Parasitology Flashcards
Symbiosis
A relationship of distinct organisms (species) in close association that can be:
of benefit to both (mutualism)
of benefit to one and neutral to the other (commensalism)
of benefit to one and harmful to the other (parasitism)
Parasitology
he study of parasitic relationships
In clinical terms = study of parasitic organisms that cause disease in humans
Historically, parasitology is confined to diseases of protozoan or helminthic (worms) etiology.
Parasite
An organism that grows, feeds, and is sheltered on or in a host organism to the detriment of that host
Parasites can be either: Endoparasites, living within their host (infection)
Ectoparasites, living on their host (infestation)
For clinical purposes, endoparasites are conveniently categorized as:
-Lumenal (intestinal or urogenital)
-Blood/tissue agents
BUT this does NOT imply a phylogenetic relationship within each group.
Host
Exploited partner in a parasitic relationship
Types:
o Definitive host is one in which the sexual stage of a parasite life cycle occurs (cat is the definite host for Toxoplasma).
o An intermediate host is one in which non-sexual parasite reproduction or development occurs. For example, humans are the definitive host and snails are the intermediate host for the trematode worm Schistosoma mansion.
o An incidental host is one that is not an obligate part of the parasite life cycle. Incidental infection may result in no disease, minor pathology (duck schistosome/swimmer’s itch), or serious disease (the nematode Trichinella).
o Parasitic diseases that are incidental to humans are called zoonoses and the animal hosts that maintain the natural cycle in the wild are called reservoir hosts.
Life Cycle
The total reproductive and developmental cycle required for the successful propagation of a parasitic species.
Some parasites have a very simple life cycle with only one morphological form and host (Trichomonas).
Others have very complicated life cycles with complicated developmental changes and requiring unique hosts at each stage (Plasmodium or Schistosoma).
Transmission
The mode in which an infectious form of a parasite is transmitted to a new host, either of the same species or another species.
4 Main Routes:
o The direct route involves transmission of a replicating form from one host to another, typically by exchange of bodily fluid (Trichomonas).
o In the ingestive route, the infectious form is typically a non-dividing or dormant cyst or egg that is ingested in food or from the environment. Cysts may form in the tissue of infected animals and be transmitted when under-cooked meat is eaten (Taenia). Alternatively, the infectious cyst/egg may be shed in feces of an infected host and subsequently transmitted to another by ingestion (fecal/oral) (Giardia or Ascaris).
o In the invasive route, the infectious form is active but non-replicating and infects by direct penetration of the skin (Schistosoma).
o The vector-borne route depends on the direct transmission of a non-replicating infectious form by a blood-sucking arthropod (Plasmodium).
Less commonly, parasite transmission can be maternal to offspring in utero. This occurs with devastating consequences in primary infections of pregnant women with Toxoplasma gondii.
Vector
A host species that directly transmits an infectious form of a parasite to another host species.
o A vector species that is an essential host in the parasite life cycle is a biological vector.
o A phoretic or mechanical vector merely transmits the parasite without maintaining an essential life cycle stage (flying syringe).
Parasite: vector relationships are usually specific (Plasmodium: Anopholes mosquitoes).
List the major groups of helminths
Cestoda
Trematoda
Nematoda
Describe the Cestoda group of helminths
Tapeworms w/flattened segmented bodies => absorb across cuticle
- adult = gut (attach w/scolex), proglottids (segments) mature from anterior to posterior
o length 2-10 m (depends on species)
• posterior gravid proglottids shed via feces => releases mature eggs into environment
Transmission: ingestion of cysticercus, mature eggs, or prglottids
o intermediate host => cysticerci
o definitive host eats intermediate host (+ cysticerci) => adults develop in gut
Serious pathology = dead-end intermediate host (Taenia solium, Echinococcus granulosus)
Describe the Trematoda group of helminths
liver, lung, blood flukes
- broad, flattened bodies w/simple digestive system (1 opening = mouth + anus)
o 2 suckers (attachment + locomotion): ventral blind, anterior = mouth + anus
Most = hermaphroditic (tissue) (exc. schistosomes [blood])
o blood flukes reproduce sexually (dimorphic elongated body forms)
o 1+ intermediate host (>1 mollusk/snail) => asexual reproduction = cercaria
• cercaria penetrate skin or encyst (metacercaria) in 2nd intermediate host
Transmission: invasive or ingestive
Ex: Schistomiasis (S. mansoni, S. japonicum, S. haematobium)
Describe the Nematoda group of helminths
roundworms of tissue + gut
- cylindrical bodies w/digestive + nervous systems
o reside free in gut, attach to gut mucosa, or in tissues
- sexual reproduction: ♀ = egg laying machines
- transmission = ingestive (No intermediate host) or vector-borne (1 intermediate host)
o humans = dead-end hosts (Toxocara, Trichinella), ¼ of pop. = 1+ infection
Ex: Pinworm (Enterobius) Whipworm (trichuris trichiuria) Hookworm (Necator americancus) Ascariasis Strongyloides Trichinella spiralis Toxocara canis
Know the difference between a hydatid cyst and a cysticercus.
Cyst:
dormant stage found encysted in host tissues (intermediate or definitive)
Cysticercus [bladder worm]:
encysted cestode larval form in intermediate hosts
Ex: Taenia solium (pork tapeworm) only [humans]
Hydatid Cyst: specialized cysticercal form of Echinococcus granulosus (cestode)
o found in intermediate hosts (humans, sheep, wild ungulates)
