47 Infectious Agents

Michelle To and Valentin Villatoro

Malarial Infection

Causative Agent:1,2

Malaria is transmitted by the female Anopheles mosquito.


There are five species of malaria:

Plasmodium falciparum – Most severe

Plasmodium vivax – Most common

Plasmodium ovale – Less common

Plasmodium malariae – Less common

Plasmodium knowlesi


Geographic Distribution:1

Tropical and subtropical areas


Clinical Features:1,2

Fever, headache, chills, sweating, splenomegaly. Patients can show cyclical patterns of fever and chills being present then absent. In severe malaria, jaundice, shock, bleeding, seizures, or coma may occur. Thrombocytopenia commonly occurs with malarial infections, and may lead to bleeding tendencies.


Hemolysis is mostly intravascular and occurs through a number of different mechanisms:1,2

  1. Direct damage to the red blood cells

  2. Immune activation of macrophage activity (Increased extravascular hemolysis)

  3. Release of cytokines that inhibit erythropoiesis (inhibit erythropoietin)

  4. Increased oxidative stress on RBCs during infection

  5. Induction of apoptosis in red blood cell precursors


Laboratory Results for Malarial Infections:1,2


RBC: Decreased

WBC: Normal to Slightly Increased (neutropenia during chills)

PLT: Decreased

Hct: Decreased

RETIC: Decreased




Intracellular Malarial parasite forms (forms and stages of development seen differ by species)


Erythroid Hypoplasia

Other Tests:

Giemsa stain

Thin Smears: Morphology and parasitemia calculation

Thick Smears: Presence or absence of malaria


Flow Cytometry

Molecular Testing (PCR)


Note: See “Malaria” under RBC inclusions for additional information.


Causative agents:1,3

Babesia microti is carried by the Ixodes scapularis ticks. Babesia can also be transmitted by blood transfusions.


Geographical Distribution:3

Northeast and Midwest United States


Clinical Features:3

Clinical symptoms include: flu-like symptoms ranging from fever, headache, anorexia, fatigue. Some patients may be asymptomatic. Hepatosplenomegaly may or may not be present.


Laboratory Results for Babesiosis:1,3


WBC: Decreased

PLT: Decreased

Hb: Decreased

RETIC: Increased


RBCs shows tetrads of merozoites (Maltese cross)

Other Tests:




Molecular testing (PCR)


Note: See “Babesia” under Abnormal RBC inclusions for additional information.

Clostridial Sepsis

Causative agent:1,3

Clostridium perfringens is an anaerobic, gram-positive spore-forming bacilli that can infect tissues after events involving trauma to the body. The bacteria releases enzymes and toxins that damage the surrounding tissue and can allow the bacteria to reach the bloodstream.

Blood infection is often fatal and can lead to massive hemolysis and the activation of DIC.


Mechanism of RBC Destruction:1,3

Hemolysis by C. perfringens is intravascular and can occur due to direct effects of alpha-toxin that it releases on the red blood cell membrane. Cell becomes spherical and subject to osmotic lysis. Significant intravascular hemolysis ensues, leading to a marked decrease in hematocrit, and dark red/brown plasma and urine.


Laboratory Results for Clostridia Infections:1,3


RBC: Decreased

PLT: Decreased

Hct: Very Decreased




Ghost cells

Toxic changes with a left shift

+/- ingested bacteria within neutrophils

Other Tests:




1. Harmening DM, Yang D, Zeringer H. Hemolytic anemias: extracorpuscular defects. 5th ed. Philadelphia: F.A. Davis Company; 2009. p. 250-79).

2. Akinosoglou KS, Solomou EE, Gogos CA. Malaria: a haematological disease. Hematology [Internet]. 2012 Mar 1 [cited 2018 Jul 9];17(2):106–14. Available from: http://www.tandfonline.com/doi/full/10.1179/102453312X13221316477336

3. Keohane EM. Extrinsic defects leading to increased erythrocyte destruction – nonimmune causes. In: Rodak’s hematology clinical applications and principles. 5th ed. St. Louis, Missouri: Saunders; 2015. p. 394-410.


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Infectious Agents Copyright © 2019 by Michelle To and Valentin Villatoro is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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