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Lyme Disease Increasing More Rapidly in the North

by lmatthews on April 29, 2013

lyme disease latitude link climate change and ticks heading north

The northward march of ticks is increasing Lyme disease rates.

A recently published report from Canadian Lyme disease researchers suggests that latitude is a significant factor in the rate at which Lyme disease cases increase, with ramifications for disease rates as climate change occurs. Southern states in the US will potentially see fewer cases as climate change progresses and more northerly states are likely to be harder hit by the infectious disease.The analysis found that there was an 80% increase in Lyme disease in the US between 1993 and 2007 but the rate of disease was markedly different across regions of the country.

How Climate Change Impacts Lyme Disease Rates

Lyme disease is a zoonosis and a tick-borne illness that is tied to climate change through factors such as animal migration, tick survival and activity, human migration and the development of rural areas for housing and recreation. Climate change has the potential to vastly increase the areas that are hospitable to ticks, thus increasing the possibility of humans and non-human family members being infected with Lyme disease.

Factors in Lyme Disease Epidemic

Using regression analysis, the researchers in Toronto and Ottawa came up with incidence rate ratios (IRRs) to describe the year-on-year increase (or not) of Lyme disease cases in different US states. This then allowed them to look at specific factors shared by states with a growing incidence of Lyme disease, or a declining rate of growth in cases, and to determine which seemed most likely to be contributing to increased numbers of cases of the tick-borne illness.


Lyme Disease Increasing in the North

Two clear factors that emerged were state latitude (how far north or south a state is) and the number of people in the state per square kilometer (population density). These two factors alone explained some 27% of the inter-state differences in Lyme disease rates. No other independent factors were found that explained the change in incidence. During the study period the states that saw the biggest increases were those in the north, whereas southern states had stable rates or declining rates of Lyme disease.

Lizards and Lyme Disease

The decline in the southern states was suggested to be due to an increasing population of lizards, courtesy of climate change, that act as ‘dead-end’ hosts to Lyme disease bacteria. In the north, the opposite is true, with longer periods of warmer weather expanding the range of animals that act as hosts to ticks carrying Lyme disease bacteria and generally expanding the area in which ticks can survive.

Lyme Disease in the North

This kind of Lyme disease research may be beneficial for those working in the field of public health, allowing them to consider future financial planning for an anticipated rise in Lyme disease cases as climate change goes unchecked. The research also has ramifications for countries at more northerly latitudes, including Canada, where Lyme disease is still considered a non-issue for many physicians. Learning from their more southerly counterparts now might help prevent future problems for healthcare workers and policy makers dealing with Lyme disease and climate change.

Reference

Tutte, A.R., Greer, A.L., Fisman, D.N., Effect of latitude on the rate of change in incidence of Lyme disease in the United States, cmajo April 16, 2013 vol. 1 no. 1 E43-E47


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