A collection of parasitic worms was made from Rattus rattus jarak (Bonhote) from Pulau Jarak, “which lies in the middle of the Malacca Straits between Penang and Port Swettenham and some 85 miles from the Sembilan Islands opposite the Dindings.” (Audy, 1950). It was collected by Dr. J. R. Audy, Senior Research Officer of the Division of Virus Research and Medical Zoology, Institute for Medical Research, Kuala Lumpur while investigating scrub-typhus on the island.
After two years' use of hexachlorocyclohexane (BHC) as a larvicide in Georgetown, on Penang Island, control of Culex fatigans breeding became unsatisfactory. Two laboratory colonies of fatigans were established, one from Georgetown, and one from Kuala Lumpur where no insecticides had been used; tests were then made to determine the median lethal concentrations (MLC) of BHC, dieldrin, and DDT for the larvae of the two strains. The Georgetown strain was found to have acquired a tenfold resistance to BHC, and also to dieldrin to which it had not been exposed, but it showed no significant increase of resistance to DDT, to which it had also not been exposed. A year later, when both strains had passed through some ten generations in the laboratory without exposure to insecticides, the Georgetown strain was found to have lost much of its resistance to BHC, although the MLC was still twice that of the non-resistant Kuala Lumpur strain.