AssessmentsTrypanosoma cruzi (Chagas disease)
Trypanosoma cruzi (Chagas disease)
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 24 year-old woman comes to the clinic with fever, myalgias, dyspnea, fatigue, and painless unilateral periorbital swelling. She recently returned from a mission trip to Brazil. Her temperature is 38.6°C (101.4°F), pulse is 112/min, respirations are 22/min, and blood pressure is 124/84 mm Hg. Physical examination shows a holosystolic murmur radiating to the axilla. There is a hardened, erythematous insect bite on her right leg. The left eye has periorbital edema but no discharge, and the right eye is unremarkable. Electrocardiogram shows sinus tachycardia with low voltage. Echocardiogram shows pleural effusion. Peripheral blood smear shows amastigotes of T. cruzi. Which of the following findings is most consistent with chronic infection with this parasite?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Pauline Rowsome, BSc (Hons), Tanner Marshall, MS, Evan Debevec-McKenney, Amanda J. Grieco, PhD
T. cruzi is transmitted through the feces of the insect triatominae.
Triatominae is a type of reduviid bug also called the kissing bug because it typically bites people on the face as they sleep at night - one heck of a good night kiss, huh?
The disease gets its name from Carlos Chagas, the physician who first described it.
The life cycle of T. cruzi starts with the epimastigote T. cruzi which sits in the lumen of the Reduviid bug’s midgut.
–Mastigote refers to the whip-like structure called a flagellum which protrudes from the center of the T. cruzi and helps it move around.
While in the midgut, the epimastigote multiplies through binary fission.
Over time, the epimastigote transforms into a trypomastigote and at that point it loses its ability to divide, but the trade-off is that it gains the ability to invade human cells.
In fact “trypo” means to bore or punch into.
Reduviid bugs feed off the blood of humans, and they prefer biting a person’s face, which is why they’re also called kissing bugs.
But unlike a normal kiss, the reduviid bug then defecates at the bite site, and if the reduviid bug is infected with T. cruzi, the feces can contain trypomastigotes.
These trypomastigotes can then infect human skin cells at the bite location; or at mucous membranes, particularly the conjunctiva of the eyes.
That can happen if a person unknowingly transfers the trypomastigotes by rubbing the bite site on their face and then touching their eyes.
Once the trypomastigote invades a human cell, it transforms into an amastigote meaning that it loses its flagellum.
The amastigotes multiply intracellularly, again through binary fission, and then transform into blood trypomastigotes which can move through the blood and lymph to other tissues.
The blood trypomastigotes then invade more cells, and then again turn into amastigotes to multiply intracellularly again - and that’s how the cycle goes.
Now, a person infected by a T. cruzi infected reduviid bug, can then get bitten by a brand new reduviid bug.
In that case, that reduviid bug might get infected by the trypomastigotes, and those trypomastigotes would make their way into the reduviid bug’s midgut and then differentiate into epimastigotes - completing the life cycle.
Transmission from the reduviid bug to a person can also happen through infected organ and blood donations, which is why blood screening is super important, and it can also spread from mother to child during a pregnancy.
Now when a person is infected with Chagas disease, there’s initially an incubation period of up to two weeks.
During that incubation period, T. cruzi trypomastigotes invade host cells, amastigotes multiply, and blood trypomastigote levels begin to increase in the blood.
Usually there’s local inflammation as immune cells move towards the area of tissue damage.
When this happens at the bite site it’s called a chagoma, and if it happens around the eye, it can cause eyelid swelling which is called a Romaña's sign.