Assessing malaria transmission in a low endemicity area of north-western Peru

Abstract

Background: Where malaria endemicity is low, control programmes need increasingly sensitive tools for monitoring malaria transmission intensity (MTI) and to better define health priorities. A cross-sectional survey was conducted in a low endemicity area of the Peruvian north-western coast to assess the MTI using both molecular and serological tools.

Methods: Epidemiological, parasitological and serological data were collected from 2,667 individuals in three settlements of Bellavista district, in May 2010. Parasite infection was detected using microscopy and polymerase chain reaction (PCR). Antibodies to Plasmodium vivax merozoite surface protein-119 (PvMSP1₁₉) and to Plasmodium falciparum glutamate-rich protein (PfGLURP) were detected by ELISA. Risk factors for exposure to malaria (seropositivity) were assessed by multivariate survey logistic regression models. Age-specific antibody prevalence of both P. falciparum and P. vivax were analysed using a previously published catalytic conversion model based on maximum likelihood for generating seroconversion rates (SCR).

Results: The overall parasite prevalence by microscopy and PCR were extremely low: 0.3 and 0.9%, respectively for P. vivax, and 0 and 0.04%, respectively for P. falciparum, while seroprevalence was much higher, 13.6% for P. vivax and 9.8% for P. falciparum. Settlement, age and occupation as moto-taxi driver during previous year were significantly associated with P. falciparum exposure, while age and distance to the water drain were associated with P. vivax exposure. Likelihood ratio tests supported age seroprevalence curves with two SCR for both P. vivax and P. falciparum indicating significant changes in the MTI over time. The SCR for PfGLURP was 19-fold lower after 2002 as compared to before (λ1 = 0.022 versus λ2 = 0.431), and the SCR for PvMSP1₁₉ was four-fold higher after 2006 as compared to before (λ1 = 0.024 versus λ2 = 0.006).

Conclusion: Combining molecular and serological tools considerably enhanced the capacity of detecting current and past exposure to malaria infections and related risks factors in this very low endemicity area. This allowed for an improved characterization of the current human reservoir of infections, largely hidden and heterogeneous, as well as providing insights into recent changes in species specific MTIs. This approach will be of key importance for evaluating and monitoring future malaria elimination strategies.

Figure 1

Study area in Bellavista district (Piura department, northwestern coast of Peru) including three settlements. Pavletich (PAV), Jose Carlos Mariátegui (JCM) and Nuevo Porvenir (NP).

Figure 2

Malaria seroprevalence curves. A: Seroprevalence curve for Plasmodium vivax. B: Seroprevalence curve for Plasmodium falciparum. Points represent observed data points, whilst the blue lines represent the maximum likelihood model and 95% confidence intervals. Estimated force of infection (λ) is plotted on the graphs. Two forces infection are plotted by likelihood ratio tests indicating change at a certain point in calendar time.

Figure 3

Evolution of the annual malaria incidence, and rainfall, in Piura department: 1990–2010. Information on annual malaria cases, registered by Regional Health Direction of Piura, was obtained from the National Institute of Statistics and Informatics [27]. Numbers in the timeline point out important events that influenced malaria incidence: 1) moderate ENSO phenomenon (1991–1992), 2) first reports of chloroquine (CQ) failure for Plasmodium falciparum (since 1994), 3) very strong ENSO phenomenon (1997–1998), 4) confirmed CQ resistance for Plasmodium falciparum (1999), 5) ACT implementation (2001–2003), 6) intense droughts (2004–2007).