medwireNews: Researchers have found that over the past 20 years, the incidence of Lyme disease has increased in the northern-most states of the USA, while declining in more southerly states.
"These differences in trends are consistent with expectations under climate change projections," say David Fisman (University of Toronto, Ontario, Canada) and colleagues.
"They suggest that global warming may have already affected the ecology of this important infectious disease."
Using data from the 50 US states and the District of Columbia, Fisman and colleagues found that the overall rate of Lyme disease increased in the USA by around 80% between 1993 and 2007 to an annual incidence of 6.2 cases per 100,000 people.
However, they note significant heterogeneity in rates between states which ranged from 0.008 cases in Colorado to 75 per 100,000 people in Connecticut. Overall, 21 states and the District of Columbia showed significant increases in disease incidence over the study period, 14 states showed a significant decrease, and 15 states showed no significant change.
In multivariate analysis, state latitude and mean population density accounted for a significant proportion of between-state variation in disease incidence trends.
"Given the strong correlation between latitude and annual temperature, this association could also be described as an apparent decrease in Lyme disease incidence in warmer states and increase in cooler states," the authors write in CMAJ Open.
They say that their findings mirror projections made in 2003 using an ecologic model and also support observations that Lyme disease and other vector-borne diseases have been documented in Canada which was previously thought too cold to support the tick lifecycle.
The authors also note that the association between population density and the rate of increase in Lyme disease incidence may reflect encroachment of human habitation into wooded areas that serve as habitats for animals that host the disease.
Fisman and colleagues say that the rapid change in Lyme disease incidence and geographic spread should serve as a wakeup call to policymakers.
"Public health agencies should consider whether existing surveillance systems are sufficiently flexible and sensitive to identify climate change-driven changes in infectious disease epidemiology," the authors conclude.
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