Monthathip Kongmee

Behavioral Responses of Aedes aegypti (Diptera: Culicidae) Exposed to Deltamethrin and Possible Implications for Disease Control

Abstract Behavioral responses of nine Aedes aegypti (L.) strains, six from recent field collections and three from the long-established laboratory colonies, were tested under laboratory-controlled conditions by using an excito-repellency test system. All nine strains showed significant behavioral escape responses when exposed to deltamethrin at the standard field dose (0.02 g/m2), regardless of background insecticide susceptibility status (susceptible or tolerant/resistant). Insecticide contact irritancy played a predominate role in overall female mosquito escape responses, whereas noncontact repellency was not observed at levels significantly different from paired noncontact control tests (P > 0.01). Among the six field populations, the Jakarta (Indonesia) Toba (north Sumatra), and Bangkok female mosquitoes showed rapid exit (>78%) during 30 min of direct contact with insecticide-treated surfaces, whereas the other three strains demonstrated only moderate escape responses (32–56%) from the chambers. Moderate escape responses during direct insecticidal contact also were observed in the three laboratory test populations (44–60%). Higher percentage of mortality was observed from laboratory strains (8–33%) that failed to escape compared with nonescape females of field strains (2–16%), possibly a reflection of background deltamethrin susceptibility status. We conclude that contact irritancy is a major behavioral response of Ae. aegypti when exposed directly to deltamethrin and that rapid flight escape from areas exposed to space sprays or surfaces treated with residual pyrethroids could have a significant impact on the effectiveness of adult mosquito control and disease transmission reduction measures.

Challenges and prospects for dengue and malaria control in Thailand, Southeast Asia

Despite significant advances in the search for potential dengue vaccines and new therapeutic schemes for malaria, the control of these diseases remains difficult. In Thailand, malaria incidence is falling whereas that of dengue is rising, with an increase in the proportion of reported severe cases. In the absence of antiviral therapeutic options for acute dengue, appropriate case management reduces mortality. However, the interruption of transmission still relies on vector control measures that are currently insufficient to curtail the cycle of epidemics. Drug resistance in malaria parasites is increasing, compromising malaria control and elimination. Deficiencies in our knowledge of vector biology and vectorial capacity also hinder public health efforts for vector control. Challenges to dengue and malaria control are discussed, and research priorities identified.

Influence of nutritional and physiological status on behavioral responses of Aedes aegypti (Diptera: Culicidae) to deltamethrin and cypermethrin.

ABSTRACT Excito-repellency responses of Aedes aegypti (L.) exposed to deltamethrin and cypermethrin were assessed using an excito-repellency test system. Contact irritancy and non-contact repellency assays compared non-bloodfed (unfed) parous (post-gravid), nulliparous, early blood-fed, late blood-fed, sugar-fed, and unmated female mosquitoes for behavioral responses based on nutritional and physiological conditions at the time of testing. Rates of escape during contact exposure with either compound were most pronounced in parous mosquitoes, followed by unmated mosquitoes, when compared to other conditional states. Significantly higher numbers of parous females also escaped from control chambers compared to other cohorts (P <0.05). Irritability of blood- and sugar-fed mosquitoes was noticeably suppressed. We conclude that nutritional and physiological conditions (including age) of mosquitoes at the time of testing can significantly influence behavioral responses (excito-repellency) to insecticides. The findings indicate that whether due to chronological age, nutrition, physiological state, or innate (circadian) activity patterns, careful consideration must be given to the selection of appropriate conditioned mosquitoes for testing.

Anopheles species diversity and distribution of the malaria vectors of Thailand

In Thailand, seven Anopheles species within three species assemblages have been incriminated as important malaria vectors. Comprehensive maps showing the countrywide geographical distribution of primary and secondary malaria vector species have not yet been developed; the maps that do exist are typically restricted to specific areas or are out of date. In addition, with the advent of molecular-based species-identification tools, the geographical locations of various sibling species have been more clearly defined in the country. This review has compiled published data on the known distribution of important malaria vectors in Thailand based on genetic and molecular identification assays from 1994 to 2014, with maps of relevant species distribution provided herein.

Biting Activity and Host Preference of the Malaria Vectors Anopheles maculatus and Anopheles sawadwongporni (Diptera: Culicidae) in Thailand

ABSTRACT: Adult mosquitoes in the Anopheles maculatus group were surveyed from different regions of Thailand and five different species were morphologically identified, including Anopheles maculatus, Anopheles sawadwongporni, Anopheles notanandai, Anopheles dravidicus, and Anopheles willmori. Blood-feeding activity and host preference of two species, Anopheles maculatus and Anopheles sawadwongporni, were observed during a one-year period at Pu Teuy Village, Sai Yok District, Kanchanaburi Province, west-central Thailand. Both species were more prevalent during the wetter period of the year and each had a greater predilection to feed on cattle than humans. Primary feeding activity occurred between 20:00–23:00 and a smaller peak at 01:00–03:00. Findings are discussed relative to the importance of these two vectors for malaria transmission in Pu Teuy.

Seasonal Abundance and Bloodfeeding Activity of Anopheles dirus Sensu Lato in Western Thailand

Abstract Bloodfeeding activity, host preference, and seasonal abundance of the Anopheles dirus complex in relation to environmental factors were measured during a 2-yr period at Pu Teuy Village, Sai Yok District, Kanchanaburi Province, western Thailand. Anopheles dirus s.l. was found more abundant during the wet season compared with the dry and hot seasons. Bloodfeeding by An. dirus s.l. commenced immediately after sunset with a distinct peak of activity at 2000 h. Anopheles dirus s.l. fed more readily on cattle than on humans, demonstrating a significant but slightly stronger zoophilic behavior (P  =  0.02). The biting activity of An. dirus s.l. in Pu Teuy appears to differ from other localities in Thailand, suggesting the existence of significant biological variability within the species complex depending on the local circumstances.

Host feeding responses of Aedes aegypti (L.) exposed to deltamethrin

ABSTRACT: Escape responses of mated and unmated nulliparous Aedes aegypti mosquitoes were compared using three different concentrations of deltamethrin in the presence or absence of a live animal host using an excito-repellency (ER) test system. Both insecticide contact (excitation) and non-contact (repellency) test configurations were compared. For contact trials, mated mosquitoes showed similar escape movements among the three concentrations when host stimuli were absent. Significant differences in responses were seen between the lower concentrations of (LC50 and LC75) deltamethrin with and without hosts present (P<0.05). Presence or absence of host stimuli produced no significant differences in escape response for unmated females when exposed to the highest concentration (LC90) of deltamethrin. Our findings indicate that as deltamethrin concentrations decrease to sublethal levels, mating status and host cues play a more significant role in escape behavior. Therefore, insemination can influence the outcome of feeding success and flight movement of nulliparous female Ae. aegypti in contact with deltamethrin and in the presence of live host stimuli. The ER assay system serves as a useful tool for observing excitation and repellency responses of Ae. aegypti to insecticides in the presence or absence of other environmental and biological cues that can affect mosquito behavior.

Comparison of Field and Laboratory-Based Tests for Behavioral Response of Aedes aegypti (Diptera: Culicidae) to Repellents.

ABSTRACT The repellent and irritant effects of three essential oils—clove, hairy basil, and sweet basil—were compared using an excito-repellency test system against an insecticide-resistant strain of Aedes aegypti (L.) females from Pu Teuy, Kanchanaburi Province. DEET was used as the comparison standard compound. Tests were conducted under field and controlled laboratory conditions. The most marked repellent effect (spatial noncontact assay) among the three test essential oils was exhibited by sweet basil, Ocimum basilicum L. (53.8% escaped mosquitoes in 30-min exposure period) under laboratory conditions while hairy basil, Ocimum americanum L. and clove, Syzygium aromaticum (L.) Merill et. L.M. Perry from laboratory tests and sweet basil from field tests were the least effective as repellents (0–14%). In contrast, the contact assays measuring combined irritancy (excitation) and repellency effects found the best contact irritant response to hairy basil and DEET in field tests, whereas all others in laboratory and field were relatively ineffective in stimulating mosquitoes to move out the test chambers (0–5.5%). All three essential oils demonstrated significant differences in behavioral responses between field and laboratory conditions, whereas there was no significant difference in contact and noncontact assays for DEET between the two test conditions (P>0.05).

Irritant and repellent responses of Anopheles harrisoni and Anopheles minimus upon exposure to bifenthrin or deltamethrin using an excito-repellency system and a live host.

Abstract Feeding responses of Anopheles harrisoni and An. minimus were evaluated following exposure to 2 pyrethroid insecticides, bifenthrin or deltamethrin, using an excito-repellency test system in the presence and absence of live host cues. The results demonstrated that contact irritancy was the primary action of bifenthrin or deltamethrin in both mosquito species. There was no noncontact repellency effect elicited by either insecticide. Anopheles minimus showed rapid escape response with high mortality rates following direct contact with deltamethrin in the absence of a host and delayed escape responses when a host was present. Similarly, exposure of An. minimus to bifenthrin also elicited a delayed escape response in the presence of a host but with lower mortality rates. In experiments using An. harrisoni, the presence or absence of a host had no significant effect on behavioral responses to either insecticide (P > 0.05). We conclude that deltamethrin elicited stronger irritant chemical effects than bifenthrin but that behavioral responses in vector populations are dampened in the presence of an available host. This information is useful for estimating probability of pathogen transmission when using irritant chemicals in proximity to a blood-meal source.

Evaluation of a Noncontact, Alternative Mosquito Repellent Assay System

Abstract A novel noncontact repellency assay system (NCRAS) was designed and evaluated as a possible alternative method for testing compounds that repel or inhibit mosquitoes from blood feeding. Deet and Aedes aegypti were used in a controlled laboratory setting. Using 2 study designs, a highly significant difference were seen between deet-treated and untreated skin placed behind the protective screens, indicating that deet was detected and was acting as a deterrence to mosquito landing and probing behavior. However, a 2nd study showed significant differences between protected (behind a metal screen barrier) and unprotected (exposed) deet-treated forearms, indicating the screen mesh might restrict the detection of deet and thus influences landing/biting response. These findings indicate the prototype NCRAS shows good promise but requires further evaluation and possible modification in design and testing protocol to achieve more desirable operational attributes in comparison with direct skin-contact repellency mosquito assays.