Hello Everyone, i am grateful to be in the group here.

My question is,

What could be the contributing factors leading to re-enter fence of vectors and unexpected increase in malaria cases after a very long dry spell. And yet you expect almost no vectors and potential breeding sites were dried and when you conduct human landing catches you see not vectors. But upon receiving one round of rain vectors are very available.

Hello Daniel, and thanks for this very interesting question. I will try to give an answer, but invite other entomologists to also give opinion. Many people think that vectors only flourish during periods of rainfall, but this is not always true, especially for some vector species that prefer to breed in small pools, such as you get during drought periods when streams stop flowing and form pools. I will use an African example, which is Anopheles arabiensis. These mosquitoes thrive in small pools and they benefit from dry-season shrinkage of streams and ponds, especially when you have formation of a large pond (from what was a stream before) and cattle etc converge at this one available pool of water where they then trample the muddy edge to form small footprint pools, where water-seepage leads to small collections of water free from aquatic insect predators and are ideal for breeding by Anopheles arabiensis. So you can actually get a build-up of some species of vectors in the dry season. However, because these sites tend to be separate and sometimes well-removed from each other, you get focal points of such Anopheles populations, so if you conduct your human landing catch sampling in the wrong area, you will not pick up on these focal points of abundance. You need to look at the population as a large mosaic, over a large area, where some areas will have pockets of abundance while other places may not show vectors. Hope this helps answer your question at least partially. And then, with return of rainy season, these vectors then spread rapidly because now there are pools forming all over the place between the core sites that existed during the dry season.

Hello Daniel, and thanks for this very interesting question. I will try to give an answer, but invite other entomologists to also give opinion. Many people think that vectors only flourish during periods of rainfall, but this is not always true, especially for some vector species that prefer to breed in small pools, such as you get during drought periods when streams stop flowing and form pools. I will use an African example, which is Anopheles arabiensis. These mosquitoes thrive in small pools and they benefit from dry-season shrinkage of streams and ponds, especially when you have formation of a large pond (from what was a stream before) and cattle etc converge at this one available pool of water where they then trample the muddy edge to form small footprint pools, where water-seepage leads to small collections of water free from aquatic insect predators and are ideal for breeding by Anopheles arabiensis. So you can actually get a build-up of some species of vectors in the dry season. However, because these sites tend to be separate and sometimes well-removed from each other, you get focal points of such Anopheles populations, so if you conduct your human landing catch sampling in the wrong area, you will not pick up on these focal points of abundance. You need to look at the population as a large mosaic, over a large area, where some areas will have pockets of abundance while other places may not show vectors. Hope this helps answer your question at least partially. And then, with return of rainy season, these vectors then spread rapidly because now there are pools forming all over the place between the core sites that existed during the dry season.

Hi again Daniel. Here is a further response I received from senior medical entomologist Dr Tarekegn Abeku, who has decades of experience in vector control in Africa. He says:

Vectors taking refuge during dry seasons in a few spots where the microclimate and breeding conditions are suitable for maintaining low densities (in mild environments) until the rainy season starts, or aestivation (especially in hot and dry environments) are probably the most likely explanations. Intermittent pools in riverbeds have been shown to drive malaria transmission by Anopheles arabiensis during the dry season (see: https://pubmed.ncbi.nlm.nih.gov/33731115/). While hibernation in anophelines over winter months in temperate regions is well-established, aestivation is not as clearly well understood. There have been attempts to induce aestivation in controlled laboratory conditions (see: https://parasitesandvectors.biomedcentral.com/articles/10.1186/s13071-020-04276-y).

<span style=”font-family: Arial, sans-serif; font-size: 12pt;”>Hello Daniel</span>
<p class=”MsoNormal”><span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>A recent report by Charlwood et al (2021) did not find evidence of gonotrophic dissociation among 15 Anopheles species (including dominant vectors) that were dissected (n=2,883 ovaries) during sustained hot, dry periods. This was an unexpected outcome especially in the deforested environments of Cambodia which could be partly explained by the presence of wider spiracles among SE Asian Anopheles compared to African ones. </span></p>
<p class=”MsoNormal”><i><span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>“They may invest in large numbers of eggs per batch to the detriment of longevity, and thereby vectorial capacity. This is likely to reduce their ability to survive in hot dry environments, typically associated with gonotrophically discordant Anopheles. Indeed, the small number of gravid insects collected during the study implies that, for all species, including the least common ones, gonotrophic discordance was not the survival strategy adopted during the long, hot, dry season. Moreover, species such as An. maculatus s.l. and An. aconitus, previously reported as becoming discordant, did not show evidence of discordance. Thus, for all locations and all species, and at all times, gonotrophic concordance appeared to be the rule.” </span></i></p>
<p class=”MsoNormal”><span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>https://malariajournal.biomedcentral.com/articles/10.1186/s12936-016-1389-0</span></p>
<p class=”MsoNormal”><span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>In our recent field evaluation of BITE prevention tools in a forested area of Cambodia, heavier rainfall (i.e., higher numbers of ‘rain instances’) was a dominant driver of low mosquito abundance during the first 21 sampling nights followed by peak numbers in the subsequent 4 weeks. Dry season transmission was recently reported in Cambodia (Vantaux A et al 2021; accessed </span><span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>https://www.nature.com/articles/s41598-021-85628-1</span&gt;<span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>) and previously in Thailand where breeding occurred at the bottom of a deep, concrete-lined well (Rosenberg R et al 1990; accessed </span><span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>https://pubmed.ncbi.nlm.nih.gov/2189240/</span&gt;<span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>) and in Bangladesh where, 47 years ago, Ron made very close-up, mud-on-your-knee observations of An dirus (previously An balabacensis) breeding, viz:</span></p>
<p class=”MsoNormal”><i><span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>“The only dry season breeding was in a stream 3 km from the focus. The effective flight range in forest of An. dims was deduced to be about 1.5 km. Immatures were found to be peculiarly adapted to, and possibly dependent on, small, temporary pools. Eggs were deposited above the water line where they embryonated and where they could remain viable for 2 weeks. Heavy rain produced a synchronous hatch that completed immature development in 5-6 days and could eventually produce waves of parous females biting man. Larvae were observed to leave a draining pool before it completedly emptied and to crawl overland as far as 53 cm, sometimes to another pool. Larvae stranded by receding water often escaped predation that eliminated cohabiting species. They could be recovered alive from drained pool bottoms up to 94 hours after being stranded.</span></i><span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>” Quote-unquote; accessed </span><span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>https://pubmed.ncbi.nlm.nih.gov/7072884/</span&gt;</p>
<p class=”MsoNormal”><span style=”font-size: 12.0pt; line-height: 107%; font-family: ‘Arial’,sans-serif;”>Based on these close-up observations, numbers of fully embryonated eggs resting on a muddy depression that is suddenly flooded by a monsoon shower may explain both the recovery of adult mosquitoes in emergence nets only 5 days after a previously drained pool was flooded, and the unusual synchrony between total numbers biting and % parous seen in the biting catches, Fig 5 below clearly illustrates these relationships.</span></p>
 

Trying again…..
<p class=”MsoNormal”><span style=”font-family: ‘Arial’,sans-serif;”>Hello Daniel</span></p>
<p class=”MsoNormal”><span style=”font-family: ‘Arial’,sans-serif;”>A recent report by Charlwood et al (2021) did not find evidence of gonotrophic dissociation among 15 Anopheles species (including dominant vectors) that were dissected (n=2,883 ovaries) during sustained hot, dry periods. This was an unexpected outcome especially in the deforested environments of Cambodia which could be partly explained by the presence of wider spiracles among SE Asian Anopheles compared to African ones. </span></p>
<p class=”MsoNormal”><i><span style=”font-family: ‘Arial’,sans-serif;”>“They may invest in large numbers of eggs per batch to the detriment of longevity, and thereby vectorial capacity. This is likely to reduce their ability to survive in hot dry environments, <span style=”mso-spacerun: yes;”> </span>typically associated with gonotrophically discordant Anopheles. Indeed, the small number of gravid insects collected during the study implies that, for all species, including the least common ones, gonotrophic discordance was not the survival strategy adopted during the long, hot, dry season. Moreover, species such as An. maculatus s.l. and An. aconitus, previously reported as becoming discordant, did not show evidence of discordance. Thus, for all locations and all species, and at all times, gonotrophic concordance appeared to be the rule.” </span></i></p>
<p class=”MsoNormal”><span style=”font-family: ‘Arial’,sans-serif;”>https://malariajournal.biomedcentral.com/articles/10.1186/s12936-016-1389-0</span></p>
<p class=”MsoNormal”><span style=”font-family: ‘Arial’,sans-serif;”>In our recent field evaluation of BITE prevention tools in a forested area of Cambodia, heavier rainfall (i.e., higher numbers of ‘rain instances’) was a dominant driver of low mosquito abundance during the first 21 sampling nights followed by peak numbers in the subsequent 4 weeks. Dry season transmission was recently reported in Cambodia (Vantaux A et al 2021; accessed </span><span style=”font-family: ‘Arial’,sans-serif;”>https://www.nature.com/articles/s41598-021-85628-1</span&gt;<span style=”font-family: ‘Arial’,sans-serif;”>) and previously in Thailand where breeding occurred at the bottom of a deep, concrete-lined well (Rosenberg R et al 1990; accessed </span><span style=”font-family: ‘Arial’,sans-serif;”>https://pubmed.ncbi.nlm.nih.gov/2189240/</span&gt;<span style=”font-family: ‘Arial’,sans-serif;”>) and in Bangladesh where, 47 years ago, Ron made very close-up, mud-on-your-knee observations of An dirus (previously An balabacensis) breeding, viz:</span></p>
<p class=”MsoNormal”><i><span style=”font-family: ‘Arial’,sans-serif;”>“The only dry season breeding was in a stream 3 km from the focus. The effective flight range in forest of An. dims was deduced to be about 1.5 km. Immatures were found to be peculiarly adapted to, and possibly dependent on, small, temporary pools. Eggs were deposited above the water line where they embryonated and where they could remain viable for 2 weeks. Heavy rain produced a synchronous hatch that completed immature development in 5-6 days and could eventually produce waves of parous females biting man. Larvae were observed to leave a draining pool before it completedly emptied and to crawl overland as far as 53 cm, sometimes to another pool. Larvae stranded by receding water often escaped predation that eliminated cohabiting species. They could be recovered alive from drained pool bottoms up to 94 hours after being stranded.</span></i><span style=”font-family: ‘Arial’,sans-serif;”>” Quote-unquote; accessed </span><span style=”font-family: ‘Arial’,sans-serif;”>https://pubmed.ncbi.nlm.nih.gov/7072884/</span&gt;</p>
<p class=”MsoNormal”><span style=”font-family: ‘Arial’,sans-serif;”>Based on these close-up F2F observations, numbers of fully embryonated eggs resting on a muddy depression that is suddenly flooded by a monsoon shower may explain both the recovery of adult mosquitoes in emergence nets only 5 days after a previously drained pool was flooded, and the unusual synchrony between total numbers biting and % parous seen in the biting catches, Fig 5 below clearly illustrates these relationships. <span style=”mso-spacerun: yes;”> </span></span></p>