New research aims to stymie the spread of malaria -- by keeping the parasite contained inside its mosquito host.
Each mosquito can carry thousands of the malaria-causing (genus Plasmodium), but only a fraction of these are transmitted to the victim of their bites. This suggests the presence of a 'bottleneck' somewhere inside the mosquitoes' bodies that retain most of the parasite.
Researchers from Johns Hopkins Medicine that looked into the issue say that the parasites are stopped in the exit channels from the mosquitoes' salivary glands. This newly-found barrier could help reduce or prevent malaria infections, the authors explain.
No spitting, please
"Our findings add substantial detail to the role of mosquito salivary glands as the gateway organs for diseases spread by these insects," says Deborah Andrew, M.S., Ph.D., professor of cell biology at the Johns Hopkins University School of Medicine.
"By enhancing transmission barriers that naturally exist in mosquitoes, we potentially can block the spread of malaria and other deadly mosquito-borne diseases, like Zika fever."
An estimated 220 million people worldwide are infected with malaria, according to the World Health Organization, most of them in tropical and subtropical regions. The disease can be treated with drugs and its spread can be prevented through mosquito eradication programs. Both approaches are quite expensive, however, which hampers overall efforts to contain malaria.
Plasmodium relies on female Anopheles mosquitoes to spread. Its life cycle begins when the insect ingests male and female parasite sex cells during a blood meal from an infected animal host (male mosquitoes eat flower nectar). These cells form fertilized eggs inside the mosquito's gut, form cysts inside its body, and then start reproducing. The final stage of their life cycle sees the parasites move into the insect's salivary glands to infect a new host when the mosquito feeds.
However, it has been noted that most of these parasites never make it out of the mosquito.
"Even though thousands of parasites invade the salivary gland, less than a 10th of them are transmitted during a mosquito bite," says Michael Wells, Ph.D., postdoctoral researcher in Andrew's laboratory and the study's lead author.
"So, we knew that the salivary gland is blocking the parasites from getting out, but we didn't know exactly how."
The Anopheles mosquito's salivary gland is made up of three lobes, each encased in a protective sheet called the basement membrane. Long ducts connect each lobe to the insect's mouth. To reach a new host, the parasites need to penetrate through the basement membrane, then through a layer of salivary cells, and then swim through a space called the secretory cavity to reach the salivary duct.
For the study, the team first allowed Anopheles mosquitoes feed on rodent blood spiked with Plasmodium. Since the mosquitoes decided how much they ate, each one ingested a different quantity of parasites. This offered the researchers data for different quantities of parasitic infection from hundreds of mosquito salivary glands.
Next, the researchers mapped out the parasites' locations by dissecting salivary glands from their mosquitoes and looking for the parasites under high-powered microscopes. Most parasites were either inside the basement membrane or in the secretory cavity, the report, with only a few parasites making it to the salivary ducts.
"The parasites seem to have no trouble getting into the salivary glands," says Wells. "So, this told us that the obstruction happens later, when parasites are trying to get to the salivary duct."
When zooming in on each cell layer in the glands' three lobes, the team found that most parasites were unable to leave the secretory cavity and were congregating at a fibrous, sturdy wall made of a substance called chitin that forms around the salivary ducts. Chitin is a biopolymer and the same substance crustaceans and insects use to make their shells.
Some parasites were able to pass through the chitin wall and reach the salivary ducts. However, much like a bottleneck that creates an entire traffic jam, the narrow openings they passed through could only accomodate a few parasites at a time. Wells says that the few parasites which make it through the tough duct wall are likely the ones that are released during a bite.
Reinforcing this wall could help limit the spread of malaria, or perhaps block them altogether, the team says.
"Our study is a first step in better understanding how salivary glands in malaria-carrying mosquitoes limit the transmission of disease parasites," says Andrew.
"In the future, we hope this information will advance strategies to limit transmission and uncover how other insects have evolved ways to affect disease transmission."
The paper "Anopheles Salivary Gland Architecture Shapes Plasmodium Sporozoite Availability for Transmission" has been published in the journal mBio.