By Greg Kincheloe

As the winter finally draws to an end (albeit a long and drawn-out end), it is time to start looking forward to warmer weather, sunshine, and leaves on the trees once again.  Accompanying this warmer weather are more opportunities to reconnect with nature and the great outdoors.  Soon, trails and parks will no longer be icy and the great Appalachian Trail, which runs continuously from Georgia to Maine and is located no more than 20 minutes from Harrisburg, will begin filling with hikers.  But while spring is a time to finally go outside, catch some fresh air, and enjoy the outdoors, it is also the time to be once again wary of other creatures that may be doing the same thing. 

If you are from the Mid-Atlantic or Northeast, the odds are that someone you know has gotten Lyme disease at some point.  This is because Pennsylvania has one of the highest incidence rates of Lyme disease in the country, with 6,763 confirmed cases in 2019¹ (Fig. 1). 

Lyme disease is a tick-borne illness most often caused by the bacteria Borrelia burgdorferi and is highly associated with Ixodes scapularis, otherwise known as deer ticks^1,2.  If a tick carrying B. burgdorferi bites you and is left undisturbed for 36 to 48 hours, the bacteria will be able to enter your body and you may begin to experience symptoms such as fever, chills, fatigue, muscle aches, and more serious nerve pain (Notable person affected by Lyme: Justin Bieber).  Sometimes, a rash may appear at the site of the bite that looks like a bullseye, but if this is absent, Lyme disease may be difficult to diagnose without further testing¹. Though Lyme disease can be treated with simple antibiotics, some people experience chronic symptoms long after the bacteria is eradicated from their system (termed Post-Treatment Lyme Disease Syndrome)⁵.  These patients often keep experiencing symptoms such as arthritis, headaches, heart palpitations, and nerve pain up to months or years after initial treatment.  Though this is scary, the ecology behind the spread of this disease is pretty neat.

Ecology and Life Cycle

The deer tick has four distinct stages in its life cycle (egg, larva, nymph, adult), and each stage occurs after the tick has had a full bloodmeal (gross)³.  When deer ticks hatch from their eggs, they come out as no more than teeny tiny larvae less than 1mm in length.  These li’l nuggets’ immediate task is to eat their first bloodmeal.  If you happen to find a larva on you at this stage, you will be relatively safe from Lyme disease, as ticks do not carry B. burgdorferi until they obtain it after a bloodmeal from another animal that is a carrier.  Carrier animals can be a number of “reservoir species,” or species that are seemingly unaffected by the bacteria but still harbor and spread B. burgdorferi to any deer tick that eats from it^2,3.  The most common reservoir species in the Northeast are mice and small mammals, though birds can often be reservoirs for Lyme as well³. 

After the first bloodmeal, the deer tick molts into a slightly larger nymph and begins searching for its next host animal.  If the first host animal contained B. burgdorferi, then the tick nymph will be free to spread it to every subsequent species it feeds from³.  Most humans get Lyme disease from nymphs because they are much smaller and harder to detect than adults.  After the second bloodmeal, the nymph molts into a full-sized adult, which can then lay eggs and start the cycle all over again (Fig. 2).  During spring, deer tick nymphs rapidly rise in number, so it is especially important to be vigilant on those early-year outings^1,3.

Infection

When a B. burgdorferi-carrying tick is not actively feeding, the bacteria slows down and stops replicating inside the tick’s midgut.  However, once the tick begins feeding again and blood reaches its midgut (usually around 24 hours after the tick begins), B. burgdorferi will kick into action and begin replicating quickly.  It is then that the bacteria migrate, making their way into the new host aided by the tick’s saliva, which contains immunosuppressants and other bioactive agents that help the tick feed, as well as the bacteria to colonize to a new host³.  Most symptoms of Lyme disease are a result of the body’s immune response to the bacteria, such as inflammation and fever^3,6,7.  Effectively targeting the bacteria can be difficult for the immune system, however, since B. burgdorferi has been shown to utilize molecular mimicry, a form of disguising itself as native cells⁸.  This molecular mimicry may also be an explanation for why some patients have continuing long term symptoms, as the immune system may begin targeting and attacking its own cells, potentially resulting in autoimmune disorders similar to rheumatoid arthritis^6,8

Prevention

Every time you go on a hike, walk in long grass, or really do anything involving the outdoors, you should give yourself a quick tick check at the end of the day.  This is the easiest and least chemical-ridden method of preventing yourself from getting Lyme disease.  However, if you want another level of protection, the chemical bug repellent DEET can work well when sprayed on your skin.  But forewarning, high concentrations of DEET does melt plastic, spandex, and other synthetic fibers so you may not want to spray it directly on your clothes. 

Closing Thoughts

Cases of Lyme disease are rising in the United States to the point where there are nearly double the number of cases as there were in 1991⁴.  Climate change is believed to play a major cause (warmer temperatures means ticks can be out for longer), though undoubtedly another cause is simply people not checking themselves thoroughly for ticks after being outside (Fig. 3).  While Lyme disease makes headlines most often, there are also other tick-borne illnesses in the Northeast like STARI (which has similar symptoms to Lyme disease) and alpha-gal syndrome, which is transmitted by the lone star tick and makes people allergic to a carbohydrate found in red meat and dairy products⁹.  Some places around the world are known for having ticks that can cause fatal paralysis even after detection and removal (we’re looking at you, Australia). 

Bottom line, be careful this season and check yourself for skin parasites!

TL:DR

• Lyme disease is heavily intertwined with the ecosystem and at worst can cause symptoms far into the future. 
• Check yourself for ticks.

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References

1. https://www.cdc.gov/lyme/index.html
2. Lo Re V 3rd, Occi JL, MacGregor RR. Identifying the vector of Lyme disease. Am Fam Physician. 2004 Apr 15;69(8):1935-7. PMID: 15117014.
3. Radolf, J. D., Caimano, M. J., Stevenson, B., & Hu, L. T. (2012). Of ticks, mice and men: understanding the dual-host lifestyle of Lyme disease spirochaetes. Nature reviews. Microbiology, 10(2), 87–99.
4. https://www.epa.gov/climate-indicators/climate-change-indicators-lyme-disease#:~:text=The%20incidence%20of%20Lyme%20disease,2018%20(see%20Figure%201).
5. Marques A. (2008). Chronic Lyme disease: a review. Infectious disease clinics of North America, 22(2), 341–viii. https://doi.org/10.1016/j.idc.2007.12.011
6. Singh SK, Girschick HJ. Lyme borreliosis: from infection to autoimmunity. Clin Microbiol Infect. 2004 Jul;10(7):598-614. doi: 10.1111/j.1469-0691.2004.00895.x. PMID: 15214872.
7. Ramesh G, Alvarez AL, Roberts ED, Dennis VA, Lasater BL, Alvarez X, Philipp MT. Pathogenesis of Lyme neuroborreliosis: Borrelia burgdorferi lipoproteins induce both proliferation and apoptosis in rhesus monkey astrocytes. Eur J Immunol. 2003 Sep;33(9):2539-50. doi: 10.1002/eji.200323872. PMID: 12938230.
8. Raveche, E. S., Schutzer, S. E., Fernandes, H., Bateman, H., McCarthy, B. A., Nickell, S. P., & Cunningham, M. W. (2005). Evidence of Borrelia autoimmunity-induced component of Lyme carditis and arthritis. Journal of clinical microbiology, 43(2), 850–856. https://doi.org/10.1128/JCM.43.2.850-856.2005
9. https://www.cdc.gov/ticks/index.html