Lecture 13 Flashcards
(17 cards)
What Is a Parasite?
Parasites are eukaryotic organisms (i.e., their cells contain a nucleus and organelles) that live in or on another living organism (the host) and cause harm to the host.
Two Main Parasite Groups
Protozoa – single-celled eukaryotes
Helminths – multicellular worms (range from microscopic eggs to large adults)
Examples of Protozoan Parasites
Plasmodium species: Cause Malaria
Leishmania species: Cause Leishmaniasis
Giardia: Lives in the gut, causes giardiasis
Examples of Helminth Parasites:
Schistosomes: Cause Schistosomiasis
Roundworms (Ascaris lumbricoides):
Cause ascariasis
Diagnosis of Parasitic Infections: Direct
Direct Methods
Aim to visualise the parasite or its components:
Classical microscopy.
Detection of DNA using PCR (Polymerase Chain Reaction).
Detection of parasite antigens (proteins unique to the parasite).
Diagnosis of Parasitic Infections: indirect
Detect effects of the parasite rather than the parasite itself:
Clinical diagnosis: Based on signs and symptoms (e.g. fever, diarrhoea).
Biochemical tests: Detect altered blood metabolites.
Serological tests: Detect antibodies made against the parasite.
Common sample types in diagnosis of parasitic infection
Blood and faeces (stools) – most important.
Others: Urine, sputum (phlegm), tissue biopsies – chosen based on suspected parasite and disease
Sensitivity and Specificity in Diagnostic Tests
Sensitivity:
Proportion of true positives identified.
A highly sensitive test will have few false negatives.
Useful when it’s crucial to not miss any infected person (e.g., severe diseases).
Specificity:
Proportion of true negatives identified.
A highly specific test will have few false positives.
Useful when unnecessary treatment should be avoided (e.g., toxic side effects).
What species cause malaria
P. falciparum – most deadly, most common in Africa.
P. vivax – less deadly, widely distributed.
P. malariae
P. ovale
P. knowlesi – zoonotic (from macaque monkeys).
Resovoirs and transmission of malaria
Reservoirs:
Humans for most species.
P. knowlesi can be carried by macaque monkey
Transmission:
Via bite of female Anopheles mosquitoes (only females blood-feed).
Life Cycle and Diagnostic Stages of Malaria (Blood Stage)
In humans, diagnostic stages appear in the blood (erythrocytic cycle):
A. Rings – early trophozoites inside red blood cells (RBCs).
B. Trophozoites – feeding forms, larger than rings.
C. Schizonts – divide into multiple merozoites (burst RBC).
D. Gametocytes – sexual forms (male and female) that mosquitoes ingest.
Malaria Diagnosis: Indirect Tests
Presumptive clinical diagnosis:
Based on fever and symptom patterns.
Used in emergency settings but not reliable on its own.
Antibody detection (serology) – not suitable for acute diagnosis:
ELISA, IFAT (Indirect Fluorescent Antibody Test).
Why not used routinely:
Antibodies take time to develop (delayed detection).
Antibodies persist, so can’t confirm active infection.
Serology is useful for:
Screening blood donors (when parasite level is very low).
Chronic infection (e.g., tropical splenomegaly syndrome).
Post-treatment confirmation (if diagnosis is uncertain).
Malaria Diagnosis: Direct tests: Microscopy
Microscopy (Gold Standard):
Thick Blood Film:
High sensitivity – detects low levels of parasite (~0.0001% parasitaemia).
Good for detection.
Thin Blood Film:
High specificity – identifies Plasmodium species.
Allows parasite counting.
Malaria Diagnosis: Direct tests: Quantitative Buffy Coat (QBC)
Uses acridine orange dye and fluorescence microscopy.
Enhanced detection of different blood stages (explained below).
If a sample contains P. falciparum, you may see:
- Crescent-shaped gametocytes
2.Ring-shaped immature trophozoites - Schizonts and mature trophozoites
Faster than films; moderate complexity
Malaria Diagnosis: Direct tests: PCR
Detects Plasmodium DNA.
High sensitivity and specificity but expensive and lab-based.
Malaria Diagnosis: Direct tests: Rapid Diagnostic Tests (mRDTs)
Use immunochromatography to detect malaria antigens.
Quick, field-usable; variable sensitivity
List the direct tests
Microscopy (thick & thin films)
Quantitative Buffy Coat (QBC)
PCR
Rapid Diagnostic Tests (RDTs)
Clinical Presentation of Malaria
No unique symptom set — overlaps with other infections. Symptoms appear after incubation (~2 weeks).
Classic fever pattern (in blood stage):
Cold stage: shivering, rising temperature
Hot stage: vasodilation, flushed skin, fast pulse, high fever (~41°C)
Sweating stage: intense sweating, temperature drops
Other symptoms:
Anaemia (due to red blood cell destruction)
Splenomegaly (spleen enlargement)
Jaundice (yellowing skin/eyes due to bilirubin, a haem breakdown product)
Serious complication:
Cerebral malaria: from P. falciparum → coma, potentially fatal
