiparasitic effect when combined with chloroquine, with an efficacy that was significantly more favorable than the efficacies of each drug given as monotherapy [13]. Similarly, a noncontrolled clinical study of 10 malaria patients (age range, 12–52 years) conducted in Pakistan, reported antimalarial activity of PZQ, with a cure rate of 80% within 4–6 days [14]. However, the study had important limitations in its methodological quality, due to inconsistencies in the duration of PZQ treatment administration (ie, 30 mg/kg/day in 3 divided doses for a maximum of 8 days, no distinction between participants with Plasmodium falciparum and Plasmodium vivax, the limited sample size, lack of randomization, and the absence of a control group). The lack of research in this field, paired with methodological insufficiencies of currently existing preliminary evidence, underscores the need for robust, well-designed studies to clarify the potential antimalarial activity of PZQ. Therefore, this study was designed as a randomized, placebo-controlled, double-blinded clinical trial to systematically investigate the efficacy of PZQ as a monotherapy for the treatment of P. falciparum infections.

METHODS

Study Design and Participants

This phase 2b study investigated the efficacy, safety, and tolerability of PZQ when given as monotherapy to treat asymptomatic P. falciparum infections in semi-immune adults. It was designed as a single-center, randomized, placebo-controlled, double-blinded study and was conducted at the Centre de Recherches Médicales de Lambaréné (CERMEL) in Gabon [15]. Key inclusion criteria comprised age ≥18 years with a microscopically confirmed asymptomatic P. falciparum parasitemia between 200 and 5000 parasites/μL. Key exclusion criteria were clinical malaria, pregnancy, recent antimalarial treatment, and underlying health conditions such as severe anemia or hepatic disease.

The trial was conducted following the International Council for Harmonisation guidelines on Good Clinical Practice and received approval by the institutional review board (Comité d’Ethique Institutionel) of CERMEL (registration number, CEI-002/2022). The study protocol was registered online before the initiation of participant recruitment for this clinical trial (pactr.samrc.ac.za, registration number PACTR202206584817951). Written informed consent was obtained from all participants before enrollment, and consent for publication was obtained from each study participant.

Randomization and Blinding

Participants were randomized (1:1) to a placebo or PZQ treatment group. Random permuted blocks of variable block size were created by the trial statistician to compute allocation sequence. Treatment allocation was concealed using opaque, sealed envelopes with a predefined sequence number, which were opened sequentially by an unblinded pharmacist after confirming participant eligibility. All study physicians and participants were blinded to group allocation.

Procedures

The screening process involved a detailed medical assessment, light microscopical analysis of blood smears, and analysis of hematology and biochemistry. PZQ was administered at a dosage of 40 mg/kg once daily for 3 consecutive days under direct supervision, with a standardized meal. Following drug administration, participants were actively followed up for 7 days. P. falciparum parasitemia was quantified by microscopy and quantitative polymerase chain reaction (qPCR) (Supplementary Material, Laboratory Procedures) [16]. Also, vital signs and laboratory safety were ascertained and adverse events (AEs) were solicited. On day 7, all participants received standardized antimalarial treatment with artemether-lumefantrine ensuring definitive cure from submicroscopic parasitemias. A rescue treatment plan ensured that all participants developing clinical malaria received an antimalarial treatment course.

Statistical Analysis and Outcomes

The sample size calculation was based on comparing the proportions of being parasite free on day 7 after treatment between the PZQ and placebo groups using the χ² test. We expected parasitemia clearance rates of 10% over 7 days in the placebo group compared to 50% in the PZQ group. With a power of 80% and an α error of 5%, considering a 1:1 allocation ratio, 40 subjects were necessary. Accounting for a dropout rate of 10% a total of 44 participants was targeted for recruitment.

Statistical analysis was performed using Stata18 (StataCorp). The intention-to-treat (ITT) analysis set included all who received at least 1 dose of PZQ or placebo. The per-protocol population analysis set was limited to participants who adhered to the complete 3-day treatment course.

The primary efficacy end point was the proportion of participants achieving complete parasite clearance (cure), as determined microscopically, on day 7. Participants requiring rescue treatment before day 7 and those who tested positive for P. falciparum on day 7 were classified as treatment failures. Parasite clearance rates on day 3 and day 7 were compared between study groups using χ² tests for hypothesis testing.

Several secondary efficacy end points were assessed including the proportion of participants with qPCR-determined parasite clearance, time to microscopically and qPCR-determined parasite clearance, as well as the description of parasite kinetics over time. The time to parasite clearance was defined as the time to the first negative result that stayed negative up to day 7, and was analyzed using Wilcoxon rank-sum tests and Kaplan-Meier survival analysis with differences assessed by log-rank tests. Total parasite reduction in percentage by day 3 and day 7 was calculated for each group and compared via Wilcoxon rank-sum tests. Microscopically and qPCR-determined parasitemia was log-transformed to approximate a normal distribution and analyzed via linear regression to evaluate hourly parasite reduction over the 7-day follow-up period. Using the trapezoidal rule, the area under the curve (AUC) of log₁₀ transformed parasitemia was computed from baseline until day 7 to estimate the total cumulative parasite mass. The AUC was compared between treatment groups using t tests.

Participants who required antimalarial rescue treatment during follow-up due to development of clinical malaria received special attention in statistical analysis to avoid bias. Thereby, we ensured that rapid parasite clearance properties of the established antimalarial treatment would not falsely indicate that participants with treatment failure would seemingly have the most favorable parasite clearance characteristics. Different strategies were employed for different statistical models: in linear regression and AUC analysis, for all participants requiring antimalarial rescue treatment, the parasitemia of all time points prior to administration of the rescue treatment was averaged and imputed onto subsequent time points. For other statistical models all time points after administration of antimalarial rescue treatment were censored. Missing data did not play a role in this study. Safety and tolerability end points included incidence of AEs and serious AEs (SAEs). Only drug-related AEs are discussed in this article.

RESULTS

Participant Disposition

Between June 2022 and February 2023, 70 individuals were screened for eligibility; 44 participants fulfilled all inclusion and exclusion criteria accordingly and were subsequently recruited in this study (Figure 1). The median age of enrolled participants was 23 years (interquartile range [IQR], 19–30 years), with 73% of the participants being male. The median height and weight were 168 cm (IQR, 168–172 cm) and 61 kg (IQR, 57–65 kg), respectively. Median baseline parasitemia was comparable in the PZQ and the placebo groups with 737 (IQR, 336–1424) parasites/μL and 612 (IQR, 367–1050) parasites/μL, respectively. All participants received at least 1 dose of the study drug and over 93% of participants received all 3 doses. Overall, 91% (40/44) of participants completed the study up to day 7 without requiring an established antimalarial rescue treatment (Table 1).

Figure 1.

Flow chart and trial profile. Abbreviations: ITT, intention to treat; PP, per protocol.

Table 1.

Demographics and Baseline Characteristics

Characteristics	Placebo	PZQ	Total
Sex
Male, n (%)	15 (68)	17 (77)	32 (73)
Female, n (%)	7 (32)	5 (23)	12 (27)
Age, y, median (IQR)	22 (19–30)	25 (19–39)	23 (19–30)
Height, cm, median (IQR)	168 (162–170)	166 (160–176)	168 (161–172)
Weight, kg, median (IQR)	60 (57–65)	62 (57–67)	61 (57–65)
Baseline parasitemia/µL, median (IQR)a	612 (367–1050)	737 (336–1424)	648 (353–1344)
Baseline Plasmodium gene copies/µL, median (IQR)b	234 (131–550)	393 (150–800)	284 (139–777)
At least 1 study drug dose received, n (%)	22 (100)	22 (100)	44 (100)
All study drug doses over 3 days received, n (%)	20 (91)	21 (95)	41 (93)
Reached day 7 without requiring antimalarial rescue treatment, n (%)	19 (86)c	21 (95)c	40 (91)

Abbreviations: IQR, interquartile range; PZQ, praziquantel; qPCR, quantitative polymerase chain reaction.

^aDetermined by microscopy.

^bDetermined by qPCR.

^cAmong the 4 individuals receiving rescue treatment, all had uncomplicated malaria.

Parasitemia Reduction Over Time

Linear regression analysis of log₁₀-transformed parasitemia indicated a statistically significant hourly reduction of parasitemia until day 7 in the PZQ group (ITT, β=−0.006, P < .001) compared to a nonsignificant reduction in the placebo group (ITT, β=−0.0013, P = .30) (Figure 2A). A comparison of regression slopes was highly statistically significant (ITT, P = .003). The observed effect within the PZQ group became even more pronounced when restricting the analysis to the first 3 days (ie, the days of treatment administration; ITT, β=−0.010, P = .003) (Figure 2B). Using such linear regression equations of the ITT analysis set to estimate the geometric mean parasitemia, results were 69.2 parasites/µL in the PZQ group versus 276.6 parasites/µL in the placebo group on day 3, and 19.1 parasites/µL versus 146.6 parasites/µL on day 7, respectively. Evaluation of the reduction in parasitemia between baseline and later follow-up points confirmed such significant differences identified by linear regression models (Supplementary Table 1). This trend of more favorable yet statistically nonsignificant reduction was further corroborated by Kaplan-Meier and AUC analysis, when using both light microscopic or qPCR data (Figure 3, Supplementary Figures 1 and 2, and Supplementary Tables 2 and 3).

Figure 2.

Comparison of log₁₀-transformed parasitemia (parasites/µL) over time between placebo and praziquantel (PZQ): (A) intention to treat group, day 0–7; (B) intention to treat group, day 0–3; (C) per protocol group, day 0–7; (D) per protocol group, day 0–3. Time points after antimalarial rescue treatment administration were imputed based on mean parasitemia until antimalarial rescue treatment administration. Comparison of slopes: (A) P = .003, (B) P = .003, (C) P = .005, (D) P = .011.

Figure 3.

Kaplan-Meier plots showing time to parasite clearance: (A) clearance determined by light microscopy; (B) clearance determined by qPCR. Numbers in parentheses show cumulative censorings. Abbreviations: CI, confidence interval; qPCR, quantitative polymerase chain reaction.

Parasite Clearance on Day 7

Lastly, using the ITT analysis set only, 23% (5/22) of participants in the PZQ group had microscopically determined parasite clearance without previously requiring antimalarial rescue treatment on day 3 versus 14% (3/22) in the placebo group (P = .43) (Table 2). On day 7 there was yet again a higher proportion of participants with parasite clearance in the PZQ group without previously requiring antimalarial rescue treatment compared with the placebo group; however, this did not reach statistical significance (ITT, 41% [9/22] vs 27% [6/22], respectively; P = .34). These results were concordant in the qPCR analysis (Table 2).

Table 2.

Comparison of Proportions of Microscopically Determined Parasite Clearance on Day 3 and Day 7

Population	Outcome	Placebo, n (Column %)	Praziquantel, n (Column %)	χ2 Test
Light microscopy, day 3
ITT (n = 44)	Absence of parasitemia at day 3 without requiring antimalarial rescue treatment
	No	19 (86)	17 (77)	0.43
	Yes	3 (14)	5 (23)
PP (n = 41a)	Absence of parasitemia at day 3 without requiring antimalarial rescue treatment
	No	17 (85)	16 (76)	0.48
	Yes	3 (15)	5 (24)
Light microscopy, day 7
ITT (n = 44)	Absence of parasitemia at day 7 without requiring antimalarial rescue treatment
	No	16 (73)	13 (59)	0.34
	Yes	6 (27)	9 (41)
PP (n = 41a)	Absence of parasitemia at day 7 without requiring antimalarial rescue treatment
	No	14 (70)	12 (57)	0.39
	Yes	6 (30)	9 (43)
qPCR, day 3
ITT (n = 44)	Absence of parasitemia at day 3 without requiring antimalarial rescue treatment
	No	21 (95)	20 (91)	0.55
	Yes	1 (5)	2 (9)
PP (n = 41a)	Absence of parasitemia at day 3 without requiring antimalarial rescue treatment
	No	19 (95)	19 (91)	0.58
	Yes	1 (5)	2 (9)
qPCR, day 7
ITT (n = 44)	Absence of parasitemia at day 7 without requiring antimalarial rescue treatment
	No	16 (73)	15 (68)	0.74
	Yes	6 (27)	7 (32)
PP (n = 41a)	Absence of parasitemia at day 7 without requiring antimalarial rescue treatment
	No	14 (70)	14 (67)	0.82
	Yes	6 (30)	7 (33)

Abbreviations: ITT, intention to treat; PP, per protocol.

^aExcluding those who did not receive the full dose (n = 2 placebo [received 2 doses]; n = 1 praziquantel [received 1 dose]).

Safety and Tolerability

A total of 5 drug-related AEs of either mild (n = 1) or moderate (n = 4) severity were observed during follow-up. All were resolved by the end of the study (Supplementary Table 4). There were no deaths or SAEs in this study.

DISCUSSION

This phase 2b study aimed at investigating the activity of PZQ against P. falciparum infections in semi-immune adults, to better understand the potential for synergistic effects of PZQ-based schistosomiasis control programs for simultaneous control of malaria. The findings of this study indicate a statistically significant antiplasmodial effect of PZQ. Specifically, linear regression analysis revealed a highly significant hourly decrease in parasitemia between day 0 and day 7 in the PZQ group. Comparing regression slopes further corroborates the statistically significantly higher geometric mean parasitemia in the PZQ group, although this group had a higher baseline parasitemia than the placebo group. However, among all participants who received PZQ only about 40% of participants managed to clear parasitemia by day 7. Not only was this not statistically different from the placebo group, with cure rates of about 25%, also, the magnitude of the antiparasitic effect was much lower when compared with the current antimalarial therapeutic gold standard of artemisinin combination therapy, demonstrating efficacies of more than 95% [17].

The strongest reduction of parasitemia occurred during the 3-day treatment administration period. This suggests a direct antiparasitic activity of PZQ for the duration of treatment administration, aligning with preliminary evidence provided by the P. berghei mouse model study from 1998, in which parasitemia was observed to consistently decrease under PZQ treatment; however, this rose again when PZQ administration was stopped [13]. Supportive of these findings, we found additional nonsignificantly higher cure rates, a higher total parasite reduction by day 7, and a shorter time to parasite clearance in the PZQ arm compared to the placebo arm. The precise molecular mechanisms and pharmacodynamic mechanisms responsible for this effect are not fully understood. It is plausible that the known ability of PZQ to disrupt calcium homeostasis in helminths may contribute to this antiplasmodial effect [18]. In addition to impacting voltage-operated calcium channels, research in the context of schistosomiasis highlighted pleiotropic effects of PZQ, including interactions with other schistosome molecules, such as myosin regulatory light chain, glutathione-S-transferase, and transient receptor potential channels [19]. Also, increased T regulatory type 1 cell differentiation and decreased inflammation were observed following PZQ administration, suggesting potential immunomodulatory pathways of PZQ [19]. Although speculative, similar pleiotropic effects could play a role in explaining the antiplasmodial activity of PZQ. However, the authors of the P. berghei model article note that PZQ antimalarial action manifested by morphological changes on the parasites, similar to those induced by dihydrofolate reductase inhibitors like pyrimethamine [13].

Apart from assessing any direct antimalarial effect of PZQ, there has been a debate on a potential indirect antimalarial effect of PZQ. This debate is predicated on the question of whether infection with a certain parasite makes an infected individual more (or less) susceptible to infection by another parasite. In this regard, some studies assessed the complex interaction between Plasmodium and Schistosoma in coinfected individuals. A Senegalese study found significantly higher susceptibility to malaria attacks among children with Schistosoma mansoni infection compared to uninfected children (27% vs 16%), hypothesizing that schistosomiasis might increase the risk for malaria [20]. A subsequent Tanzanian study proceeded to testing this hypothesis that schistosomiasis increases the risk for malaria. They designed an anthelmintic treatment study and examined the impact of PZQ (40 mg/kg) and albendazole (400 mg) on malaria prevalence and parasite density among 756 children with schistosomiasis and found no additional benefits of 4 annual doses (every 3 months) in comparison to a single annual dose [21]. However, given that the control group still received an annual single-dose treatment of PZQ due to ethical considerations, it is likely that the difference may have been lower than if compared to an untreated control group. In contrast, a randomized trial in Nigeria recruited 1228 children and found a significantly slower increase of Plasmodium infections in children over a 12-month period when treated with albendazole (200 or 400 mg) every 4 months compared to children in the placebo group, indicating a protective antimalarial effect of regular deworming [22].

Importantly, regardless of any role that PZQ might potentially play in the future in the management of individual malaria patients, at present PZQ is widely used within mass drug administration programs for schistosomiasis control in malaria endemic areas. In such disease control programs, the moderate direct antimalarial activity of PZQ paired with the above-described indirect effect resulting from helminth clearance could have a relevant additive impact on malaria control efforts. This was suggested by a modeling study assessing the impact of S. mansoni on malaria transmission, which showed reduced malaria transmission in the presence of PZQ mass administration programs [23]. Furthermore, leveraging the already existing infrastructure of PZQ-based schistosomiasis control programs for malaria control would be particularly interesting from an implementation perspective. To estimate the synergistic effect of combined mass treatment against helminths and malaria, a study was performed with primary school children in Zimbabwe. The authors found that malaria and schistosomiasis prevalence decreased by approximately 70% and 60%, respectively, while coinfections of P. falciparum and other helminths decreased by 80%–90% [24]. However, while these numbers are promising, they stem from an uncontrolled trial and therefore need to be interpreted with due caution. All this underlines the important role of further research. In doing so, future research projects should aim at assessing the synergistic potential of intermittent preventive treatment programs against malaria (eg, Intermittent Preventive Treatment in infants, Intermittent Preventive Treatment in schoolchildren, Intermittent Preventive Treatment in pregnancy) and PZQ, as well as albendazole mass treatment programs for helminthic disease control.

However, in the light of only moderate antimalarial efficacy of PZQ treatment, as observed in this study, it becomes apparent that this study may have been underpowered to detect statistically significant effects at the 0.05% level for some end points. First, the anticipated 40% difference in efficacy between PZQ and placebo, as assumed in sample size calculations, was not achieved given that the observed difference was roughly 15%. Second, semi-immunity is a potential confounder, as it allows individuals to control parasitemia more effectively [25, 26]. Therefore, the rather high proportion of adults with an undetectable parasitemia in the placebo group is likely attributable to semi-immunity, which independently contributed to parasite clearance and diminished the comparative treatment effect between the study arms. Third, another limitation of our study is the accuracy of malaria light microscopy when applied to detect a very low parasitemia, due to the subjective nature of this diagnostic method [27]. We tried to account for this variability by employing only WHO-certified expert readers and a 2-3 reader approach, which is the gold standard in pharmacological studies on new antimalarial drugs. Fourth, we also applied molecular detection methods, which are highly sensitive but might have yielded a false-positive diagnostic result if DNA was detected from dead parasites [28]. However, the parasitological results from light microscopy and qPCR are comparable, and therefore the limitations of each method can very likely be assumed to be negligible in this case. Fifth, the observation period of this study only comprised 7 days and did not ascertain robust end points commonly used in antimalarial drug trials such as the adequate clinical and parasitological response at 28 or 42 days after treatment. However, based on the only moderate antimalarial efficacy of PZQ when given as a single-drug regimen, as observed in this study, we would not advise such a study design given the availability of highly effective first-line antimalarial treatments. Lastly, we assessed the efficacy of a 3-day once-daily PZQ dosing regimen, which is different from the single-dose PZQ treatment that is commonly administered within mass drug administration programs for control of schistosomiasis [5]. The magnitude of an antiplasmodial effect of single-dose PZQ treatment needs to be assessed by future research.

CONCLUSIONS

The findings of this clinical trial corroborate the existing preliminary evidence, demonstrating a statistically significant antimalarial activity of PZQ; however, the observed effect is only moderate in magnitude and falls short compared to the efficacy of current first-line antimalarial treatment regimens. Therefore, PZQ monotherapy is not of value in individual malaria case management. PZQ may, however, act synergistically on Plasmodium infections when administered in combination with antimalarial treatment. This may—at least in theory—lead to higher overall cure rates and protection from the development and spread of antimalarial drug resistance. Furthermore, a combination regimen containing PZQ in addition to other antimalarials could also be potentially beneficial in the treatment of schistosomiasis given current preliminary evidence in favor of anti-schistosomal activity of anti-malarial drugs. In any case, combining preventive antimalarial treatment regimens with PZQ may lead to important operational advantages of integrated control programs for malaria and schistosomiasis compared to the current vertical approaches. Therefore, future research should try to further investigate the synergistic potential of PZQ-based helminthic disease control programs for malaria control efforts, and to investigate the potential of antimalarials in schistosomiasis treatment, by assessing parasitological end points for malaria and schistosomiasis, respectively, in current disease control programs. Robust and favorable research results could help unify the current fragmentation of single-disease control programs into a unified comprehensive approach, which will facilitate the implementation of highly cost-effective, integrated control programs.

Supplementary Data

Supplementary materials are available at The Journal of Infectious Diseases online (http://jid.oxfordjournals.org/). Supplementary materials consist of data provided by the author that are published to benefit the reader. The posted materials are not copyedited. The contents of all supplementary data are the sole responsibility of the authors. Questions or messages regarding errors should be addressed to the author.