UNIT 2: PARASITISM

Overview

• Parasitism: Study of the relationship between parasites and their hosts.

PARASITISM (Definition)

• Parasitism is defined as:

  • "Heterotypic negative, temporal or permanent, external or internal association, among a species, the PARASITE (usually smaller and with a lower zoological level) and other species, the HOST (bigger and with a higher level)."

• Key Elements of Definition:

  • Heterotypic: Different species involved.

  • Negative: Typically detrimental to the host.

  • Temporal or Permanent: Can be for a fixed duration or throughout the host's life.

  • External or Internal: Location of the parasite relative to the host.

TYPES OF HOSTS AND PARASITES

TYPES OF HOSTS

1. Regarding the stage (of the parasite) that harbors:

   • DEFINITIVE Host (DH) or Final Host (FH):

     • Harbors adult stages of a parasite.

     • Engages in sexual reproduction.

     • Example: Humans for Taenia solium.

   • INTERMEDIATE Host (IH):

     • Harbors immature stages of a parasite.

     • Engages in asexual reproduction.

     • Example: Pigs for Taenia solium.

2. Regarding the specificity of the Parasite-Host relation:

   • TYPE Host:

     • The first host where the parasite species was described.

     • Example: Humans for Fasciola hepatica.

   • PRINCIPAL Host:

     • Most specific to parasites affecting a broad host range.

     • Example: Sheep for Fasciola.

   • OBLIGATORY Host:

     • Essential for completing the life cycle.

     • Example: Galba truncatula (aquatic snail) for Fasciola hepatica.

   • ACCIDENTAL Host:

     • Usually not the typical host but can still be parasitized.

     • Example: Humans for Anisakis spp..

   • PARATENIC Host:

     • Not necessary for the life cycle but aids in transmission.

     • Example: Different fishes for Anisakis.

   • ARTIFICIAL Host:

     • Reproduces the biological cycle in non-natural hosts experimentally.

     • Example: Gerbils for Haemonchus contortus (ovine abomasum).

3. Regarding Epizootiologic characteristics:

   • VECTOR: Invertebrate carrying a parasite to the DH.

     • Mechanic Vector: Carries the parasite without development.

     • Biological Vector: Completes its development within the vector.

     • Multiplicative Vector: Parasite multiplies in the host.

     • Example: Babesia transmitted by ticks.

     • Cycloevolutive Vector: Evolution occurs without multiplication.

     • Example: Dirofilaria in mosquitoes.

     • Cyclomultiplicative Vector: Both evolution and multiplication occur.

     • Example: Fasciola in aquatic snail (Galba truncatula).

   • RESERVOIR: Animals harboring parasites without severe disease but contribute to transmission.

     • Example: Armadillos and opossums for Chagas disease.

   • BRIDGE Host: Connects different biological cycles.

     • Example: Rats for Trichinella connecting domestic and sylvatic cycles.

TYPES OF PARASITES

A. Regarding their NATURE:

• FITOPARASITES (plant parasites)

• ZOOPARASITES:

  1. Protozoa

  2. Helminths (worms):

     • Plathelminthes: Cestodes, Trematodes.

     • Nemathelmintes: Nematodes.

     • Acanthocephalans, annelids, etc.

  3. Arthropoda:

     • Arachnids, Insects.

B. Regarding relations with the Host:

• ECTOPARASITES: External parasites.

• ENDOPARASITES: Internal parasites.

• ERRATICS: Mismatched hosts.

  • Arthropods: Monogenea (fish & amphibians parasites).

  • Coelozoics: Found in coelom.

  • Histozoics: Found in tissues.

  • Citozoics: Found in cells.

  • Cariozoics: Found in nucleus.

  • Plasmozoics: Found in cytoplasm = outside their usual place.

C. Regarding metabolic dependence:

• OBLIGATED: Dependent on host.

  • Example: Hypoderma.

• FACULTATIVE: Can survive without a host.

  • Example: Calliphora.

• ACCIDENTALS: Rarely parasitic.

  • Example: Musca.

D. Regarding the Host spectrum:

• EURIXENIC: Wide spectrum hosts.

• STENOXENIC: Narrow spectrum hosts.

• OLIGOXENIC: Limited host range.

• MONOXENIC: Unique host in the life cycle.

E. Regarding the number of Hosts in the Life Cycle:

• MONOXENIC:

  • Example: Eimeria spp.

• HETEROXENIC:

  • Example: Dirofilaria immitis.

• AUTOHETEROXENIC: Same species can act as DH or IH.

  • Example: Trichinella.

F. Regarding pathogenicity:

• PATHOGENIC:

  • Example: Malaria.

• FACULTATIVELY PATHOGENIC: Can be pathogenic under certain conditions.

  • Example: Cryptosporidium.

• NON PATHOGENIC: Minimal to no pathogenic effects.

G. Regarding length of stay:

• TEMPORARY: Short-lived parasites.

• PERMANENT: Long-term or lifelong parasites.

H. Regarding the stage:

• PERIODICAL: Occurs at specific times.

• CONSTANT: Always present.

  • IMAGINALS: Adults.

  • PROTELICS: Larval.

TYPES OF LIFE CYCLES

LIFE CYCLE

1. DIRECT:

   • Parasite requires only one type of host in the life cycle (LC).

   • MONOXENIC Parasites.

     • Example: Trichostrongylus axei (DH).

2. INDIRECT:

   • Requires more than one host in the life cycle.

   • HETEROXENIC Parasites:

     • DIHETEROXENIC: 1 DH and 1 IH, like Fasciola hepatica.

     • POLIHETEROXENIC: 1 DH and multiple IHs, like Dicrocoelium dendriticum.

DIHETEROXENIC CYCLE

• Requires 1 DH and 1 IH.

  • HD (Definitive Host)

  • HI (Intermediate Host)

  • Example: Fasciola hepatica.

POLIHETEROXENIC CYCLE

• Requires 1 DH and 2 or more IHs.

  • HI (Intermediate Host)

  • HD (Definitive Host)

  • Example: Dicrocoelium dendriticum.

1. AUTOHETEROXENIC:

   • Same host can act as IH and DH simultaneously.

     • Example: Trichinella spiralis.

LIFE CYCLE OF TRICHINELLA SPIRALIS

• Initial stage: Ingestion of undercooked meat containing encysted larvae.

• Cycle includes multiple hosts, with infection stages classified and progressing.

HOMOGONIC vs. HETEROGONIC CYCLES

• HOMOGONIC CYCLES: Parasitic life with parthenogenetic females.

• HETEROGONIC CYCLES: Free-life generations with sexual reproduction.

• Example: Strongyloides spp. depends on environmental conditions.