One of the major mosquito-borne diseases plaguing the world is malaria and thus protection from mosquitoes becomes of optimum importance in our fight against the disease. Notably, mosquitoes can harbour thousands of malaria-causing parasites in their bodies, yet they transmit only a tiny fraction of the virus while feeding blood from a victim. In a recent study to define precisely the location of the parasite bottleneck, scientists from the John Hopkins Medicine say they have discovered the parasites are stopped by a roadblock along the escape route in the insect’s spit glands.
According to researchers, the barrier could potentially serve as a novel target for protection from mosquitoes and malaria prevention and reduce the malarial infection as well as other mosquito-borne diseases, like Zika.
Speaking about the study Deborah Andrews, one of the study authors revealed that their findings add substantial detail to the role of mosquito salivary glands as the "gateway organs" for diseases spread by the insects. Andrews added that by "enhancing transmission barriers" that exist naturally in mosquitoes, one can block the spread of malaria and other deadly mosquito-borne diseases, like Zika.
According to WHO, an estimated 220 million people worldwide, mostly in tropical and subtropical regions, have malaria, and more than 400,000 die of the parasitic infection each year. The infection is marked by disabling fever, chills, fatigue and sweating while the disease can be treated with drugs and prevented with mosquito eradication programmes.
A description of the research was published in the August 6 issue of the journal mBio.
Notably, Malaria parasites are dependent on female Anopheles mosquitoes to spread in a complex life cycle that begins with them eating male and female parasite sex cells during a blood meal from an infected animal host. The cell then winds up in the mosquito's gut, where they fuse to form fertilized eggs that then squeeze through the gut's lining and become encased in cysts inside the insect's body.
The parasites begin a reproductive frenzy, creating multiple copies of themselves. When the cysts finally burst, the parasites raid the salivary gland, ready to be squirted out when the mosquito takes its next blood meal. But scientists have observed that most of them never make it out of the mosquito.
Speaking about the same, Michael Well's the lead author of the study, said, "Even though thousands of parasites invade the salivary gland, less than a 10th of them are transmitted during a mosquito bite," adding, "So, we knew that the salivary gland is blocking the parasites from getting out, but we didn’t know exactly how.”
According to scientists, the Anopheles mosquito's salivary gland is made up of three lobes of saliva-producing cells which are encased in a protective sheet called the basement membrane, and in each lobe there are long ducts that extend into the insect's mouth. For release, the parasites must first go through the basement membrane, penetrate a layer of salivary cells and then swim across a space called the secretory cavity to reach the salivary duct.
To study how the salivary gland might obstruct malaria transmission, the researchers first let Anopheles mosquitoes feed on rodent blood enriched with malaria parasites. Since the mosquitoes decided how much they ate, each one consumed a different quantity of parasites. This offered the researchers data for different quantities of parasitic infection from hundreds of mosquito salivary glands.
The researchers then systematically mapped out the parasites' location by dissecting salivary glands from these mosquitoes and looking for parasites under high-powered microscopes.
Researchers found that most parasites were either inside the basement membrane or in the secretory cavity. Only a few parasites were in the salivary ducts.
According to Wells, since there seemed to be no problem with the parasites entering the salivary glands, the obstruction happens later, when the parasites are trying to get to the salivary duct.
Researchers further found that most parasites appeared to be unable to leave the secretory cavity and were congregating at a fibrous wall made of chitin that forms around the salivary ducts.
However, some of the parasites were able to tunnel through, but the bottlenecks, only allowed a few parasites to pass through. These are the ones, which were released during a bite.
According to researchers, if the chitin wall around salivary ducts can be fortified, infections may be thwarted. They hope that in the future, the information will advance strategies to curb transmission and uncover how other insects have evolved ways to affect disease transmission.
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