The chloroquine resistance of Plasmodium falciparum in Vanuatu (1980-1984): appearance, evolution, distribution

Chloroquine resistance of P. falciparum was studied in Vanuatu from March 1981 to July 1984, with limited means, and a non-systematic procedure; it was first evidenced in 1980. In vivo chloroquine resistance criteria were those established by W. H. O. In vitro testing methods were the standard W. H. O. macro-test and micro-test. 124 in vivo chloroquine resistant cases were seen: 111 cases in hospital (63 R III, 28 R II and 20 R I) and 13 cases in field studies (1 R III, 4 R II and 8 R I). 87 in vitro chloroquine sensitivity tests were carried out. A high failure rate was apparently due to a defective batch of lyophilised culture medium. Out of the 25 isolates successfully tested, 22 showed in vitro chloroquine resistance (88%). Correlation between in vivo and in vitro resistance was good in 13 cases studied by both methods. Moreover, 13 in vitro mefloquine sensitivity tests evidenced a high sensitivity to this drug. Chloroquine resistant malaria thus appears to be extended to the whole country. Its prevalence remains unknown but was estimated at 60% at least in 1984. Moreover, a geographical spread of chloroquine resistance from north to south of the group was evidenced between 1980 and 1984, identical to that of the dramatic increase of P. falciparum incidence at the same period of time. Possible mechanisms of the advent of chloroquine resistance in Vanuatu are discussed.