Insecticide-induced behavioral responses of Anopheles minimus, a malaria vector in Thailand.

This study was designed to determine the behavioral responses of 2 test populations of Anopheles minimus females to DDT at 2 g/m2, deltamethrin at 0.0625 g/m2, and lambdacyhalothrin at 0.0369 g/m2 using an improved excito-repellency escape chamber. One test population was colonized in 1993 and referred to as a young colony. The 2nd field test population was collected from Ta-Soa County, Tri-Yok District, Kanchanaburi Province. in western Thailand and referred to as a wild population. Results showed that females of both young and wild test populations rapidly escaped from direct contact with DDT, deltamethrin, and lambdacyhalothrin. Lambdacyhalothrin exhibited the strongest irritant effect on female mosquitoes, followed by DDT and deltamethrin. Fewer females escaped from test chambers without direct contact with treated surfaces but the response was significantly different from that of the controls (P < 0.05). The noncontact response is indicative of a noncontact repellent action. Both contact irritancy and noncontact repellency are involved in An. minimus escape responses. Experimental hut studies that include monitoring of house-entering populations of An. minimus are needed for a meaningful assessment of noncontact repellent actions.

Effects of nutritional and physiological status on behavioral avoidance of Anopheles minimus (Diptera: Culicidae) to DDT, deltamethrin and lambdacyhalothrin.

The monitoring of behavioral responses of mosquitoes to insecticides are critical to the understanding of how chemicals function in the control of disease transmission. The excito-repellency avoidance responses of laboratory-reared Anopheles minimus females exposed to diagnostic concentrations of DDT (2 g/m2), deltamethrin (0.0625 g/m2), and lambdacyhalothrin (0.0369 g/m2) were observed using an excito-repellency escape chamber. Insecticide contact (measuring irritancy) and non-contact (measuring repellency) behavioral assays were conducted on non-blood-fed (unfed), sugar-fed, early blood-fed (recently engorged) and late blood-fed mosquitoes. Rates of escape from the contact and non-contact chambers, regardless of chemical compounds, were most dramatic in unfed mosquitoes compared to other nutritional states (P < 0.05). In general, across all 3 chemicals, slower escape response was observed in sugar-fed and early blood-fed specimens, whereas late blood-fed showed an intermediate response. Relative suppression of escape flight response in comparison to matched non-insecticide treated controls and the unfed condition is likely the result of normal reduced flight activity among recent blood and sugar-engorged mosquitoes. We conclude that nutritional states and physiological conditions of mosquitoes as a result of blood feeding can dramatically influence excito-repellency test results. Therefore, for interpretive purposes, studies on chemical irritancy and repellency must account and control for the inherent variability of avoidance responses to insecticides influenced by nutritional and physiological conditions of the mosquitoes at the time of test.

Status of malaria in Thailand.

Despite decades of control success and a competent network of country-wide health infrastructure, malaria remains an important health threat in rural Thailand. All 4 known human malaria parasites have been reported present, with Plasmodium falciparum and Plasmodium vivax predominant. The expansion and intensity of multi-drug resistant Plasmodium falciparum is the most serious development to occur the last several decades. Members of 3 anopheline species complexes, Anopheles dirus, Anopheles minimus, and Anopheles manculatus, are considered to be primary malaria vectors in the country. Representatives within all 3 taxa are difficult or impossible to separate morphologically from one another, and insufficient information exists about population genetics between sibling species and vector status. Vector control in Thailand has been the primary means of malaria control, mainly by the use of routine residual insecticide spray inside houses. The use of DDT in vector control has resulted in measurable successes to interrupt malaria transmission in many parts of the country. Since 1949, DDT has been the predominant compound used: however, its public health use has continued to decline as a result of perceived operational difficulties, political issues and environmental concerns. The increased use of pyrethroids to impregnate bednets and for intradomiciliary spraying are generally more accepted by rural populations and are rapidly replacing the use of DDT. Organized malaria control activities have reduced malaria morbidity from 286/1,000 population in 1947 to 1.5/1,000 population by 1996. Despite encouraging trends in dramatically reducing malaria, the rates of disease may be re-emerging in the country as evidence from an increased annual parasite index from 1.78/1,000 in 1997 to 2.21 in 1998. The possible reasons for the apparent increase in incidence are discussed in terms of the technical, operational and social obstacles in malaria control in Thailand.

Current insecticide resistance patterns in mosquito vectors in Thailand.

Chemical pesticides are still commonly used in Thailand for control of agricultural pests and disease vectors. Organophosphates, carbamates and synthetic pyrethroids are commonly used for agricultural purposes, whereas synthetic pyrethroids have become more popular and predominate for public health use. The genetic selection of insecticide resistance (whether physiological, biochemical or behavioral) in pests and disease vectors has been extensively reported worldwide (Brown and Pal, 1971). The long-term intensive use of chemical pesticides to control insect pests and disease vectors is often cited as the reason behind the development of insecticide resistance in insect populations. Unfortunately, reliable information on vector resistance patterns to pesticides in Thailand is sparse because of a remarkable shortage of carefully controlled, systematic studies. This review gathers useful information on what is presently known about disease vector resistance to chemical pesticides in Thailand and provides some possible management strategies when serious insecticide resistance occurs.

A new ecology for scrub typhus associated with a focus of antibiotic resistance in rice farmers in Thailand.

Following the documentation of chloramphenicol-resistant and doxycycline-resistant strains of Orientia tsutsugamushi (Hyashi) in northern Thailand, we conducted ecological and epidemiological studies near the houses of patients hospitalized with antibiotic-resistant infections. New associations between chiggers, rodents, and O. tsutsugamushi in active rice agriculture areas, an ecological habitat not described previously, are reported. Rattus rattus (L.) was the most common species (representing 85.8% of the 1,433 rodents processed), followed by Rattus losea (Swinhoe) (9.4%), Bandicota indica (Bechstein) (3.6%), and Rattus argentiventer (Robinson and Kloss) (1.3%). O. tsutsugamushi was isolated from 30% of the R. rattus and R. losea, 29% of the B. indica, and 33% of the R. argentiventer collected. Mean minimum infection rates were 0.03 in Leptotrombidium chiangraiensis Tanskul & Linthicum, a new species of chigger, and 0.002 in Leptotrombidium imphalum (Vercammen-Grandjean & Langston), a chigger species not previously associated with scrub typhus transmission. Efficient vertical and horizontal transmission of O. tsutsugamushi by L. chiangraiensis and L. imphalum was demonstrated. During a 19-mo period from October 1993 to April 1995, the overall prevalence of human IgM and IgG antibody to O. tsutsugamushi was 25.5 and 47.3%, respectively. L. chiangraiensis and L. imphalum are incriminated as vectors of O. tsutsugamushi in a rice field habitat associated with a focus of antibiotic resistance.

Scanning electron microscopic study of microfilaria and the third stage larva of Wuchereria bancrofti.

The surface structures of microfilaria and of the third stage larva of Wuchereria bancrofti were studied by scanning electron microscopy. Distinct features were observed that could be used for differentiating species of this parasite. Specifically, the sheath of microfilariae of W. bancrofti projected beyond the head. The head region of the microfilaria was composed of a cephalic cap with hook, mouth and amphidial opening, and its cuticle showed annulation. Spines were absent at the first transverse annulation, and the tail end showed a slight constriction. In the infective stage larva, characters which are used for differentiating species, such as the two bubble-like ventro-lateral papillae and one dorso-terminal papilla were rather similar to each other in size, but the grooves seen around the base were absent. A previously unreported feature of the third stage larva of W. bancrofti that was discovered in this study is a papilliform process on the left side of the posterior region, between the anus and the tail end.

[Comparison of mosquito larvicidal efficacy between microbial encapsulated BTI (EBTI) and standard BTI (SBTI)].

A comparative study was conducted on the efficacy of mosquito larvicides EBTI and SBTI. The results showed that: 1. The residual efficacy of EBTI was remarkably higher than that of SBTI at 0.007-0.002ppm BTI concentration (P < 0.01), the mosquito larvae mortality of EBTI remained 100% until the 4th week; in contrast, the mosquito larva mortality of SBTI gradually and obviously declined from the 2nd to 4th week after treatment (x2 = 8.17-24.08), and reached 0 at the 4th week. 2. The lowest BTI concentration of EBTI at which a high efficacy persisted for 4 weeks was 0.007ppm BTI, the mosquito larva mortality remaining 100%.

Bionomics of Anopheles maculatus complex and their role in malaria transmission in Thailand.

The bionomics of Anopheles maculatus complex and its role in malaria transmission were conducted in Pakchong and Sadao districts, Nakhon Ratchasima and Songkhla provinces, respectively, from January 1984 to July 1985. In Pakchong, An. maculatus species A was the most dominant species, followed by species B form F and species C which was rare. The densities of species A and species B form F were high between July and November, with their peaks in October. Biting activities of both species occurred through out the night, with a major peak during the first quarter of the night on all seasons. In Sadao, only An. maculatus species B form E was detected with peak densities between February and June. Biting activities of this species varied according to seasons. The prevalence of mosquitoes was influenced by monthly rainfall, relative humidity and air-temperature. All species of female An. maculatus complex studied prefered to feed on animal rather than on human, and tended to bit human more outdoors than indoors, and thus exhibiting a zoophilic and exophagic behaviour. Life expectancies of An. maculatus species A ranged from 1.6 to 6.6 days, species B form F from 1.1 to 8.1 days, and species B form E from 0.7 to 21.2 days. The natural malaria infection rate was very low. Out of 4,430 guts dissected, only 0.23% were found infected with oocysts. There were no sporozoites detected in the 4,472 dissected salivary glands.

Bionomics of Anopheles minimus and its role in malaria transmission in Thailand.

The bionomics of Anopheles minimus, one of the main malaria vectors in Thailand, were conducted in Pakchong district, Nakhon Ratchasima province, from January 1984 to June 1985. The prevalence of An. minimus was influenced by monthly rainfall, relative humidity, temperature and wind velocity, with a major peak of density from September to November. An. minimus preferred to feed on animal rather than on human, tended to bite human more outdoors than indoors, and thus exhibiting zoophilic and exophilic behaviour. The biting activity of the mosquitoes on animal exhibited high densities throughout the night in all seasons, whereas on human they tended to be an early evening biter in the dry cool season, and early morning biter in the wet season, and thus increasing the chance of man-vector contact. The life expectancy of An. minimus varied from month to month, ranging from 2.7 to 11.5 days, with the longest longevity during the dry cool season. The natural malaria infection rate of this species was very low. Out of 1,518 dissected guts, only 0.4% were found infected with oocysts. There were no sporozoites detected in the 1,560 dissected salivary glands.