A single dose of 200 mg of doxycycline was shown to be as effective as a seven day course of tetracycline, in patients suspected of having scrub typhus. 65 (44%) of the 149 patients studied fulfilled the criteria for definite diagnosis of scrub typhus; 10 had an additional diagnosis. Rickettsia tsutsugamushi was isolated from 49 (75%) patients. There was no difference between the two treatment groups in time to defervescence, abolition of cough and headache, or in the time taken to recover well-being. There were no relapses in either group. Of the remaining 84 patients, a causal diagnosis was achieved in 52. Irrespective of a diagnosis there was no difference in apparent response to either doxycycline or tetracycline.

Serological surveillance for up to two years of 114 patients with laboratory confirmed scrub typhus showed that antibody to Rickettsia tsutsugamushi as demonstrated by the indirect fluorescent antibody test is short-lived. The mean reversion time from mean peak titre (1:499) was 48.9 weeks and the calculated annual reversion rate to a titre less than 1:50 was 61%. This can be used to estimate attack rates based on point prevalence of antibody. The relationship between antibody prevalence and attack rates observed by other workers was confirmed using this model. The possible uses of the finding and its implications in Malaysia are briefly discussed.

The use of doxycycline, as a prophylactic antibiotic against scrub typhus was investigated in a prospective, randomized, double-blind study. Twenty volunteers were divided into two similar groups. Beginning three days before exposure to Leptotrombidium fletcheri chiggers infected with Rickettsia tsutsugamushi, and continuing for six weeks after exposure, one group received weekly 200-mg oral doses of doxycycline and the other group received a placebo. Nine of 10 doxycycline-treated subjects remained well during prophylaxis but developed antibody to scrub typhus, whereas nine of 10 subjects given the placebo required treatment for scrub typhus. Therefore, the efficacy of the regimen in preventing scrub typhus was 89% (eight cases prevented of nine expected). Ten days after successfully completing prophylaxis, eight of nine subjects reported minor self-limiting symptoms. A single dose of doxycycline was given on day 3 of illness to volunteers who developed scrub typhus. Such therapy was initially effective but was frequently followed by relapse and cannot be recommended.

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