Resistance in malaria vectors is likely to be caused by the massive use of insecticides in agriculture. Anopheles gambiae s.l. collected from breeding grounds in two cabbage growing areas within Accra were assessed for levels of resistance to 0.75% permethrin, 0.05% deltamethrin, 5% malathion and 4% DDT using standard WHO susceptibility test kits. Pyrethroid and organophosphate residue levels in soil and runoff water from these cabbage farms were determined and possible association between resistance and residue levels were established. Compared to the susceptible 'Kisumu' strain, both Korle-Bu and Airport populations were highly resistant to DDT and gave resistance levels which were over nine-fold for permethrin and over 2.5-fold for deltamethrin. Both wild and susceptible populations showed full susceptibility to malathion. The S and M forms of A. gambiae s.s. were found to occur in sympatry in the two study sites with a higher frequency of S form in the Airport area. Toxicity testing of extracts of soil and runoff water from these cabbage farms, using brine shrimp lethality tests, showed high level of toxicity, indicative of the presence of residues of insecticides. Differential fractionation of these extracts using solid phase extractor (SPE) suggests that the bulk of residues in these extracts may be pyrethroids and organophosphates. No correlation was observed between either residue levels or residual bioactivity in soil and runoff water, and resistance levels in A. gambiae s.l. populations, collected from breeding grounds within the farms under investigation. It is proposed that resistance in A. gambiae larvae in these breeding sites contaminated with agricultural insecticides may have occurred over time due to continuous exposure to sub-lethal doses.
SUMMARYBuruli ulcer disease caused by Mycobacterium ulcerans results in extensive destruction of skin and soft tissue and long-term functional disabilities that ultimately require surgery and rehabilitation. The disease is associated with aquatic and swampy environments with the mycobacterium occurring in biofilms, soil, aquatic insects, fish and wildlife however, the mode of transmission to humans remains an enigma. Current transmission ideas including bites from predatory water bugs and mosquitoes, do not explain satisfactorily the spasmodic disease distribution in human populations. Here we argue that Acanthamoeba species are the natural hosts of M. ulcerans and are mainly responsible for disease transmission because; (i) Acanthamoebae are known natural hosts of several microbial pathogens including M. marinum, M. avium and Legionella pneumophila, (ii) culture of slow-to-grow microbial pathogens hosted in nature by Acanthamoeba spp is enhanced when the media is seeded with the protozoa, (iii) acanthamoebae and M. ulcerans share similar bio-ecological and epidemiological settings, (iv) documented evidence that prior growth of L. pneumophila and M. avium in acanthamoebae influences entry mechanisms, intracellular growth and virulence in human monocytes, (v) Acanthamoeba spp also infect humans and cause diseases via routes of openings including broken skin and sites of trauma similar to M. ulcerans and (vi) M. ulcerans is rather a fastidious intracellular organism as recent analysis of the genome indicate. We argue further that temperature plays a significant role in transmission determining the fate of either the intracellular microbe or the host cells. Also, Acanthamoeba-pathogen association has a long evolutionary history because the same set of bacterial genes and gene products e.g. in L. pneumophila are required for survival in both mammalian and protozoan host cells. We suggest that the involvement of Acanthamoeba in the transmission of M. ulcerans to humans better explains the disease's epidemiology.
Resistance of diamondback moth (DBM), Plutella xylostella (L.) to insecticides applied for its control on cabbage was evaluated. DBM populations were tested for susceptibility to three pyrethroids (delatamethrin, lambdacyhalothrin, cypermethrin) and an organophosphate (chlorpyrifos-methyl) insecticide using larvae immersion dose-response technique. Non-specific carboxylesterase activity was used as a measure of DBM resistance. Partial characterization of carboxylesterase by zymogram studies was also undertaken using polyacrylamide gel electrophoresis (PAGE). Susceptibility studies revealed that DBM was resistant to the four insecticides assayed. DBM was least susceptible to chlorpyrifos-methyl, but most susceptible to deltamethrin. Esterase zymogram studies revealed between 2 to 7 bands, each with different characteristics. Higher frequency of the slow-moving esterases associated with high resistance was recorded in Mampong DBM population than the Accra suburbs. This finding confirmed results for carboxylesterase activity. Deltamethrin and cypermethrin demonstrated high negative correlation between fold resistance and carboxylesterase activity for β-isozyme. Chlorpyrifos-methyl, on the other hand showed positive correlation between fold resistance and α-carboxylesterase activity. The high carboxylesterase activities recorded for Mampong DBM, however, did not correlate with results of susceptibility studies. It is suggested that farmers should adopt an integrated approach using the IPM principles coupled with judicious use of biopesticides such as Bacillus thuringiensis.
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