Blood & Tissue Protozoa

This section focuses on Plasmodium (the malaria culprits) and Trypanosoma (responsible for diseases like sleeping sickness and Chagas disease). Understanding their etiology (cause), epidemiology (how they spread), and transmission (how they infect) is key to identifying them in the lab and understanding the diseases they cause

Blood and Tissue Protozoa

Plasmodium spp. (Malaria)

• Etiology: Malaria is caused by protozoan parasites of the genus Plasmodium. The major species that infect humans are:
  • Plasmodium falciparum (most deadly)
  • Plasmodium vivax
  • Plasmodium ovale
  • Plasmodium malariae
  • Plasmodium knowlesi (zoonotic, found in Southeast Asia)
• Epidemiology
  • Global Distribution: Malaria is endemic in many tropical and subtropical regions of the world, particularly in Africa, Asia, and South America
  • Risk Factors: Living in or traveling to endemic areas, inadequate mosquito control, poverty, lack of access to healthcare
  • Impact: Malaria is a major global health problem, causing significant morbidity and mortality, especially in young children and pregnant women
• Transmission
  • Vector: Malaria is transmitted to humans through the bite of infected female Anopheles mosquitoes
  • Life Cycle
    • Mosquito Stage: The mosquito ingests Plasmodium gametocytes from an infected human. Sexual reproduction occurs in the mosquito, producing sporozoites that migrate to the salivary glands
    • Human Stage: When the mosquito bites a human, sporozoites are injected into the bloodstream. They travel to the liver and infect liver cells (hepatocytes), where they multiply asexually (exoerythrocytic cycle). Merozoites are released from the liver and infect red blood cells (erythrocytic cycle). In the red blood cells, merozoites multiply asexually, eventually causing the red blood cells to rupture and release more merozoites, leading to the characteristic symptoms of malaria. Some merozoites develop into gametocytes, which can be ingested by mosquitoes, completing the life cycle

Trypanosoma brucei (African Trypanosomiasis, Sleeping Sickness)

• Etiology: African trypanosomiasis is caused by Trypanosoma brucei. There are two main subspecies that infect humans:
  • Trypanosoma brucei gambiense (chronic form, West and Central Africa)
  • Trypanosoma brucei rhodesiense (acute form, East Africa)
• Epidemiology
  • Geographic Distribution: Sub-Saharan Africa, particularly in rural areas
  • Risk Factors: Living in or traveling to areas with tsetse flies, working in agriculture or other outdoor occupations
  • Impact: Sleeping sickness affects the nervous system and can be fatal if untreated
• Transmission
  • Vector: Transmitted by the bite of infected tsetse flies (Glossina spp.)
  • Life Cycle
    • Tsetse Fly Stage: The tsetse fly ingests trypomastigotes from an infected mammal (human or animal reservoir). The parasite undergoes development in the fly
    • Human Stage: When the tsetse fly bites a human, metacyclic trypomastigotes are injected into the skin. They enter the bloodstream and lymphatic system, eventually crossing the blood-brain barrier and infecting the central nervous system

Trypanosoma cruzi (American Trypanosomiasis, Chagas Disease)

• Etiology: Chagas disease is caused by Trypanosoma cruzi
• Epidemiology
  • Geographic Distribution: Primarily in Latin America, from Mexico to Argentina. Increasing cases are being seen in the United States and other countries due to migration
  • Risk Factors: Living in rural areas with poor housing conditions, exposure to triatomine bugs, blood transfusions, congenital transmission
  • Impact: Chagas disease can cause chronic heart and gastrointestinal problems
• Transmission
  • Vector: Transmitted by triatomine bugs (also known as kissing bugs or assassin bugs)
  • Life Cycle
    • Triatomine Bug Stage: The bug ingests trypomastigotes from an infected mammal (human or animal reservoir). The parasite undergoes development in the bug
    • Human Stage: When the bug bites a human (often at night), it defecates near the bite wound. The metacyclic trypomastigotes in the feces are rubbed into the bite wound or mucous membranes. They enter cells near the bite and transform into amastigotes, which multiply intracellularly. Amastigotes transform back into trypomastigotes, which are released into the bloodstream and can infect other cells or be ingested by triatomine bugs
    • Other Routes: Blood transfusions, organ transplantation, congenital transmission, and ingestion of contaminated food or drink

Key Takeaways

• Vector-borne: Both Plasmodium and Trypanosoma are transmitted by insect vectors (mosquitoes and tsetse/triatomine bugs, respectively)
• Complex Life Cycles: These parasites have complex life cycles involving both invertebrate and vertebrate hosts
• Geographic Considerations: Understanding the geographic distribution of these parasites is crucial for diagnosis and prevention
• Disease Manifestations: The diseases caused by these parasites can range from mild to life-threatening, affecting various organ systems

Key Terms

• Etiology: The cause or origin of a disease or condition
• Epidemiology: The study of the distribution and determinants of health-related states or events (including disease), and the application of this study to the control of diseases and other health problems
• Transmission: The way a disease is spread from one person or organism to another
• Vector: An organism (e.g., mosquito, tsetse fly, triatomine bug) that transmits a disease agent from one host to another
• Protozoa: Single-celled eukaryotic microorganisms
• Trypomastigote: A flagellated form of trypanosomes found in the blood of infected hosts
• Amastigote: A non-flagellated form of trypanosomes found intracellularly in infected tissues
• Sporozoite: A motile, spore-like stage in the life cycle of some parasitic sporozoans (e.g., Plasmodium)
• Merozoite: A cell that results from asexual reproduction of protozoa (e.g., Plasmodium) that is capable of initiating a new sexual or asexual cycle of development
• Gametocyte: A cell that develops into a gamete (sexual reproductive cell) in protozoa (e.g., Plasmodium)
• Endemic: A disease that is constantly present in a particular geographic region or population