Causes, origins and possible solutions to insecticide resistance in Aedes aegypti from the Cayman Islands

• Main Author: Harris, Angela Frances
• Published: University of Liverpool 2011
• Subjects: 614.4323
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.569793

Aedes aegypti is the principle vector of dengue and yellow fever. This mosquito species is widely distributed throughout the tropics and international travel of goods and people have helped spread the mosquito and the diseases it transmits. Ae. aegypti has been recorded in the Cayman Islands four times in the past 45 years and each time efforts by the Mosquito Research and Control Unit have seemingly served to eliminate it. However, the current population of Ae. aegypti in Grand Cayman, believed to have been introduced in 2002, has proved refractory to control with current insecticide based methods. This study was conducted to assess the extent and potential causes of insecticide resistance in Ae. aegypti in Grand Cayman, investigate the likely origin of the 2002 introduction, and explore alternative methods of control. Ae. aegypti from Grand Cayman are highly resistant to DDT and pyrethroid insecticides but show only low level resistance to the organophosphate temephos. Glutathione-5- transferase, cytochrome P450 and esterase activities were elevated in adults of the Cayman Islands strains compared to a susceptible strain although use of the synergist piperonyl butoxide did not implicate increased insecticide detoxification as the major mechanism of resistance. Partial sequencing of the voltage gated sodium channel, the target site of DDT and pyrethroid insecticides, identified two single nucleotide polymorphisms, one within domain 11 56 region (V10161) and the other within domain III 56 (F1534C). A tetraplex PCR assay was designed to detect the latter mutation and its association with the insecticide resistance to permethrin and DDT was confirmed (Fisher's exact test P=O). Microsatellite analysis was employed to investigate the similarities between Cayman Islands Ae. aegypti and other populations collected from around the Caribbean. One hundred and fifty mosquitoes from three sites in the Cayman Islands and a total of 180 mosquitoes from four sites in neighbouring countries were genotyped at 12 loci. The allelic diversity suggests that the Cayman Islands population is well established, most likely receiving new introductions regularly. Fst analysis indicates closest comparative similarity to Jamaican populations of Ae. aegypti (Fst = 0.059-0.083). The high level of insecticide resistance prompted the search for alternative methods of Ae. aegypti control. The first field trial of a genetically modified sterile strain of mosquito (OX513A) was performed in collaboration with the company Oxitec Ltd. This strain carries a dominant lethal and mating with wild mosquitoes induces sterility. In the first field trial, this technology proved capable of significantly suppressing a wild Ae. aegypti population in a 16 Ha site in Grand Cayman (reduction in positive ovitraps in relation to control site, P<O.OOOl). The work carried out for this thesis has impacted upon decisions made by the department regarding insecticide choice for Ae. aegypti control. Knowledge of the mechanisms of resistance will further serve to assist in resistance management, and further studies of genetic control methods are planned.