Search PubMed⌕ Search

Biomedical subjects

H K Webster

Publications and source records attributed to H K Webster.

At least 73 records · Page 4Linked to original sources
PubMed1992

Emergence of multidrug-resistant Plasmodium falciparum in Thailand: in vitro tracking.

Mefloquine was introduced into Thailand in 1985 for the treatment of Plasmodium falciparum infection. Recently, clinical failure of mefloquine was observed in southeastern Thailand, where an epidemic of falciparum malaria occurred. Beginning in 1984 and continuing until 1989, in vitro monitoring of P. falciparum isolates from Borai, a border district in the southeastern part of the country, showed a progressive decrease in mefloquine sensitivity until 1989; in 1990, the degree and prevalence of resistance accelerated. A similar pattern of resistance was observed for halofantrine, an antimalarial drug not yet commercially available in Thailand. In vitro sensitivity patterns of mefloquine and halofantrine elsewhere in the country remained relatively unchanged. These observations suggest a serious deterioration in available drugs for the treatment of falciparum malaria in southeastern Thailand that is predicted to spread throughout the country and Southeast Asia.

Animals↗
PubMed1992

Characterization of naturally acquired antibodies to the non-repetitive flanking regions of the circumsporozoite protein of Plasmodium falciparum.

Antibody responses to the circumsporozoite (CS) protein of Plasmodium falciparum have previously been reported against the central repeating tetrapeptides of this protein. Segments of the protein flanking the repeat region also contain B-cell epitopes, but specific antibody responses have not been previously characterized. Longitudinal serum sets from 16 Thai adults who developed acute falciparum malaria were selected to represent a spectrum of antibody response to the repeat region (R32). These sera were assayed by enzyme-linked immunosorbent assay using as capture antigen a recombinant fusion protein, NS1(81)RLF, which contains both flanking regions, but lacks the NANP and NVDP repeats of the P. falciparum CS protein. Antibody responses to the repeatless flanking (RLF) regions were observed in all subjects, including five individuals who lacked detectable anti-R32 antibody responses. Anti-RLF antibody responses induced by natural infection appear to be short-lived and of low-to-moderate magnitude. Thus, if anti-RLF antibodies prove to be protective, derived vaccine candidates may require presentation of these epitopes with adjuvants or delivery systems that enhance immunogenicity.

Adult↗
PubMed1992

Circumsporozoite antibody as a serologic marker of Plasmodium falciparum transmission.

In a longitudinal study of a malaria-endemic village in southeastern Thailand, circumsporozoite (CS) antibody to sporozoites of Plasmodium falciparum was measured by an enzyme-linked immunosorbent assay to determine its usefulness as a seroepidemiologic marker of malaria transmission. The CS anti-(NANP)n antibody level and prevalence during a 25-month period paralleled the pattern of seasonal transmission consistent with conventional parasitologic and entomologic measurements. The prevalence and level of antibody decreased during the non-transmission wet season, and increased over a 1-2-month transition period between the end of monsoon rains and the onset of dry conditions, an interval of maximum vector activity. Antibody increased with age in the population. The prevalence of antibody to the asexual blood stage as measured by conventional indirect fluorescent antibody assay did not coincide with changes in transmission and was sustained throughout the study period. Thus, CS antibody appeared to reflect the relative population exposure to mosquito inoculation of P. falciparum sporozoites and provided a useful measure of malaria transmission dynamics.

Adolescent↗
PubMed1992

Treatment of patients with recrudescent falciparum malaria with a sequential combination of artesunate and mefloquine.

A sequential combination of artesunate followed by mefloquine was evaluated prospectively in 24 patients with acute recrudescent falciparum malaria. The sequential combination was used to minimize possible side effects and to take advantage of the ability of artesunate to rapidly clear parasitemia and the prolonged effect of mefloquine to clear residual parasites. All patients had experienced one or more treatment failures with one or more courses of the following drugs (administered alone or in combination): quinine, tetracycline, mefloquine, artesunate, and sulfadoxine/pyrimethamine. Sequential treatment with artesunate (600 mg over five days) followed by mefloquine (750 mg and 500 mg six hours apart) cured all 24 patients. Each patient was followed for 28 days and 10 were observed for at least 35 days without clinical or parasitologic evidence of recrudescence. Fever and parasite clearance times after treatment with the sequential combination were 32.8 +/- 19.3 hr (mean +/- SD) and 40.0 +/- 16.2 hr, respectively. Susceptibility testing of selected parasite isolates indicated that all of the isolates tested were resistant to one or more antimalarial drugs. These results suggest that sequential treatment with artesunate followed by mefloquine is effective and well-tolerated in patients with recrudescent falciparum malaria.

Acute Disease↗
PubMed1992

Practical uses of acridine orange fluorescence microscopy of centrifuged blood (QBC Malaria Test) and the QBCII Hematology System in patients attending malaria clinics in Thailand.

We evaluated the detection of malaria parasites using acridine orange fluorescence microscopy of centrifuged blood (AOFM/CB or "QBC Malaria Test") at two government malaria clinics in rural Thailand. In a subgroup of the patients, a QBC Hematology System for the determination of complete blood counts was also utilized. A Giemsa-stained thick smear (GTS) reading of 100 (1,000x) microscopic fields was used as standard. The AOFM/CB sensitivities were 97% overall and 95% for P. falciparum (Pf). Sensitivity was lower for P. vivax (Pv) (76%). Pv sensitivity depended largely on ameboid form density. A threshold for AOFM/CB to consistently detect Pv ameboid forms was estimated to be 10/100 WBC (700/microliters blood). AOFM/CB was capable of detecting Pf gametocytes and schizonts more frequently than GTS. The total Pf rings per microliter blood estimated from GTS was highly correlated with the number of Pf rings per Paralens microscopic field (PMF) suggesting that AOFM/CB could be used quantitatively. From a technical standpoint, the rural tropical settings of Thailand in this study were not an obstacle to the use of QBC Hematology. The system was found to be useful in conjunction with AOFM/CB. However, in patients heavily infected with Pf gametocytes of Pv ameboid forms, their total WBC and lymphocyte counts needed to be appropriately corrected. Overall, AOFM/CB appears to be a promising tool for field diagnosis of malaria if it is affordable to developing countries.

Acridine Orange↗
PubMed1991

Mefloquine-resistant falciparum malaria on the Thai-Burmese border.

Mefloquine is the treatment of choice for uncomplicated multiresistant falciparum malaria, and in combination with sulphadoxine and pyrimethamine (MSP) at a single dose of 15/30/1.5 mg/kg, respectively, has been used in Thailand for the past 6 years. In 1985-86, MSP cured over 98% of 5192 patients with falciparum malaria on the Thai-Burmese border. 4 years later we studied the efficacy of MSP in 395 patients at the same location. The cure rate at 28 days was 70.8% (95% Cl 67-77.2%). The proportion of early treatment failures (in whom parasitaemia did not clear) had risen from 0.27 to 3.7% (p less than 0.0001). Failure rates were 50% in children under 6 years old, 29% in the 6-15 age group, and 19% in adults (p less than 0.001). Patients with early treatment failure were retreated with 25 mg/kg mefloquine, but 27% had a further recrudescence of infection within 28 days. The mean (95% Cl) serum mefloquine concentration at the time of first recrudescence was 638 (546-730) ng/ml, a value previously associated with successful treatment. Mefloquine concentrations were no lower in those with recrudescent infections than in age-matched successfully treated patients, suggesting that pharmacokinetic factors were not responsible for the high treatment-failure rate. Plasmodium falciparum has developed resistance to mefloquine rapidly, despite the addition of sulphadoxine and pyrimethamine and strict control of drug administration. The MSP combination should now be abandoned.

Adolescent↗
PubMed1991

Immune cell activation in melioidosis: increased serum levels of interferon-gamma and soluble interleukin-2 receptors without change in soluble CD8 protein.

To evaluate immune cell activation in patients with melioidosis, serum samples were assayed for interferon-gamma (IFN-gamma), soluble interleukin-2 receptors (sIL-2R), and soluble CD8 protein (sCD8). Forty patients with sepsis (23 fatal cases, 17 survivors) and 13 with localized disease were studied during acute illness; 12 additional patients were studied after discharge while on maintenance antimicrobial therapy. Serum concentrations of IFN-gamma and sIL-2R were greatly elevated, but sCD8 concentrations were not. These levels increased with disease severity and were associated with fatal outcomes. Macrophage activation by high concentrations of the cytokine IFN-gamma may contribute to pathophysiology and death in septicemic patients. Both IFN-gamma and sIL-2R seem to be predictive of outcome in patients with severe melioidosis and may prove useful in detection of relapse.

Adolescent↗
PubMed1991

Molecular basis of sequestration in severe and uncomplicated Plasmodium falciparum malaria: differential adhesion of infected erythrocytes to CD36 and ICAM-1.

The CD36 and ICAM-1 glycoproteins on vascular endothelial cells have been implicated as cytoadherence receptors for Plasmodium falciparum-infected erythrocytes (IRBC). Adhesion of IRBC from Thai patients with uncomplicated and severe falciparum malaria to purified CD36 or ICAM-1 and to C32 melanoma cells was compared. All malaria isolates bound to solid phase-adsorbed CD36 and to fluid-phase 125I-labeled CD36. IRBC adhesion to purified ICAM-1 varied widely, and no correlation with clinical severity of disease was observed. The cytoadherent phenotype of IRBC was modulated by selective panning on plates coated with purified CD36 or ICAM-1. IRBC selected by panning on CD36+, ICAM-1+ melanoma cells bound to cells that express surface CD36 but not to CD36-deficient cells, indicating that CD36 exerts a strong selective pressure on the IRBC cytoadherent phenotype. IRBC adhesion to CD36 and ICAM-1 suggests that P. falciparum parasites may use these receptors in vivo to promote parasite survival and immune evasion.

Animals↗
PubMed1991

Ciprofloxacin treatment of drug-resistant falciparum malaria.

A randomized, open study of high-dose ciprofloxacin (750 mg every 12 h) in uncomplicated falciparum malaria was conducted in Thailand. No patient completed the planned 1-week treatment course. Because of rising parasitemia (threefold higher at 36 h than on admission) and deterioration of clinical status, three individuals required quinine treatment 36 h after commencing ciprofloxacin; a fourth was given quinine at 54 h. The study was terminated early for safety reasons after only four ciprofloxacin and four control patients had been enrolled. Ciprofloxacin was well absorbed and efficiently entered erythrocytes; median plasma and red cell concentrations 90 min after the first dose were 4.0 (range, 3.7-6.8) and 5.1 (3.8-6.0) micrograms/ml, respectively. However, 50% inhibition of parasite growth in vitro required 6.6 micrograms/ml, (5.6-9.6). Ciprofloxacin should not be used alone to treat chloroquine-resistant falciparum malaria.

Adult↗
PubMed1991

An improved high-performance liquid chromatographic method for the simultaneous measurement of halofantrine and desbutylhalofantrine in human serum.

A rapid, accurate, and sensitive high-performance liquid chromatographic (HPLC) method, with ultraviolet detection, for simultaneous measurement of halofantrine (HAL) and desbutylhalofantrine (BHAL) in human serum is described. Sample preparation involved protein precipitation, followed by a solid-phase clean-up and a liquid-liquid extraction. Chromatographic separation was carried out on an Ultrasphere C8 column (25 cm x 4.6 mm I.D., 5 microns particle size) using a mobile phase of acetonitrile:water, 75:25, (vol/vol), with 0.2% (w/vol) sodium dodecyl sulfate and 0.2% (vol/vol) glacial acetic acid. The serum sample volume used was 0.5 ml, with concentrations normalized to 1 ml. Retention times for BHAL, HAL, and the internal standard were 5.5, 8.3, and 11.5 min, respectively, with a total run time of 13.5 min. The average extraction recovery for HAL was 85.6% and 100.5% for BHAL. Inter- and intra-assay precisions of HAL and BHAL were less than or equal to 7.5%, with an accuracy of less than or equal to 10.8% over three concentrations (0.02, 0.5, 2.0 micrograms/ml). Detection limits of HAL and BHAL were 5 and 3 ng/ml, respectively. The new HPLC method resulted in cleaner chromatograms and a 1.7-fold faster run time than previous HPLC methods. Application of the method with clinical specimens was demonstrated.

Antimalarials↗
PubMed1991

Quantitation of Fansimef components (mefloquine + sulfadoxine + pyrimethamine) in human plasma by two high-performance liquid chromatographic methods.

Two simple, precise, and selective high-performance liquid chromatographic methods are described for the simultaneous quantitation of mefloquine (MQ) plus pyrimethamine (PYR) or sulfadoxine (SDX) plus its principal metabolite N4-acetylsulfadoxine (N4SDX) in human plasma. After a single-step extraction, MQ plus PYR and SDX plus N4SDX including internal standards were separated using ion-paired and ion-suppression chromatography. Total run times for the assays were less than 12 min. Intraassay and interassay precision of the methods expressed as the coefficients of variation were less than 9% in plasma for the four compounds. The extraction recovery averaged 98% for MQ, 97% for PYR, 96% for SDX, and 81% for N4SDX. Plasma concentrations of the four compounds in a pediatric patient after a single oral dose of Fansimef (MQ + SDX + PYR) were determined to demonstrate the clinical application of the methods.

Antimalarials↗
PubMed1991

Measurement of ciprofloxacin in human plasma, whole blood, and erythrocytes by high-performance liquid chromatography.

A simple, rapid, and accurate high-performance liquid chromatographic method using fluorescence detection is described for the measurement of ciprofloxacin in plasma, whole blood, and erythrocytes. Ciprofloxacin and the internal standard difloxacin were separated on a mu-Bondapak C18 column (30 cm x 3.9 mm inside diameter, 10 microns particle size), using a mobile phase of 0.1 M phosphate buffer (pH 2.5):acetonitrile (75:25, vol/vol). The retention times were 5.1 min for ciprofloxacin and 7.9 min for difloxacin. The compounds were extracted from the three biological fluids using protein precipitation followed by a single-step liquid-liquid extraction. The assay is precise, with interassay coefficients of variation of less than or equal to 9.1% and an accuracy of less than or equal to 7.4% at 0.5 and 5.0 micrograms/ml (n = 5). The mean extraction recoveries of ciprofloxacin in plasma, whole blood, and erythrocytes were 84.4, 63.9, and 48.0%. The limit of detection for ciprofloxacin is 25 ng/ml. Ciprofloxacin concentrations in the three biological fluids were measured in patients with uncomplicated falciparum malaria to demonstrate the application of the method.

Adult↗
PubMed1991

Macrophage activation in vivax malaria: fever is associated with increased levels of neopterin and interferon-gamma.

In order to evaluate the relationship between fever and macrophage activation in vivax malaria, serum interferon-gamma (IFN-gamma) and urinary neopterin concentrations were determined in Thai adults with Plasmodium vivax parasitaemia. Magnitude of fever, after controlling for parasite density, was found to be positively correlated with both IFN-gamma (r = 0.47) and neopterin (r = 0.57). In the 26 febrile patients, neopterin excretion increased further during the first two days of chemotherapy (P = 0.0002). Mean neopterin values for both groups had fallen to within the normal range by the sixth day post-treatment. Thus, the fever of vivax malaria was associated with IFN-gamma induced macrophage activation, as reflected by neopterin excretion.

Adult↗
PubMed1991

Acridine orange fluorescent microscopy and the detection of malaria in populations with low-density parasitemia.

Detection of low-density malaria parasites with Giemsa-stained thick smears (G-TS) requires time and experience and becomes impractical with high sample loads. Acridine orange fluorescent microscopy (AO/FM) of capillary centrifuged blood may offer an alternative technique. We compared AO/FM readings with G-TS in 290 specimens from asymptomatic people in Thai villages endemic for malaria. AO/FM specimens were prepared in modified capillary tubes coated with acridine orange (Quantitative Buffy Coat or "QBC tubes") and examined under a fluorescent microscope. Twenty-three (85.2%) of the 27 specimens found positive by G-TS had under 100 parasites/microliters blood (less than 35 parasites/200 microscopic fields). The overall AO/FM sensitivity was 78.9% [range: 66.7% (10/15)-86.7% (13/15)]. For Plasmodium falciparum, regardless of stages, the sensitivities varied from 66.7% (8/12) to 91.7% (11/12). AO/FM performed better for P. falciparum than for Plasmodium vivax and for asexual than for sexual stages of the parasite. However, the species- and stage-specific results must be interpreted with caution because of the small sample sizes and very low parasite densities involved. The test specificity was 96.6% [range: 95.6% (263/275)-97.1% (263/271)]. These levels of accuracy plus the known advantages of AO/FM suggest that the test, supplemented with G-TS to improve species and stage differentiation, is also useful for screening low-density parasitemias.

Acridine Orange↗
PubMed1991

Naturally acquired circumsporozoite antibodies and their role in protection in endemic falciparum and vivax malaria.

The role of naturally acquired circumsporozoite (CS) antibodies in protection against falciparum and vivax malaria was evaluated in a group of Thai endemic villagers using a prospective cohort and a case-control study design. There was no evidence of protection by either the presence of positive CS antibody levels at the presumed time of sporozoite exposure or in individuals who persistently had measurable levels of the antibodies. The study defined levels of CS antibodies that were not protective in natural infection.

Adolescent↗
PubMed1991

Characteristics of natural antibody responses to the circumsporozoite protein of Plasmodium vivax.

The antibody response to the prototype circumsporozoite (CS) protein of Plasmodium vivax (CSPV) was studied in Thai soldiers experiencing occupational malaria. Seventy-four (65%) of 114 men followed during assignment to a malaria transmission area developed blood-stage infection with P. vivax. IgG antibodies against the central repeat region of the CSPV protein were quantitated by ELISA using the recombinant protein, NS181V20, as the capture antigen. One quarter of the subjects had detectable anti-CSPV antibodies at the beginning of the study. CSPV antibody seroconversion was documented in 16 of 26 subjects assessed during their first observed episodes of vivax malaria. This antibody response was of moderate magnitude, fell off after the first week post-diagnosis and appeared, at the low levels observed, to be unassociated with protection. Continued assessment of anti-CSPV antibody after subjects left the transmission area found no increase associated with release of P. vivax. These findings indicate that CS antibody responses to P. vivax during occupational malaria share many characteristics with responses to P. falciparum.

Acute Disease↗
PubMed1991

Rosette formation by Plasmodium coatneyi-infected red blood cells.

Animal models are needed for the study of cytoadherence in falciparum malaria. Red blood cell (RBC) rosette formation is one type of cytoadherence and appears to be associated with knob formation, endothelial cell adhesion and sequestration of Plasmodium-infected RBCs. Since Plasmodium coatneyi-infected RBCs develop knobs and sequester, we hypothesized that they also form rosettes. RBCs from P. coatneyi-infected rhesus monkeys (Macaca-mulatta) were collected, allowed to mature overnight in vitro and found to form rosettes as hypothesized. This observation adds to the known falciparum-like characteristics of P. coatneyi, and suggests that the Macaca mulatta-P. coatneyi model may be appropriate for pathophysiologic studies of cytoadherence.

Animals↗