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

IntroductionAsymptomatic 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.MethodsWe 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.ResultsA 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.ConclusionPyrogenic 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.引言 恶性疟原虫（Plasmodium falciparum）无症状携带在疟疾流行区十分常见，而临床疟疾发作的个体诊断仍存在难度。本研究针对季节性疟疾传播区域，探究疟疾流行特征变化对疟疾发作诊断标准的影响。 方法 我们对塞内加尔中部乡村社区恩迪奥普（Ndiop）全体居民开展了为期20年的每日随访调查，收集得到纵向数据，并通过逻辑回归模型分析恶性疟原虫血症水平与发热风险之间的关联，旨在确定将发热事件归因于疟疾的最优虫血症阈值。 结果 本研究纳入了1993年7月至2013年12月期间，对850名年龄跨度为1天至87岁的居民收集的共计34136条观测数据。1993年至2013年间，恶性疟原虫无症状携带率从36%降至1%。共计9819次发热事件与恶性疟原虫血涂片阳性结果相关。通过采用基于年龄的虫血症阈值将发热事件归因于疟疾，本研究周期内共记录到6006次疟疾发作。在低传播至零传播季节，虫血症阈值似乎更低，且该阈值随疟疾防控政策的调整呈下降趋势。若将所有与恶性疟原虫血症相关的发热事件均定义为疟疾发作，则整个研究周期内所有年龄组的临床疟疾发作数均被高估。 结论 致热阈值对疟疾流行病学特征的变化尤为敏感，因此在疟疾传播水平下降的背景下，该指标可作为精准评估疟疾疾病负担的有效工具。