L2. The General Principles of Epidemiology/Epizootiology of Parasitic Disease, The Basic Mechanisms of Immunity, the Role of Immunity in Epidemiology

The General Principles of Epizootiology of Parasitic Diseases

Epizootiology:

  • the science concerned with the factors involved in the occurence and spread of pathogens of animal diseases
  • explains the emergence, expansion, delevopment specificity, course and occurence of parasitosis (the parasitic/aetiological agent/pathogen-induced disease process)

Infection: often used in connection with endoparasites

Infestation: often used in connection with ectoparasites

Contamination: presence of parasite and its developmental forms in the external environment (water, food, soil)

Epizootiologic Triad-Related Concepts

  1. Infectivity (ability to infect)

    1. (number of infected / number of susceptible) X 100

  2. Pathogenicity (ability to cause disease)

    1. (number with clinical disease / number of infected) X 100

  3. Virulence (ability to cause death)

    1. (number of deaths / number with disease) X 100

All are dependent on host factors

Ecological Factors in Infections

  1. Altered environment

    1. e.g. air conditioning

  2. Changes in food production & handling

    1. e.g. intensive husbandry with antibiotic protection; deep-freeze; fast food industry

  3. Climate changes

    1. e.g. global warming

  4. Deforestation

  5. Ownership of (exotic) pets

  6. Air travel & exotic movements

  7. INCREASED USE OF IMMUNOSUPPRESSIVES / ANTIBIOTICS

Classifications Levels of Disease

Sporadic Level: occasional cases occurring at irregular intivals

Endemic Level: persistent occurrence with a low to moderate level

Epidemic or Outbreak: occurrence clearly in excess of the expected level for a given time period

Pandemic: epidemic spread over several countries or continents, affecting a large number of people

Parasitological Status Expression

  1. Prevalence (%)

    1. proportion of infected individuals showing the disease in a group of animals in a certain time period
    2. (number of infected hosts / number of hosts examined) X 100

  2. Moderate Intensity

    1. represents the average number of specimens of the parasite species on/in one infected host
    2. (total number of parasites of the species in the host sample / number of infected individuals in the hose sample)

  3. Incidence (%)

    1. number of new disease cases in the population over a given period

  4. Morbidity

    1. the proportion of individuals with disease to a particular group ( or the whole population) to a certain time period

  5. Mortality

    1. the number of deaths of individuals with a disease in a particular group ( or the whole population) and in a certain period of time

  6. Lethality

    1. the number of deaths of individuals with disease to number of individuals with disease in a particular group

Epizootiology of Parasitoses

  1. Abundance

    1. total number of parasites of a given species per test host

  2. Average abundance

    1. total number of individuals of the given parasite species on the total number of all tested hosts

  3. Density

    1. the number of individuals of a given parasite per unit area, volume or weight of the host organism

  4. Pre-patented time

    1. the time interval from the infestation of the parasite to host and beginning of excretion of the propagation stages of the parasite from the host organism

  5. Incubation time

    1. time from the infestation of the parasite (pathogen) to the host following the appearance of the initial symptoms of the disease

Life Cycles

Life cycle: ontogenesis, development and reproduction of the parasite, tracking it through the various phases of its life history which will encompass both parasitic and non-parasitic stages

Generalised Stages of Life Cycles

  1. stage in human/animal host (linking to pathogenesis)
  2. stage of discharge (diagnostic stage)
  3. stage developing outside human/animal host (in external environment, intermediate host, or insect host) (linking to transmission)
  4. stage infecting human/animal (infective stage)

How Parasites Enter Their Hosts

Passive Infection

  • faecal-oral contamination with food, water, hands by cysts
  • uncooked or undercooked foods
  • eating paratenic or intermediate hosts
  • inhalation

Active Infection

  • penetration of the skin
  • inoculation of infective stages by vector

Transmission Mechanisms

Horizontal - between individuals of the same generation

  • transmission through contact (e.g. sexual)
  • transmission through vectors
    • contamination (mechanical)
    • inoculation (bite; blood)

Vertical - from one host generation to the following generation

  • transplacental
  • lactogenous

Iatrogenic - transmission by physician or veterinarian e.g. through contaminated needle

Adverse Effects of Parasites on Hosts - PATHOGENICITY

  1. Competition for host nutrients

  2. Destruction of host tissues

    1. direct damage

      1. mechanically, blockade, tissue destruction, compression of organs, destruction of cells

    2. indirect damage - parasites signal with host reactions

      1. loss of nutrients
      2. inflammatory response
      3. immunopathological effects
      4. cytopathogenicity
      5. immune responses
      6. blood loss
      7. toxins e.g. Trypanosoma

  3. Tissue changes

  4. Toxins, poisons and excretions

  5. Mechanical interference

Immunology of Parasitosis

Vertebrates are equipped with two functionally closely related defence systems to fight pathogens

  1. innate immune system - non-specific

    1. includes physical and chemical barriers

      1. first non-specific line of defence = skin, mucosa
      2. secretions = saliva, digestive enzymes, bile acids

    2. cellular components

      1. phagocytes, natural killer cells, T-cells

    3. earliest effects

      1. inflammation: vasodilation, increasing blood flow and vascular permeability
      2. phagocytosis: neutrophils and macrophages

  2. acquired/adaptive immune system - specific

    1. regulate and control the course of primary infections and protect an organism completely or partially from

      1. re-infection (anti-infectious immunity)
      2. clinical consequences of an infection (protection from the disease)

    2. cellular immune reactions: T-lymphocytes

    3. humoral immune reactions: B-lymphocytes, plasma cells, antibodies

Zoonosis

Zoonotic Disease: any disease which may be passed from animals to people or from people to animals

  • approximately 60% of all human pathogens are zoonotic
  • 75% of emerging infectious diseases have an animal origin
  • growing need for veterinarians and physicians to work together

Causes of Zoonotic Diseases:

  • bacteria
  • fungi
  • viruses
  • parasites

Vector-Borne Parasites

  • a vector is an agent which transfers a parasite from one host to another
  • typical parasite vectors: flies, fleas, mites, mosquitoes, and other insects
  • people become infected when a vector picks up the parasite from an infected animal and infects a human

Water-Borne Parasites

  • spread when humans/animals come into contact with water that has been contaminated by an infected animal/human
  • common waterborne parasites:
    • Giardia
    • Cryptosporidium

Faecal-Oral Transmission

  • most common way people/animals become infected with zoonotic parasites
  • parasites are spread to humans/animals when they ingest the eggs from the faeces of an infected animal
  • parasites spread by faecal-oral transmission generally live in the intestinal tract
    • hookworms
    • carried by dogs and cats
    • kids often get infected by playing in sandboxes in which animals have defecated or by putting toys in their mouth which have been on the ground
    • roundworms
    • (see hookworms above)
    • hydatid disease
    • after humans consume eggs passed by dogs, the worms can form huge cysts in the body
    • toxoplasmosis

Contaminated Meat

  • meat can be contaminated with harmful bacteria and can also contain parasitic cysts which may infect people
  • common parasites found in meat
    • Toxoplasma
    • toxoplasmosis is the most commonly transmitted through undercooked meat
    • Trichinella
    • Taenia
    • Gnathostoma

Introduction to Protozoology

General Concepts

  • eukaryotic organisms
  • generally unicellular
  • found in every conceivable damp habitat
  • approximately 60,000 living species
  • largest is visible to the naked eye
  • smallest only seen with an EM
  • have all necessary life activities

Protozoa

  • most species are free living, but all higher animals are infected with one or more species of protozoa
  • infections range from asymptomatic to life-threatening, depending on the species and strain of the parasite and the resistance of the host

Structure

  • microscopic, unicellular eukaryotes with relatively complex internal structures and carry out complex metabolic activities
  • some protozoa have structures for propulsion or other types of movement
  • Basic structure
    • cell membrane (glycolax)
    • cytoplasm organelles: pseudopodium flagellum, cilium
    • nucleus: vesicular or compact

Reproduction

  1. Asexual

    1. binary fission - division into two organisms by way of nuclear division and cytokenesis (e.g. flagellates [longitudinal], Ciliates [transverse], sarcodins [random])
    2. budding - portions of a parent cell differentiates and develop into a new individual

  2. Sexual

    1. fusion of gametes
    2. conjugation

Protozoan Life Styles

  • absorb solutes (osmotrophy)
  • ingest particles (phagotrophy)
    • predation on bacteria or other protozoa
    • pinocytosis (fluid uptake)
  • photosynthetic (autotrophy)
  • combinations (heterotrophy)

free-living vs. symbiosis

Life Cycle Stages

  • The stages of parasitic protozoa that actively feed and multiply are frequently called trophozoites; in some protozoa, other terms are used for these stages
  • Cysts are stages with a protective membrane or thickened wall.
    Protozoan cysts that must survive outside the host usually have more resistant walls than cysts that form in tissues