Longitudinal survey of malaria morbidity over 10 years in Saharevo (Madagascar): further lessons for strengthening malaria control

Abstract

Background: Madagascar has been known for having bio-geo-ecological diversity which is reflected by a complex malaria epidemiology ranging from hyperendemic to malaria-free areas. Malaria-related attacks and infection are frequently recorded both in children and adults living in areas of low malaria transmission. To integrate this variability in the national malaria control policy, extensive epidemiological studies are required to up-date previous records and adjust strategies.

Methods: A longitudinal malaria survey was conducted from July 1996 to June 2005 among an average cohort of 214 villagers in Saharevo, located at 900 m above the sea. Saharevo is a typical eastern foothill site at the junction between a costal wet tropical area (equatorial malaria pattern) and a drier high-altitude area (low malaria transmission).

Results: Passive and active malaria detection revealed that malaria transmission in Saharevo follows an abrupt seasonal variation. Interestingly, malaria was confirmed in 45% (1,271/2,794) of malaria-presumed fevers seen at the health centre. All four Plasmodia that infect humans were also found: Plasmodium falciparum; Plasmodium vivax, Plasmodium malariae and Plasmodium ovale. Half of the malaria-presumed fevers could be confirmed over the season with the highest malaria transmission level, although less than a quarter in lower transmission time, highlighting the importance of diagnosis prior to treatment intake. P. falciparum malaria has been predominant (98%). The high prevalence of P. falciparum malaria affects more particularly under 10 years old children in both symptomatic and asymptomatic contexts. Children between two and four years of age experienced an average of 2.6 malaria attacks with P. falciparum per annum. Moreover, estimated incidence of P. falciparum malaria tends to show that half of the attacks (15 attacks) risk to occur during the first 10 years of life for a 60-year-old adult who would have experienced 32 malaria attacks.

Conclusion: The incidence of malaria decreased slightly with age but remained important among children and adults in Saharevo. These results support that a premunition against malaria is slowly acquired until adolescence. However, this claims for a weak premunition among villagers in Saharevo and by extension in the whole eastern foothill area of Madagascar. While the Malagasy government turns towards malaria elimination plans nowadays, choices and expectations to up-date and adapt malaria control strategies in the foothill areas are discussed in this paper.

Figure 1

Climatic data from Moramanga, 12 km away from Saharevo. Profiles extracted from data collected over the past 30 years (1961–1990).

Figure 2

Monthly incidence of fever (uncorrected axillary temperature ≥37.5°C) and P. falciparum malaria attacks (fever + asexual parasites) for 1,000 inhabitants. According to the annual censuses: 182 inhabitants in 1996, 188 (1997), 195 (1998), 205 (1999), 216 (2000), 215 (2001), not done in 2002, 224 (2003), 236 (2004), and 261 (2005). The average monthly incidence of malaria P. falciparum attacks was 5.6%.

Figure 3

Curves of the annual age-specific incidence of fever (uncorrected axillary temperature ≥37.5°C) and P. falciparum malaria attacks (fever + asexual parasites). Histograms represent the parasite densities for P. falciparum (proportion of the parasite densities in number of parasites per μl of blood, for 100% of the malaria attacks). Values are drawn from passive monitoring of entire years (1997 to 2004, except 2002). Using only subjects with a recorded age from those years, the average number of inhabitants was 201 (ranged from 153 in 1997 to 236 in 2004). For each sample, the age of the subject used is the age at the time of the blood collection.

Figure 4

Monthly prevalence rate (%) for P. falciparum measured in active detection cases compared to cumulative P. falciparum incidence (Figure 2) from February 2003 to June 2005.

Figure 5

Asymptomatic carriage of P. falciparum parasites according to age. Data come from the monthly malaria active detection. Age distribution of symptomatic carriage among positive cases during systematic sampling (black circle) and parasite densities (histograms) in the 541 cases of asymptomatic carriage of P. falciparum observed from February 2003 to June 2005. Prevalence of asymptomatic carriages (grey rhombus) is calculated by the ratio between positive slides for P. falciparum and total number of slides (from active detection cases in the absence of fever). Among the 541 positive slides, 57 (10.53%) had P. falciparum gametocytes (black square). For each sample, the age of the subject used was the age at the time of the blood intake.