From causing diseases to destroying crops, parasites wreak havoc across the globe. However, one way to fight them is to figure out what attracts them to their hosts, which is exactly what Elissa Hallem ’99 does for a living. As a neuroscientist who focuses on odor detection, Hallem studies the behavior of parasitic worms called nematodes. Currently, she is researching the neuron circuits that allow these worms to seek their hosts.
Hallem is particularly interested in a parasite commonly known as threadworm. Threadworm is a major health issue in rural areas of developing nations. According to Hallem, an estimated 25 percent of the global population is infected. Additionally, a similar species of nematode infects crops, which causes economic losses in agriculture every year. If scientists can figure out what chemicals attract these parasites, then they could potentially develop traps for them.
Hallem has been interested in this area of neurobiology since attending graduate school at Yale University, where she studied odor detection in mosquitos. Mosquitos are of particular interest to biologists because of their link to malaria. She helped discover that mosquito olfactory receptors respond to human sweat. “A lot of people are studying host-seeking behaviors in mosquitos, but I realized that, in worms, essentially nothing was known about how they find hosts,” Hallem said.
Parasites like threadworm need hosts to reproduce and complete their life cycle. When young, they move through the soil until they find a host. They usually enter humans through bare feet, often between the toes. Once inside their host, the parasites develop, reproduce and lay eggs. When the eggs hatch, the young exit the host’s body and the life cycle begins again.
In past research, Hallem found that threadworms respond to various stimuli when host-seeking. She discovered that temperature is one way the worms find their hosts; they move much faster at 37 degrees Celsius, or body temperature. Hallem also discovered that the parasites respond to human body odors – they are attracted to odors of skin and sweat, while they are repelled by carbon dioxide.
Many of the odors that attract threadworms are also known to attract mosquitos. Because of this overlap, Hallem wondered if insect repellants would also repel threadworms. However, the repellants she tested did not repel threadworms, and conversely, the common repellant DEET actually attracted them.
Once she knew more about the chemicals that attracted the worms, Hallem began studying the neural basis of host-seeking. She discovered a pair of neurons in the worms’ head called BAG neurons, which are neurons connected to the worm’s nose and responsible for detection of carbon dioxide and other odors like those of human skin and sweat. Hallem’s current studies aim to further elucidate the way these neural circuits work. Furthermore, she wants to understand how the neural circuits have evolved to be species-specific or, in other words, how parasites know which hosts to infect.
As Hallem and her colleagues discover more about parasitic worms, people could use their findings to fight infections in humans. “An interesting approach [to fight the parasites] would be to use odor sticks. You put wooden sticks with odors on them to serve as a trap, as opposed to taking a drug. That type of approach has been used for crops, but could also be used for humans,” Hallem said.