Frequency of pyrethroid resistance in Aedes aegypti and Aedes albopictus (Diptera: Culicidae) in Thailand

Thirty‐two Aedes aegypti populations collected throughout Thailand and five populations of Aedes albopictus from southern Thailand were subjected to standard WHO contact bioassays to assess susceptibility to three commonly used synthetic pyrethroids: permethrin, deltamethrin, and lambda‐cyhalothrin. A wide degree of physiological response to permethrin was detected in Ae. aegypti, ranging from 56.5% survival (Lampang, northern Thailand) to only 4% (Kalasin in northeastern and Phuket in southern Thailand). All 32 populations of Ae. aegypti were found to have evidence of incipient resistance (62.5%) or levels of survival deemed resistant (37.5%) to permethrin. Four populations of Ae. albopictus were found with incipient resistance (97 – 80% mortality) and one with resistance (< 80%) to permethrin. The majority of Ae. aegypti populations (68.7%) was susceptible (> 98% mortality) to deltamethrin, with incipient resistance (observed 97–82% mortality) in other localities. In contrast, all populations of Ae. aegypti were completely susceptible (100% mortality) to the recommended operational dosage of lambda‐cyhalothrin. All five populations of Ae. albopictus were found completely susceptible to both deltamethrin and lambda‐cyhalothrin. Evidence of defined incipient or resistance to synthetic pyrethroids mandates appropriate response and countermeasures to mitigate further development and spread of resistance. In light of these findings, we conclude that routine and comprehensive susceptibility monitoring of dengue mosquito vectors to synthetic pyrethroids should be a required component of resistance management policies and disease control activities.     

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

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.     

Susceptibility and avoidance behavior by Culex quinquefasciatus Say to three classes of residual insecticides.

The behavioral responses ofthree colonized strains of Culex quinquefasciatus, two from recent field collections in Thailand (Nonthaburi and Mae Sot) and one from a long-established colony from the National Institute of Health (NIH), Ministry of Public Health, Thailand, were compared during and after exposure to deltamethrin (0.02 g/m2), propoxur (0.2 g/m2), and fenitrothion (0.2 g/m2) using an excito-repellency escape chamber system. We observed striking differences in behavioral response and excito-repellency between mosquito strains and test compounds. Greater escape responses were observed in the NIH strain during direct contact with deltamethrin and fenitrothion compared with the two field populations. Deltamethrin was the most irritant, followed by fenitrothion. Escape responses with propoxur were significantly delayed but increased slightly towards the end of the 30-min exposure period, more notably in the Nonthaburi strain (P < 0.05). Non-contact repellent responses were generally much weaker than irritancy, with the greatest escape response seen with NIH and Nonthaburi. Deltamethrin showed the weakest repellent response overall (< 10% escape), while propoxur again demonstrated a delayed effect (NIH and Mae Sot) before escape occurred. We conclude that irritant and repellent behavioral responses by Cx. quinquefasciatus are important components for assessing the impact of residual spraying in mosquito control programs. A better understanding of chemical properties that elicit behavioral responses in mosquitoes should be considered in formulating control strategies designed to control mosquitoes or mitigate disease transmission risk.     

Key role of hydrazine to the interaction between oxaloacetic against phosphoenolpyruvic carboxykinase (PEPCK): ONIOM calculations

The interactions between oxaloacetic (OAA) and phosphoenolpyruvic carboxykinase (PEPCK) binding pocket in the presence and absence of hydrazine were carried out using quantum chemical calculations, based on the two-layered ONIOM (ONIOM2) approach. The complexes were partially optimized by ONIOM2 (B3LYP/6-31G(d):PM6) method while the interaction energies between OAA and individual residues surrounding the pocket were performed at the MP2/6-31G(d,p) level of theory. The calculated interaction energies (INT) indicated that Arg87, Gly237, Ser286, and Arg405 are key residues for binding to OAA with the INT values of ?1.93, ?2.06, ?2.47, and ?3.16 kcal mol^?1, respectively. The interactions are mainly due to the formation of hydrogen bonding interactions with OAA. Moreover, using ONIOM2 (B3LYP/6-31G(d):PM6) applied on the PEPCKHS complex, two proton transfers were observed; first, the proton was transferred from the carboxylic group of OAA to hydrazine while the second one was from Asp311 to Lys244. Such reactions cause the generation of binding strength of OAA to the pocket via electrostatic interaction. The orientations of Lys243, Lys244, His264, Asp311, Phe333, and Arg405 were greatly deviated after hydrazine incorporation. These indicate that hydrazine plays an important role in terms of not only changing the conformation of the binding pocket, but is also tightly bound to OAA resulting in its conformation change in the pocket. The understanding of such interaction can be useful for the design of hydrazine-based inhibitor for antichachexia agents.

Genetic structure and gene flow of Anopheles minimus and Anopheles harrisoni in Kanchanaburi Province, Thailand

Isozyme frequencies were compared in seven field collections of Anopheles minimus complex using starch gel electrophoresis. Mosquito collections were sampled from four districts in Kanchanaburi Province where malaria is endemic. From eight enzyme systems, nine loci and seven polymorphisms were detected, indicating limited genetic differentiation among the seven collections (F(ST) = 0.061). The highest percent polymorphic loci were observed in Bong Ti Noi (BTN) Village (55.6%), whereas the least percent polymorphism was seen in Tha Kradan (TK) Village (22.2%). Comparing villages Pra Jedee (PJ) with Pu Teuy C (PTC) and Huai Khayeng (HK) with Pra Jedee (PJ), gene flow among collections varied from 3.72 to 62.25 reproductive migrants per generation. Among the seven collections, no correlation was seen between genetic and geographical distances (P > 0.05). Anopheles minimus (former species A) and Anopheles harrisoni (former species C) from Pu Teuy fit most closely in the same cluster, possibly indicating relatively recent divergence between taxa. The genetic and epidemiological ramifications of these findings are discussed.     

Biting activity and host preference of the malaria vectors Anopheles maculatus and Anopheles sawadwongporni (Diptera: Culicidae) in Thailand

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.     

Seasonality of group size,feeding, and breeding in wild red‐shanked douc langurs (Lao PDR)

In Asian colobines, small one‐male groups (OMG) seem to predominate alongside all‐male groups (AMG), while larger multimale groups (MMG) are rare, but are reported for Hanuman langurs and red‐shanked douc langurs. Recently, however, it has been speculated that the genus Pygathrix could have multilevel societies based on (1) a theoretical extension of the multilevel societies found in Rhinopithecus to all odd‐nosed colobines and (2) first data for black‐shanked douc langurs. This assumes bands composed of small OMG with a skewed adult sex ratio. Band size may vary with seasonal food availability resulting in smaller bands when feeding competition is increased. To investigate the social organization of red‐shanked douc langurs and potential seasonal influences, we observed 2 unhabituated groups at Hin Namno National Protected Area, Lao PDR from March 2007 to August 2008 for 803 hr. We recorded births and performed group counts and scan sampling of feeding behavior. Most births (79% of N = 15) occurred from June–September, indicating a 4‐month peak conception season from November to February. Group size averaged 24.5 individuals (range 17–45) with 2.45 adult males (range 1–4). Although the smaller group remained at a stable size (about 18 individuals), the larger group reduced from about 45 to 25 individuals during the 7‐months long lean season, when less than 50% of the feeding time was spent on fruits. This suggests feeding competition as a potential cause of seasonal variation in group size. With 1.9 females per male the skew in adult sex ratio was much lower compared with Rhinopithecus, indicating MMG rather than multilevel societies. However, data on the spacing and interaction patterns between recognized individuals need to be collected and analyzed before the social organization can be determined. Detailed ecological data are furthermore required to investigate the basis for the seasonal changes in group size found. Am. J. Primatol. 73:1134–1144, 2011. © 2011 Wiley Periodicals, Inc.