The incidence of Lyme disease increased about 80% in the United States from 1993 to 2007, according to new data published in CMAJ Open. Although the prevalence of Lyme disease varied from state to state, its increase has been linked to climate change.
"The incidence of Lyme disease is increasing in North America, possibly as a result of climate change," study researcher David N. Fisman, MD, MPH, of the University of Toronto Dalla Lana School of Public Health, told Infectious Disease News. "Physicians who practice in areas previously thought too cold to support the ticks that spread Lyme disease — including those in many heavily populated areas of Canada — need to be familiar with the diagnosis and treatment of this disease. The distribution of Lyme disease appears to be changing quickly."
David N. Fisman
Fisman and colleagues estimated the incidence rate ratios (IRRs) for Lyme disease for the 1993-2007 period on a state-by-state and year-by-year basis. Additional analyses were conducted to examine the heterogeneity of the disease between states and to identify state-level characteristics associated with its increasing prevalence.
Results indicated that the incidence of Lyme disease increased by approximately 80% during the 14-year study period (IRR=1.049; 95% CI, 1.048-1.050). The average incidence of Lyme disease differed significantly between states, ranging from 0.008 per 100,000 person-years in Colorado to 75 per 100,000 person-years in Connecticut, and in trends over time between states (P<.001).
State latitude and population density explained up to 27% of the variation in incidence between states. Southern states experienced stable or even declining incidence rates in Lyme disease while the northernmost states experienced marked increases. The relationship between population density and the rate of increase in Lyme disease incidence may be explained by human encroachment into wooded areas that support wildlife that serve as hosts for the disease, according to the researchers.
Fisman and colleagues noted that the results are consistent with ecological projections that Lyme disease and other vector-borne diseases would be documented in areas previously thought too cold to support their existence — such as Canada — as a consequence of warming temperatures.
“Public health agencies should consider whether existing surveillance systems are sufficiently flexible and sensitive to identify climate change-driven changes in infectious disease epidemiology,” they concluded.
Disclosure: The researchers report no relevant financial disclosures.