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How do these rare, rare mosquitoes survive? Notes from Pailin.

May 7, 2012 - 10:56 -- Derek Charlwood

Now as I see it a blog is sort of a diary.  So although I posted one the other day I would still like to share thoughts, bemusements, about the mosquitoes and malaria in Pailin, infamous for the fact that it was the last stronghold of the Khmer Rouge (Pol Pot died from malaria here) and the place where resistance to artemisenine originated.  More recently it should be infamous for the rate and degree of deforestation.  This has, however, probably unwittingly been an extremely effective anti-vector measure.  The vector in this neck of the woods is the forest dwelling Anopheles dirus.  The forest goes and along with a huge range of other wildlife so does An. dirus and so does malaria transmission, which anyway compared to African standards is anyway relatively low (It was also the place where resistance to chloroquine and mefloquine originated.  How come there are all these genotypes mutating in such a small gene pool?) So it is a little bit like closing the stable door after the horse has bolted.  But the almost complete absence of dirus – we have caught one in over 500 all night collections – fortunately does not compromise the work that we want to do. Which leads me to my Blog.

It has only just started raining.  This must surely be welcome both for people and mosquitoes.  For many species populations are hanging on by the skin of their teeth. We have been collecting seven out of eight days in 35 day blocks, with generally a weeks interval between them.  We undertake four landing collections between 18:30 and 22:30 (a person sitting under a raised net does part net and part landing collection) one whole night landing collection and five Furvela tent-trap collections, one running from 18:30 to 06:30 and four running from 22:30 with collections being divided into four-hourly intervals.  I also do ad hoc tent-trap collections, often away from the village. (My other blog on Barmy Barney, now eating my boot will explain why these have necessarily been reduced recently) and we have an MMX trap (one advantage of Pailin is that being close to Thailand the cooking gas is LPG whereas in the rest of Cambodia it is butane which is unsuitable for MMX trap operation).  The MMX trap catches a wider range of species than landing collections, including An. vagus We get trifling numbers of mosquitoes but the diversity is amazing.  One dirus, tiny numbers of several other anophelines, An. minimus (which incidentally does have a mating plug similar in appearance to that found in An. funestus, its African counterpart) is the most common anopheline in the village. They also get a fair number of mites (in nullipars) but they are generally old mosquitoes.  I am not dissecting for gonotrophic age but occasionally have pulled out a four par or a five par. Although they are supposed to be a secondary vector no one seems to get malaria.  Housing is very similar to that found on the island of São Tomé (wooden and raised high off the ground) and there are more dogs there per house than any other place I’ve lived. So it may be a combination of housing and going to the dogs that controls/eliminates transmission. The environment around the village in which these collections are made is a very simple one – mostly monoculture of cassava, no trees.

So we have a conundrum.

So why the diversity in the mosquitoes and what sustains it?  Are some species relics that will die out or are they immigrants from other populations where the mosquito is abundant.  What does one mosquito in 90 nights of collection mean in terms of possible population size? In an equivalent area of Mozambique we collected just two species of anopheles.  Here we have collected 10 species. Maybe I am missing something – help gratefully received.

Having started it should continue raining for a while.  We wait to see what effect this might have on our different mosquito populations.  I will keep