Here come the ticks: is global warming leading to an increase in Lyme disease?

This is the last of 16 student posts, guest-authored by Jessica Waters. 

Climatologists have been warning us about the ongoing and impending consequences of global warming for years. But the results of climate change affect more than just polar bears and penguins  – if you live anywhere in the northeastern, north-central or west coast states of the U.S.., you could be at a greater risk for contracting Lyme Disease.

Lyme disease is an infection of the Borrelia burgdorferi bacterium that is spread through black legged ticks (otherwise known as deer ticks) who feed on the white footed mouse species, also known as the wood mouse, which carries the bacteria.  The symptoms of the disease itself include fever, headache, fatigue, and a telltale “bulls eye” rash near the site of the tick-bite. Left untreated, Lyme disease can spread to affect the joints (causing arthritis), heart, and nervous system – often causing irritability and mood swings.

Lyme disease transmission occurs in a Reservoir à Vector à Host cycle.  A Reservoir is the habitat in which an infectious agent normally lives, grows and multiplies – in this case, it is the white-footed mouse. A disease vector is a carrier animal (usually an arthropod) that transfers an infective agent from one host to another- i.e. the blacklegged tick.  And the host in this scenario is an organism that harbors an infective agent – us, our pets, and other animals.

Lyme disease is transmitted when a nymphal (young) tick feeds on a B. burgdorferi carrying white-footed mouse. The contaminated bloodmeal that it ingests allows the bacterium to live on in the tick (the vector), and the infected tick can then transmit the bacteria to its next host – a dog, your child, you, or any other animal roaming around in a wooded area.

Nearly a quarter of all Lyme disease cases are in children, as they play near to the ground, where host-seeking ticks are often waiting.  The CDC reports that pet owners and outdoorsy types are also at higher risk, as dogs and people traipsing through thick brush can easily pick up a tick or two without realizing it.

So how does climate change factor into this? According to ecologist Rick Osfeldt, a small mammal expert in Millbrook , New York, it all comes down to acorns.

“ Acorn abundance gives rodents a jump start on breeding. By the next summer, mice numbers are through the roof”.

This phenomenon gave rise to a “mouse-boom” in 2010, a low-acorn year in 2011, and what promises to be a busy summer for public health officials in 2012.  As the theory goes, as nymphal ticks wake up to a low mouse count (from 2011), they will feed on the existing mice and then turn to the next best thing – humans.

While the exact science behind what causes oak trees to produce more acorns is not yet identified, studies suggest that plants in warmer climates produce more seeds.

More acorns means a bumper crop for hungry mice, and milder winters mean higher breeding rates and higher survival rates for the B. burdorferi carrying rodents.

Maria Diuk-Wasser, an assistant professor of epidemiology at the Yale school of public health also attributes an increase in Lyme disease to higher average temperatures, but for a different reason.

“One possible way in which temperature may limit tick populations is by increasing the length of their life cycle from two to three years in the north, where it is colder.”  As average temperatures increase, climate change could be reverting the normal temperature pattern and increasing the production Lyme disease carrying ticks.

If both hypothesis prove to be true (and so far, CDC reported cases of Lyme disease have increased from 15,000 in the mid 1990s to over 40,000 today), an increase in both mouse and tick populations could indicate an increased prevalence of Lyme disease in years to come.

It may also be that the number of (geographically) susceptible people will increase as well. Nick Ogden, a zoonoses  researcher with the Public Health agency of Canada recently published a paper suggesting that the tick-inhabitable regions of North America may be increasing – in Eastern Canada, the tick inhabitable region will expend from 18% to over 80% by 2020, while the average temperatures in Canada have simultaneously increased by 2.5 degrees Fahrenheit over the past 60 years.

While some measures can be taken to prevent infection of Lyme disease once a tick has made a meal of you, cautionary measures are the best way to prevent you and your loved ones from becoming hosts.

The CDC recommends using insect repellant, applying pesticides, reducing tick habitat (i.e. cutting down heavy brush areas in your yard), and wearing long sleeves and pants when in wooded areas.  Prompt removal of ticks is also necessary, so continually check exposed skin areas when you are outdoors  -the backs of your legs, the back of your neck, the ears of your dog, etc.

One creepy-but-saving grace in tick removal may be that once a tick has landed on you, it will not immediately attach, instead crawling around for up to three hours to find an ideal location to feed. While not pleasant to imagine, it may give you enough time to jump in a hot shower after time outdoors and wash off any unattached ticks. Even attached ticks still require 24 to 36 hours to spread the B. burgorferi bacteria into your blood – if you remove a tick within 24 hours, you are greatly reducing your chances of getting Lyme disease.   Attached ticks  should be removed gently with tweezers.

If diagnosed early, Lyme disease can be cured with antibiotics. If you find an attached tick, see a general practitioner. You may be offered a single dose of antibiotics if you were bitten by a Lyme disease carrying tick species and the tick has probably been attached for at least 36 hours.

So, perhaps most importantly, if you suspect that you may have been bitten by a tick or have symptoms of Lyme disease – get thee to a doctor, and consider saving the planet from further warming by riding your bike there.

Online Sources:

http://www.cdc.gov/lyme/

http://www.cdc.gov/lyme/transmission/blacklegged.html

http://www.who.int/topics/zoonoses/en/

http://www.huffingtonpost.com/2012/04/04/global-warming-lyme-disease-west-nile_n_1400692.html

http://www.mnn.com/health/fitness-well-being/blogs/experts-predict-major-increase-in-lyme-disease-for-2012

Patrick A. Leighton, Jules K. Koffi, Yann Pelcat, L. Robbin Lindsay, Nicholas H. Ogden. Predicting the speed of tick invasion: an empirical model of range expansion for the Lyme disease vector Ixodes scapularis in Canada.Journal of Applied Ecology, 2012; DOI: 10.1111/j.1365-2664.2012.02112.x