The correct answer is E. The description of Schüffner dots tells us this patient is infected with Plasmodium vivax or ovale malaria. Schüffner dots are morphologic changes that occur in infected host erythrocytes. They are visible by light microscopy in blood smears and appear as multiple brick-red dots. The larger, infected cell in the center of the figure below shows an oval body and numerous Schüffner dots. Both P. vivax and P. ovale are capable of causing relapse following recovery from the primary infection. Both organisms leave dormant forms in the liver, called hypnozoites. Of these two, however, only P. vivax occurs in the western hemisphere.
P. falciparum and P. malariae (choices C and D) do not possess these dormant forms and thus are incapable of causing relapses, but if drug treatment of the erythrocytic stages is inadequate, the symptoms can "recrudesce" due to renewed multiplication in the bloodstream. Relapses and recrudescences look the same symptomatically, because once parasites begin multiplying again in the bloodstream, the cyclic fevers return.
Anaplasma phagocytophilum (choice A) is the cause of human granulocytic anaplasmosis (previously known as human granulocytic ehrlichiosis). Anaplasma phagocytophilum is an obligate intracellular parasite transmitted by Ixodes ticks. It infects neutrophils, not erythrocytes. Symptoms include fever and chills, headache, and myalgias.
Babesia microti (choice B) causes malaria-like symptoms and is transmitted by the same tick that spreads Lyme disease. It does not occur in Panama.
This is a multi-step question.
What is the question asking?
You are being asked to identify the most likely causal organism of this patient's condition.
What is the first step?
The first step is to identify important findings regarding the patient's condition.
- The patient is from Panama, so you need to consider what diseases are endemic to this region.
- Has recurring high-grade fevers
- Blood smear shows oval bodies with punctate granulations (Schüffner dots)
What is the next step?
The next step is to make a diagnosis. Recurring high fevers should raise suspicion for malaria, especially in patients originating from endemic regions. The oval body pictured within the red blood cell represents the gametocye form of malaria. The coarse brown punctate granulations known as Schüffner dots are a hematologic finding associated with Plasmodium vivax or P. ovale, the two species known to have a dormant form that can cause relapsing disease. P. vivax (choice E) is most applicable to this patient because it is transmitted in Panama, whereas P. ovale is not.
Can other answers be eliminated?
- Anaplasma phagocytophilum (choice A) can be eliminated because it infects neutrophils, not erythrocytes.
- Babesia microti (choice B) can be eliminated because it does not occur in Panama.
- P. falciparum and P. malariae (choices C and D) can be eliminated because they do not have a dormant form and thus are incapable of causing relapsing symptoms.
What is the single best answer and why?
The single best answer is Plasmodium vivax (choice E) because it matches this patient's presentation with high-grade, recurrent fever, along with punctate granulations and oval bodies on a peripheral blood smear (representing Schüffner dots), as well as her geographic origin from Panama.
MedEssentials (4th Ed.): pp. 125
First Aid (2019): pp. 157.1
First Aid (2018): pp. 157.1
First Aid (2017): pp. 153.1
Pathoma (2018-2019): pp. 51.2
Pathoma (2014-2017): pp. 51.2
