Pathologic Parasites: Overview
Pathologic parasites belong to two major groups:
Protozoa include the amoebozoa, flagellates, ciliates, and apicomplexa.
Be aware that, whereas many biologists consider the term "Protozoa" to be outdated or problematic, "Protozoa" is still used informally, especially by authors in the medical sciences, so we'll use it, here.
Animalia include the worms and arthropods.
Key determinants of parasite pathogenicity:
Parasite dose and exposure
How many parasites is a person exposed to, and for how long do the parasites have access?
The parasite's ability to breach host barriers
Can the parasite breach the skin? How does it get access to the host's internal environment?
Parasite attachment & tropism
Is the parasite tissue trophic? To which cells can it attach or enter?
Replication and life cycle strategies
Does the parasite replicate intracellularly? Does it lyse the host cells?
Mechanisms and extent of damage to the host
Does the parasite cause chemical or mechanical damage?
Parasite's ability to avoid the host's immune defenses.
Mechanisms of damage:
Some parasites produce toxins, including various enzymes and endotoxins.
Parasites, especially large ones, can cause mechanical damage as the result of their movement through tissues, pressure, or obstruction of anatomical sites.
Parasites can produce immunopathy by triggering auto-immunity or hypersensitivity reactions.
Parasites have various ways of evading the host's immune system, including:
- Surviving inside the host's cells
- Antigenic variation and molecular mimicry, which deceive the immune system into ignoring the parasite
- By inhibiting the immune response itself
Be aware that the following characteristics may not be applicable to non-parasitic protozoans, worms, and arthropods.
Single-celled, anaerobic protists.
Most reproduce asexually, but some can replicate via sexual reproduction, as well.
They have varied life cycles and stages, some of which are quite complicated.
Trophozoite stage is the active feeding and replicating form; thus, it is most directly responsible for pathology.
Many protozoa have a cyst or cyst-like stage, in which the organism is in a relatively inactive state and is protected by a hard outer covering. This form provides protection during unfavorable environmental conditions and aids in transmission.
Learn about
apicomplexa life cycles
Intra- and extracellular parasites.
Some require multiple hosts – for example, plasmodia, the causative agents of malaria, spend a portion of their life cycles in mosquitos before infecting human hosts.
- Parasitic protozoans are motile.
– Can be classified by type of motility.
Amoebozoa
Entamoeba
Use pseudopods for movement, which are temporary cytoplasmic extensions produce "crawling" motility.
Flagellates
Giardia lamblia aka,
Giardia duodenalis, aka,
Giardia intestinalis which causes gastrointestinal illness.
Pear-like shape with two nuclei at one end.
Multiple hair-like flagella extend laterally from the midline; flagella make whip-like motions that produce movement
Ciliates
Balantidium coli is the only ciliate known to infect humans.
Ovoid shape with
cilia that wave in unison to produce movement.
Balantidium coli has two nuclei: one macronucleus and one micronucleus
Apicomplexa
Be aware that some authors refer to this group as the Sporoza.
Plasmodium
Oblong shape with a slightly pointed end with "apical complex," a collection of organelles unique to the apicomplexa.
Cytoskeletal components of the apical complex contribute to gliding motility in parasitic Apicomplexa.
Be aware that some texts state that apicomplexa are non-motile.
HELMINTHS = Worms
Multicellular, and most adults are anaerobic.
Reproduction may be hermaphroditic or involve separate male and female sexes.
Nemathelminthes comprise the Nematodes, aka, the roundworms.
Platyhelminthes comprise the flat Cestodes and Trematodes.
Helminths use their muscular body walls for motility.
They can grow to be quite large within their human hosts, which can lead to mechanical damage and obstruction.
Roundworms = Nematodes
To remember this, know that "nemo" means "thread".
Example:
Trichinella spiralis
Round and coiled, with outer covering (cuticle), and definitive mouth and anus.
Flatworms
Cestodes aka, tapeworms
Ex: Echinococcus
Cestodes comprise segments covered in a tough tegument, with two suckers on one end.
Cestodes are hermaphroditic.
Trematodes aka, flukes
Ex: Paragonimus, the lung flukes
Leaf-shaped, with sucker at one end, tegument covering.
Also Hermaphroditic.
Ex: Shistosoma
Separate male and female sexes that live as a unit; it's helpful to know that "shitosoma" means "split body."
The male has a snow-pea like shape; the female is smaller, slenderer, and resides within the male's gynecophoral canal.
ARTHROPODS
We focus on the "true" or "inherent" parasites that reside long-term on their human hosts.
Segmented bodies with hard exoskeletons.
Mites are members of the subphylum Chelicerata (formerly the arachnids).
For our mite example, we draw
Sarcoptes scabiei, the causative agent of scabies.
The adult form has a dorsal shell with four pairs of legs; long, bristle-like setae project posteriorly.
Lice are members of Insecta.
Parasitic lice are named for the region of the body they inhabit: the body, scalp, or pubic region.
For our example, we show the body louse Pediculus humanus humanus; it has three pairs of legs that terminate in claws that help them to grip the skin and hair.
Many other arthropods, such as ticks, mosquitoes, and fleas, play important roles in disease as vectors and intermediate hosts for other bacterial, viral, and parasitic pathogens.
