Lyme Disease Rate Increasing in the Northern United States

Janis C. Kelly

April 17, 2013

Lyme disease incidence increased about 80% in the United States between 1993 and 2007, and the increase correlated with latitude and with population density, Ashleigh R. Tuite, MPH, from the Dalla Lana School of Public Health, Toronto, Ontario, Canada, and colleagues report in an article published online April 16 in the Canadian Medical Association Journal.

Lyme disease incidence increased in the northern-most states while remaining stable or declining in the southern states.

The researchers used publicly available data to estimate state-level year-on-year incidence rate ratios for Lyme disease, using Poisson regression methods. They then evaluated between-state heterogeneity and identified state-level characteristics that could be associated with increasing incidence, such as geography, environment, demography, and politics and economy.

The researchers found that Lyme disease incidence increased by about 80% between 1993 and 2007, that the average incidence ranged from 0.008 per 100,000 person-years in Colorado to 75 per 100,000 in Connecticut, and that increasing incidence showed a linear association with state latitude and population density, which together explained 27% of the difference in incidence rate ratios between states.

The authors note that the presence of a spring–autumn interval that was long and warm enough to permit Ixodes ticks to complete their life cycles became more common at northern latitudes during the study period.

"We hypothesized that, if climate change is affecting the risk of tick-borne illness, a north–south gradient in year-on-year trends would be seen, such that the most rapid changes in risk would occur in areas that have traditionally been too cold to support robust local transmission of Lyme disease," the researchers note.

That is what they found, although they warn, "We would caution against definitive conclusions about causality being drawn on the basis of our study."

The researchers suggest that the decreasing Lyme disease incidence in some southern states might reflect "an expanded range of habitat for lizards, which serve as 'dead-end' hosts for Borrelia burgdorferi, the causative agent of Lyme disease."

"Our findings are consistent with the hypothesis that increases in Lyme disease incidence in recent decades are attributable at least in part to the effects of climate change, with increasing rates of change observed at more northerly latitudes, and declines in disease incidence in the southernmost states," the authors add.

"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 conclude.

The authors have disclosed no relevant financial relationships.

CMAJ. Published online April 16, 2013. Full text