Borrelia burgdorferi (Lyme disease)


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Borrelia burgdorferi (Lyme disease)

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Borrelia burgdorferi (Lyme disease)


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Borrelia burgdorferi (Lyme disease)

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USMLE® Step 1 style questions USMLE

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A 22-year-old woman comes to your clinic with complaints of low-grade fever, malaise, and fatigue. The symptoms started two days ago and are only partially responsive to acetaminophen. The patient returned from a vacation in Pennsylvania 3 weeks ago where she hiked, camped, and went to the beach with friends. Physical examination of the left upper extremity is shown below. If not adequately treated, this condition can cause which of the following complications?  

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Borrelia burgdorferi

animal transmission p. 147

coinfection with p. 154

facial nerve palsy p. 183

Lyme disease p. 144

tetracyclines p. 189


Content Reviewers

Yifan Xiao, MD

Robert S. Porter, MD, Editor-in-Chief, Merck Manual

Scott Line, DVM, PhD, DACVB, Editor-in-Chief, Merck Veterinary Manual


Evan Debevec-McKenney

Jerry Ferro

Zachary Kevorkian, MSMI

Amanda J. Grieco, Ph.D.

Lyme disease, or Lyme borreliosis, is an infectious, blood-borne bacterial disease that is transmitted by ticks.

It’s caused by Borrelia burgdorferi species.

Now, the genus Borrelia contains several species.

Lyme disease in people is caused primarily by Borrelia burgdorferi in North America and by B. afzelii, B. garinii, and B. burgdorferi in Europe and Asia.

In domestic animals, only B burgdorferi is confirmed to cause Lyme disease..

Borrelia are spirochetes, which means spiral-shaped bacteria.

They have outer surface proteins, abbreviated as Osp, which play a role in virulence; and sets of flagella that run between the cell wall and outer membrane, which they use to spin or twist to move in a wave-like motion.

Hard-shelled, Ixodes ticks, or deer ticks, are the vector for B. Burgdorferi, meaning they are the intermediate organism that spreads the bacteria.

In the northeast and Midwest USA, I. scapularis, the black-legged deer tick is the main vector; while on the Pacific coast, it’s I. pacificus, the western black-legged tick.

In Europe and Asia I ricinus and I. persulcatus are the primary vectors.

Ticks like environments with moderate humidity and temperature so they’re often found in wooded areas, thick brush, marshes, and tall grass.

The ticks are small, and even adults are only about 3 mm long, so they can be hard to notice.

Now Ixodes ticks feed on the blood from hosts throughout their life stages of larva, nymph, and adult.

When they hatch as larvae, they are uninfected.

When they feed on infected hosts as larvae or nymphs, they can pick up the B. burgdorferi bacteria.

In the younger stages of their life, they often feed on smaller animals like rodents, birds and even lizards.

When they grow into adults, they move on to larger mammals like dogs, cats, or horses.

A tick infected with B. Burgdorferi can transmit the bacteria to humans and animals through their saliva during feeding.

In the first few hours after attachment, the bacteria in the tick’s midgut switch their outer surface protein from OspA to OspC, which helps with transmission and provides protection against the host’s immune system.

After 24 hours of attachment, the bacteria passes from the ticks digestive system to the host’s skin, and then into the blood vessels that the tick is feeding on.


Lyme disease is a bacterial infection that is transmitted to humans through the bite of an infected deer tick. It can cause a wide range of symptoms, including fever, headache, fatigue, and a characteristic rash. If left untreated, it can lead to more serious complications such as joint inflammation (arthritis), heart problems (carditis), and even death.


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