Spatiotemporal patterns of malaria incidence in northern Thailand.

We present a detailed analysis of long-term time series of malaria incidence in northern Thailand. Positive cases for Plasmodium falciparum and P. vivax have been recorded monthly from 1977-2002 at 13 provinces in the region. Time series statistical methods are used to examine the long-term trends and seasonal dynamics of malaria incidence at regional and provincial scales. Both malarial types are declining throughout the region, except in the two provinces that share a large border with Myanmar. The rate of decline in P. vivax has decreased across the region since the end of the 1980s, and this may be a signal of developing resistance or changing vector potential. Both species display a two-peak annual seasonality that may be attributed to patterns of vector occurrence, farming practice and migration of individuals across international borders. In a number of provinces, the importance of the first seasonal peak has grown in recent years, possibly owing to increases in vector densities. The medium-term fluctuations of both species exhibit a clear spatial organisation. There is some evidence of a subtle close to 4-year super annual cycle in P. falciparum, which we suggest is driven by extrinsic factors relating to the climate of the region.

Determining cost-effectiveness and cost component of three malaria diagnostic models being used in remote non-microscope areas.

This cross-sectional experimental study developed a methodology to analyze the cost-effectiveness of three malaria diagnostic models: microscopy; on-site OptiMAL; and on-site Immunochromatographic Test (on-site ICT), used in remote non-microscope areas in Thailand, from both a public provider and patient perspective. The study covered six areas in two highly malaria-endemic areas of provinces located along the Thai-Myanmar border. The study was conducted between April and October 2000, by purposively recruiting 436 malaria suspected cases attending mobile malaria clinics. Each patient was randomly selected to receive service via the three diagnostic models; their accuracy was 95.17%, 94.48% and 89.04%, respectively. In addition, their true positive rates for all malaria species were 76.19%, 82.61% and 73.83%; for falciparum malaria 85.71%, 80.95% and 80.00%, and for vivax malaria 57.14%, 100% and 50%, respectively, with the parasitemia ranging from 80 to 58,240 microl of blood. Consequently, their costs were determined by dividing into provider and consumer costs, which were consequently classified into internal and external costs. The internal costs were the costs of the public providers, whereas the external costs were those incurred by the patients. The aggregate costs of these three models were 58,500.35, 36,685.91, and 40,714.01 Baht, respectively, or 339.53, 234.39, and 243.93, in terms of unit costs per actual case. In the case of microscopy, if all suspected malaria cases incurred forgone opportunity costs of waiting for treatment, the aggregate cost and unit cost per actual case were up to 188,110.89 and 944.03 Baht, respectively. Accordingly, the cost-effectiveness for all malaria species, using their true positive rates as the effectiveness indicator, was 446.75, 282.40, and 343.56 respectively, whereas for falciparum malaria it was 394.80, 289.37 and 304.91, and for vivax malaria 595.67, 234.39 and 487.86, respectively. This study revealed that the on-site OptiMAL was the most cost-effective. It could be used to supplement or even replace microscopy for this criteria in general. This study would be of benefit to malaria control program policy makers to consider using RDT technology to supplement microscopy in remote non-microscope areas.

Desbutyl-benflumetol, a novel antimalarial compound: in vitro activity in fresh isolates of Plasmodium falciparum from Thailand.

Desbutyl-benflumetol (DBB) is a novel antimalarial compound closely related to benflumetol (lumefantrine), of which it is a putative metabolite. The in vitro response of Plasmodium falciparum to DBB was studied in Mae Hong Son and Mae Sot, in northwest Thailand, in 1997 and 1998. In total, 155 fresh isolates were successfully tested using the World Health Organization standard in vitro microtest system (Mark II). The mean 50% effective concentration (EC(50)) and 90% effective concentration of DBB were 6.36 and 31.09 nmol/liter, respectively. The comparison of the activity of DBB and benflumetol yielded a highly significant potency ratio of 4.52, corresponding to a more than four times higher efficacy of DBB. A considerable potency difference was found between isolates from Mae Hong Son and those from Mae Sot, reflecting lesser sensitivity in the area with marked resistance to mefloquine and quinine. This observation is also supported by a highly significant activity correlation with benflumetol (P < 0.001) and to a similar degree with mefloquine (P < 0.001), reflecting a close relationship of DBB with the class II aryl amino alcohol blood schizontocides. A less distinct association was also found with artemisinin, which was significant only at the EC(50) level, and there was no correlation at all with chloroquine. DBB is a promising antimalarial compound that merits further investigation in order to define its practical therapeutic potential.

Variation in microfilariae and infective stages of two types of Wuchereria bancrofti from the Thai-Myanmar border.

Comparative morphometric and morphological studies of microfilariae and infective stages were undertaken in nocturnally periodic and subperiodic Wuchereria bancrofti. For microfilariae, the body dimensions of nocturnally periodic (NP) were significantly smaller than nocturnally subperiodic (NSP), i.e., body length 268.03+/-14.75 microm (NP), 307.61+/-11.52 microm (NSP); cephalic space length 4.21+/-0.62 microm (NP), 5.32+/-0.79 microm (NSP); head to nerve ring 49.39+/-5.43 microm (NP), 57.40+/-4.46 microm (NSP); innenkörper length 33.05+/-5.89 microm (NP), 44.02+/-8.71 microm (NSP); cephalic space width 4.28+/-0.59 microm (NP), 6.04+/-0.68 microm (NSP); body width at nerve ring 5.01+/-0.57 microm (NP), 7.45+/-0.75 microm (NSP). The number of nuclei between the cephalic space and nerve ring of NP (66.67+/-5.19) was also significantly less than in NSP (94.74+/-6.95). For infective stages, the body dimensions of NP were significantly smaller than NSP, i.e., body length 1632.50+/-131.48 microm (NP), 2002.63+/-222.60 microm (NSP); head to nerve ring 103.09+/-7.47 microm (NP), 122.44+/-9.62 microm (NSP); head to oesophago-intestinal junction 567.69+/-94.84 microm (NP), 666.75+/-110.08 microm (NSP); body width at oesophago-intestinal junction 23.15+/-1.55 microm (NP), 26.78+/-1.62 microm (NSP). It is too early to infer the NP type as an additional sibling species of W. bancrofti but it is reasonable to treat it as a new variety and additional work is needed to clarify its status.

High prevalence of Plasmodium malariae and Plasmodium ovale in malaria patients along the Thai-Myanmar border, as revealed by acridine orange staining and PCR-based diagnoses.

The prevalence of the four human malaria parasites was investigated among malaria patients at northern, central and southern towns in Thailand along the border with Myanmar between September 1995 and May 1996. Thin smears obtained from 548 Thai and Burmese patients were reviewed by an acridine orange staining method, and many mixed infections with two to four species, including P. malariae and P. ovale, were detected. These diagnostic results were compared with those by two PCR-based diagnoses, microtitre plate hybridization (MPH) and a nested PCR method, both of which targets the same, species-specific regions in the 18S rRNA genes. In both PCR diagnoses, many P. malariae and P. ovale infections were also detected. Detection sensitivity of P. malariae infection was higher in nested PCR than MPH, and a total prevalence of P. malariae infection estimated by nested PCR reached 24.3% (133/548). In 16 of them, the size of PCR products amplified by the P. malariae-specific primer was about 20-bp shorter than the expected size of 115-bp. Four of 16 possessed two different bands with normal and shorter sizes, suggesting that P. malariae isolates may be separated into two types, and that those with shorter products may be new variant form (s) with a nucleotide deletion in the target region. On the other hand, 21 P. ovale infections (3.8%) were detected by nested PCR, but four of them were MPH-negative because of the sequence variation at the probe region. These results indicated that the prevalence of P. malariae and P. ovale along the Thai-Myanmar border may be substantially higher than previously reported.

Operational trial of ParaSight-F (dipstick) in the diagnosis of falciparum malaria at the primary health care level.

The rapid manual ParaSight-F test of Plasmodium falciparum malaria, an antigen capture test for detecting trophozoite-derived histidine rich protein-2 (PF HRP-2), is simple to perform and provides a definite diagnosis within 10 minutes. During an operational trial at health centers and mobile malaria units where microscopical diagnosis is not available and using defined symptom screening criteria, 3,361 subjects were tested yielding 618 positives (18.4%) for PF-HRP-2 by ParaSight-F. Microscopic examination of the same subjects by thick blood film examined 7 days later at a malaria clinic showed 578 falciparum, and 349 vivax and mixed infection (F+V) 41. The technology proved highly effective in detecting falciparum malaria at the peripheral levels where access to malaria laboratory services are difficult, thus allowing immediate administration of a complete course of treatment in the absence of a microscopic examination.

Entomological evaluation of community-wide use of lambdacyhalothrin-impregnated bed nets against malaria in a border area of north-west Thailand.

This paper reports 2 studies. (i) After a year of baseline data collection, lambdacyhalothrin-treated bed nets were introduced into 3 of 5 villages in north-west Thailand, the remaining 2 being treated with placebo. Human bait collections were carried out in each village on 2 nights per month, for 8 months of each year, and the biting densities were compared between the first year and the second year. The treated bed nets did not have any significant impact on the density or parous rates of Anopheles sawadwongporni and A. maculatus s.s. populations. The results for A. dirus s.l. were not conclusive because of the low number caught. Significant reductions in biting and parous rates of A. minimus species A were observed in only one of the 3 treated villages, and there was no overall difference between treated and control groups. However, the trial suffered from the washing of nets by villagers and the low rate of reimpregnation. (ii) A short-term study involved 4 villages in a cross-over design, and lasted 48 d. For the first 24 d, residents of 2 villages were given new treated nets while the other 2 villages retained their own untreated nets. For the second 24 d, this situation was reversed. Daily light-trapping revealed no significant difference in the indoor densities or parous rates of A. minimus species A between the periods with treated or untreated nets.(ABSTRACT TRUNCATED AT 250 WORDS)