0603 - Parasitic Infections Flashcards
What are some lab tests to diagnose parasitic infections.
Macro Examination micro examination Culture Antigen detection (ICT, EIA, NAAT/PCR) Antibody detection Imaging.
Describe Macro exam of parasites
Round (nematode - Ascaris) vs tapeworm (taenia). Also whipworm (whip shaped) or pinworm (very small - Enterobius). Rarely liver flukes.
Describe Microscopic Exam of parasites
Look at blood parasites (malaria), fecal parasites (giardia, crypto, strongyloides, hookworm), other fluids and tissues (hydatid, trichomonas).
Can do thin or thick films - Thin are good for morphology/speciation, thick for increasing sensitivity but getting more.
Describe Culture of parasites
Grow the pathogen, including via an arthropod that has fed on patient to detect low levels of parasites (xenodiagnosis).
Advantages and disadvantages of ICT
More sensitive than microscopy, rapid, easy to perform and can be perfomed as a one-off
Expensive ($15-20/test)
Good for Giardia and Crypto
Advantages and disadvantages of Enzyme immunoassay (EIA)
Advantages - more sensitive than microscopy, easy, and more cost effective than ICT
Disadvantages - needs to be batched.
Advantages and disadvantages of antibody detection
Useful for diagnosis and monitoring post-treatment.
Good for infections where you can’t get a specimen/other methods have low sensitivity
Not available for all parasite, and sensitivity and specificity are variable. Can’t always distinguish past from current infection, and relatively expensive, requires reagents and equipment.
Advantages and disadvantages of Macro
Advantages - Rapid, spot-diagnosis.
Disadvantages - Only a few ectoparasites/helminths. Often found fortuitously. Still need to do microscopy to differentiate some species.
Advantages and disadvantages of Micro
Most widely used technique
Fecal parasites - may have low sensitivity due to intermittent release. Short processing time before they die.
General advantages - can diagnose many common parasites by simple methods, cheap, and can be done in the field.
General disadvantages - Parasites may be excreted periodically (e.g. faecal), or in hard to reach tissue, reducing sensitivity. Requires reasonable expertise and experience. Special stains which increase sensitivity also increase cost and complexity.
What factors influence the immune response to parasites?
Size of organism (KEY) - can’t be phagocytosed
Different antigens exposed at different life stages/locations
Some live in the GIT without invasion
Chronic/persistent infection due to immune evasion
Rarely eliminated - protects against lethal infection, but causes significant immunopathology.
What are our Layers of defence
to parasites?
External - mechanical, chemical (stomach acid, lysozyme etc), microbiological (normal flora).
Innate immunity
Acquired immunity
What is the difference between Th1 and Th2 T-cells?
Th1 - Triggered by IL-12, IL-2, and effect via IFN-Gamma. Work against intracellular bacteria and protozoa, and activate predominantly macrophages, and CD8 T-cells. Stimulate cellular immunity.
Th2 - Triggered by IL4-, effect via IL4, 5, 9, 10, and 13. Work against extracellular parasites including helminths, and activate eosinophils, basophils, and mast cells, as well as B-cells. Stimulate humoral immunity.
What is the body’s response to parasite infection?
If protozoa, Th1-moderated, if helminth, Th2 moderated.
Body aims to suppress or eliminate the parasite with minimal damage to the host, often rebalancing towards the middle ground - chronic, low-grade infection with persistent but low-level controlled inflammation.
