Informational Tables

6.2. Parasites found in blood: characteristicsa

Protozoa

Diagnostic stage

Comments

Malaria

Plasmodium vivax (benign tertian malaria)

Ameboid rings; Schüffner's dots, beginning in older rings (appear later than those in P. ovale); all stages seen in peripheral blood; mature schizont contains 16–18 merozoites

Infects young cells; 48-h cycle; large geographic range; may have true relapse from residual liver stages, enlarged RBCs; infection considered more serious than previously thought

Plasmodium ovale (ovale malaria)

Nonameboid rings; Schüffner’s dots, beginning in young rings (appear earlier than those in P. vivax); all stages seen in peripheral blood; mature schizont contains 8–10 merozoites; RBCs may be oval and have fimbriated edges

Infects young cells; 48-h cycle; narrow geographic range; may have true relapse from residual liver stages, enlarged RBCs

Plasmodium malariae (quartan malaria)

Thick rings; no stippling; all stages seen in peripheral blood; “band forms” and “rosette”-shaped mature schizont; lots of malarial pigment

Infects old cells; 72-h cycle; relatively rare, but widely distributed; associated with recrudescence and nephrotic syndrome, no true relapse, normal or small RBCs

Plasmodium falciparum (malignant tertian malaria)

Multiple rings; appliqué-accolé forms; no stippling (rare Mauer’s clefts); rings and crescent-shaped gametocytes seen in peripheral blood (no other developing stages; rare exception, mature schizont); developing stages may also be seen in peripheral blood films in severe infections (high parasitemia)

Infects all cells, 36- to 48-h cycle; large geographic range; no true relapse; most pathogenic of five species; plugged capillaries can cause severe symptoms and sequelae (cerebral malaria, RBC lysis, etc.) Rarely, merozoites may be seen outside of the RBCs (may be high parasitemia when this occurs). Stages seen outside of RBCs is more common in Babesia spp. infections.

Plasmodium knowlesi (simian malaria)

Ring forms have multiple nuclei; multiple rings (mimic P. falciparum); late trophozoites tend to form bands; schizonts and gametocytes similar to P. malariae; (parasitemia, 0.03%, band forms); stippling present but probably not easily recognized; often misdiagnosed as a Plasmodium falciparum/Plasmodium malariae mixed infection.

Infects all cells, 24-h cycle (most rapid life cycle). Geographic range includes Thailand, Myanmar, peninsular Malaysia, Malaysian Borneo, Palawan Island in the Philippines, and Vietnam; can be as pathogenic as P. falciparum—should be considered a medical emergency

Babesia spp.

Ring forms only (resemble P. falciparum rings); seen in splenectomized patients as more severe disease; endemic in United States (no travel history necessary); if present, “Maltese cross” configuration is diagnostic, but it is not always seen; in some areas ticks that carry Babesia spp. may also be infected with Borrelia burgdorferi, the cause of Lyme’s disease.

Tick-borne infection associated with Nantucket Island; infection mimics malaria; ring forms are more pleomorphic than those in malaria; more rings/cell (usually) than in malaria; endemic in several areas within the United States (not limited to eastern U.S.), Europe, and Asia; organisms occasionally seen outside RBCs (unlike malaria merozoites, which rarely may be seen outside RBCs in heavy infections)

Trypanosomes

Trypanosoma brucei gambiense (West African sleeping sickness)

Trypomastigotes long and slender, with typical undulating membrane; very small kinetoplast; lymph nodes and blood can be sampled; microhematocrit tube concentration helpful; examine spinal fluid

Tsetse fly vector; tends to be chronic infection, exhibiting real symptoms of sleeping sickness; multiplication occurs in peripheral blood

Trypanosoma brucei rhodesiense (East African sleeping sickness)

Trypomastigotes long and slender, with typical undulating membrane; very small kinetoplast; lymph nodes or blood can be sampled; microhematocrit tube concentration helpful; examine spinal fluid in later stages of infection (however, death may occur prior to actual sleeping sickness symptoms).

Tsetse fly vector; tends to be more severe, more acute infection (particularly in children); patient may expire before progressive symptoms of sleeping sickness appear; multiplication occurs in peripheral blood

Trypanosoma cruzi (Chagas’ disease; American trypanosomiasis)

Trypomastigotes short, stumpy, often curved in C shape; very large kinetoplast (key differentiation point between these trypomastigotes and those of the African trypanosomes) sample blood early in infection; trypomastigotes enter striated muscle (heart, GI tract) and transform into amastigote form (multiply in this form)

Reduviid bug (“kissing bug”) vector; chronic in adults, severe in young children; great morbidity associated with cardiac failure and loss of muscle contractility in heart and GI tract

Leishmania spp. (cutaneous) (not actually a blood parasite but presented for comparison with L. donovani complex)

Amastigotes found in skin macrophages; intracellular forms containing nucleus and kinetoplast are diagnostic; NAAT recommended for species identification.

Not actually blood parasite but presented for comparison with Leishmania donovani complex; sand fly vector; organisms recovered from site of lesion only; stain specimens or culture in NNN and/or Schneider's medium; animal inoculation (hamster) rarely used

Leishmania braziliensis (mucocutaneous) (not actually a blood parasite but presented for comparison with L. donovani)

Amastigotes found in macrophages of skin and mucous membranes; intracellular forms containing nucleus and kinetoplast are diagnostic; NAAT recommended for species identification.

Not actually blood parasite but presented for comparison with L. donovani; sand fly vector; organisms recovered from site of lesion only; stain specimens or culture in NNN and/or Schneider's medium; animal inoculation (hamster) rarely used

Leishmania donovani complex (visceral)

Amastigotes found throughout reticuloendothelial system (spleen, liver, bone marrow, etc.); intracellular forms containing nucleus and kinetoplast are diagnostic; NAAT recommended for species identification.

Sand fly vector; organisms recovered from buffy coat (rarely found), bone marrow aspirate, spleen or liver puncture (rarely performed); stain specimens or culture in NNN and/or Schneider's medium; animal inoculation (hamster) rarely used; cause of kala-azar

Helminths

Wuchereria bancrofti

Microfilaria sheathed, clear space at end of tail; nocturnal periodicity; elephantiasis seen in chronic infections

Pathogenicity due to adult worms; mosquito vector; microfilariae recovered in blood (membrane filtration, Knott concentrate, thick films); hematoxylin stains sheath; sheath does not stain well with Giemsa, but the Innenkörper (inner body) stains a pink color

Brugia malayi

Microfilaria sheathed, subterminal and terminal nuclei at end of tail; nocturnal periodicity; elephantiasis seen in chronic infections

Pathogenicity due to adult worms; mosquito vector; microfilariae recovered in blood (membrane filtration, Knott concentrate, thick films); hematoxylin stains sheath; sheath tends to stain pink with Giemsa

Loa loa (African eye worm)

Microfilaria sheathed; nuclei continuous to tip of tail; diurnal periodicity; adult worm may cross conjunctiva of the eye

Pathogenicity due to adult worms; Chrysops (mango fly) vector; history of Calabar swellings, worms in the eye; microfilariae difficult to recover from blood; hematoxylin stains sheath

Mansonella spp.

Microfilaria unsheathed, nuclei may or may not extend to tip of tail (depending on species); nonperiodic; symptoms usually absent or mild

Pathogenicity mild and due to adult worms; Cullicoides (midge or blackfly) vector; microfilariae recovered in blood (membrane filtration, Knott concentrate, thick films)

Mansonella streptocerca
Not generally found in blood

Microfilaria unsheathed, nuclei extend to tip of tail; when immobile, curved like shepherd's crook; adults in dermal tissues

Pathogenicity mild and due to adult worms and/ or microfilariae; midge vector; microfilariae found in skin snips; microfilarial tails split rather than blunt

Onchocerca volvulus
Not generally found in blood.

Microfilaria unsheathed, nuclei do not extend to tip of tail; adults in nodules; routine histology of nodules characteristic and diagnostic

Pathogenicity due to microfilariae; Simulium (blackfly) vector; microfilariae found in skin snips; microfilariae migrate to optic nerve; cause of river blindness