Microbiology: Parasites

Question 1

Intermediate vs. Definitive host

Answer 1

• Definitive host: parasite undergoes sexual reproduction in host
• Intermediate host: parasite undergoes a larval stage, but not sexual reproduction

Question 2

Infection vs. infestation

Answer 2

• Infection: parasite is present in the internal organs or tissues (entry usually oral or skin). Organisms called endoparasites
• Infestation: Parasites are on hair, skin, or clothing. Called ectoparasites

Question 3

Protozoa

Answer 3

• Unicellular eukaryotes found world-wide
• Can be in trophozoites (active) or cyst (less active) stages
• Intestinal: entamoeba, giardia (monkey face), cyptosporidium
• Urogenital: trichomonas vaginalis
• Free-living: naegleria fowleri, acanthamoeba
• Intestinal/tissue: toxoplasma gondii (cats to humans)
• Blood and tissue: hemoflagellates (trypanosoma, leishmania), malaria (plasmodium)

Question 4

Hemoflagellates

Answer 4

• Trypanosoma cruzi causes chagas disease (eventual myocarditis), from America (kissing bug)
• Trypanosoma brucei causes sleeping sickness (eventual meningoencephalitis), from Africa (tset fly)
• Leishmania causes cutaneous and mucocutaneous lesions, transmitted from sand fly

Question 5

Malaria

Answer 5

• Caused by plasmodium species, transmitted by anopheles mosquitoes
• Once injected into human the protozoa replicates in the liver parenchymal cells (1st round of replication)
• Liver cells burst to release merozoites, which infect RBCs and mature into gametocyte or schizont (which contains merozoites and continue the inter-RBC replication)
• Gametocyte taken up by mosquito to be passed on to next host
• Mosquito is definitive host, human is intermediate host

Question 6

Methods for Dx of protozoa

Answer 6

• Intestinal parasite: stool sample analysis (for cysts)
• Intestinal/tissue: ELISA of blood
• Hemoflagellates and malaria: blood smear

Question 7

Helminths

Answer 7

• Can be tapeworms (flat), roundworms (round, nematodes), or flukes (flat)
• Relatively long and often long-lived
• Develop thru egg-> larva-> adult
• Dx is established through demonstration of parasite or its eggs in stool

Question 8

Tapeworms

Answer 8

• Taenia saginata (beef) or solum (pork)
• Saginata is better to have (fewer complications)
• Each segment is an egg (seen in feces)
• Get them from eating undercooked meat that was infected

Question 9

Nematodes

Answer 9

• Includes roundworms, hookworms, pinworm
• Dx is usually through detecting eggs in feces (except pinworm)
• Pinworm is Dx through the scotch tape test: tape against anus to grab some of the eggs or worms
• Female pinworms migrate out of anus at night and deposit eggs on perianal skin

Question 10

Flukes

Answer 10

• Includes hermaphroditic flukes and bisexual (blood flukes or schistosoma)
• Schistosoma lay eggs in lumen of blood vessels, eggs disseminate and can cause granulomas in liver (contributes to obstruction of portal circulation)
• Chronic hyper-responsiveness to egg Ags leads to clay pipestem fibrosis: fibrosis of portal vein
• This is the third stage in the lifecycle of schistosoma, after the two sexes have mated and the eggs have been laid

Question 11

Rx for parasitic worms

Answer 11

• Use Praziquantel, which disrupts tegument of skin on the worm
• Will kill living worms but do nothing for damage already done/eggs left deposited or in circulation
• Killing all worms at once can result in hypersensitivity rxn (must know worm burden)