Impact of ivermectin components on Anopheles dirus and Anopheles minimus mosquito survival

Background: Ivermectin mass drug administration to humans or livestock is a potential vector control tool for malaria elimination. Racemic ivermectin is composed of two components, namely a major component (> 80%; ivermectin B1a), which has an ethyl group at C-26, and a minor component (< 20%; ivermectin B1b), which has a methyl group at C-26. There is no difference between the efficacy of ivermectin B1a and ivermectin B1b efficacy in nematodes, but only ivermectin B1b has been reported to be lethal to snails. The ratios of ivermectin B1a and B1b ratios in ivermectin formulations and tablets can vary between manufacturers and batches. The mosquito-lethal effects of ivermectin B1a and ivermectin B1b have never been assessed. As novel ivermectin formulations are being developed for malaria control, it is important that the mosquito-lethal effects of individual ivermectin B1a and ivermectin B1b compounds be evaluated. Methods: Racemic ivermectin, ivermectin B1a or ivermectin B1b, respectively, was mixed with human blood at various concentrations, blood-fed to Anopheles dirus sensu stricto and Anopheles minimus sensu stricto mosquitoes, and mortality was observed for 10 days. The ivermectin B1a and B1b ratios from commercially available racemic ivermectin and marketed tablets were assessed by liquid chromatography-mass spectrometry. Results: The results revealed that neither the lethal concentrations that kills 50% (LC50) nor 90% (LC90) of mosquitoes differed between racemic ivermectin, ivermectin B1a or ivermectin B1b for An. dirus or An. minimus, confirming that the individual ivermectin components have equal mosquito-lethal effects. The relative ratios of ivermectin B1a and B1b derived from sourced racemic ivermectin powder were 98.84% and 1.16%, respectively, and the relative ratios for ivermectin B1a and B1b derived from human oral ivermectin tablets were 98.55% and 1.45%, respectively. Conclusions: The ratio of ivermectin B1a and B1b does not influence the Anopheles mosquito-lethal outcome, an ideal study result as the separation of ivermectin B1a and B1b components at scale is cost prohibitive. Thus, variations in the ratio of ivermectin B1a and B1b between batches and manufacturers, as well as potentially novel formulations for malaria control, should not influence ivermectin mosquito-lethal efficacy

Present status of the insecticide susceptibility of Aedes mosquitoes in Thailand

デング熱の伝播を止めるのに現在使われている方法は、流行地域に殺虫剤を噴霧することである。そのような場合、媒介昆虫の殺虫剤感受性についての情報があれば、適切な殺虫剤を散布できるであろう。このことは対策プログラムの重要な要素でもある。そこで、WHO 感受性評価キットを使って、ネッタイシマカAedes aegypti とヒトスジシマカAe.albopictus の殺虫剤に対する感受性を調べた。2008 年から2010 年にかけて、バンコク、Phra Nakhon Si Ayutthaya(プラ・ナコーン・シー・アユタヤー)、Chantha Buri(チャンタブリー)、Prachin Buri(プラチンブリー)で採集し、実験室で飼育した蚊の成虫または幼虫に、WHO が推奨している識別判定が可能な投与量の殺虫剤に1 時間暴露させ、24時間後の死亡率を調べた。おおむね、ネッタイシマカはマラチオンを除く全ての殺虫剤に対して耐性あるいは抵抗性を示した。殺虫剤抵抗性の程度は、バンコク、プラ・ナコーン・シー・アユタヤー、プラチンブリー、チャンタブリーで、それぞれ高いレベルから低いレベルまで分布していた。バンコクのBang Khae(バンケー)区およびThung Khru(トゥンクル)区では、殺虫剤に対する抵抗性が毎年増してきていた。ピレスロイド抵抗性と関係があるノックダウン型抵抗性(kdr)型抵抗性が、ネッタイシマカで観察された。一方、プラチンブリー、バンコク、Phang-nga(パンガー)から採集したヒトスジシマカは、DDT に対して耐性を示した。しかし、ピレスロイドあるいはマラチオンに対して耐性/ 抵抗性を示したヒトスジシマカはいなかった。The current method for interrupting dengue fever transmission is spraying insecticide in the area affected by the epidemic. In such cases, information about the insecticide susceptibility of the vector would allow an appropriate insecticide to be applied. It is also a key factor for control programs. Therefore, the insecticide susceptibility of Aedes aegypti and Ae. albopictus were investigated using the WHO susceptibility test kit. Adult mosquitoes or larvae collected from Bangkok, Phra Nakhon Si Ayutthaya, Chanthaburi, and Prachin Buri from 2008-2010 and reared in a laboratorywere exposed to insecticide for 1 hour at discriminating diagnostic doses recommended by the WHO, and mortality was examined after 24 hour. In general, Ae. aegypti were tolerant or resistant to all insecticides except malathion. Their degrees of insecticide resistance ranged from high to low in Bangkok, Phra Nakhon Si Ayutthaya, Prachin Buri, and Chanthaburi, respectively. In the Bang Khae and Thung Khru districts of Bangkok, insecticide resistance increased each year. knock down resistance, which is related to pyrethroid resistance, was observed in Ae. aegypti. On the other hand, Ae. albopictus from Prachin Buri, Bangkok, and Phang-nga were able to tolerate DDT. However no Ae. albopictus that were tolerant/resistant to pyrethroid or malathion were found in this study