Laboratory Diagnosis of Parasitic Infection Flashcards
T or F. Every 30s someone dies from malaria
T
2 main classes of parasites
- protozoan
= multiply within host, intracellular or extracellular - metazoan
= do not multiply within host, extracellular existence
- majority of infections = light and asymptomatic in non-immune host such as pinworm or ascaris (nematodes*
prasitism
parasite has higher reproductive potential than host and parasite has potential of harming the host
types of hosts
- definitive host:
- host in which parasite sexuall matures and resproduces sexually - intermediate host:
- host in which parasite is sexually immature, reproduces asexually and are sometimes dead-end hosts - reservoir host:
- host form which humans acquire infection or in which agent normally cycles
what are vectors?
- any agent (person, animal or microorganism) that carries and transmits an infectious parasite to another living organism
- usually, parasites have adapted their life cycle to a particular vector
- arthropods are usually implicated (e.g. flies, ticks or mosquitoes)
diagnosis of most intestinal parasitic infections is dependent upon finding (3)
- eggs or ova
- larvae of helminths
- trophozoites or cysts of protozoa in feces
these fecal specimens are of limited value and may lead to inaccurate diagnosis
old
inadequate amount
poorly preserved
ingestion of some medications prior to collection of fecal specimens, may interefere with examination and detection of parasites. which compounds do these?
anti-diarrheal compounds such as:
- antacids
- bismuth
- mineral oils
- barium sulfate
- gallbladder dyes
wait three weeks after ingestion
this antibiotic may eliminate or reduce the number of organisms in feces for several weeks
tetracycline
in addition to date of coll and relevant patient history, clinical findings such as these are important for parasite investigation:
diarrhea
abdominal discomfort
eosinophilia
immunocompromised status
foreign travel (indicate countries)
ingestion of raw/undercooked meat/fish
close contact w pets or farm animals
prior parasitic infections
collection of feces
specimens should not be contaminated with urine or water
should be collected directly into container (NOT from toilet bowl, soil or grass)
number and type of stool specimens
three stool specimens, two to three day intervals is recommended
- multiple specimens collected on same day = REJECTED
**additional or alternate procedures for amoebiasis or giardiasis (duodenal sampling)
time factor in examination of specimens
- unpreserved stool = examined ASAP
> trophs will be killed by refrigeration
> helminth eggs and protozoan cysts will retain their morphology - specimens that will be delayed for more than a day should be placed in an appropriate preservative
preservation of stool specimens
- unpreserved stools = care if delayed
- preservatives provided in 15-30 mL plastic or glass screw-capped vials
- ratio of formed stool to preservative = 1:3
- liquid or water stools = 1:1
- DO NOT FREEZE or INCUBATE specimens
commonly used stool perservatives:
sodium acetate-acetic acid-formaldehyde (SAF)
modified polyvinyl alcohol (PVA)
Schaudinn’s fixative
5% or 10% formalin
merthiolate-iodine-formalin (MIF)
transport and mailing of stool specimens
- if sent through mail should conform to postal shipping regulations
- specimens must be packaged carefully to prevent leakage and accidental infection of handlers
loose diarrheic and liquid stool samples more likely to contain ….
trophs
cysts less likely
unpreserved vs preserved stool specimens
unpreserved = evaluates gross specimens
preserved = provides information as is
which type of stool are cysts most likely to be found?
formed stool
macroscopic findings of stool examination
- dark blood = bleeding high up in GI
- bright blood = bleeding lower or around the rectum
- fresh blood or mucus = always examine for trophozoites
- yellow/foul-smelling stool indicates fat malabsorption, common sequela of Giardiasis
- gross examination may provide opportunity to recover tapeworm proglottids, roundworms, or pinworms
microscopic findings of stool examination
normal constituents of feces may resemble parasitic organisms
- referred to as artifacts or pseudo-parasites
objects that may be found are:
- animal and plant materials
- red/white blood cells
- Charcot-Leyden crystals
- macrophages
- epithelial cells
- eggs of arthropods and plant nematodes
direct wet mounts
Simple and efficient procedure for the detection of motile trophozoite in liquid or diarrheic specimens
- should be done on all fresh specimens but time constraints interferes = practice more in field labs
two categories of concentration procedures for stool specimens
flotation
sedimentation
sedimentation
- achieved by centrifugation/gravity
- most frequently used
- sediment = all the parasites
- used with both fresh and preserved stool
- disadvantage = excessive debris present may mask the parasites
Flotation
- Zinc sulphate or sucrose
- solutions of high specific gravity will float parasites
- protozoan cysts and most nematode eggs will float
- heavier eggs of trematodes and many cestodes may not float and will NO be detected
- disadvantage = walls of eggs and cysts can collapse and organism can become distorted
