DS

Chapter 9: Apicomplexa—Malaria, Babesia, and Theileria (Vocabulary Flashcards)

Chapter 9 Notes: Phylum Apicomplexa — Malaria Organisms and Piroplasms

Family Plasmodiidae — Genus Plasmodium spp.

  • Impact of malaria

    • Disease is among the most devastating in human history, killing millions and causing widespread suffering.
    • Global burden: worldwide prevalence of malaria is estimated at 6.50\times 10^{8} people.
    • Endemic regions: 1.50\times 10^{9} people live in endemic areas; many lack resources to control the parasite.
    • Note: Transcript lists intermediate vs definitive hosts and vectors; some spellings in the source differ (e.g., Aonopheles).

  • Hosts (life cycle hosts)

    • Definitive host (vector): Anopheles (mosquito).
    • Intermediate hosts: mammals, birds, and reptiles.

  • Life cycle (Plasmodium; general outline)

    • Pre-erythrocytic (hepatic) stage:
    • Sporozoites enter the bite wound and travel to hepatocytes.
    • Hepatic schizogony produces merozoites.
    • Note: The transcript uses-terminology like “pre erythrocytic stage” and “schizogomy”; standard terms are pre-erythrocytic stage and schizogony.
    • Erythrocytic (blood) stage:
    • Merozoites break out of liver cells and invade red blood cells (RBCs).
    • In RBCs, schizogony produces more merozoites.
    • Within RBCs: gametocytogenesis occurs—merozoites become gametocytes.
    • Mosquito ingest RBCs containing gametocytes during a blood meal.
    • In the mosquito gut: gametes fuse (syngamy) to form zygotes; zygote develops into an ookinete.
    • Ookinete migrates and forms an oocyst in the gut wall, which releases sporozoites that migrate to the mosquito’s salivary glands.

  • Ring stage

    • Within the RBC, the host cytoplasm is consumed by the developing trophozoite, forming a large food vacuole.
    • The ring stage appears as a ring with the nucleus.

  • Which Plasmodium species is the most virulent? Why?

    • Plasmodium falciparum is the most virulent.
    • Reasons:
      1) Disease duration can be prolonged (often many months; transcript notes “8-10 mths”).
      2) High parasite burden: many RBCs infected; multiple pathways of RBC entry.
      3) Infected RBCs develop surface knobs that promote cytoadherence to vessel walls, blocking microcirculation.

  • Major clinical signs: two contributing factors

    • Host inflammatory response.
    • Anemia due to parasite-induced destruction of RBCs.

  • Hypnozoites

    • Definition: a “resting stage” in some Plasmodium species.
    • Clinical significance: can cause disease relapse after apparent clearance.

  • Transmission and epidemiology (factors affecting transmission)

    • Key factors determining transmission level in a given area (synthesized from the prompt):
    • Vector density and biting rate (Anopheles population dynamics).
    • Parasite prevalence in human and animal hosts.
    • Human behaviors that influence exposure (e.g., use of nets, indoor stay).
    • Environmental and climatic conditions (temperature, rainfall) that affect vector breeding and survival.
    • Immunity levels in the human population (acquired partial immunity with age and exposure).
    • Access to prevention, diagnosis, and effective treatment.
    • Compare stable endemic malaria vs unstable malaria:
    • Stable endemic malaria:
      • High and persistent transmission; partial immunity develops in the population, especially adults.
      • Lower incidence of severe disease in adults but ongoing burden in children.
    • Unstable malaria:
      • Irregular or unpredictable transmission; immunity is low across ages.
      • Higher risk of severe disease outbreaks across all age groups.

  • Measures to reduce transmission (practical interventions)

    • Vector control:
    • Insecticide-treated bed nets (ITNs).
    • Indoor residual spraying (IRS).
    • Environmental management to reduce breeding sites.
    • Chemoprophylaxis and treatment:
    • Prophylactic antimalarials for travelers or high-risk groups.
    • Prompt diagnosis and effective treatment to reduce parasite reservoir.
    • Surveillance and public health measures:
    • Surveillance of vector populations and malaria cases.
    • Community education on preventive practices.

  • Sickle cell trait and malaria resistance

    • The sickle cell trait (heterozygous HbS) provides partial protection against falciparum malaria.
    • Implication: explains higher frequencies of HbS in malaria-endemic regions due to balanced polymorphism.

  • Sources of malaria cases diagnosed in the United States

    • Predominantly imported cases:
    • Travelers returning from endemic regions.
    • Immigrants and people visiting friends and relatives in endemic areas.
    • Military personnel and others who travel to endemic regions.
    • Other sources (less common in the transcript):
    • Possible transfusion of infected blood/products in rare cases.

  • Family Babesiidae — Genus Babesia spp.

  • Babesia in cattle (Texas red-water fever)

    • Causative agent: Babesia spp. (classical cattle babesiosis agents include Babesia bigemina; also commonly discussed is Babesia bovis).
    • Vector (major tick vector): Boophilus annulatus (cattle fever tick); notes: the historical primary vector for cattle babesiosis in North America. Other related ticks can also act as vectors in different regions.

  • Babesia life cycle (high-level outline)

    • Transmission to cattle is tick-borne.
    • Sporozoites from tick saliva enter bovine RBCs and initiate merozoite replication (merogony) within RBCs, causing hemolysis.
    • Infected RBCs produce merozoites that disseminate and infect more RBCs.
    • Gametocytes form in RBCs and are ingested by a feeding tick.
    • Sexual reproduction occurs in the tick gut, leading to zygotes and formation of new sporozoites that migrate to the tick’s salivary glands to infect a new host on the next bite.

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Babesia vertical transmission in the tick

  • Vertical transmission in the female tick

    • Transmission from the mother tick to her offspring is transovarial (vertical) transmission, enabling infection of larval offspring without a second host.
    • This mode maintains Babesia in tick populations across generations.

  • Babesia canis and dogs

    • Canine babesiosis (Babesia canis) causes hemolytic anemia and related clinical signs in dogs; may present with fever, lethargy, pale mucous membranes, jaundice, and splenomegaly depending on severity.
    • Canine babesiosis is a zoonotic-relevant parasite via ticks but is primarily a veterinary issue in dogs.

  • Why zoonotic babesiosis cases are increasing in the United States

    • Expansion of tick vectors and habitats (e.g., Ixodes spp.) into new regions increases human exposure.
    • More human-tick encounters due to outdoor activity and ecological changes.
    • Improved awareness and diagnostic capabilities lead to more reported zoonotic cases.

Family Theileriidae — Genus Theileria spp.

  • East Coast Fever (ECF)

    • Causative agent: Theileria parva.
    • Vector: Rhipicephalus (formerly Boophilus) appendiculatus (brown ear tick).
    • Affected animals: primarily cattle in endemic regions; disease also affects susceptible bovine species.

  • Notes on scope

    • Theileria spp. infections differ from Babesia in life cycle details and pathology, but both are tick-borne apicomplexan parasites affecting ruminants.

Summary of Key Points (quick reference)

  • Malaria global burden and transmission cycle involves Anopheles mosquitoes as definitive hosts; humans and other vertebrates serve as intermediate hosts.
  • P. falciparum is the most virulent malaria parasite due to high parasitemia and cytoadherence causing microvascular obstruction.
  • Hypnozoites cause relapse in P. vivax/ovale; not all species form hypnozoites.
  • Transmission depends on vector density, climate, human behavior, and immunity; stable endemic vs unstable malaria differ in immunity and outbreak risk.
  • Sickle cell trait offers protective advantage against falciparum malaria.
  • Babesia spp. cause bovine babesiosis with cattle fever ticks as vectors; vertical transmission in ticks sustains infection across generations.
  • Canine babesiosis (B. canis) affects dogs with hemolytic anemia and systemic illness.
  • Zoonotic babesiosis is becoming more common in the U.S. due to tick range expansion and increased human exposure.
  • Theileria parva causes East Coast Fever in cattle, transmitted by the brown ear tick (Rhipicephalus appendiculatus).

Editor’s note: The source uses some nonstandard spellings (e.g., Aonopheles, syngomy, schizogomy). In standard terms: Anopheles, syngamy, schizogony. These notes preserve the meaning while aligning with conventional nomenclature for study purposes.