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Emerging Trends in Tick-borne Diseases in the United States

In a webinar presented on June 9 by The Department of Health and Human Services (HHS) Working Group on Lyme and Other Tick-borne Diseases, a panel of speakers discussed trends in tick-borne diseases (TBDs) in the United States.
Ben Beard, PhD, Chief of the Bacterial Diseases Branch, US Center for Disease Control and Prevention (CDC), Fort Collins, Colorado, introduced and moderated the webinar, highlighting the large number of TBDs that comprise a significant public health problem in the United States. Dr. Beard noted the increased incidence of TBDs in recent decades and added that, although Lyme disease remains the most common of these diseases in this country, novel TBDs also continue to emerge.  

Lyme Disease

Rebecca Eisen, PhD, and Kiersten Kugeler, PhD, MPH, both also from the CDC, Fort Collins, Colorado, discussed the geographic expansion of Lyme disease and its vectors.
According to Dr. Eisen, Lyme disease is a systemic disease, caused by the spirochete Borrelia burgdorferi, which is spread to humans by the bite of two types of infected ticks—the blacklegged tick (Ixodes scapularis) and the western blacklegged tick (Ixodes pacificus). The infected nymphal tick is responsible for most cases of Lyme disease in humans because this tick stage is very small and often goes unnoticed after it attaches to a person’s skin.
Dr. Eisen emphasized the changing landscape of risk for transmission of Lyme disease to humans. Although about 95% of Lyme disease cases occur in the northeastern and northcentral states, she discussed how its range is now expanding geographically in all directions from these foci, adding that oceans provide natural landscape barriers to expansion. Indeed, research has identified the two tick vectors in 49% of continental US counties, Dr Eisen said. This represents a 45% increase in the number of US counties that have reported the presence of these ticks since 1998.
In conjunction with this shifting distribution of tick vectors, Dr. Kugeler discussed how the distribution of Lyme disease in humans has also changed over time. She shared data from a study conducted to define counties in which residents have a high risk of acquiring Lyme disease. In particular, in high-risk areas in the United States, the results quantified a substantial increase over time in the number of counties identified as having high incidence of Lyme disease. In the northeastern states, the incidence of Lyme disease increased by greater than 320% over a 20-year period, rising from 43 cases between 1993 and 1997, to 182 between 2008 and 2012; similarly, in the northcentral states, its incidence increased by about 250%, rising from 22 cases to 78 cases over the same time period.
Nevertheless, it is important to remember that human Lyme disease risk is influenced not only by the presence of tick vectors, but by numerous other factors, Dr. Eisen concluded. These factors include awareness and use of tick precautions.

Borrelia mayonii—a New Cause of Lyme Disease

Borrelia burgdorferi was originally believed to be the only borrelial species to cause Lyme disease in North America. However, Elitza Theel, PhD, from Mayo Clinic, Rochester, Minnesota, discussed the recent identification  of Borrelia mayonii, a novel cause of Lyme disease in humans.
This discovery unfolded when scientists at Mayo Clinic tested diagnostic specimens from 6 patients with suspected Lyme disease. They obtained atypical polymerase chain reaction (PCR) results in the specimens, indicating that the causative bacterium in these cases was genetically distinct from B. burgdorferi. The greatest genetic similarity occurred in one specimen that showed 95% sequence homology to B. burgdorferi, said Dr. Theel. As a consequence, because at least 98.3% nucleotide sequence similarity between 2 organisms is required to identify them as the same species, Dr Theel added that the causative agent in these 6 cases was designated as a new species and the name B. mayonii was proposed for it. 
According to Dr. Theel, the 6 patients involved were residents of Minnesota, North Dakota, and Wisconsin. They ranged from ten to 65 years old, and four of them were male. Clinical features in these cases included fever, headache, rash, and arthralgia—symptoms similar to those of Lyme disease caused by B. burgdorferi. However, cases of Lyme disease caused by B. mayonii involved more severe disease that required hospitalization, with nausea, vomiting, and diffuse rash. In contrast to Lyme disease cases due to B. burgdorferi, these cases due to B. mayonii were also associated with a much higher spirochetemia, and the causative organism was also found in whole blood specimens. Five of these patients recovered completely with antibiotic therapy. Dr. Theel also noted that B. mayonii DNA was detected in 19 of 658 ticks collected from one patient exposure site in Wisconsin. Ixodes scapularis ticks are therefore believed to be the likely vector for B. mayonii, she concluded.

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