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By Kashmira Gander, Newsweek, July 24, 2019 Scientists believe they are a step closer to preventing the spread of Lyme disease after they mapped the genes of a species of mouse that helps to spread the illness. Caused by the Borrelia burgdorferi bacterium, Lyme disease is the most common vector-borne illness in the U.S. Those bitten by ticks which transmit the disease can develop a fever, headache, chills, fatigue, joint and muscle aches, and swollen lymph nodes. In many cases, patients experience an erythema migrans rash that can grow up to 12 inches around the area of the tick bite. Around 329,000 cases are thought to occur each year, although the true number of infections is thought to be higher. In most cases, patients recover after a round of antibiotics. However, for others the symptoms can linger in  what is known as post-treatment Lyme disease (PTLD) or post-treatment Lyme disease syndrome. The condition is characterized by cognitive dysfunction, incapacitating fatigue and chronic pain, according to the authors of a study published in the journal BMC Public Health. The white-footed mouse, Peromyscus leucopus (P. leucopus), is the most common rodent species in North America, and carries Lyme disease as well as other conditions including babesiosis, anaplasmosis Powassan encephalitis, and hantavirus. When a tick bites an infected mouse, and the tick then bites a human, the individual can fall sick. In the past researchers investigating the conditions have studied the unrelated Mus musculus, or house mouse, species which is often used in medicine and whose genome has been sequenced. The authors of the study published in the journal Science Advances sequenced the genome of the white-footed deer mouse in the hope of uncovering the genetic differences between the species, and understanding why it is the primary carrier of Lyme disease. Co-author Anthony D Long, professor of ecology and evolutionary biology at University of California, Irvine, told Newsweek: "P. leucopus is really quite different in many ways, including how it responds to infection. So now we can use this genome to understand those differences." Long explained: "One important observation is that the bacteria is pretty good at getting to a place in Peromyscus [deer mice] where it can be picked up by another tick and complete its life cycle: the skin, not the blood." The team found differences in the gene expressions associated with hair follicles and skin in the different mice species. They believe that after a few weeks, the white-footed deer mouse's skin reacts to the infection, and changes may enable the bacteria to multiply to levels that can help it can spread. That's important as the bacterium which causes Lyme disease usually lives in the skin, not the blood, and the skin is where the tick picks up the infection. Researchers also found that P. leucopus don't have a part of the gene which helps the body to tackle infections diseases. Long said he and his colleagues have provided a genetic roadmap which makes it easier to carrier out more research. "This isn't 'the' answer, but it opens up a powerful toolkit we couldn't apply to this system before," he said. Future studies might look at how to rid P. leucopus of the bacteria or the creation of a vaccination. "A long term goal would be having lower infection rates in the wild so that if you are bitten the tick is less likely to be carrying the disease," said Long. Sam R. Telford III, Professor of Vector-Borne Infections at Public Health at Tufts University, was not involved in the project but is working with a team to genetically modify the white-footed mouse. A combination of suburbanizaiton, the overabundance of deer, and ticks being transported by animals including birds, as well as humans and their pets, usually dogs, have contributed to a rise in Lyme disease, Telford explained. "Having a characterized genome will allow for future analyses of its evolutionary history, comparative studies to try to determine the basis for its longevity, and why it seems to tolerate a variety of infections well," he said. Scientists could use the CRISPR gene editing tool to modify the mice, Telford suggested. However, Durland Fish, Professor Emeritus of Epidemiology at Yale School of Public Health, who also didn't work on the study, told Newsweek genetic alteration "has many limitations." "The primary limitation is that P. leucopus is not the only source of B burgdorferi for ticks and targeting this single species alone will not eliminate the risk for Lyme disease. Also, the release of genetically modified organisms in the natural environment will be opposed by environmentalists and conservationists." Read the entire article at Newsweek.com.
More on the white-footed mouse and Lyme disease: GLA Research Blog: The Mighty White-Footed Mouse PBS on CRISPR