AssessmentsBorrelia species (Relapsing fever)
Borrelia species (Relapsing fever)
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 24-year-old woman is brought to the emergency department following a 2-week history of intermittent fevers. The last time she felt well was 3 weeks ago when she returned from Ethiopia. She then developed a high-grade fever along with dizziness, headaches, and joint pains. Two days after the fever started, she experienced a sudden spike in her temperature followed by an episode of shivering and sweating. The fever then appeared to resolve, but returned again after 6 days, which prompted her to come to the emergency department. Her temperature is 39.9°C (103.8°F), pulse is 90/min, respirations are 20/min, and blood pressure is 135/86 mm Hg. She appears acutely unwell and there is splenomegaly on abdominal examination. A Giemsa stain is positive for spirochetes and labs are normal. Which of the following is the most likely causative organism for this presentation?
Content Reviewers:Viviana Popa, MD
Borrelia is a genus of spirochetes, which are long, thin, spiral shaped bacteria, that spin or twist to move around.
The second group is associated with relapsing fever and the most common species are Borrelia hermsii, Borrelia turicatae, Borrelia parkeri and Borrelia recurrentis.
Of the second group, the first three species cause tick-borne relapsing fever, while Borrelia recurrentis causes louse-borne relapsing fever.
By the way, you may know this already, but “louse” is actually singular for “lice” - so body lice are the vector that transmits Borrelia recurrentis.
Now, all Borrelia species have an outer membrane that contains a substance similar to lipopolysaccharides, an inner membrane, and a layer of peptidoglycan in a periplasmic space, which is the space between the inner and the outer membrane.
Since the peptidoglycan layer is thin, it doesn’t retain the purple dye used during Gram staining, so they’re classified as Gram-negative.
However, they can’t be easily visualized with Gram staining. Instead, they’re best seen with the Wright or Giemsa stain, which make the bacteria appear purple, and with dark-field microscopy, which make the bacteria appear white against a dark background.
Now, Borrelia species are motile, which means they can move around using long, thin filaments called endoflagella, which are located in their periplasmic space.
The filaments rotate in this space, between the outer membrane and the peptidoglycan layer, propelling the bacterium forward in a corkscrew-like motion.
Finally, the bacteria can be cultivated on Barbour-Stoenner-Kelly medium or by intraperitoneal inoculation of immature laboratory mice, but these tests are not widely available.
Now, Borrelia can enter the body one of two ways, depending on the type of relapsing fever.
So, in tick-borne relapsing fever, or TBRF, the bacteria enters the bloodstream through the bite of an infected tick.
With louse-borne relapsing fever, or LBRF, the bacteria is transmitted by lice.
When lice feed on infected humans, they acquire Borrelia which then multiply in the lice gut and hemolymph - which is like their blood.
And then, infected lice can feed on an uninfected human, and the bacteria enters the bloodstream when the person crushes the louse or scratches the area where the louse is feeding.
Once in the bloodstream, Borrelia species can escape the immune system through antigenic variation.
Ok, so the outer membrane of Borrelia has surface proteins called variable small proteins, or Vsp, and variable large proteins, or Vlp.
These surface proteins are constantly changing, and this allows Borrelia to trick the immune system over and over again.
So, by the time the immune system has developed antibodies against Vsp or Vlp, they’ve already changed, allowing Borrelia to escape immune recognition.
Once it’s safe from immune destruction, Borrelia replicates in the blood through binary fission.
This means the bacteria splits in two identical copies - and if it sounds similar to mitosis...well, it is!
But the term binary fission is used to describe division of prokaryotic cells, which don’t have a nucleus, and therefore some steps in replication are different from mitosis.
When they replicated enough, they spread from the bloodstream to various organs such as liver, heart, lungs, bone marrow or central nervous system.