The American Journal of Tropical Medicine and Hygiene - Volume 68, Issue 5, May 2003

Previous studies in animal models have revealed an association between interferon-γ (IFN- γ), produced by CD8+ T cells and irradiated sporozoite-induced sterile immunity. To determine whether IFN-γ can serve as a marker of pre-erythrocytic protective immunity in individuals naturally exposed to malaria, we characterized IFN-γ and lymphocyte proliferative responses to previously defined CD8+ cytotoxic T lymphocyte (CTL) epitopes from six pre-erythrocytic stage antigens in 107 children six months to two years old from a community-based birth cohort in western Kenya. We found that IFN- γ positive responders had higher hemoglobin (Hb) levels and significantly reduced prevalence of severe malarial anemia one month after the test compared with IFN- γ non-responders, suggesting that IFN- γ immune responses to these pre-erythrocytic antigens were associated with protection against malarial anemia. Children who responded by lymphocyte proliferation had a significantly longer time to first documented malaria parasitemia after birth; however, there was no correlation between the presence of lymphocyte proliferative response and higher Hb levels. We propose that IFN- γ production could be used as a potential marker of protective immunity against malaria associated anemia in young children living in malaria holoendemic areas.

From 1992–1999, we have assessed the therapeutic efficacy of three malaria treatment regimens (chloroquine 25 mg/kg over three days, pyrimethamine/sulfadoxine 1.25/25 mg/kg in one dose, and quinine 25–30 mg/kg daily in three oral doses over a four-, five-, or seven-day period) in 1,189 children under age 10 at Malabo Regional Hospital in Equatorial Guinea. Of those children, 958 were followed up clinically and parasitologically for 14 days. With chloroquine, the failure rate varied from 55% in 1996 to 40% in 1999; the early treatment failure rate increased progressively over the years, from 6% in 1992 to 30% in 1999. With pyrimethamine/sulfadoxine, the failure rate varied from 0% in 1996 to 16% in 1995. The short quinine treatment regimens used in 1992 and 1993 (4 and 5 days, respectively) resulted in significantly higher failure rates (19% and 22%, respectively) than the 7d regimen (3–5.5%). We conclude that: a) failure rates for chloroquine are in the change period (>25%), and urgent action is needed; b) pyrimethamine/ sulfadoxine failure rates are in the alert period (6–15%), and surveillance must be continued; and c) quinine failure rates are in the grace period (<6%), so quinine can be recommended.

Parasitologic and entomologic cross-sectional surveys were carried out during an outbreak of malaria between December 1998 and August 2000 in forest villages near the Mohkhed Primary Health Center in the Chhindwara District of Madhya Pradesh in central India. In December 1998, surveys showed that more than 70% of the fever cases had malaria, with 87% of the malaria caused by Plasmodium falciparum. The rate of enlarged spleens in children was 74.5%. In November 1999, 58% of the inhabitants were infected with malaria, with 80% of these cases caused by P. falciparum. Chloroquine resistance was seen in 23% of the cases. Anopheles culicifacies was the dominant mosquito species in all surveys (70–85%) and was resistant to DDT. The results indicate that the incidence of malaria in Chhindwara has increased gradually from 0.31 per 1,000 in 1990 to 6.75 per 1,000 in 2000. Improved access to treatment facilities, combination therapy, and vector control using an effective insecticide appear to be the most promising methods for controlling malaria in this region.

In the Amazon Basin of Peru, more than 50% of patients with uncomplicated Plasmodium falciparum malaria fail to respond to treatment with chloroquine or sulfadoxine-pyrimethamine. To assist the National Malaria Control Program in identifying an alternative first-line therapy for this region, we conducted a trial of the safety and efficacy of mefloquine (MQ) compared with mefloquine-artesunate (MQ-AS) combination therapy. Patients with uncomplicated P. falciparum infections between the ages of 5 and 50 years were randomly assigned to be treated with either MQ (15 mg/kg in a single oral dose) or MQ (15 mg/kg) plus AS (4 mg/kg/day for three days). A total of 98 patients were enrolled and followed for 28 days. None of the 47 patients who received MQ alone or the 51 patients who received MQ-AS combination therapy had recurrences of parasitemia during the 28-day follow-up period. Asexual parasite densities decreased significantly more rapidly and the proportion of patients with gametocytes was significantly lower on days 3–21 in the MQ-AS group than in patients treated with MQ alone. All patients tolerated the medication well. Based on the results of this study and with the objective of slowing the development of resistance, the Peruvian Ministry of Health has decided to revise its malaria treatment policy and recommend combination therapy with MQ-AS as the new first-line treatment of uncomplicated P. falciparum malaria in the Amazon region.

Using two polymorphic genetic markers, the merozoite surface protein-3α (MSP-3α) and the circumsporozoite protein (CSP), we investigated the population diversity of Plasmodium vivax in Mae Sod, Thailand from April 2000 through June 2001. Genotyping the parasites isolated from 90 malaria patients attending two local clinics for the dimorphic CSP gene revealed that the majority of the parasites (77%) were the VK210 type. Genotyping the MSP3-α gene indicated that P. vivax populations exhibited an equally high level of polymorphism as those from Papua New Guinea, a hyperendemic region. Based on the length of polymerase chain reaction products, three major types of the MSP-3α locus were distinguished, with frequencies of 74.8%, 18.7%, and 6.5%, respectively. The 13 alleles distinguished by restriction fragment length polymorphism analysis did not show a significant seasonal variation in frequency. Genotyping the MSP-3α and CSP genes showed that 19.3% and 25.6% of the patients had multiple infections, respectively, and the combined rate was 35.6%. Comparisons of MSP-3α sequences from nine clones further confirmed the high level of genetic diversity of the parasite and also suggested that geographic isolation may exist. These results strongly indicate that P. vivax populations are highly diverse and multiple clonal infections are common in this malaria-hypoendemic region of Thailand.

An open randomized controlled study of mefloquine-artesunate and mefloquine-primaquine for the treatment of uncomplicated Plasmodium falciparum malaria was carried out in Kanchanaburi in the Saiyok District in western Thailand. Weekly parasite counts from thick and thin blood films were done for six weeks. The gametocyte carriage rate was calculated and compared between the two treatment groups. Gametocytes on presentation, recrudescent infection, and reinfection were the significant factors associated with subsequent development of gametocytemia. It is the increased propensity of recrudescent infections to produce gametocytes that drives drug resistance. The results of this study confirmed that the complete eradication of asexual forms of P. falciparum by effective antimalarial treatment, but not by combination treatment with primaquine, is the most effective means to prevent subsequent gametocytemia.

The OptiMAL test detects both Plasmodium falciparum and P. vivax malaria infections. In this study, we evaluated the performance of the OptiMAL test at the Basic Health Units (BHUs) and the District Health Quarter (DHQ) Center in rural villages of Punjab, Pakistan that provide minimal health services. Two sets of blood specimens obtained from 930 suspected malaria patients attending these BHUs were tested at BHUs and the DHQ Center by microscopy and the OptiMAL test. At the BHUs, 231 (25%) of the patients were positive by microscopy and 278 (30%) patients tested positive by the OptiMAL test. At the DHQ Center, microscopic analysis of a second set of specimens from the same patients confirmed the malaria infection in 386 (42%) patients and the OptiMAL test result was positive in 300 (32%) patients. To determine the performance of OptiMAL test at the BHUs and the DHQ Center, all data were compared with microscopy results obtained at the DHQ Center. The OptiMAL test results for P. falciparum at the BHUs were comparable to those of the OptiMAL test at the DHQ Center. However, the sensitivity, positive predictive value (PPV), and negative predictive value (NPV) of the OptiMAL test were considerably lower for P. vivax infections than for P. falciparum infections, irrespective of whether the test was performed at the BHUs or at the DHQ Center (P. falciparum: sensitivity = 78–85%, PPV = 89–97%, NPV = 96–98%; P. vivax: sensitivity = 61–76%, PPV = 88–95%, NPV = 90–93%). The OptiMAL test also detected a number of false-positive and false-negative results at both the BHUs and the DHQ Center. The false-positive results ranged from 1% to 2%; however, the number of false-negative results was much higher (BHUs: P. falciparum = 22%, P. vivax = 39%; DHQ Center: P. falciparum = 15%, P. vivax = 24%). In conclusion, these results, when combined with other advantages of the OptiMAL test, suggest that this test can be used by relatively inexperienced persons to diagnose malaria infection in rural areas where facilities for microscopy are not available.