An international team of scientists led by Purdue University has sequenced the genome of the tick that transmits Lyme disease, the most common vector-borne illness in North America. Ixodes scapularis, known as the blacklegged tick or the deer tick, is the first tick species to have its genome sequenced.
The decade-long project, involving 93 authors from 46 institutions, decodes the biology of an arachnid with sophisticated spit, barbed mouthparts, and millions of years of successful parasitism. The genome of Ixodes scapularis also sheds light on how ticks acquire and transmit pathogens and offers tick-specific targets for control.
“The genome provides a foundation for a whole new era in tick research,” said Catherine Hill, lead author of a paper that was published in Nature Communications. “Now that we’ve cracked the tick’s code, we can begin to design strategies to control ticks, to understand how they transmit disease and to interfere with that process.”
Tick-borne illnesses cause thousands of human and animal deaths annually, and ticks transmit a wider variety of pathogens and parasites than any other arthropod. They primarily spread disease by creating a feeding wound in the skin of their hosts, regurgitating infected saliva into the wound as they ingest blood.
Despite ticks’ capacity to acquire and pass on an array of pathogens, research on ticks has lagged behind that of other arthropod vectors, such as mosquitoes, largely because of a lack of genetic and molecular tools and resources.
“Ticks are under-appreciated as vectors — until you get Lyme disease,” Hill said.
About 30,000 cases of Lyme disease cases are reported in the U.S. annually, most concentrated in the Northeast and upper Midwest. But the Centers for Disease Control estimates the actual number of cases is 329,000 a year, many of which are unreported or misdiagnosed.
While not fatal, Lyme disease can be permanently debilitating if the infection is not treated before it reaches the chronic phase. The deer tick also vectors human granulocytic anaplasmosis, babesiosis and the potentially lethal Powassan virus.
“Genomic resources for the tick were desperately needed,” Hill said. “These enable us to look at tick biology in a systems way.”
The genome provides two lines of valuable biological resources: the genes and proteins that make ticks successful parasites and excellent vectors of parasites and pathogens. Identifying the proteins involved in the transmission of tick-borne diseases could help researchers develop strategies to halt this process.
The genome also provides insights into unique aspects of tick biology. Tick saliva, for example, teems with antimicrobials, pain inhibitors, cement, anticoagulants and immune suppressors, all designed to help the tick feed on its host undetected for days or weeks.
The genome reveals that tick saliva contains thousands of compounds — compared with mere hundreds in mosquito saliva — a diversity that presumably allows ticks to exploit a wide range of hosts and stay attached for a long time. The researchers also identified genes that could be linked to ticks’ ability to synthesize new armorlike cuticle as they feed, allowing them to expand over 100 times.
The team searched for clues to how ticks digest blood, a toxic food source due to its high concentrations of iron. The genome points to a number of proteins that link with iron-containing heme molecules, the byproducts of blood digestion, to make them less toxic.
“Ticks have an amazing number of detoxification enzymes, and we don’t know why,” Hill said. “We’ve got our eye on this because these enzymes are also involved in detoxifying insecticides. As we develop new chemicals to control ticks, we’ll be going up against this massive arsenal of detoxification enzymes, far more than insects have.”
One of the major findings of the genome project is that about 20 percent of the genes appear to be unique to ticks. These genes could provide researchers with tick-specific targets for control.
“We don’t see the equivalent of these genes in a mosquito or human,” Hill said. “That’s a fascinating collection of molecules, and as a scientist, I can’t wait to get into that pot of gold and find out what these are and what they do.”
The project also included the first genome-wide analysis of tick population structure in North America, resolving a long-standing debate over whether deer ticks in the North and South are actually two different species. According to Hill, the genome offers convincing evidence that the two populations are the same species, despite their genetic differences. Because the majority of Lyme disease cases occur in the North, there might be a genetic component to ticks’ ability to transmit Lyme disease that a comparison of the two populations could illuminate.
“Now we’ve got the script to help us work out what proteins the tick’s genes are making, what these proteins do, and whether we can exploit them to control the tick,” Hill said.
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