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A study to determine the pattern and persistence of malarial antibody in infected United States citizens was devised to evaluate these parameters for use in routine diagnosis of malaria by the indirect fluorescent-antibody test. Two hundred thirty-one serum samples were collected at intervals up to 1 year from 69 men who had clinical attacks of Plasmodium vivax and Plasmodium falciparum infection in the United States after exposure in Vietnam. The specimens were tested with both P. vivax and P. falciparum washed-cell, thick-smear antigens. By 60 days after onset of symptoms, antibody titers with homologous antigens ranged from negative to 1:4,096; 98.5% had positive reactions of 1:16 or above. Serum drawn 6 months after radical cure had a significant drop in titer of 16-fold or greater; 53% had no detectable antibody. Persons with titers of 1:256 to 1:4,096 can be said to have had a recent infection 98.7% of the time. A titer of 1:16 or 1:64 is only indicative of experience with malaria. x2 analysis showed no significant re-enforcement of titers in patients with history of previous malaria attacks when compared with patients with no history of malaria.
Present address: United States Army Environmental Hygiene Agency, Edgewood Arsenal, Edgewood, Maryland 21010.