MICROBIO 4.6 - PARASITES (BLOOD & TISSUE PROTOZOA)

Plasmodia Overview

• Plasmodia have a sexually reproductive stage in mosquitoes and an asexually reproductive stage in humans
• Malaria is caused by a Plasmodia infection of red blood cells (RBCs)
• The infection results in cyclical episodes of acute fever, chills, headache, and flu-like symptoms
• Pregnant women, young children, the elderly, and immunosuppressed individuals have increased malaria vulnerability
• Malaria is transmitted to humans by female Anopheles mosquitoes from tropical and subtropical areas during feeding
• Transmission can also occur through organ transplants, from mother to fetus, or via infected needles
• Reducing mosquito populations through repellants and barriers can help prevent malaria
• Certain blood cell polymorphisms, which alter surface antigens or the shape of RBCs, may protect against some malarial parasites
• Malaria is treated with antimalarial drugs

Key Terms

• Schizont (Sky-zont): A cell formed from a trophozoite during the asexual stage of sporozoan protozoans,
• It divides by schizogony to form daughter cells
• Schizogony (skits-ohg-o-knee): Asexual reproduction by multiple fission
• Sporozoite (spore-o-zo-ite): A motile spore-like stage in the life cycle of some parasitic sporozoans, such as during a malaria infection
• It is typically the infective agent introduced into a host
• Trophozoite (Trof-o-zo-ite): The active, feeding, and multiplying stage of most protozoa that absorbs nutrients from the host, In parasitic species, this is associated with pathogenesis

Apicomplexa Life Cycle

• Malaria infection begins when an infected mosquito transfers parasites from saliva during feeding
• Parasites travel to the liver and invade hepatocytes
• Asexual reproduction occurs in the liver with parasites being released into circulation and invading RBCs
• Inside RBCs, the erythrocytic phase begins, and asexual reproduction happens again
• Replicating parasites cause RBCs to rupture, releasing more parasites and toxins into circulation, leading to clinical malaria symptoms
• Newly released parasites invade new RBCs and asexually reproduce
• Each round of asexual reproduction results in one of the cyclical malarial episodes
• Some parasites leave the asexual cycle and enter the sexual reproduction pathway
• Gametocytes (precursors to gametes) travel freely in blood
• Another mosquito ingests the gametocytes during feeding
• After ingestion a new sexual reproductive phase can infect a new human host

Plasmodium falciparum

• Plasmodium falciparum is the predominant malaria species in Africa and causes the most severe form of malaria and the highest mortality rates
• P. falciparum can invade any and all red blood cells
• It has a short reproductive cycle, multiplies rapidly, and causes significant parasitemia
• Destruction of red blood cells can lead to anemia, and released toxins can produce severe clinical symptoms
• Cells infected by P. falciparum adhere to the walls of microvasculature, obstructing blood flow and damaging organs
• Cerebral malaria is a potential and often fatal complication
• Damage to kidneys or lungs caused by cerebral malaria can also be fatal
• Mature forms of falciparum are isolated in microvasculature and appear as immature "ring stage" in blood smears
• On edges of red blood cells ("appliqué position") because they appear stuck on the edges
• There is little to no red blood cell distortion
• Mature gametocytes are crescent shaped and travel freely in the blood
• Multiple parasites may be found in a single cell, along with Maurer's clefts

Maurer's Clefts

• Maurer's clefts are membrane-limited vacuoles or sack-like structures found in the cytosol of erythrocytes

Plasmodium malariae

• Less common cause of malaria
• Found worldwide
• Invades senescent (fully differentiated and mature) red blood cells
• RBCs are not distorted and remain rigid after infection
• Schizonts exhibit a rosette pattern in blood smears
• Trophozoites tend to appear as "bands" across the red blood cell

Plasmodium vivax

• Can be a significant cause of malaria
• Found primarily in Asia, Latin America, and some parts of Africa
• Capable of causing relapsing malaria due to dormant forms residing in the liver
• Invades young, immature red blood cells
• Can be seen in peripheral blood during any stage of its development
• Mature trophozoites exhibit an amoeboid shape
• Infected red blood cells become enlarged and distorted
• Using Giemsa staining, Schüffer's dots may be visible, giving cell a "stippled" look

Duffy Antigen/Chemokine Receptor (DARC), AKA CD234 (Cluster of Differentiation 234)

• Duffy antigen is located on the surface of RBCs
• Named after the patient in whom it was discovered
• Protein acts as the receptor for Plasmodium vivax
• Duffy-negative individuals whose erythrocytes do not express the receptor are believed to be resistant to invasion of Plasmodium vivax
• P. vivax infection has been reported in Duffy-negative children in Kenya, suggesting a role in resistance to disease but not infection

Plasmodium ovale

• Found in Sub-Saharan Africa and the western Pacific Islands
• Similar to Plasmodium vivax
• It causes relapsing malaria via dormant forms in the liver and infects and distorts immature red blood cells
• Schüffer's dots can be seen with Giemsa staining, giving cells a characteristically "stippled" look
• Infected cells can have jagged edges, referred to as fimbriae
• The trophozoite form is relatively compact
• Multiple ring forms may be present within a single cell, similar to Plasmodium falciparum

Plasmodium Species Treatment

• Plasmodium falciparum:
• Treatment involves Chloroquine, Mefloquine or Quinine + antibiotics (in chloroquine-resistant areas), Artemisinin (severe infections), Atovaquone-Proguanil
• Plasmodium malariae:
• Found worldwide and is treated with Chloroquine
• Plasmodium vivax
  • Found in Asia, Latin America, and parts of Africa
  • Treatment involves Chloroquine + Primaquine
• Plasmodium ovale
  • (similar to Plasmodium vivax)
  • Treatment involves Chloroquine + Primaquine

General Objectives

• Compare and contrast the differences between intracellular and extracellular protozoa
• Identify free-living amoebae
• Classify different parasites, their associated diseases, and their presentations
• Identify when to prescribe specific antiparasitic drugs
• The last two points mentioned are reoccurring themes

Key Parasites

• Intracellular:
  • Babesia microti: Causes Babesiosis
  • Leishmania donovani: Causes Leishmaniasis
  • Toxoplasma gondii: Causes Toxoplasmosis
  • Trypanosoma cruzi (American): Causes American Trypanosomiasis, or Chagas Disease
• Extracellular:
  • Trypanosoma brucei (African): Causes African Trypanosomiasis, or Sleeping Sickness
• Free-Living Amoebae:
  • Naegleria fowleri: Causes Primary Amebic Meningoencephalitis

Babesiosis- Babesia microti (US)

• Symptoms can include fever, headache, chills, and fatigue
• The disease is transmitted by hard ticks, the same ones that transmit Lyme disease
• Babesia invades human red blood cells and replicates
• Infected red blood cells rupture and release parasites
• Parasitemia produces clinical symptoms
  • Serious infection can result in hemolytic anemia and renal failure Released parasites invade new red blood cells and repeat the cycle of replication and cell rupture
• Forms distinctive trophozoite tetrad in blood smears
• Treatment involves Atovaquone + azithromycin (antibiotic) or Quinine + Clindamycin (antibiotic, severe cases)

Leishmaniasis- Leishmania donovani

• Leishmaniasis is transmitted via the sandfly
• There are three types of Leishmaniasis:
  • Cutaneous leishmaniasis (CL) is the most common form
    • It is characterized by one or more papules that eventually become ulcers (typically painless)
    • The ulcers are usually on exposed skin and are characterized by central depressions covered with crusts or scarring
    • Ulcers can facilitate bacterial infections
  • Muco-cutaneousleishmaniasis (ML)
    • Infects the nose, mouth, and pharynx
    • Tissues often destroyed without treatment
  • Visceral leishmaniasis (VL) is the most severe form
    • Characterized by splenomegaly, hepatomegaly, a reduction in red blood cells, white blood cells, and platelets
    • It is often fatal if untreated
    • Dermal lesions can develop in treated patients 6-12 months after symptoms subside
• Treatment involves VL with IV Ampho B or CL, ML, & VL with oral miltefosine

Toxoplasmosis- Toxoplasma gondii

• Transmitted via ingestion of cysts in undercooked meat or shellfish, or ingestion of oocysts in cat feces
• Advise pregnant women to avoid unfamiliar cats due to the risk of congenital infection:
  • During the first trimester, infection leads to stillbirth
  • After the first trimester, congenital infection leads to a triad of Chorioretinitis (most common outcome), Hydrocephalus, and Intracranial calcifications
• Clinical presentations vary by severity: - Mild symptoms include flu-like symptoms (swollen glands and muscle aches lasting more than a month) - Severe symptoms include: affected brain and eye, as well as dissemination to the CNS in patients with immunosuppression
• Toxoplasmosis is a common cause of encephalitis in AIDS patients due to reactivation of earlier infection
• 30% of the human population is infected with Toxoplasmosis, but few people experience illness
• Treatment involves Pyrimethamine & sulfadiazine (+folinic acid to counteract the anti-folate action of pyrimethamine)

Trypanosomiasis (American)- Trypanosoma cruzi

• Symptoms include cardiac enlargement, arrhythmias, arrest, or other heart issues & gastrointestinal tract
• American form known as Chagas Disease, primarily transmitted in feces of triatomine bugs ("kissing bugs") in Latin America
• Acute phase includes high parasitemia, fever, aches, rashes, and characteristic swelling of the eyelids, called Romaña's sign
• Chronic phase (if untreated) includes cardiac enlargement, arrhythmias, arrest, as well as other heart and gastrointestinal issues
• Benznidazole & nifurtimox can be used as treament, but should not be confused with azoles or benzimidazole

Trypanosomiasis (African)- Trypanosoma brucei

• African form is also known as "Sleeping sickness"
• Transmitted by the tsetse fly in Sub-Saharan Africa
• Individuals may be asymptomatic for years
• Clinical presentation:
• Chancre phase: area around bite becomes red, swollen, & may form an ulcer
• Hemolymphatic phase: intermittent fever, malaise, myalgia, arthralgia, edema & lymphadenopathy
• Meningoencephalitic phase: sensory, motor, psychiatric, & sleep disturbances
• If untreated then it could lead to coma & death
• Fexinidazole can be used as treatment

Primary Amebic Meningoencephalitis- Naegleria fowleri

• Transmitted through swimming in freshwater, usually when contaminated water goes up into the nose
• Clinical Presentation includes sudden headache, photophobia, stiff neck, altered sense of taste or smell, nose bleeds, and altered mental status (meningoencephalitis)
• Fatal in the majority of cases within 1 week
• Only 4 out of 148 known infected individuals in the US (1962 to 2019) have survived
• Can be treated with Ampho-B