United States and their impact on patients. These researchers also explain what efforts to track these diseases among people, and the movement of the pathogens in the environment, reveal about how infection moves from animals to people, especially among vulnerable populations.

Gary P. Wormser, M.D., New York Medical College

Lyme disease is the most commonly reported vector-borne infection in the United States. B. burgdorferi is the only recognized pathogen to cause Lyme disease in the United States, and can be differentiated into 16 to 45 subtypes that may vary in infectivity and/or in pathogenicity (Wormser et al., 2008a; Crowder et al., 2010). In Europe, B. burgdorferi also causes Lyme disease, but other species of Borrelia, such as Borrelia afzelii and Borrelia garinii, also are responsible for Lyme disease. Because different species are responsible for infection in the two locations, the clinical syndromes associated with Lyme disease also differ between the United States and Europe.

The reported incidence rate of Lyme disease has increased steadily from 10,000 cases in 1992 to approximately 30,000 cases in 2009 (CDC website) (Figure 5-1). Twelve states in the Northeast, Mid-Atlantic, and North Central regions of the United States account for nearly 95 percent of these reported cases. New Hampshire (a 37-fold increase) and Maine (a 19-fold increase) have seen the largest proportionate increases in the number of cases during the past 10 years. New York has the largest absolute number of reported cases of Lyme disease, but is only fifth in the incidence of Lyme disease—the number of cases per 100,000 residents. Connecticut has the highest reported incidence.

In the United States, B. burgdorferi are transmitted exclusively by Ixodes ticks, which may transmit pathogens that cause other infections as well, including babesiosis, human granulocytic anaplasmosis, and flavivirus Powassan-like encephalitis. Of the diseases transmitted by I. scapularis, flavivirus Powassan-like encephalitis virus is the least well characterized. One recent study suggested that 2–5 percent of adult Ixodes scapularis ticks collected from two sites in Westchester County, New York, in 2008 contained Powassan virus (Tokarz et al., 2010). Approximately 4 percent of those ticks also contained B. miyamotoi, a relapsing fever-like Borrelia. Whether Borrelia miyamotoi causes human infection is unknown, and the clinical manifestations, if it does, are likewise unknown. Both Powassan virus and B. miyamotoi deserve of additional research efforts.

A number of animals act as reservoirs for Borrelia species, including mice, other small mammals, and some birds. Although deer serve as hosts