Last year when we visited my native country India, my 2yr old daughter was bitten by a lot of mosquitos while I, who slept right next to her, was avoided by them! While discussing this, my mom-in-law casually remarked that mosquitos’ preference for O blood group individuals rendered her a victim of their vicious bites too. I was intrigued and found that a previous study had indeed found that the Aegus albopictus mosquito which can harbor viruses that cause chikungunya, dengue etc, does have a preference for certain blood types. The female Anopheles mosquito, a carrier of the malarial parasite, is apparently more attracted to take its blood meal from people already infected with the malarial parasite and it was not clear why. That is the central point of interest in a new short and neat study by Emami and colleagues (1). They found that the female Anopheles mosquitos prefer Red Blood Cells (RBCs) that contain a key compound secreted by the malaria causing parasite Plasmodium falciparum. It has a crazy long name and so for short it will be referred to as HMBPP and the RBCs containing it as HmbRBCs. HMBPP is a compound that contributes to the production of many other chemical compounds including aromatic ones (think compounds that give eucalyptus and ginger their unique smell).
When mosquitos were given a choice of consuming regular RBCs verses HmbRBCs, 95% of mosquitos chose the latter option. The malarial parasite Plasmodium falciparum goes through a complex life cycle that is shared between humans and mosquitos. When presented with RBCs laden with the Plasmodium parasite in two different developmental stages that it goes through in the human host, the likelihood that a mosquito fed on these RBCs doubled. When they blocked HMBPP synthesis by Plasmodium, the mosquitos did not care for such RBCs any more than the unsupplemented RBCs. The mosquitos also did not care for feeding on RBCs that were supplemented with a different compound, leading the authors to infer that HMBPP secreted by the malarial parasite needs to be present in the mosquito feeding attraction concoction.
HMBPP mixed with salt water or serum still attracted about 80% of mosquitos to check out these RBC-free meals. It is clear that HMBPP is an attractant but since mosquitos like to take blood meals and not RBC-free meals, what is the role of RBCs? RBCs usually release CO2. Now CO2 release by a host organism had been shown before to have an attractive effect on mosquitos. The authors speculated that HMBPP released by the malarial parasite sitting in the RBCs could be increasing the CO2 released by these RBCs thus making them attractive for the mosquito to consume. To test this, they captured the gaseous compounds released by the RBCs, HmbRBCs and RBCs containing the malarial parasite. As hypothesized, the amount of CO2 released increased by 16% in the HmbRBCs compared to the unsupplemented RBCs. Apart from CO2, a few other compounds were also found to be released in significantly higher amounts by HmbRBCs and parasite-containing RBCs. Interestingly, a synthetic concoction of these compounds and CO2 was as attractive to the mosquitos as HmbRBCs indicating that the right mixture of these compounds is all it takes for the mosquito to land and take its meal. Presence of HMBPP also moderately but significantly increased the meal size per mosquito.
So what is the consequence of mosquitos being attracted to take larger blood meals of RBCs containing HMBPP? Does it affect the mosquito in anyway? According to this study, no. The mosquitos survived and reproduced just fine even though they had now consumed RBCs laden with this compound. You might wonder why the mosquitos don’t get malaria? Read my other blog post to know why! Very briefly, the mosquito’s defense system makes sure that most of the parasites that land in the mosquito gut after it takes a blood meal, are cleared out. The few that escape are able to lodge themselves in the mosquito salivary gland and are transformed into a developmental phase that is now ready to infect humans and other hosts. According to this study, more of the mosquitos that consume HMBPP supplemented, parasite-laden RBCs are infection ready. That means, a higher percentage of the mosquitos that took this type of blood meal have their salivary glands teeming with the malarial parasite.
In the very last segment, they test for changes in the kind of genes expressed in the mosquito at various time points post consumption of HmbRBCs. They observed changes in genes that send messages between neurons and infer that these neurological changes might be directing the mosquito towards a malarial parasite infected host. They also observed changes in genes that control the mosquito’s innate defense response indicating that their immune response could be changed to allow a greater number of malarial parasites to survive, multiply and reside in the mosquito salivary gland. So the big take home story for us is that when given a choice of taking a blood meal from a person who is affected with malaria and an unaffected person, the mosquito might end up choosing the former 95% of the time. Such mosquitos are highly likely to pass on the parasite to whoever they take their next blood meal from. This work by Emami and colleagues uncovers HMBPP secretion as yet another trick in the malarial parasite Plasmodium falciparum’s survival tool kit that cleverly scapegoats the mosquito!