A culex mosquito
Konstantin Nechaev / Alamy
If a malaria-infected bird is bitten by mosquitoes within 3 hours, the first insects to feed carry fewer malaria parasites than those that feed later – and the same can be true if infected humans are bitten.
Malaria is caused by microbes belonging to the Plasmodium group. It can cause fever and vomiting, and in extreme cases it can be fatal: in 2018 alone, almost 400,000 people died from the disease.
Malaria usually spreads when mosquitoes drink an infected person’s blood. This is because the Plasmodium parasites are in the digested blood and can later be passed on to an uninfected person, who will keep biting the mosquito.
However, some mosquito bites seem more likely to lead to infections than others, says Romain Pigeault of the University of Lausanne in Switzerland.
To investigate this, Pigeault and his colleagues exposed the legs of malaria-infected Atlantic canaries (Serinus canaria) to 100 young, hungry Culex pipiens mosquitoes for 3 hours. Every time a mosquito fed on a bird’s leg, the researchers captured the insect and either immediately frozen it in liquid nitrogen or kept it alive in isolation for a week. They then dissected the digestive tracts of each mosquito to quantify the parasite load.
The birds have now been treated and released into large, enriched aviaries, where they “lead a very good and comfortable life,” says Pigeault.
The researchers found similar numbers of Plasmodium parasites in all mosquitoes that were frozen immediately. In other words, regardless of whether a mosquito bit an infected canary for 5 minutes or 175 minutes in the 3-hour window, it has ingested a similar number of malaria-causing parasites.
However, when Pigeault and his colleagues dissected the mosquitoes, which had been kept in isolation for a week, they noticed dramatic differences. Among these insects, the later-biting mosquitoes – those that bite the canaries towards the end of the 3-hour window – carried many more parasite eggs in their bodies than the early-biting mosquitoes. This suggests that later biting mosquitoes are more likely to spread malaria if they bite an uninfected person.
Although the study was conducted in the Canaries, a similar phenomenon could occur in humans, Pigeault says.
“A large number of mosquitoes biting the same person can result in a high infection rate in the mosquitoes that bite later. So it is very important to reduce the number of bites per person, ”says Pigeault.
Because Plasmodium reproduces in mosquitos’ digestive tracts, the later biting mosquitoes could provide a more favorable environment for parasites to reproduce, Pigeault says, although it is not entirely clear why this is the case.
One idea the team is considering is that when an organism – in this case a canary – is bitten by a mosquito, the host’s immune system reacts in a way that somehow benefits the Plasmodium parasites in its blood . When another mosquito bites the organism, it takes on these amplified parasites, which can thrive and multiply better in the mosquito.
However, the birds showed no visible stress responses to the bites during the study, Pigeault says, making it unclear whether there actually is a significant immune response that the Plasmodium parasites could somehow benefit from.
Alternatively, the result could be some sort of evolutionary adaptation within the parasite. It could respond to signals that there are a lot of biting mosquitoes in the area by increasing the rate of reproduction as this could ensure that it is going through the energy-intensive reproductive process at the moment it is most likely to move from host to host, says host Pigeault.
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098 / rspb.2020.2615
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