Ion exchange at the critical point of solution.

A mixture of isobutyric acid (IBA)+water has an upper critical point of solution at 26.7°C and an IBA concentration of 4.40M. We have determined the Langmuir isotherms for the hydroxide form of Amberlite IRN-78 resin in contact with mixtures of IBA+water at temperatures, 27.0, 29.0, 31.0 and 38.0°C, respectively. The Langmuir plot at 38.0°C forms a straight line. At the three lower temperatures, however, a peak in the Langmuir plot is observed for IBA concentrations in the vicinity of 4.40M. We regard this peak to be a critical effect not only because it is located close to 4.40M, but also because its height becomes more pronounced as the temperature of the isotherm approaches the critical temperature. For concentrations in the vicinity of the peak, the data indicate that the larger isobutyrate ion is rejected by the resin in favor of the smaller hydroxide ion. This reversal of the expected ion exchange reaction might be used to separate ions according to size. Using the Donnan theory of ion exchange equilibrium, we link the swelling pressure to the osmotic pressure. We show that the peak in the Langmuir plot is associated with a maximum in the "osmotic" energy. This maximum has its origin in the concentration derivative of the osmotic pressure, which goes to zero as the critical point is approached.

The 'non-falciparum' malarias: the roles of epidemiology, parasite biology, clinical syndromes, complications and diagnostic rigour in guiding therapeutic strategies.

Plasmodium vivax, P. ovale, P. malariae and P. falciparum routinely infect humans. The infections caused by these parasites are loosely referred to as vivax (or benign tertian), ovale, malariae (or quartan) and falciparum (or malignant tertian) malaria, respectively. Recently, P. knowlesi, a parasite of macaque monkeys in South-east Asia, has been identified as the cause of uncomplicated and severe human malaria in Malaysian Borneo. The prescription of appropriate therapies for reliably diagnosed malaria requires a grasp of the epidemiology of the 'non-falciparum' malarias, the biology of the parasites involved, the chemotherapeutic strategies that are available and the problems of emerging drug resistance and changing clinical syndromes. This review is intended to increase clinicians' understanding of how these factors relate to the selection of the antimalarial drugs to be given to a case of 'non-falciparum' malaria, with the aims of improving outcomes and preventing relapses and recrudescences.

Onset of clinical immunity to Plasmodium falciparum among Javanese migrants to Indonesian Papua.

Onset of clinical immunity to Plasmodium falciparum occurred among Javanese migrants to Indonesian Papua. Surveillance of the 243 migrants investigated began on the day of their arrival in Indonesian Papua and continued for 33 months. Asexual parasitaemia without fever constituted objective evidence of clinical immunity. Compared with first infection, the odds ratio (OR) for not having fever at the fourth infection within 24 months was 3.2 [95% confidence interval (CI)=1.03-10.2; P=0.02]. The corresponding OR with fewer infections within 24 months was not distinguishable from 1.0. The level of the fourth parasitaemia within 24 months (N=58) was classified as 'high' or 'low' in relation to the median count at first infection (840 parasites/microl; N=187). Fourth parasitaemias that were low-but not those that were high (OR=1.8; CI=0.6-5.4; P=0.35)-were associated with dramatic protection from fever (OR=31; CI=3.5-1348; P=0.0001). Among the adult subjects, the risk of fever with low parasitaemia was significantly higher at the first infection than at the fourth (OR=12.6; CI=1.7-530; P=0.005), indicating the development of clinical immunity. A similar but less marked pattern appeared among the children investigated (OR=6.5; CI=0.8-285; P=0.06).

Malaria in a cohort of Javanese migrants to Indonesian Papua.

The epidemiology of infection by Plasmodium falciparum and P. vivax was investigated among Javanese migrants to an endemic region of Papua, Indonesia. A cohort of 243 migrants from Java was followed for malaria in a new settlement village in the endemic Armopa area of north-eastern Papua, beginning on the day each migrant arrived in the village. The subjects were monitored during home visits (three/week) and by the twice-monthly production of bloodsmears that were checked for malarial parasites. At the end of 33 months, 159 (65%) of the subjects remained under follow-up. The prevalence of parasitaemia in the village declined from 16% among those already living there when the study began in August 1996, to 5% when the study finished in June 1999. Over this period, 596 infections by P. falciparum and 723 by P. vivax occurred in the cohort, 22 and 27 of the subjects each experiencing at least six infections by P. falciparum and P. vivax, respectively. The incidence of malarial infection was higher during the first and second years post-migration (3.2 and 2.7 infections/person-year) than during the third (1.2 infections/person-year). Although the geometric mean parasite counts for P. falciparum increased over time (1209, 1478, and 1830 parasites/microl in the first, second and third years, respectively), the corresponding values for P. vivax (497, 535 and 490 parasites/microl) showed no such trend. Only one of the nine subjects who developed severe malaria (requiring intravenous quinine therapy) was a child, giving an odds ratio for a case of severe malaria being in an adult of 6.1 (P=0.08).

Primary infection by Plasmodium falciparum or P. vivax in a cohort of Javanese migrants to Indonesian Papua.

The clinical and parasitological characteristics of the first naturally acquired malarial infection have rarely been documented in humans. When 243 migrants from non-endemic Java were followed from the day of their arrival in Indonesian Papua, 217 (89%) were found to become infected with Plasmodium falciparum and/or P. vivax before they were lost to follow-up. The incidence of malarial infection in the children investigated (who were aged 6-10 years) was indistinguishable from that in the adults (aged >20 years), with 1.10 and 1.14 P. falciparum infections/person-year (relative risk=0.97; 95% confidence interval=0.72-1.29) and 1.47 and 1.49 P. vivax infections/person-year (relative risk=0.99; 95% confidence interval=0.72-1.29), respectively. During their first infections, the children had higher P. falciparum parasitaemias than the adults (with geometric means of 1318 and 759 parasites/microl, respectively; P=0.04) but similar P. vivax parasitaemias (with geometric means of 355 and 331 parasites/microl, respectively; P=0.76). At first infection, 56% of the subjects were febrile and 90% complained of symptoms. There were no differences between children and adults with respect to these two parameters, either for P. falciparum or P. vivax. These findings indicate that, with promptly diagnosed and treated uncomplicated malaria, migrant children and adults in north-eastern Indonesian Papua have an equal risk of malarial infection and of disease following their first infections with P. falciparum and P. vivax.

Adult Javanese migrants to Indonesian Papua at high risk of severe disease caused by malaria.

Migrants from Java arrive in hyperendemic Papua, Indonesia lacking exposure to endemic malaria. We evaluated records of evacuation to hospital with a diagnosis of severe malaria from a transmigration village in northeastern Papua. During the first 30 months, 198 residents with severe disease were evacuated (7.5 evacuations/100 person-years). During this period the risk of evacuation for adults (> 15 years of age) was 2.8. (95% CI = 2.1-3.8; P < 0.0001) relative to children, despite apparently equal exposure to risk of infection. Relative risk (RR) for adults was greatest during the first 6 months (RR > 16; 95% CI > or = 2.0-129; P = 0.0009), and diminished during the second 6 months (RR = 9.4; 95% CI = 2.7-32.8; P < 0.0001) and the third 6 months (RR = 3.7; 95% CI = 1.7-7.9; P = 0.0004). During the next two 6-month intervals, the RR for adults was 1.6 and 1.5 (95 % CI range 0.8-2.6; P < 0.18). Adults lacking chronic exposure were far more likely to progress to severe disease compared to children during initial exposure, but not after chronic exposure to infection.

The drug sensitivity and transmission dynamics of human malaria on Nias Island, North Sumatra, Indonesia.

Nias Island, off the north-western coast of Sumatra, Indonesia, was one of the first locations in which chloroquine-resistant Plasmodium vivax malaria was reported. This resistance is of particular concern because its ancient megalithic culture and the outstanding surfing conditions make the island a popular tourist destination. International travel to and from the island could rapidly spread chloroquine-resistant strains of P. vivax across the planet. The threat posed by such strains, locally and internationally, has led to the routine and periodic re-assessment of the efficacy of antimalarial drugs and transmission potential on the island. Active case detection identified malaria in 124 (17%) of 710 local residents whereas passive case detection, at the central health clinic, confirmed malaria in 77 (44%) of 173 cases of presumed 'clinical malaria'. Informed consenting volunteers who had malarial parasitaemias were treated, according to the Indonesian Ministry of Health's recommendations, with sulfadoxine-pyrimethamine (SP) on day 0 (for P. falciparum) or with chloroquine (CQ) on days 0, 1 and 2 (for P. vivax). Each volunteer was then monitored for clinical and parasite response until day 28. Recurrent parasitaemia by day 28 treatment was seen in 29 (83%) of the 35 P. falciparum cases given SP (14, 11 and four cases showing RI, RII and RIII resistance, respectively). Recurrent parasitaemia was also observed, between day 11 and day 21, in six (21%) of the 28 P. vivax cases given CQ. Although the results of quantitative analysis confirmed only low prevalences of CQ-resistant P. vivax malaria, the prevalence of SP resistance among the P. falciparum cases was among the highest seen in Indonesia. When the parasites present in the volunteers with P. falciparum infections were genotyped, mutations associated with pyrimethamine resistance were found at high frequency in the dhfr gene but there was no evidence of selection for sulfadoxine resistance in the dhps gene. Night-biting mosquitoes were surveyed by human landing collections and tested for sporozoite infection. Among the five species of human-biting anophelines collected, Anopheles sundaicus was dominant (68%) and the only species found to be infective--two (1.2%) of 167 females being found carrying P. vivax sporozoites. The risk of malarial infection for humans on Nias was considered high because of the abundance of asymptomatic carriers, the reduced effectiveness of the available antimalarial drugs, and the biting and infection 'rates' of the local An. sundaicus.

Chloroquine for the treatment of uncomplicated malaria in Guyana.

At a public hospital in Georgetown, Guyana, 44 patients seeking treatment for symptomatic, slide-confirmed malaria were given standard chloroquine (CQ) therapy and followed for 28 days. The patients apparently had pure infections with Plasmodium falciparum (14), P. vivax (13) or P. malariae (one), or mixed infections either of P. falciparum and P. vivax (17) or of P. falciparum, P. malariae and P. vivax (two). Each received supervised treatment with 10 mg CQ base/kg on each of days 0 and 1, and 5 mg/kg on day 2. On the day of enrollment (day 0), the patients complained of fever (100%), headache (100%), malaise (94%), myalgia (79%), nausea (67%), vertigo (49%) and vomiting (33%). Many (39%) were ill enough to confine themselves to bed. On day 4, fewer of the subjects complained of fever (15%), headache (15%), malaise (6%), myalgia (21%), nausea (6%), vertigo (24%) or vomiting (0%) despite the relatively high (>48%) risk of therapeutic failure. The cumulative incidence of parasitological failure against P. falciparum was 15% at day 4, 33% at day 7 and 48% at day 14. All of the P. vivax and P. malariae infections cleared before day 4 and none recurred by day 7. Two infections with P. vivax recurred later (on day 14 or 28) but in the presence of less than adequate, whole-blood concentrations of CQ plus desethyl-chloroquine (i.e. <100 ng/ml). Taken together, the results indicate a high risk of therapeutic failure of CQ against P. falciparum but also indicate that resistance to CQ in P. vivax occurs infrequently in Guyana.

Chloroquine or sulfadoxine-pyrimethamine for the treatment of uncomplicated, Plasmodium falciparum malaria during an epidemic in Central Java, Indonesia.

A recent malaria epidemic in the Menoreh Hills of Central Java has increased concern about the re-emergence of endemic malaria on Java, which threatens the island's 120 million residents. A 28-day, in-vivo test of the efficacy of treatment of malaria with antimalarial drugs was conducted among 167 villagers in the Menoreh Hills. The treatments investigated, chloroquine (CQ) and sulfadoxine-pyrimethamine (SP), constitute, respectively, the first- and second-line treatments for uncomplicated malaria in Indonesia. The prevalence of malaria among 1389 residents screened prior to enrollment was 33%. Treatment outcomes were assessed by microscopical diagnoses, PCR-based confirmation of the diagnoses, measurement of the whole-blood concentrations of CQ and desethylchloroquine (DCQ), and identification of the Plasmodium falciparum genotypes. The 28-day cumulative incidences of therapeutic failure for CQ and SP were, respectively, 47% (N = 36) and 22% (N = 50) in the treatment of P. falciparum, and 18% (N = 77) and 67% (N = 6) in the treatment of P. vivax. Chloroquine was thus an ineffective therapy for P. falciparum malaria, and the presence of CQ-resistant P. vivax and SP-resistant P. falciparum will further compromise efforts to control resurgent malaria on Java.

Randomized, parallel placebo-controlled trial of primaquine for malaria prophylaxis in Papua, Indonesia.

Malaria causes illness or death in unprotected travelers. Primaquine prevents malaria by attacking liver-stage parasites, a property distinguishing it from most chemoprophylactics and obviating 4-week postexposure dosing. A daily adult regimen of 30 mg primaquine prevented malaria caused by Plasmodium falciparum and P. vivax for 20 weeks in 95 of 97 glucose-6-phosphate dehydrogenase (G6PD)-normal Javanese transmigrants in Papua, Indonesia. In comparison, 37 of 149 subjects taking placebo in a parallel trial became parasitemic. The protective efficacy of primaquine against malaria was 93% (95% confidence interval [CI] 71%-98%); against P. falciparum it was 88% (95% CI 48%-97%), and >92% for P. vivax (95% CI >37%-99%). Primaquine was as well tolerated as placebo. Mild methemoglobinemia (mean of 3.4%) returned to normal within 2 weeks. Blood chemistry and hematological parameters revealed no evidence of toxicity. Good safety, tolerance, and efficacy, along with key advantages in dosing requirements, make primaquine an excellent drug for preventing malaria in nonpregnant, G6PD-normal travelers.

The T76 mutation in the pfcrt gene of Plasmodium falciparum and clinical chloroquine resistance phenotypes in Papua, Indonesia.

The T76 mutation in the pfcrt gene has been linked to chloroquine (CQ) resistance in Plasmodium falciparum. PCR-based analysis of pfcrt alleles was performed on pre-treatment samples from 107 individuals who had P. falciparum infections and lived in Papua, Indonesia. The results of a 28-day, in-vivo test revealed clinical resistance to CQ in 79 (74%) of the samples. The crude sensitivity of the pfcrt T76 assay for detecting the CQ-resistant infections in the samples was 96% and the crude specificity 52%. Discordance between pfcrt genotype and in-vivo phenotype was analysed either by genotyping of the merozoite surface protein-2 (to distinguish re-infection from recrudescence) or by amplification of the P. falciparum-specific small-subunit ribosomal RNA (ssrRNA) gene, using nested PCR (to detect any sub-patent but resistant parasites in infections misclassified as sensitive by the in-vivo test). When adjusting for the results of these analyses, the sensitivity and specificity of the pfcrt T76 assay for detecting the CQ-resistant infections became 93% and 82%, respectively. Overall, the present results indicate that the pfcrt T76 assay may be used to forecast therapeutic failure caused by CQ resistance. Validation requires exploration of the phenotype classifications based on the results of in-vivo tests, using genetic analyses that distinguish re-infection from recrudescence and detect microscopically subpatent parasitaemias.

Incidence of symptomatic and asymptomatic Plasmodium falciparum infection following curative therapy in adult residents of northern Ghana.

Adult residents of holoendemic malaria regions in Africa have a naturally acquired immunity (NAI) to malaria that renders them more resistant to new infections, limits parasitemia, and reduces the frequency and severity of illness. Given such attributes, it is not clear how one might evaluate drug or vaccine efficacy in adults without serious confounding. To determine symptomatic and asymptomatic malaria attack rates in adults of northern Ghana, 197 men and women underwent curative therapy for any pre-existing malaria infections at the start of the high transmission (wet) season. They were monitored for first parasitemia and first clinical episode of infection by Plasmodium falciparum over a 20-week period (May-October 1996). The cumulative incidence of primary infection by P. falciparum was 0.98 and incidence density of infection was calculated to be 7.0 cases/person-year. Symptoms were reported by 19.5% of the individuals at the time of first recurrent parasitemia. Incidence of infection, parasite density, and the frequency of symptoms were comparable in males and females. The results suggest that NAI did not provide these adults with significant defense against rapid re-infection and suggest that this population-infection design could serve to demonstrate the efficacy of a drug or vaccine in preventing parasitemia.

Meiotic recombination, cross-reactivity, and persistence in Plasmodium falciparum.

We incorporate a representation of Plasmodium falciparum recombination within a discrete-event model of malaria transmission. We simulate the introduction of a new parasite genotype into a human population in which another genotype has reached equilibrium prevalence and compare the emergence and persistence of the novel recombinant forms under differing cross-reactivity relationships between the genotypes. Cross-reactivity between the parental (initial and introduced) genotypes reduces the frequency of appearance of recombinants within three years of introduction from 100% to 14%, and delays their appearance by more than a year, on average. Cross-reactivity between parental and recombinant genotypes reduces the frequency of appearance to 36% and increases the probability of recombinant extinction following appearance from 0% to 83%. When a recombinant is cross-reactive with its parental types, its probability of extinction is influenced by cross-reactivity between the parental types in the opposite manner; that is, its probability of extinction after appearance decreases. Frequencies of P. falciparum outcrossing are mediated by frequencies of mixed-genotype infections in the host population, which are in turn mediated by the structure of cross-reactivity between parasite genotypes. The three leading hypotheses about how meiosis relates to oocyst production lead to quantitative, but no qualitative, differences in these results.

Multistage multiantigen heterologous prime boost vaccine for Plasmodium knowlesi malaria provides partial protection in rhesus macaques.

A nonhuman primate model for malaria vaccine development allowing reliable, stringent sporozoite challenge and evaluation of both cellular and antibody responses is needed. We therefore constructed a multicomponent, multistage DNA vaccine for the simian malaria species Plasmodium knowlesi including two preerythrocytic-stage antigens, the circumsporozoite protein (PkCSP) and sporozoite surface protein 2 (PkSSP2), and two blood stage antigens, apical merozoite antigen 1 (PkAMA1) and merozoite surface protein 1 (PkMSP1p42), as well as recombinant canarypox viruses encoding the four antigens (ALVAC-4). The DNA vaccine plasmids expressed the corresponding antigens in vitro and induced antiparasite antibodies in mice. Groups of four rhesus monkeys received three doses of a mixture of the four DNA vaccine plasmids and a plasmid encoding rhesus granulocyte-monocyte colony-stimulating factor, followed by boosting with a single dose of ALVAC-4. Three groups received the priming DNA doses by different routes, either by intramuscular needle injection, by intramuscular injection with a needleless injection device, the Biojector, or by a combination of intramuscular and intradermal routes by Biojector. Animals immunized by any route developed antibody responses against sporozoites and infected erythrocytes and against a recombinant PkCSP protein, as well as gamma interferon-secreting T-cell responses against peptides from PkCSP. Following challenge with 100 P. knowlesi sporozoites, 1 of 12 experimental monkeys was completely protected and the mean parasitemia in the remaining monkeys was significantly lower than that in 4 control monkeys. This model will be important in preclinical vaccine development.

Evidence for different mechanisms of chloroquine resistance in 2 Plasmodium species that cause human malaria.

Chloroquine (CQ)-resistant Plasmodium vivax malaria was first reported 12 years ago, nearly 30 years after the recognition of CQ-resistant P. falciparum. Loss of CQ efficacy now poses a severe problem for the prevention and treatment of both diseases. Mutations in a digestive vacuole protein encoded by a 13-exon gene, pfcrt, were shown recently to have a central role in the CQ resistance (CQR) of P. falciparum. Whether mutations in pfcrt orthologues of other Plasmodium species are involved in CQR remains an open question. This report describes pfcrt homologues from P. vivax, P. knowlesi, P. berghei, and Dictyostelium discoideum. Synteny between the P. falciparum and P. vivax genes is demonstrated. However, a survey of patient isolates and monkey-adapted lines has shown no association between in vivo CQR and codon mutations in the P. vivax gene. This is evidence that the molecular events underlying P. vivax CQR differ from those in P. falciparum.

Resurgent malaria at the millennium: control strategies in crisis.

Completion of the Panama Canal in 1914 marked the beginning of an era of vector control that achieved conspicuous success against malaria. In 1955 the World Health Organization (WHO) adopted the controversial Global Eradication Campaign emphasising DDT (dichlorodiphenyltrichloroethane) spraying in homes. The incidence of malaria fell sharply where the programme was implemented, but the strategy was not applied in holoendemic Africa. This, along with the failure to achieve eradication in larger tropical regions, contributed to disillusionment with the policy. The World Health Assembly abandoned the eradication strategy in 1969. A resurgence of malaria began at about that time and today reaches into areas where eradication or control had been achieved. A global malaria crisis looms. In 1993 the WHO adopted a Global Malaria Control Strategy that placed priority in control of disease rather than infection. This formalises a policy that emphasises diagnosis and treatment in a primary healthcare setting, while de-emphasising spraying of residual insecticides. The new policy explicitly stresses malaria in Africa, but expresses the intent to bring control programmes around the world into line with the strategy. This review raises the argument that a global control strategy conceived to address the extraordinary malaria situation in Africa may not be suitable elsewhere. The basis of argument lies in the accomplishments of the Global Eradication Campaign viewed in an historical and geographical context. Resurgent malaria accompanying declining vector control activities in Asia and the Americas suggests that the abandonment of residual spraying may be premature given the tools now at hand. The inadequacy of vector control as the primary instrument of malaria control in holoendemic Africa does not preclude its utility in Asia and the Americas.

Severe anemia in young children after high and low malaria transmission seasons in the Kassena-Nankana district of northern Ghana.

Malaria and anemia accounted for 41% and 18% respectively of hospital deaths in the Kassena-Nankana district of northern Ghana during 1996. We measured hemoglobin (Hb), malaria prevalence, and anthropometric indices of 6--24-month-old infants and young children randomly selected from this community at the end of the high (May-October, n = 347) and low (November-April, n = 286) malaria transmission seasons. High transmission season is characterized by rainfall (the equivalent of 800-900 mm/yr.), while the remaining months receive less than 50 mm/yr. Severe anemia, defined as Hb < 6.0 g/dL, was 22.1% at the end of the high transmission season compared to 1.4% at the end of the low transmission season (Odds Ratio [OR] = 20.1; 95% CI: 7.1-55.3). Parasitemia was 71% and 54.3% at these time points (OR = 2.1; 95% CI: 1.5-2.9). Nutritional anemia appeared to have little impact upon this seasonal difference since anthropometric indices were comparable. Although the relative contributions of other causes of severe anemia were not assessed, repeated malaria infections may be a primary determinant of severe anemia among infants and young children during the high transmission season.

Assessing drug sensitivity of Plasmodium vivax to halofantrine or choroquine in southern, central Vietnam using an extended 28-day in vivo test and polymerase chain reaction genotyping.

Chloroquine-resistant Plasmodium vivax malaria is emerging in Oceania, Asia, and Latin America. We assessed the drug sensitivity of P. vivax to chloroquine or halofantrine in two villages in southern, central Vietnam. This area has chloroquine-resistant Plasmodium falciparum but no documented chloroquine-resistant P. vivax. Standard dose chloroquine (25 mg/kg, over 48 hours) or halofantrine (8 mg/kg, 3 doses) was administered to 29 and 25 patients, respectively. End points were parasite sensitivity or resistance determined at 28 days. Of the evaluable patients, 23/23 100% (95% confidence interval [CI] 85.1-100) chloroquine and 21/24 (87.5%) (95% CI 67.6-97.3) halofantrine-treated patients were sensitive. Three halofantrine recipients had initial clearance but subsequent recurrence of their parasitemias. Genotyping of the recurrent and Day 0 parasitemias differed, suggesting either new infections or relapses of liver hypnozoites from antecedent infections. Among these Vietnamese patients, P. vivax was sensitive to chloroquine and halofantrine. Genotyping was useful for differentiating the recurrent vivax parasitemias.

Prevention of malaria in travelers.

Malaria occurs throughout the tropics and represents a serious health threat to people exposed to risk of infection. Health care providers may be the only link between information for their traveling patients and that threat. The likelihood of infection can be reduced drastically by deliberate measures that minimize exposure to biting mosquitoes. However, personal protective measures alone rarely suffice where exposure to anopheline mosquitoes and infected human beings is appreciable.

Chloroquine-resistant Plasmodium vivax in transmigration settlements of West Kalimantan, Indonesia.

Malariometric surveys were conducted during July 1996 in native Dayak villages and predominantly Javanese transmigration settlements in Ketapang district of West Kalimantan, Indonesia. Malaria prevalence ranged from 0.9% to 2.7% in Dayak villages and from 1% to 20% in the transmigration settlements. Plasmodium falciparum accounted for 67% of the cases among Dayaks but P. vivax was dominant among transmigrants, accounting for more than 72% of the infections. Chloroquine sensitivity/resistance was assessed by 28-day in vivo testing of uncomplicated malaria infections and measurement of chloroquine blood levels in cases where parasitemias reappeared within the 28-day test period. Resistance was based on the appearance of asexual parasites against chloroquine plus desethylchloroquine levels exceeding the minimally effective whole blood concentrations proposed for sensitive parasite strains (P. vivax, 100 ng/ml; P. falciparum, 200 ng/ml). All parasitemias cleared initially within four days of beginning supervised chloroquine therapy (25 mg base/kg over a 48-hr period), but asexual parasites reappeared within 28 days in 27 of 52 P. vivax and three of 12 P. falciparum cases. Chloroquine blood levels at the time of recurrent parasitemias revealed resistance in 12 of the 27 P. vivax cases and in one of the three P. falciparum cases. Genotypes of nine of the 12 recurrent P. vivax isolates matched with their primary isolates and ruled out reinfection. These findings establish the presence of chloroquine-resistant P. vivax on the island of Borneo. The pattern of malaria and the high frequency of chloroquine resistance by P. vivax at the West Kalimantan location may relate to demographic, ecologic, agricultural, and socioeconomic changes associated with transmigration.