ABSTRACT
We analyzed prospectively 326 laboratory-confirmed, uncomplicated malarial infections (46.3 percent due to Plasmodium vivax, 35.3 percent due to P. falciparum, and 18.4 percent mixed-species infections) diagnosed in 162 rural Amazonians aged 5-73 years. Thirteen symptoms (fever, chills, sweating, headache, myalgia, arthralgia, abdominal pain, nausea, vomiting, dizziness, cough, dyspnea, and diarrhea) were scored using a structured questionnaire. Headache (59.8 percent), fever (57.1 percent), and myalgia (48.4 percent) were the most frequent symptoms. Ninety-six (29.4 percent) episodes, all of them diagnosed during cross-sectional surveys of the whole study population (96.9 percent by molecular technique only), were asymptomatic. Of 93 symptom-less infections left untreated, only 10 became symptomatic over the next two months following diagnosis. Fever was perceived as " intense " in 52.6 percent of 230 symptomatic malaria episodes, with no fever reported in 19.1 percent episodes although other symptoms were present. We found significant differences in the prevalence and perceived intensity of fever and other clinical symptoms in relation to parasite load at the time of diagnosis and patient's age, cumulative exposure to malaria, recent malaria morbidity, and species of malaria parasite. These factors are all likely to affect the effectiveness of malaria control strategies based on active or passive detection of febrile subjects in semi-immune populations.