Monday, July 25, 2016

Increase of Lyme disease cases can be linked to climate change

Image of blacklegged tick female.

Pennsylvania has led the nation in confirmed cases of Lyme disease from 2012–2014, according to the most recent data available from the Centers for Disease Control and Prevention.

That’s not surprising given that Pennsylvania is a large state both in population and land that is 58 percent covered in forests, providing ideal habitat for blacklegged ticks—commonly called deer ticks and the carriers of Lyme disease in the northeastern, mid-Atlantic, and north-central United States.

In addition, deer ticks have been found in each of Pennsylvania’s 67 counties, and more than half of Pennsylvanians participate in outdoor recreation one or more times a week, increasing the chances of contracting Lyme disease.

Data from the Pennsylvania Department of Health shows there were 7,400 cases of Lyme disease in the commonwealth, compared with 5,900 during 2013, a 25 percent increase over the prior year. However, the actual number of illnesses is most likely greater than what is reported to health officials due to the fact that many of the symptoms of tick-borne diseases are similar to those of other conditions, which complicates the diagnoses of Lyme disease.

So what are the reasons for this spread of deer ticks and Lyme disease? Science and studies are providing evidence that climate change is one of the factors contributing to the increase in the life span and range of ticks that can spread the disease. According to the U.S. Environmental Protection Agency (EPA), the life cycle and prevalence of deer ticks are strongly influenced by temperature.

Climate change is increasing not only the range in which Lyme disease-carrying ticks can survive, but the amount of time in which ticks can feed. For example, deer ticks are mostly active when temperatures are above 45°F, according to the EPA, and they thrive in areas with at least 85 percent humidity. In addition, shorter winters also could extend the period when ticks are active each year, increasing the time that we can be exposed to contracting Lyme disease.

Reported Lyme Dissease Cases in 1996 and 2013
Data source: CDC (Centers for Disease Control and Prevention). 2015. Lyme disease data and statitstics. For more information, visit U.S. EPA's "Climate Change Indicators in the United States."

These climate patterns supporting the life cycle of deer ticks are happening in Pennsylvania in the form of higher temperatures and an increase in annual precipitation. Since the early 20th century, the commonwealth has seen a temperature increase of more than 1.8° F. Winter temperatures have risen faster, at a rate of 1.3° F per decade from 1970 to 2000 in northeastern United States.

In addition, annual precipitation has increased about 10 percent during the past 100 years. These changes in climate are predicted to increase, thereby continuing the risk of Lyme disease for Pennsylvanians in the future.

The commonwealth is projected to be as much as 5.4° F warmer by the middle of this century than it was at the end of last century. Additionally, annual precipitation is expected to increase by 8 percent. But don’t just take our word. The correlation between climate change and the spread of Lyme disease is so significant, that the EPA monitors the spread of Lyme disease as an indicator of climate change in the U.S. The majority of climate scientists agree that observed changes in climate patterns can be linked to rising levels of carbon dioxide and other greenhouse gases caused by human activities.

While climate change presents significant challenges, there is much forest landowners and managers can do to limit its effects. Developing strategies to:
•Manage our forests to sequester an increasing amount of carbon
•Ensure forest lands remain resilient
•Help landowners and communities reduce their carbon footprint and adapt to climate change

While we may not be able to limit the span of ticks that can spread Lyme disease, there are some tips you can follow from the Centers for Disease Control and Prevention to reduce your exposure to tick bites after spending time in the outdoors.

Penn State Extension Four Common Ticks in Pennsylvania
Penn State Extension Lyme Disease Brochure

1 comment:

Anonymous said...

This article addresses an interesting topic and has so much food for thought that it is difficult to pass up an opportunity to comment. While being intuitively pleasing, it contains information that makes the conclusions less than scientifically rigorous.

My personal experience with deer ticks covers the period from the late 1940s, when I first started hunting, to the present. Geographically, it relates to the high country near State Game Land 60 at the Allegheny Front .

I don’t recall ever seeing a tick of any kind on a harvested deer until the late 1970s, and they then became more common and are taken for granted in deer killed today. Most of us intuitively attributed the increase to warmer weather patterns, although this was purely a “gut feel” instead of any scientific analysis. Also, we had some winters that were extremely cold, with no apparent effect on the prevalence of ticks.

When drawing conclusions relating the increase of Lyme Disease to climate change, we need to be careful to avoid what the statisticians call “spurious correlations.” Simply defined, a spurious correlation occurs when one of two events happening at the same time seems to be caused by the other; when in fact both are unrelated. Also, as the article states, improved diagnostic and reporting methods need to be taken into account.

Other events that happened in my geographical area during the same time span were:

An increase in the black bear population from zero to significant numbers today.

The migration of coyotes to the area and a decline of the fox population to near zero.

Gypsy moths invading the area.

Rattlesnakes becoming more numerous.

Most recently, Chronic Wasting Disease coming closer.

From the point of view of scientific analysis, we need to be careful about how we determine the causes of each of these events. All possibilities should be considered and uncertainties need to be presented.

A final comment about uncertainty related to climate change. A lot of significance is attached to a temperature rise of 1.8 degrees Fahrenheit over a period of 100 years. Recognizing that the Earth’s climate and weather consist of highly complex and always varying combinations of thermodynamic and gas dynamic processes, this temperature change should be measured from absolute zero. This is the standard reference point for analysis and computations in these sciences.

Using the Rankine scale, absolute zero is minus 460 degrees Fahrenheit. A “standard day” of 60 degrees Fahrenheit at sea level is then 520 degrees Rankine. A “climate change” increase of 1.8 degrees results in the standard day rising to 521.8 degrees Rankine, or an increase of 0.346 percent.

It is difficult for an old engineer like I am to get excited over an increase of 0.346 percent over 100 years. If plotted by hand on a scale starting at zero, the change would be lost in the width of a pencil point.