AssessmentsBorrelia burgdorferi (Lyme disease)
Borrelia burgdorferi (Lyme disease)
USMLE® Step 1 style questions USMLE
A 14-year-old girl presents to the emergency department with lightheadedness. Since last night, she has felt fatigued and has noticed that every time she stands, she feels like she is about to pass out. She reports feeling short of breath after climbing a set of stairs for the past week. One month ago, she returned from summer camp in New Jersey, after which she began to develop fatigue and headaches. She takes acetaminophen for the headaches. Her temperature is 37.8°C (100°F), pulse is 40/min, respirations are 20/min, and blood pressure is 90/60 mmHg. She appears exhausted. Physical exam shows regular bradycardia without murmurs. No skin lesions are seen. An ECG is obtained and shows the following:
Reproduced from: Wikimedia Commons
Which of the following species most likely transmitted this patient’s disease?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
Lyme disease is an infectious disease that’s found around the world—in the US it’s most commonly caused by the bacterium Borrelia burgdorferi, and cousins of this bacteria, Borrelia garinii and Borrelia afzelii are causes of Lyme disease in Europe and Asia. And new ones are still being found.
In fact, in 2015, researchers from Mayo clinic discovered a new bacterial strain causing Lyme disease and decided to name it Borrelia Mayonii.
So these Borrelia bacteria are all spirochetes which means that unlike other bacteria, they are long, thin, and spiral-shaped, and they spin or twist to move around.
Lyme disease is classified as a zoonosis, meaning it’s it’s spread to humans from animals that exists as natural reservoirs, reservoir meaning that the bacteria typically doesn’t cause serious disease to the animal.
And Borrelia species can infect a wide range of animals, including small mammals like mice, lizards, and birds.
The bacterium isn’t known to spread directly from animals to humans, but instead it needs a vector, meaning some sort of intermediate organism to spread from the animal to the human.
Depending on the region, there are different vectors, for example, in the northeastern U.S. it’s spread via Ixodes scapularis, commonly known as the black-legged deer tick, but other types of Ixodes ticks transmit Lyme Borrelia in western North America and Eurasia: like the sheep tick, the taiga tick, and western black-legged tick.
Ticks need blood to survive, and I don’t mean they have blood already, they need blood from a host, or a blood-meal—just like mosquitoes, leeches, vampires, and other not-so-pleasant creatures.
Literally being blood-suckers, it makes them obvious candidates for spreading blood-borne disease, right?
And ticks need to go through life stages—larvae, nymph, and adult, not too different from us: children, adolescents, and adults.
The larval tick will most likely get the bacteria while feeding on something like a mouse, since deer ticks in the larval stage prefer smaller animals.
After larval deer ticks molt to the nymphal stage they feed on a broader range of animals, meaning that they can spread the bacteria they picked up to new reservoirs.
After molting again to the adult phase, they focus mostly on deer, which actually aren’t the best hosts for Borrelia.
So since the larval and nymphal deer ticks spread Borrelia to more suitable hosts, these two stages are very important for Borrelia.
Now, both nymphal and adult-stage deer ticks, although not their preferred meal, occasionally feed on the unknowing human.
If you’ve ever had a tick on you, you’d probably agree that the first thing you want to do is get it off.
The tough part about deer ticks, especially nymphal stage deer ticks, is that they’re really small--about the size of a poppyseed--, and therefore sometimes hard to notice.
This means they can be allowed to feed for long periods of time, thus allowing the tick to transmit the disease.
The Borrelia bacteria are transmitted within the saliva of the tick as its feeding.
If Borrelia infects a human it causes disease in three stages.
The early localized stage of Lyme disease is usually days to weeks after the initial infection.
As the bacteria spreads from this initial point, redness and inflammation expands as well.
Sometimes, the space between the initial bite and outer radius of the rash is cleared of bacteria, leading to a bulls-eye shaped rash, also known as erythema migrans—which is a classic early sign of Lyme disease.
In addition, there might be some vague flu-like symptoms at this stage as well.
Next is the early dissemination stage which is usually weeks to months later, when the bacteria starts to spread through the bloodstream, or disseminate to places like the heart, brain, and joints.
In general, relatively few bacteria actually invade these various tissues, but the immune reaction is usually pretty severe and turns these tissues into a warzone, essentially killing the bacteria but damaging the tissue in the process.
Other instances of erythema migrans might start to pop up in places that don’t relate to the original bite.
This is where Lyme disease can become serious.