Plasmodium vivax is the most prevalent
malaria infection and is an important cause of morbidity in Central and South America and Asia. P. vivax is generally sensitive to the common
antimalarial drugs but high level resistance to
chloroquine and/or
pyrimethamine has been documented in some geographic locations. In the studies reviewed here, the therapeutic responses to
antimalarial and antibacterial drugs in
vivax malaria have been assessed in the Bangkok Hospital for Tropical Diseases. The evaluated drugs consisted of the eight most widely used
antimalarial drugs and anti-bacterial drugs that possess
antimalarial activities (
tetracycline,
doxycycline,
clindamycin or
azithromycin). The activities of these drugs in descending order of parasite clearance times were
artesunate,
artemether,
chloroquine,
mefloquine,
quinine,
halofantrine,
primaquine, followed by the antibacterial drugs and lastly
sulfadoxine-pyrimethamine. Clinical responses to
sulfadoxine-pyrimethamine were also poor with evidence of high grade resistance in 42% of the patients. Of the four antibacterial drugs,
clindamycin was more effective than
azithromycin and can be an alternative to the
tetracyclines. Except for
chloroquine and
mefloquine which have long plasma half lives and may therefore suppress first relapses, the cumulative cure rates for the short acting
antimalarial drugs were similar. Double
infection with Plasmodium falciparum was common and usually manifested 3-4 weeks following clearance of
vivax malaria. The prevalence of cryptic
falciparum malaria was 8-15% and was higher in patients treated with less potent
antimalarial drugs. Follow-up studies have revealed that the relapse time in Thai patients with
vivax malaria is on average only 3 weeks, but can be suppressed by the slowly eliminated
antimalarial drugs such as
chloroquine and
mefloquine. For accurate comparison of relapse/recrudescence rates in
vivax malaria, at least 2 month's follow-up is required. It can be concluded that in malarious areas of Thailand, double
infection with P. falciparum and P. vivax is common affecting at least 25% of the patients and usually manifests as sequential illnesses. P. vivax in Thailand is sensitive to
chloroquine but has acquired high grade resistance to
sulfadoxine-pyrimethamine.