Guest post by Alan Barbour, MD (With Lyme disease on the move and making news, we invited Lyme Disease author Dr. Alan Barbour to contribute regular updates to the JHU Press blog in the coming months. His posts will highlight the latest findings on Lyme and other deer tick-associated infections and share insights on diagnosis, treatment, and prevention that are reported in the medical literature and other sources. For more frequent short updates and tips, follow Dr. Barbour on Twitter: @alanbarbour.) 
The season of tick-transmitted diseases is well underway in North America and Europe and it is timely to take stock of recent developments. In a new report, the Centers for Disease Control and Prevention (CDC) described the further expansion of Lyme disease through 2012 in the northeastern and north-central United States. During 1993–1997, 65 counties in these regions had a high incidence of Lyme disease. By the 2008–2012 period, the number of high-incidence counties had risen to 260, showing a four-fold increase in the 15 year interval that elapsed after 1997. Between 2009 and 2012 there was a concomitant increase by more than two-fold in the number of cases of Lyme disease acquired in Canada, according to a Canadian government publication. The ecology of Lyme disease is complex, and risk is not easily explained by one or two factors. Nevertheless, the northward expansion of the pathogen and the ticks that transmit it is plausibly accounted for in part by climate change. In general, milder winters allow for greater survival of deer ticks into the spring of the following year. The list of human infections transmitted by deer ticks in the eastern and central U.S. includes Lyme disease, anaplasmosis, babesiosis, relapsing fever, and a viral encephalitis. Now a sixth disease has been added. A more simple name may emerge with time, but for now it is awkwardly called “Ehrlichia muris-like agent infection.” So far, reports of these bacteria in humans and deer ticks have only come from Wisconsin and Minnesota, not elsewhere in North America. The bacteria are similar to the agent of anaplasmosis in their dependence on the inside of animal cells for life. The resemblance of the newly-described infection to anaplasmosis extends to shared symptoms including fever, headache, and muscles aches, usually without a rash. A major difference from anaplasmosis is the absence of visible Ehrlichia bacteria from blood cells. Because of this, Ehrlichia bacteria cannot be detected by examining blood under the microscope. Although some of the patients with the new infection had underlying deficiencies in their immune systems, these and other patients seem to respond to the tetracycline antibiotic doxycycline, which is also used for treatment of Lyme disease and anaplasmosis. A second type of Ehrlichia bacteria has long been known to cause a similar disease in the southeastern, south-central, and mid-Atlantic U.S. But this form of ehrlichiosis is transmitted by the lone star tick, which is distinguishable in appearance from the deer tick that carries Lyme disease. 
Alan G. Barbour, MD, is a professor of medicine and microbiology at the University of California, Irvine School of Medicine, a co-discoverer of the cause of Lyme disease, and a leading Lyme disease researcher. He is the author of Lyme Disease: Why It’s Spreading, How It Makes You Sick, and What to Do about It.
