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Tick Genome Sequenced, Control Strategies ID’ed

Black-legged or deer tick

Black-legged or deer tick (Andrew Nuss, Purdue University)

10 February 2016. A research consortium on three continents sequenced the genome of the black-legged tick, the insect responsible for Lyme disease and other conditions. The team of 93 authors from 46 institutions, led by entomologist Catherine Hill at Purdue University, published its findings in yesterday’s (9 February) issue of the journal Nature Communications, with a companion report on viral proteins associated with the tick in PLoS Neglected Tropical Diseases.

The black-legged or deer tick carries Borrelia burgdorferi bacteria, which when transmitted to humans through tick bites causes Lyme disease, with symptoms including fever, fatigue, headache, and rash. The disease can be treated with antibiotics, but if left untreated, infections can spread to the joints, heart, and nervous system, and become chronic and debilitating. Centers for Disease Control and Prevention says between 19,000 and 30,000 cases of Lyme disease are reported in the U.S. each year, mainly in northeastern states and the upper Midwest. Ticks also transmit the diseases anaplasmosis, babesiosis, ehrlichiosis, tick-borne encephalitis, and Rocky Mountain spotted fever.

Sequencing the tick genome was a decade-long project that revealed a highly complex insect with many ways of transmitting disease-causing microbes, as well as multiple defenses making it difficult to eradicate. The team discovered the tick genome has some 2.1 billion base pairs (human genomes have 3 billion base pairs), of which about 70 percent are repeated inside the genome. The authors report unraveling the order and sequence of about two-thirds of the genome, identifying more than 20,000 genes that encode proteins.

Ticks spread disease by ingesting blood from their human or animal hosts, then spread infected saliva into the wound, which can go on for hours or days at a time. The team identified proteins associated with the saliva that enable the tick to feed on the host undetected by attaching securely, suppressing immune reactions, killing pain, and thinning blood. Ticks also can create a stiff waxy shell that protects them while they expand to nearly 100 times their original size during feeding.

In addition, the tick can digest blood from the host by making iron in the blood less toxic, while removing water from the blood during digestion. This capability was one of many defenses encoded in the tick genome, which will make it more difficult to attack. “We’ve got our eye on this,” says Hill in a Purdue statement, “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.”

The analysis, however, revealed about 20 percent of its protein-coding genes are unique to ticks, which may identify targets for future control strategies.”Now we’ve got the script to help us work out what proteins the tick’s genes are making” adds Hill, “what these proteins do and whether we can exploit them to control the tick.”

Purdue biologist Richard Kuhn and colleagues, in the PLoS Neglected Tropical Diseases paper, identified proteins and molecular pathways for the Langat virus, also transmitted by ticks. The Langat virus is responsible for tick-borne encephalitis. “Once you know which host proteins are critical for virus replication,” says Kuhn, “you can manipulate those proteins to interfere with the growth and development of the virus.”

The research was funded in part by National Institute of Allergy and Infectious Diseases, an agency of National Institutes of Health, that also contributed members of the study team.

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