From left: James Gathany/CDC; Steven Glenn/Laboratory & Consultation Division/CDC
A microbe that causes malaria tricks mosquitos into helping it spread, a new study finds. The microbe is a parasite that leaves a chemical behind in the blood of the people or other animals that it infects. Mosquitoes are drawn to scent of blood hosting this chemical. It entices them to slurp up some of the infected meal. Then voila! The parasites get airlifted from their old host to new ones. And so malaria spreads.
The new study offers an “intriguing glimpse” into how the parasite manipulates the blood-suckers into fostering its spread, says Conor McMeniman. He’s a mosquito researcher at Johns Hopkins University in Baltimore, Md. He was not involved in the study.
Malaria is a serious illness. Its symptoms include fevers, chills, head and body aches, nausea and tiredness. Some people get recurring bouts of these symptoms for decades. And in young children, the disease can prove deadly.
A few mosquito species in the genus Anopheles (Ah-NAH-ful-ees) can spread malaria. They bite people in warm regions of the world, especially in Africa, South Asia and parts of Central and South America. If the insects that sip their blood had previously dined on the blood of people (or animals) already infected with malaria, the mosquito can transmit some of those disease-causing parasites.
The parasite in the new research is called Plasmodium falciparum (Plaz-MOH-dee-um Faal-SIH-puh-rum). It’s one of several single-celled microbes that cause malaria in people. And it’s the one that most often causes a severe form of the disease.
Previous research had hinted that mosquitoes seemed to target people already sick with malaria. But it was not clear why. This prompted a research team led by Noushin Emami to look into it. She’s a biologist at Stockholm University in Sweden. Her group got an unexpected tip off while studying a molecule known as HMBPP. They were studying how the molecule affects the immune system of Anopheles gambiae. These mosquitoes are a major means of transmitting malaria. And P. falciparum releases this molecule into the blood of its animal hosts.
The researchers watched mosquitoes sip blood from artificial feeders. “The mosquitoes ate a lot more,” notes Ingrid Faye, “when the HMBPP was in the blood.” Faye also works at Stockholm University. That observation, she explains, “made us think [the molecule] changes [the mosquito’s] behavior.”
HMBPP didn’t directly lure the mosquitoes. The researchers learned that when they tried mixing it with serum. That’s the watery part of blood that’s left behind when red blood cells are removed. HMBPP-laced serum just didn’t interest the mosquitoes nearly as much. But it was a different story when they added this chemical to red blood cells. These cells now released more carbon dioxide than usual. That’s a gas that people exhale.
The researchers also found that adding HMBPP to red cells led the blood to release more airborne chemicals called aldehydes (AL-duh-hides) and monoterpenes. These compounds also are released by plants. And the scent of these chemicals lures in more mosquitoes. The insects drank a bigger blood meal than normal when the blood had released all of these gases.
The team reported its results online February 9 in Science.
The new data make sense. Carbon dioxide is a gas that helps mosquitoes track down their mammal meal tickets — including people. And aldehydes and monoterpenes might attract mosquitoes by making people smell a bit like the plants from which these insects also get some nectar, says McMeniman at Johns Hopkins. He cautions, however, that HMPBB’s effect was tested, to date, only in artificial mosquito feeders. That means it’s still unclear how strongly it would lure mosquitoes to infected people.
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aldehyde A family of chemicals that can be formed by the oxidation of alcohols. These compounds, known as organic (because they contain carbon) contain a chemical group: CHO. Many members of this family are toxic, most notably formaldehyde and acetaldehyde.
behavior The way a person or other organism acts towards others, or conducts itself.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) burns. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
cell The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical can also be an used as an adjective to describe properties of materials that are the result of various reactions between different compounds.
compound (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical formula is H2O.
genus (plural: genera) A group of closely related species. For example, the genus Canis — which is Latin for “dog” — includes all domestic breeds of dog and their closest wild relatives, including wolves, coyotes, jackals and dingoes.
host (in biology and medicine) The organism in which another lives. Humans may be a temporary host for food-poisoning germs or other infective agents.
immune Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.
immune system The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.
insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.
malaria A disease caused by a parasite that invades an animal’s red blood cells. The parasite is transmitted by mosquitoes, mostly in tropical and subtropical regions.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
monoterpene A type of molecule with 10 carbon atoms and 16 hydrogen atoms; it may produce a scent.
nectar A sugary fluid secreted by plants, especially by flowers. It encourages pollination by insects and other animals. It is collected by bees to make into honey.
online (n.) Term referring to the internet. (adj.) Term for what can be found or accessed on the internet.
parasite An organism that gets benefits from another species, called a host, but doesn’t provide it any benefits. Classic examples of parasites include ticks, fleas and tapeworms.
plasmodium (in medicine) A genus of protozoa that cause the disease malaria. The Plasmodium genus includes more than 100 species; just five are responsible for most malaria in humans.
probability A mathematical calculation or assessment (essentially the chance) of how likely something is to occur.
red blood cell Colored red by hemoglobin, these cells move oxygen from the lungs to all tissues of the body. Red blood cells are too small to be seen by the unaided eye.
serum The part of blood that contains neither blood cells nor chemicals that help with clotting.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
symptom A physical or mental indicator generally regarded to be characteristic of a disease. Sometimes a single symptom — especially a general one, such as fever or pain — can be a sign of any of many different types of injury or disease.
transmission (In medicine) To spread a disease or toxic agent.
Journal: S.N. Emami et al. A key malaria metabolite modulates vector blood seeking, feeding, and susceptibility to infection. Science. Published online February 9, 2017. doi: 10.1126/science.aah4563.