One of the most important reasons for pest management is the protection of public health. A number of the pests that we are charged with managing can transmit serious diseases. But do we fully understand how those diseases are transmitted from insect to human? 

The transmission of pathogens from an invertebrate host to a vertebrate host is not always a hard and fast rule. There are several factors that can influence the success of transmission, such as the immune response of both the invertebrate and the vertebrate to the pathogen, the type of pathogen, the level of infection of the pathogen in the invertebrate, and more. However, one of the factors that has not been fully studied is the age of the insect host and how it can affect the success of transmission to the vertebrate host. Age can potentially be a huge factor in how pathogens are transmitted, as both the behavior and the biology of the insect host can drastically change across their lifetimes.  

This paper specifically looked at the effects of adult age of Anopheles colluzzii (a member of the Anopheles gambaie species complex) on the transmission of Plasmodium falciparum, or the protozoan parasite that causes malaria. Plasmodium falciparum is transmitted through the bite of female Anopheles mosquitoes when they blood feed to produce eggs and is the deadliest parasite in humans. Plasmodium falciparum is responsible for the majority of the deaths associated with malaria.* The researchers raised Anopheles colluzzii mosquitoes in the lab, and then provided them with a blood meal that was infected with Plasmodium falciparum either 12 days, 8 days, or 4 days post-emergence from the pupal stage. 

 

 

First, the researchers examined the ability of the different age groups of mosquitoes to act as a competent host for the parasite. This was done by counting the number of different life stages of the parasite that were visible when they dissected the adult mosquitoes. The researchers found that the oldest mosquitoes, which were fed the infected blood meal at 12 days, featured the lowest number of parasites compared to the two other age groups. This suggests that mosquitoes exposed at a later age to the parasites are less likely to be successful hosts for the parasite. 

Similarly, the researchers also looked at the progression of the parasite’s lifecycle within the mosquito post-feeding on the infected bloodmeal. This aspect of the study determined if age of the mosquito influenced Plasmodium falciparum’s development. The researchers found that despite the age differences with the mosquitoes, the development of Plasmodium continued at its normal pace, and did not differ between mosquitoes fed at 4-days or 12-days post-emergence.  

The researchers also examined the combined effects of age and infection status on mosquito survival. The mosquitoes that fed on an infected blood meal either 4- or 8-days post-emergence showed no difference in survival compared to their uninfected counterparts. However, the mosquitoes that fed on the infected bloodmeal at 12-days post-emergence had a significantly longer lifespan compared to their uninfected counterparts. This suggests that Plasmodium may have a “fountain of youth” effect on older mosquitoes that gave them a median of an extra two days of survival. 

After taking all this data into account, the researchers then created a model to estimate the ability of each age group to transmit Plasmodium falciparum. Unsurprisingly, the model revealed that the mosquitoes that were exposed at the youngest age had the highest potential to transmit the parasite. But the mosquitoes that were exposed at an older age of 12 days post-emergence were not that far behind them, and still had a higher potential for passing on the parasite than their uninfected counterparts.  

So what does all this data tell us about age and how it affects transmission of Plasmodium falciparum in Anopheles colluzzii? Mostly, much like a Facebook status in approximately the late 2000s: it’s complicated.  While mosquitoes that were exposed to the parasite at a younger adult age still hold the crown for the highest transmission potential, it appears that there are confounding factors that can still make mosquitoes exposed at an older adult age just as capable of transmission, but at a lower potential. Although the mosquitoes exposed at an older age don’t appear to be great hosts for the parasite, the extended lifespan offers more potential for the mosquito to spread Plasmodium falciparum. This may be especially true as previous studies have shown that Anopheles mosquitoes will not only increase their bloodmeal intake, but will also increase the number of times that they seek out a bloodmeal.^ 

If anything, this study has proven that age is not just a number, but an important factor to consider in disease transmission. While this study focused on the transmission of one disease, it emphasizes the complicated factors that can contribute to how a disease is passed on from invertebrate to a vertebrate. Understanding how a disease may be passed on from an invertebrate host to a vertebrate host can dictate management strategies of not only the pathogen, but also can potentially dictate how and when we treat for these public health pests.  

Article by Laura Rosenwald, BCE

References

Somé BM, Guissou E, Da DF, Richard Q, Choisy M, Yameogo KB, Hien DF, Yerbanga RS, Ouedraogo GA, Dabiré KR, Djidjou-Demasse R, Cohuet A, Lefèvre T. Mosquito ageing modulates the development, virulence and transmission potential of pathogens. Proc Biol Sci. 2024 Jan 10;291(2014):20232097. doi: 10.1098/rspb.2023.2097. Epub 2024 Jan 3. PMID: 38166422; PMCID: PMC10762442.

*Severe malarial anemia: Innate immunity and pathogenesis. Perkins, D. J.; Were, T.; Davenport, G. C.; Kempaiah, P.; Hittner, J. B.; Ong'Echa, J. M. (2011). International Journal of Biological Sciences. 7 (9): 1427–1442. doi:10.7150/ijbs.7.1427. PMC 3221949. PMID 22110393. 

^The malaria parasite, Plasmodium falciparum, increases the frequency of multiple feeding of its mosquito vector, Anopheles gambiae J.C. Koella , F.L. Sørensen and R.A. Anderson. Proceedings of the Royal Society of London. Series B: Biological Sciences Volume 265, Issue 1398 Published:07 May 1998 https://doi.org/10.1098/rspb.1998.0358 

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