Parasitic Infections: Types, Life Cycles, and Impact

Posted on Mar 14, 2025 in Naval Architecture and Marine Engineering

Infection vs. Parasitic Disease:

  • Infection (without symptoms): Presence of a parasite without clinical manifestations.
  • Parasitic Disease: Presence of a parasite with clinical symptoms.

Zoonotic Parasites: Parasitic diseases that impact human health. Consequences include:

  • Low or high mortality
  • Disease outbreaks
  • Debilitating illnesses
  • Economic losses due to hospitalization

Understanding Biological Associations

Parasitism: A biological association where one individual (the parasite) benefits by obtaining habitat and protection, always causing harm to the host. It involves close and prolonged contact, and the parasite is always smaller in size.

Mutualism: A biological association where both organisms (integrants) benefit.

Commensalism: A biological association where one organism benefits, and the other is neither harmed nor helped.

The Role of Environment

Environment: Almost always external. Conditions such as sewage, contaminated water, and garbage contribute to parasitic infections. It’s the habitat or site where a population fulfills its biological functions.

Types of Environment:

  • Aquatic / Terrestrial
  • Extensive / Confined
  • With / Without treatment
  • Geographical features and climatic aspects

Hosts and Parasites

Host: Individuals, often children (due to a less developed immune system), that harbor parasites at different stages of development.

Types of Hosts:

  • Definitive (Primary, Secondary): Host in which the parasite reaches sexual maturity.
  • Intermediate: Host in which the parasite develops its larval or asexual stage.
  • Paratenic: A ‘waiting’ or circumstantial host.
  • Accidental or Trap: Host that shortens the parasite’s life cycle.

Parasite: An organism that lives temporarily or permanently at the expense of another organism of a different species, obtaining nutrients and shelter, causing varying degrees of damage.

Degree of Contact

  • Accidental: Erratic and missing.
  • Facultative: Can adapt to both free-living and parasitic lifestyles.
  • Obligate: Requires a host for all or part of its life cycle.
    • Temporary: Part of the cycle is spent in the host.
    • Permanent: Requires living within the host continuously.

Host-Parasite Relationship: Parasitic adaptation to their host, including morphological, physiological, and behavioral changes. Parasitic specificity can be:

  • Stenogenic (narrow host range)
  • Oligogenic (few host species)
  • Eurygenic (broad host range)

Parasite Life Cycles

Direct or Monoxenic Cycle: Involves only one host (e.g., human-to-human transmission through fecal-oral route).

Indirect or Heteroxenic Cycle: Involves one or more intermediate hosts (e.g., a pig ingesting feces containing parasite larvae, then a human consuming undercooked pork). Further classifications include:

  • Diheteroxenic
  • Polyheteroxenic
  • Autoheteroxenic

Stages of the Parasite Within the Cycle: Adult, larval, infecting, and diagnosed stages.

Factors Determining Parasitic Intensity and Disease

Factors influencing the intensity of parasitic infection and disease include:

  • Nutritional activity
  • Size
  • Parasite load
  • Final location (tropism)
  • Migratory route
  • Path of invasion
  • Transmission mechanism
  • Pathogenicity mechanisms
  • Domestication
  • Age, size, sex, and race of the host
  • Nutritional and immune status
  • Presence of previous diseases
  • Hygiene habits
  • Socio-cultural habits
  • Environmental conditions (weather, O2 presence, sunlight, pH)
  • Soil characteristics
  • Vegetation
  • Presence of other flora and fauna

Forms of Parasitic Disease Presentation

  • Acute
  • Chronic
  • Subclinical

Routes of Entry and Exit

Access Routes to Host:

  • Oral
  • Dermal
  • Ocular
  • Transplacental
  • Transmammary
  • Mucous membranes
  • Respiratory

Routes of Exit from Host:

  • Anal
  • Skin
  • Oral
  • Genitourinary
  • Respiratory
  • Transmammary
  • Transplacental

Parasite Infection Mechanisms

  • Direct or indirect fecal-oral
  • Carnivorism
  • Vectors (biological, mechanical)
  • Congenital
  • Venereal
  • Direct contact
  • Iatrogenic

Fecalism: Direct contact with human or animal feces.

Direct Defecation: An animal (e.g., a fly) lands on feces and then on food, contaminating it.