INOCULUM SIZE, INCUBATION PERIOD AND SEVERITY OF MALARIA - ANALYSIS OF DATA FROM MALARIA THERAPY RECORDS

The size of the infecting inoculum may influence the severity of malar ia, but evidence is scarce. Malaria therapy records provide a unique s ource of information on induced malaria in people. The therapy was giv en to large numbers of neurosyphilis patients and the malaria was left untreated as long as possible. Data from patients treated at the Hort on Hospital, Epsom 1923-60 with a single strain of vivax malaria were analysed to assess the influence of inoculum size on severity of disea se. Malaria was induced by mosquito bite, blood inoculation or direct sporozoite inoculation. The different measures of inoculum size were i nversely correlated with pre-patent period, as expected. Overall, info rmation was available on a total of 563 non-immune patients who were n ot treated during the first 5 days of patent parasitaemia. No strong o r consistent relationships were found between measures of inoculum siz e and any of the measures of severity used: neither parasitaemia level s, nor peak fevers, nor number of paroxysms of fever. In the largest d ata set, longer pre-patent periods were associated with tertian fever, spontaneous recovery and less use of modifying treatment. Difficultie s in interpreting the results are discussed, particularly with respect to misclassification of both exposure and outcome variables. While an inoculum size-severity relationship cannot be ruled out, a strong rel ationship is very unlikely.