Ans. a. P. falciparum (Ref: Robbins 9/e p390, 8/e p387)Person having heterozygous sickle cell trait is protected from infection of P. falciparum."People who are heterozygous for the sickle cell trait (HbS) become infected with P. falciparum, but they are less likely to die from infectionQ. The HbS trait causes the parasites to grow poorly or die because of the low oxygen concentrationsQ."- Robbins 8/e p387Host Resistance to PlasmodiumTwo general mechanisms of host resistance to Plasmodium:Inherited alterations in red cells make people resistant to PlasmodiumQ.Repeated or prolonged exposure to Plasmodium species stimulates an immune response that reduces the severity of the illness caused by malariaQ.People who are heterozygous for the sickle cell trait (HbS) become infected with P. falciparum, but they are less likely to die from infectionQ.The HbS trait causes the parasites to grow poorly or die because of the low oxygen concentrationsQ.The geographic distribution of the HbS trait is similar to that of P. falciparumQ, suggesting evolutionary selection of the HbS trait in people by the parasite.HbC, another common hemoglobin mutation, also protects against severe malaria by reducing parasite proliferationQ.People can also be resistant to malaria due to the absence of proteins to which the parasites bindQ.P. vivax enters red cells by binding to the Duffy blood group antigenQ.Many Africans, including most Gambians, are not susceptible to infection by P. sivax because they do not have the Duffy antigenQ.Antibodies and T lymphocytes specific for Plasmodium reduce disease manifestations.Cytotoxic lymphocytes may also be important in resistance to P. falciparum.In the given image, irregularly shaped large rings and trophozoites are seen with enlarged erythrocytes, which are the characteristic features of Plasmodium vivax.Examination of Blood Films for Malaria ParasiteMalaria parasites pass through a number of developmental stages. In all stages, however, the specific parts of the parasite will stain a specific colour.Parts of parasiteStaining characteristicChromatin (parasite nucleus)* Usually round in shape & stains a deep red.Cytoplasm* Blue, although shade of blue may vary between malaria species.Recognition of a Malarial Parasite* Malaria parasite takes up Giemsa stain in a special way in both thick & thin blood films. Blood Smears of PlasmodiumFeaturesP. falciparumP. vivaxP, malariaeP. ovaleFeatures of red cellsSizeAll sizes/ normalLarge (young) paleSmall (Old), normalLarge (young)ShapeRound, may be crenatedRound or ovalRoundRound or pear- shaped fimbriatedStipplingMaurer's cleftsQ Large;red up to 20 Basophilic stippling +-Schuffner's dotsQ;numerous small redNone, Occasionally Zieman's dotsQSchuffner's dots, James dotQFeatures of ParasiteRing (early trophozoite)Threadlike, multiple infections, double chromatin dots form accoleQThickerQCompactQCompactQMature/Late trophozoites (amoeboid form)Absent/ occasionally seenAmeboid may fill cell-- More regular, smaller, Band formQLess ameboid &smaller than those of P vivaxDiagnostic keys Gametocyte, multiple rings, double chromatin dots, accoie forms, heavy infectionQSchizont, large RBCs, amoeboid formsQSchizont, small RBCs, band formsQSchizont and large RBCs; pear-shaped, fimbriated RBCsQ Characteristics of plasmodium species infecting HumansCharacteristicP. falciparumP. vivaxP. ovaleP. malariaeIncubation period12 days (shortest) Q14 days14 days30 days (longest) QDuration of erythrocytic cycle (hours)48 (malignant tertian malaria) Q48 (benign tertian malaria)50 (ovale tertian malaria)72 (Guatran malaria) QRed cell preferenceYounger cellsQ (but can invade cells of all ages), >2% of RBC infectedQRed cells upto 14 days oldQ, <1% of RBC infectedReticulocytesQOlder cellsQMorphologyUsually only ring forms, banana shaped gametocytesQIrregularly shaped large rings and trophozoites; enlarged erythrocytes; Schuffner's dotsQInfected erythrocytes enlarged and oval with tufted endsQ; Schuffners dotsSand ofrectangular formsof trophozoites commonPigmentBlackQYellow-brownQDark brownQBrown-blackQRelapse (hypnozoits or exo- erythrocytic schizogony)NoYesQYesQNo