Answer to Case 558

Answer toParasite Case of the Week 558:Plasmodium falciparummalaria,>10% parasitemia. NEGATIVE rapid antigen.Sowhy is the rapid antigen test negative???As noted by our readers, there are many possible reasons for apositive blood smear and negative rapid malaria antigen test (RDT). Here are our options, along with the reasons why each is or isn ' t a likely explanation in this case:This is babesiosis, and not malaria. This is a very important consideration given the morphologic similarities betweenBabesiaspp. andPlasmodium falciparum.However, the moprhologic features in this case are highly consistent withP. falciparum, including the presence of relatively-homogenous rings, without the size and shape pleomorphism usually seen withBabesiaspp. There are also applique forms and headphone forms (arrows, below) which are characteristic, but not definitive, forP. falciparuminfection. There may also be a hint of hemazoin (malaria pigment), but it is not obvious. Overall, we can rule out babesiosis based on the microscopic morphology.The negative RDT is due to deletion of theP. falciparum histidine rich protein II repeat region in the parasite infecting this patient. This deletion has been reported in some African and South American countries, including Kenya where this patient had recently traveled. While this is a good thought, it would not explain why the pan-malaria antigen band (in this case, aldolase) is also absent, resulting in a completely negative RDT result. Th...
Source: Creepy Dreadful Wonderful Parasites - Category: Parasitology Source Type: blogs