Research Article: Measuring malaria morbidity in an area of seasonal transmission: Pyrogenic parasitemia thresholds based on a 20-year follow-up study

Date Published: June 27, 2019

Publisher: Public Library of Science

Author(s): Marion Dollat, Cheikh Talla, Cheikh Sokhna, Fatoumata Diene Sarr, Jean-François Trape, Vincent Richard, Thomas A. Smith.


Asymptomatic carriage of P. falciparum is frequent in areas endemic for malaria and individual diagnosis of clinical malaria attacks is still difficult. We investigated the impact of changes in malaria endemicity on the diagnostic criteria for malaria attacks in an area of seasonal malaria transmission.

We analyzed the longitudinal data collected over 20 years from a daily survey of all inhabitants of Ndiop, a rural community in central Senegal, in a logistic regression model to investigate the relationship between the level of Plasmodium falciparum parasitemia and the risk of fever, with the aim of determining the best parasitemia thresholds for attributing to malaria a fever episode.

A total of 34,136 observations recorded from July 1993 to December 2013 from 850 individuals aged from 1 day to 87 years were included. P. falciparum asymptomatic carriage declined from 36% to 1% between 1993 and 2013. A total of 9,819 fever episodes were associated with a positive blood film for P. falciparum. Using age-dependent parasitemia thresholds for attributing to malaria a fever episode, we recorded 6,006 malaria attacks during the study period. Parasitemia thresholds seemed to be lower during the low-to-zero transmission season and tended to decrease with changes in control policies. The number of clinical malaria attacks was overestimated for all age groups throughout the study when all fever episodes associated with P. falciparum parasitemia were defined as malaria attacks.

Pyrogenic thresholds are particularly sensitive to changes in malaria epidemiology and are therefore an interesting tool to accurately assess the burden of malaria in the context of declining transmission.

Partial Text

In recent years, the scale-up of new treatments and effective prevention tools has led to major advances in the fight against malaria [1,2]. However, it is still difficult to precisely assess the population-scale impact of these various strategies on malaria morbidity, due to the lack of reliable surveillance data, the varying diagnosis criteria, and the limited epidemiological methods used to estimate the disease burden. Indeed, most individuals in areas endemic for malaria progressively acquire partial and labile immunity, which allows them to tolerate low to moderate levels of parasitemia without experiencing clinical symptoms [3]. Thus, the detection of parasites in the blood film from a febrile individual is not sufficient to distinguish a malaria attack from other causes of fever.

In this cohort of villagers living in an initially moderate and seasonal malaria transmission area, we observed that malaria epidemiology and the pyrogenic parasitemia thresholds guiding the definition of malaria attacks tend to change over time. These results corroborate those observed in Dielmo, a neighboring locality with initial intense and perennial malaria transmission and where the same treatment policies and control measures were implemented in parallel [13,19].

The pyrogenic parasitemia threshold model is applicable to cohort studies in the context of seasonal malaria transmission, as previously documented for intense and perennial transmission settings. Pyrogenic thresholds are not fixed and particularly sensitive to the evolution of the epidemiological profile, and are therefore an interesting tool to accurately assess the burden of malaria in the context of declining transmission.




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