Medical Parasitology Lecture 4 - Zoonosis and Vector-borne parasitic infection (Notes)

Zoonosis and Vector-borne Parasitic Infection

Lecturer: Nana Yaw Barimah

University of Health and Allied Science

February, 2019

Objectives

By the end of this class, students should:

  • Understand what zoonosis is.
  • Understand vector-borne parasitic infections.
  • Be acquainted with some examples.
  • Know the diagnosis, treatment, and prevention of these infections.

General Information and Definitions

  • Vector: Insects (mosquitoes, ticks, fleas, black flies, and sandflies) that carry infectious agents (protozoa, bacteria, and viruses).
  • Arthropod: From Greek (arthron/joint + podus/footed), any invertebrate of the phylum Arthropoda, having a segmented body, jointed limbs, and usually a chitinous shell that undergoes molting. Includes insects, spiders, arachnids, and crustaceans.

The Arthropod as a Vector

  • Arbo-diseases: Diseases transmitted by arthropods.
  • Evidences for judgment when an arbo-disease occurs:
    • Biological Features: The arthropod should:
      • Be in close relationship with humans (e.g., biting or sucking man, contaminating foods).
      • Be a common species at the locality, or its population is dense.
      • Have a lifespan long enough for pathogen development or proliferation.
    • Epidemiological Evidence: The geographic and seasonal distribution of the arthropod should be the same as the arbo-disease.
    • Laboratory Evidence: The arthropod can be infected with the pathogen experimentally, and the pathogen can develop into an infective stage in the arthropod in the laboratory.
    • Natural Infection Evidence: In the epidemic season, the pathogen can be examined from the arthropod in the field. This is the most important evidence to judge the vector.

What Zoonosis Is

  • A zoonotic disease is any disease that may be passed from animals to people or from people to animals.
  • Reverse zoonosis: A disease transmissible from humans to animals.

Types of Zoonosis

  • Infections transmitted directly from animals to humans.
  • Vector-borne infections in which an animal or human is infected by the vector (can be arthropod).
  • Infections in which animals act as a reservoir for disease transmission, including having the potential for contaminating human food and water sources.
  • Not every species in animals is necessarily infective in humans (wide host range).

Transmission of Zoonotic Infections

  • Airborne
  • Fecal-oral
  • Direct contact
  • Foodborne
  • Arthropod Vector

General Classification of Infectious Zoonotic Agents

  • Viruses
  • Bacteria
  • Fungi
  • Parasites

Parasitic Zoonotic Diseases

  • Giardiasis
  • Cryptosporidiosis (mammals, birds, cattle)
  • Amoebiasis
  • Balantidiasis
  • Toxoplasmosis
  • Roundworm
  • Hookworm
  • Whipworm
  • Hydatid Disease
  • Naegleria fowleri
  • Malaria, etc.

Examples of parasitic zoonotic diseases and their life cycles

  • Toxoplasmosis

    • Infective Stage: Oocysts and tissue cysts
    • Diagnostic Stage: Serum, CSF
    • Both oocysts and tissue cysts transform into tachyzoites shortly after ingestion.
    • Tachyzoites localize in neural and muscle tissue and develop into tissue cyst bradyzoites.
    • If a pregnant woman becomes infected, tachyzoites can infect the fetus via the bloodstream.
    • Diagnosis: Serological diagnosis or direct identification of the parasite from peripheral blood, amniotic fluid, or in tissue sections.
    • Reservoir: Pigs, Cats, Rats, Deer, Lamb

  • Giardiasis

    • Infective Stage: Mature cysts
    • Diagnostic Stage: Cysts and Trophozoites
    • Excystation of cysts into trophozoites
    • Multiplication of trophozoites
    • Non-Invasive Colonization (A)
    • Intestinal Disease (B)
    • Extra-Intestinal Disease (C)

  • Naegleria fowleri

    • Infection by intranasal instillation of amebae
    • Enflagellation of amebae into flagellate form
    • Encystment of amebae into cyst form
    • Amebae in CSF

  • Leishmania donovani

    • Sandfly Stages:
      • Sandfly takes a blood meal (injects promastigote stage into the skin)
      • Promastigotes transform into amastigotes inside macrophages
      • Sandfly takes a blood meal (ingests macrophages infected with amastigotes)
      • Amastigotes transform into promastigote stage in midgut
    • Human Stages:
      • Promastigotes are phagocytized by macrophages
      • Amastigotes multiply in cells (including macrophages) of various tissues
    • Infective Stage: Promastigotes
    • Diagnostic Stage: Amastigotes

  • Trypanosomiasis, brucei

    • Tsetse fly Stages:
      • Epimastigotes multiply in salivary gland and transform into metacyclic trypomastigotes.
      • Procyclic trypomastigotes leave the midgut and transform into epimastigotes.
      • Bloodstream trypomastigotes transform into procyclic trypomastigotes in tsetse fly's midgut, which multiply by binary fission.
      • Tsetse fly takes a blood meal (ingests trypomastigotes)
    • Human Stages:
      • Tsetse fly takes a blood meal (injects metacyclic trypomastigotes)
      • Injected metacyclic trypomastigotes transform into bloodstream trypomastigotes, which are carried to other sites.
      • Trypomastigotes multiply by binary fission in various body fluids (e.g., blood, lymph, and spinal fluid).
    • Infective Stage: Metacyclic trypomastigotes
    • Diagnostic Stage: Trypomastigotes in blood

  • Trypanosomiasis, cruzi

    • Triatomine Bug Stages:
      • Metacyclic trypomastigotes in hindgut
      • Multiplies in midgut
      • Triatomine bug takes a blood meal (trypomastigotes ingested)
      • Epimastigote stage in midgut
    • Human Stages:
      • Triatomine bug takes a blood meal (passes metacyclic trypomastigotes in feces, trypomastigotes enter bite wound or mucosal membranes)
      • Metacyclic trypomastigotes penetrate various cells at bite wound site and transform into amastigotes.
      • Amastigotes multiply by binary fission in cells of infected tissues.
      • Intracellular amastigotes transform into trypomastigotes, then burst out of the cell and enter the bloodstream.
    • Infective Stage: Metacyclic trypomastigotes
    • Diagnostic Stage: Trypomastigotes

  • Plasmodium spp (Malaria)

    • Mosquito Stages (Sporogonic Cycle):
      • Microgamete entering macrogamete
      • Ookinete formation
      • Oocyst development
      • Ruptured oocyst releases sporozoites
      • Mosquito takes a blood meal (ingests gametocytes)
    • Human Liver Stages (Exo-erythrocytic Cycle):
      • Mosquito takes a blood meal (injects sporozoites)
      • Infected liver cell
      • Ruptured schizont
    • Human Blood Stages (Erythrocytic Cycle):
      • Immature trophozoite (ring stage)
      • Mature trophozoite
      • Schizont
      • Ruptured schizont
      • Gametocytes
    • Infective Stage: Sporozoites
    • Diagnostic Stage: Gametocytes

  • Wuchereria bancrofti (Filariasis)

    • Mosquito Stages:
      • Mosquito takes a blood meal (ingests microfilariae)
      • Microfilariae shed sheaths, penetrate mosquito's midgut, and migrate to thoracic muscles
      • L1 larvae develop
      • L3 larvae migrate to head and mosquito's proboscis
      • Mosquito takes a blood meal (L3 larvae enter skin)
    • Human Stages:
      • Adults in lymphatics
      • Adults produce sheathed microfilariae that migrate into lymph and blood channels
    • Infective Stage: L3 larvae
    • Diagnostic Stage: Microfilariae

  • Trichinella spiralis

    • Pigs/Rodents ingest encysted larva in striated muscle via carnivorism or meat scraps
    • Humans ingest undercooked meat (especially pork) containing encysted larva
    • Larva released in small intestine
    • Adults in small intestine
    • Larva deposited in mucosa
    • Circulation
    • Encysted larva in striated muscle
    • Infective Stage: Encysted larva in striated muscle
    • Diagnostic Stage: Encysted larva in striated muscle

Examples of Arthropods and Associated Diseases

  • Hard tick (Ixodidae): Forest encephalitis, Xing jing hemorrhagic fever
  • Soft tick (Argasidae): Tick-borne recurrent fever
  • Itch mite (Sarcoptidae mite): Scabies
  • Demodicidae mite: Folliculitis
  • Dust mite (Pyroglyph idea): Asthma, allergic rhinitis, allergic dermatitis
  • Mosquito: Malaria, filariasis, yellow fever, etc.
  • Fly (Musca): Dysentery, typhoid fever, poliomyelitis, amoebic dysentery, myiasis
  • Sandfly: Kala-azar disease/ visceral leishmaniasis
  • Flea: Plague
  • Lice: Epidemic typhus, lice-borne relapsing fever

Factors Influencing Vector-Borne Parasitic Diseases

  • Climate (warmer temperatures, e.g., Naegleria fowleria)
  • Altitude
  • Travel
  • Diet
  • Culture (blood meals)
  • Poor care of animals (vaccines, deworming, etc.)

Control of Vector-Borne Diseases

  • Environmental Management: Reduces or controls resting/growing fields or breeding sites and reduces the arthropod population through environmental modifications and sanitation.
  • Physical Measures: Control or drive away medically important arthropods (e.g., bed-nets to avoid mosquito bites).
  • Chemical Measures: Use of chemicals (e.g., DDT).
  • Biological Measures: The bacterium Bacillus thuringiensis and other species can infect mosquito larvae and kill them. Breeding fish in rice fields is also useful for controlling mosquito larvae.
  • Genetic Measures: Use molecular biological methods such as mutation or gene transfer to produce infertility in males of medically important arthropods and let them mate with wild females, preventing reproduction.