0305 - Biology of Parasites Flashcards
What is a parasite?
A eukaryotic pathogen (protozoa, metaozoa) that has a complex lifecycle and requires at least one host for nourishment and/or protection (and to complete lifecycle), without giving a benefit in return.
Describe protozoa, including the classes
Free-living, unicellular eukaryotes. Generally live in water and form cysts in unfavourable conditions. Disease caused by a very low infective dose.
How are protozoa classified?
Classified based on motility:
Flagellates and Mastigophorans - flagella - Giardia
Amoebae and Sarcodines - Pseudopods - Entamoeba Histolytica
Ciliates - Cilia - Balantidium Coli
Coccidia and Apicomplexans- Immobile - Cryptosporidium
What are the different forms of parasite host?
Host specificity - Range of different hosts in which a parasite can mature
Definitive Host - Parasite matures into adult form and undergoes sexual reproduction
Intermediate host - parasite development occurs, but no sexual maturity (may have asexual reproduction)
Paratenic/Transport Host - Remains alive and infective, but doesn’t undergo any development
Incidental/dead end host - intermediate host that doesn’t allow transmission to definitive host.
Describe Helminths, including the classifications
Multicellular worms, bilaterally symmetrical, with a distinct head and tail, and 3 layers of tissue (ecto, meso, endoderm).
Classified into
Trematodes (flukes) - lung/liver
Nematodes (roundworms) - Pinworm, hookworm
Cestodes (flatworms) - Pig tapeworm, dog tapeworm
What requirements exist for parasites to cause human disease?
Parasites need to:
Gain entry (either endogenous or exogenous)
Replicate/disseminate (generally extracellular)
Damage the host; and
Spread to a new victim
How can parasites damage the host?
Specifics are parasite-specific, but include
Nutritional stress (deplete nutrients/damage absorptive surfaces)
Physical tissue damage resulting from blockage during growth, replication/migration, and pressure
Secondary infections
Toxicity from parasite products or metabolites
Immunopathological reactions - chronic inflammation, immunodeficiency.
Discuss how an understanding of parasitic lifecycles can influence our knowledge of parasitic infection, disease, diagnosis, treatment, and prevention (key concept).
Life cycle determines how you get infected, what symptoms you get, diagnosis, treatment, and prevention
Describe the features of cryptosporidium infection
Self-limiting gastroenteritis, lasting 1-2 weeks and generally characterised by watery diarrhoea.
In immunocompromised, can cause cholera-like illness, or chronic diarrhoea and is extremely difficult to treat.
Describe the features of amoebiasis
Variable symptoms, from asymptomatic to dysentery, appendicitis, liver abscess
Requires metronidazole for the trophozoites, and iodoquinol for the cysts.
Describe giardia infection.
Excystation occurs due to stomach acid, releasing for trophozoites. These colonise the small intestine, undergo rapid asexual reproduction, and encyst in the large intestine.
Damage includes flattened villi, inflamed mucosa (damage), altering absorption, and inducing a host immune response.
Symptoms start 6-15 days after exposure, range from asymptomatic to chronic diarrhoea, pain, malabsorption, and weight loss. Generally last several days but are self-limiting.
