Introduction to Parasitology

• • Pathogens can be microbes or parasites.
  • Microbes include bacteria, viruses, and fungi.
  • Parasites include protozoa, helminths, and arthropods.

  Why Study Parasitology?

  • Parasites are causative agents of major public health problems worldwide.

  • Parasites cause varying morbidities and even mortalities.

  • Recent global prevalence estimates include:

    • Ascaris: 1.5 billion

  • Pathogens can be microbes or parasites.

  • Microbes include bacteria, viruses, and fungi.

  • Parasites include protozoa, helminths, and arthropods.

  Why Study Parasitology?

  • Parasites are causative agents of major public health problems worldwide.

  • Parasites cause varying morbidities and even mortalities.

  • Recent global prevalence estimates include:

    • Ascaris: 1.5 billion

  • Hookworms: 1.3 billion

  • Whipworms: 1 billion

  • Filarial worms: 657 million

  • Malaria: 500 million

  • Schistosomes: 210 million

  • Amebiasis: 50 million

  • Taenia tapeworms: 50 million

  • Clonorchis: 20 million

  • Chagas' Disease: 15 million

Burden of Major Parasitic Infections

• Plasmodium causes malaria, affecting 273 million people.
• Soil-transmitted helminths affect 2 billion people.
  • Roundworm (Ascaris) causes pnemonitis and intestinal obstruction.
  • Whipworm (Trichuris) causes bloody diarrhea and rectal prolapse.
  • Hookworm (Ancylostoma and Necator) causes coughing, wheezing, abdominal pain, and anemia.
• Schistosoma causes renal tract and intestinal disease, affecting 200 million people.
• Filariae cause lymphatic filariasis and elephantiasis, affecting 120 million people.
• Trypanasoma cruzi causes Chagas disease (cardiovascular), affecting 13 million people.
• African trypanosomes cause African sleeping sickness, affecting 0.3 – 0.5 million people.
• Leishamania causes cutaneous, mucocutaneous, and visceral leishmaniasis, with 12 million affected and 2 million new cases per year.

Key Areas in Human Parasitology

• Definition of parasitology.
• Understanding parasitism, parasites, and hosts.
• Studying the life cycle of parasites.
• Understanding the host-parasite interplay.
• Diagnosing infections caused by parasites.
• Understanding the epidemiology of parasitic infections.
• Applying principles of control and management of parasitic diseases.

Introduction to Parasitology

• Parasitology studies the dependence of one living organism on another.
• Medical parasitology focuses on parasites infecting humans, related diseases, host responses, and methods of diagnosis and prevention.

Definition of Terms

• Parasite: an organism that receives nourishment and shelter from another organism.
• Host: an organism that harbors the parasite.
• Symbiosis: an association where both organisms are dependent on each other for survival.
• Commensalism: an association where the parasite benefits without harming its host.

Forms of Dependence

• Commensalism occurs when one organism benefits and the other is neither harmed nor helped.
• Mutualism/symbiosis occurs when both organisms benefit from the relationship.
• Parasitism (disease causing) occurs when one organism benefits at the expense of the other.
• Obligate parasites cannot survive without a host, while free-living organisms do not require a host.

Kinds of Parasites

• Ectoparasites: live on the outer surface of a host, such as lice, ticks, and mites.
• Endoparasites: live inside the body of a host, such as Entamoeba histolytica.
• Obligate parasites: completely dependent on the host during part or all of their life cycle, such as Plasmodium spp.
• Facultative parasites: can exhibit both parasitic and non-parasitic modes of living, such as Naegleria fowleri.
• Accidental parasites: attack and survive in an unnatural host, such as Hymenolepis diminuta.
• Erratic parasites: wander into an organ in which they are not usually found, such as Entamoeba histolytica in the liver or lung.

Clinical Parasitology

• Clinical parasitology focuses on parasites of humans and their medical significance.

Life Cycle

• The life cycle describes the ontogenesis, development, and reproduction of a parasite.
• It tracks the parasite through its parasitic and non-parasitic stages.

Parasite Life Cycle - A Generalized Mode

• Cycle includes human residing stage, and extra-human development.
• Invading stage - oral, skin, vector contact.
• Infective stage
• Pathogenesis: Site and number.
• Stage to discharge: feces, urine, sputum, vector, blood.
• Epidemiology and Diagnosis are also components.

Specificity of Humans as Hosts

• Very specific: parasite can only infect humans
• One of several hosts: parasite can infect humans and other animals
• Incidental host: human is not the normal host, but can be infected

Kinds of Hosts

• Definitive host: harbors the parasite in the adult stage or where sexual reproduction occurs.
• Intermediate host: harbors the larval stages or where asexual development takes place.
• Paratenic host: intermediate host necessary for the completion of a parasite's life cycle, but no development occurs in this host.
• Reservoir host: makes the parasite available for transmission to another host is usually not affected; animals harboring parasites of man.
• Accidental host: a host that is not normally infected with the parasite.

Zoonoses and Vectors

• Zoonoses: diseases of animals transmissible to humans
• Vectors: hosts that transmit parasites to humans

Effect of Parasites on the Host

• Direct effects include mechanical injury and deleterious effects of toxic substances, and parasites competing for nutrients.
• Deleterious effects of toxic substances in Plasmodium falciparum.
• Mechanical injury examples include Hydatid cysts.
• Indirect effects include immunological reactions and excessive proliferation of certain tissues.
• Immunological reactions include nephritic syndrome following Plasmodium infections
• Excessive tissue proliferation is Wuchereria bancrofti elephantiasis

Sources of Infection

• Soil
• Water
• Food
• Blood-sucking insects
• Animals
• Another person
• Yourself (autoinfection)

Entry points for Parasites

• Mouth (Taenia spp) is the most common
• Skin (hook worm)
• Transplacental (Toxoplasma gondii)
• Sexual intercourse (Trichomonas vaginalis)

What to know for parasitic infections

• Travel history (multi years)
• Immune status of patient
• Clinical symptoms
• Previous parasitic infections
• Contacts and outbreaks

Diagnosis of Parasitic Infections

• Clinical diagnosis.
• Laboratory diagnosis. -Confirmation of clinical suspicion.
• Identification of unsuspected infection.

Laboratory Diagnosis

• Specimens will vary based on parasitic infection
  • Blood (serum and plasma)
  • Stool
  • Urine
  • Sputum
  • Biopsy material and tissue aspirates
  • Urethral or vaginal discharge
  • Anal swab, duodenal aspirate

Classification of Parasites

• Parasites are classified into 2 sub-kingdoms: Protozoa (unicellular) and Metazoa (multicellular helminths)
• Protozoan parasites are classified via morphology and means of locomotion.
• Protozoa species that cause human disease belong to sarcomastigophora and apicomplexa.

Cont'd Classification

• Metazoa (multicellular parasites): include helminths (worms) and arthropods.
• Genus starts with a capital letter and the species is always in italics.

Nomenclature of Parasites

• Each parasite has two names (generic and specific).
• The generic name begins with an initial capital letter.
• The specific name begins with an initial small letter, after which comes the designator's name, a punctuation mark, and the year.
• Generic and specific names are in italics.

Classification of Parasites Based on Medical Importance

• Phylum Protozoa: single-celled organism; multiplies in human host (Protozoology)
• Phylum Platyhelminthes and Phylum Nemahelminthes-multicellular worms, do not normally multiply in host (Helminthology).
• Phylum Arthropoda–Medical Entomology.

Parasites of Humans

• Protozoa examples are amoebae, flagellates, ciliates, sporozoans, coccidia, and microsporidia.
• Nematodes are roundworms.
• Platyhelminthes are flatworms: cestodes and trematodes.
• Pentastomids are tongue worms.
• Acanthocephala are thorny headed worms.
• Arthropods examples are insects, mites, ticks, etc.

Taxonomic Classification of Protozoa

• Protozoa - Sarcomastigophora, Apicomplexa, Ciliophora, Microspora.
• Entamoeba - E. histolytica
• Giardia - G. lamblia
• Plasmodium - P. falciparum, P. vivax, P. malariae, P. ovale
• Balantidium - B. coli
• Enterocytozoa- E. bienusi

Mastigophora

• All are flagellates; they have one or more whip-like flagella for locomotion. In some cases, there’s presence of an undulating membrane.
• e.g., Trypanosoma
• Mastigophora includes intestinal and genitourinary flagellates & blood/tissue flagellates.
• Intestinal and genitourinary flagellates include: Giardia, Trichomonas, Dientamoeba, Chilomastix
• Blood and tissue flagellates includes: Trypanosoma and Leishmania

Sarcodina

• Typically amoeboid.
• Entamoeba, Endolimax, Iodamoeba, Naegleria, and Acanthamoeba are examples.
• Amoebae consist of shapeless mass of moving cytoplasm divided into granular endoplasm and clear ectoplasm.
• They move by pushing out ectoplasm to form pseudopodia (false feet).
• reproduce asexually by binary fission.

Sporozoa

• Members undergo complex life cycle with alternating sexual and asexual reproductive phases involving two different hosts.
• Coccidia are intracellular parasites that reproduce asexually/sexually
  • via schizogony (merogony) and sporogony
• Class Coccidia - Isospora and Toxoplasma
• Class Haematozoa - malarial parasites (Plasmodium species)

Ciliophora

• Protozoa bearing cilia (short hairs) in rows or patches - move with cilia.
• Two kinds of nuclei (macronucleus/micronucleus)
• Have a large contractile vacuole
• Balantidium coli (giant intestinal ciliate -humans and pigs) are only human parasite representative of this group.

Helminths

• Parasitic worms that belong to Platyhelminthes (flatworms) & Nemathehelminthes (round worms).
• Nematoda are the round worms
• Platyhelminthes include; -Cestoda (Tapeworms) -Trematoda (Flukes)

Cestodes:

• Tape-like, segmented, hermaphrodite organisms.
• Have suckers in their head: some species also have hooks that attach tapeworm to host.
• Consists of a head (scolex) and many proglottids.
• Alimentary canal and body cavity are absent. Diphyllobothrium latum, Taenia spp, Echinococcus spp, Hymenolepsis nana are examples

Nematodes

• Elongated, cylindrical, unsegmented. Sexes are separate [diecious].
• Lack hooks/suckers.
• Possess a complete alimentary canal and body cavity.
• Small intestine nematodes are Ascaris lumbricoides, Ancyclostoma duodenale), and Necator americanus.
• Caecum and vermiform appendix: Enterobius vermicularis and Trichuris trichuria
• Lymphatic system: Wuchereria bancrofti and Brugia malayi
• Subcutaneous tissue: Onchocerca volvulus, and Dracunculus medinensis
• Lungs: Strongyloides stercoralis
• Conjunctiva: Loa loa

Trematodes:

• Leaf-like unsegmented organism. Sexes not separate except Schistosomes.
• Alimentary canal is present – not complete (anus absent): body cavity is absent.
• Schistosoma spp, Fasciolopsis buski, Fasciola hepatica, Clonorchis sinensis, and Heterophyes spp are examples.