Researchers have recently discovered that a common fungus makes mosquitoes more susceptible to transmitting the malaria parasite. [U.S. National Library of Medicine]
Researchers have recently discovered that a common fungus makes mosquitoes more susceptible to transmitting the malaria parasite. [U.S. National Library of Medicine]

We often don’t think of what insects, such as mosquitoes, are exposed to in their environment or how it affects their primitive immune systems. A mosquito’s sheer annoyance factor is more than enough to overshadow any sympathy one might generate toward any other species that is negatively affected by external factors. Yet, when these deleterious influences ultimately have a detrimental human health, it may warrant, at the very least, some in-depth analysis of what is going on within the mosquito’s realm.

Now, researchers at the Johns Hopkins Bloomberg School of Public Health have just identified a fungus that compromises the immune system of mosquitoes, making them more susceptible to infection with the parasite that causes malaria. The scientists believe that their discovery may help explain variations in the prevalence of malaria in different geographic areas.

Much like humans, mosquitoes are continuously exposed to a variety of microbes in their environment, and these bacteria and fungi can influence the health of mosquitoes in many ways. While malaria researchers have in the past identified microbes that block the Anopheles mosquito from being infected by the parasite that causes malaria, this is the first time they have found a microorganism that instead appears to make the mosquito more likely to become infected with—and then spread—malaria.

“This very common, naturally occurring fungus may have a significant impact on malaria transmission: It doesn’t kill the mosquitoes, it doesn’t make them sick, it just makes them more likely to become infected and thereby to spread the disease,” explained senior study investigator George Dimopoulos, Ph.D., professor in the department of molecular microbiology and immunology at the Bloomberg School and a deputy director of its Johns Hopkins Malaria Research Institute. “While this fungus is unlikely to be helpful as part of a malaria control strategy, our finding significantly advances our knowledge of the different factors that influence the transmission of malaria.”     

Malaria is transmitted through the saliva from the bite of an infected female mosquito. There are several strains of malaria parasites that affect humans and are carried by a variety of mosquito species. However, Plasmodium falciparum is the most pathogenic human strain, causing the majority of the almost 500,000 fatalities annually, and is transmitted by the Anopheles mosquitoes.

In the current study, the Hopkins team isolated the Penicillium chrysogenum fungus—a member of the same family of fungi that gives us the antibiotic penicillin—from the gut of field-caught Anopheles mosquitoes. They determined that its presence made the mosquitoes much more susceptible to being infected by the parasite that causes malaria. The fungus, they found, was compromising the immune system of the mosquitoes, allowing the malaria parasite to infect them more easily.

“We report the isolation of a common mosquito-associated ascomycete fungus, Penicillium chrysogenum, from the midgut of field-caught Anopheles mosquitoes,” the authors wrote. “Although the presence of Pe. chrysogenum in the Anopheles gambiae midgut does not affect mosquito survival, it renders the mosquito significantly more susceptible to Plasmodium infection through a secreted heat-stable factor. We further provide evidence that the mechanism of the fungus-mediated modulation of mosquito susceptibility to Plasmodium involves an upregulation of the insect’s ornithine decarboxylase gene, which sequesters arginine for polyamine biosynthesis.”

The findings from this study were published recently in Scientific Reports in an article entitled “A Natural Anopheles-Associated Penicillium chrysogenum Enhances Mosquito Susceptibility to Plasmodium Infection.”

According to the World Health Organization, there were roughly 214 million malaria cases and an estimated 438,000 malaria deaths around the world in 2015. Sub-Saharan Africa continues to carry a disproportionately high share of the global malaria burden. In 2015, the region was home to 89% of malaria cases and 91% of malaria deaths.

Though malaria vaccine development is a high priority, researchers have also focused their attention on preventing mosquitoes from transmitting malaria infections, such as renewed efforts of insecticide spraying to control the quantity of malaria-carrying mosquitoes, as well as developing ways to genetically modify mosquitoes to make them immune from the malaria parasite.   

The Johns Hopkins team had initially anticipated the fungus would act like several other bacteria identified by researchers that block mosquitoes from becoming infected with the parasite that causes malaria. Even though this fungus actually appears to worsen infections, it can still help researchers in their fight against malaria.

“We have questions we hope this finding will help us to answer, including why do we have increased transmission of malaria in some areas and not others when the presence of mosquitoes is the same?” Dr. Dimopoulos concluded. “This gives us another piece of the complicated malaria puzzle.”