This document discusses the relationship between climate change and the spread of Lyme disease. It begins with an introduction about how a colleague contracted Lyme disease from a tick bite. The rest of the document is divided into three sections: 1) Biological background on Lyme disease, how it is transmitted by deer ticks, and its current prevalence. 2) A study that found using climate models that deer tick habitat is expected to expand 213% by 2080 due to warming temperatures and increased humidity from climate change. 3) Limitations of the climate model which only considered temperature and humidity and not biological factors like the distributions of deer and mice hosts that also impact tick ranges. More accurate risk projections will require models considering both

1. Project: Climate Change and Lyme Disease
Introduction
Lyme disease caught my attention several years ago when a colleague became seriously ill after
being bitten by a deer tick while mowing his lawn. Shortly thereafter, he complained about flulike symptoms and constant tiredness, but his doctor did not suspect Lyme disease until weeks
later, when he began showing heart abnormalities. The disease had begun to affect the nerve
regulating his heartbeat, and had the doctor not diagnosed the disease, it could have been fatal. It
appears the risk of tick-borne diseases is about to worsen. This paper assesses the scientific
evidence for the linkage between climate change and Lyme diseasein light of what we’ve learned
so far in the MOOC.
Biological Background
Lyme diseasein the United States was discovered in 1977 in Old Lyme, Connecticut and since
then has spread throughout the northeastern US and the mid-Atlantic States. It has also been
reported in the Great Lakes region, northern California and the Pacific Northwest. The disease is
notoriously difficult to diagnose given that infected individuals can display a wide range of
symptoms. It is transmitted by the bite of adeer tick (Ixodes scapularis), which has a two year
life cycle that passes through larval, nymph and adult stages. The tick serves as a “vector” for
Lyme disease, meaning that it transmits the active agent that causes the disease (a bacteria) to
humans. The tick requires the presence of two host species – the white tailed deer and white
footed field mouse – in order to complete its life cycle. General information about the disease,
its causes, symptoms, treatment and prevention are given in Figure 1. Currently Lyme disease
infects an estimated 300,000 people a year, including in the United States, Europe and Asia.
Climate Linkage
According to a study funded by the US Centers for Disease Control and Prevention (CDC), we
can expect to see a big jump in those numbers as the mid-Atlantic and northeastern states
experience warmer and wetter weather with climate change. The peer-reviewed scientific study
(J. Brownstein, T. Holford, and D.Fish, Ecohealth, March 2005) reached that conclusion by
using a climate model to predict the distribution of the deer tick based on the two environmental
factors that are critical to its survival – temperature and humidity. Correlating temperature and
humidity data with the known geographical distribution of the deer tick, researchers developed a
computer model that predicted the 2005 distribution of I. scapularis with 89% accuracy. They
then extrapolated the temperature and humidity data to 2080 to map suitable new habitat areas
for the deer tick. The result showed an overall expansion of the tick’s geographic range by
213%, with new habitat extending northward into Canada and westward into the American
Midwest (See maps at http://climatechangeanddisease2011.weebly.com/index.html).
Limitations of the Model
It is important to note that the researchers only took into account two environmental factors –
temperature and humidity – in order to develop their tick suitability maps. Biological factors
including the geographic distribution of the tick’s two host species could obviously also affect

2. tick distribution. In order to complete its life cycle, the tick requires the presence of white-tailed
deer and white-footed field mice, whose populations fluctuate in response to a variety of factors
beyond temperature and humidity. For example, the model does not take into account the
explosion of the deer population throughout the mid-Atlantic, creating more opportunities for
adult female ticks to survive the winter and to lay their eggs the following spring. Nor does the
model address the possibility of extreme low temperatures (such aswe have seen recently) that
could lower survival rates for both tick larvae and theirmice hosts. These complex interactions
among biological and climate variables suggest that the temperature-humidity computer model
may yield only a crude approximation of how tick habitat is likely to expand with climate
change, and that more accurate risk maps will have to await the development of more
sophisticated computer models that take both biological and climate variables into account.

3. Figure 1 (Source: Lyme Disease Primer, Washington Post)