Lecture 13 Flashcards
What are parasites?
Eukaryotic organisms which live in or on another organism and cause harm to host
Explain pathogenicity
The ability for an organism to cause disease
Explain virulence
Degree to which pathogenic organisms can cause disease
2 main groups of parasites
Protozoa and helminths (worms)
What are protozoa and helminths
Protozoa - Single-celled organisms
Helminths - Multicellular - very small to very large - often have small eggs
Important protozoan parasites
Plasmodium species - Cause malaria
Leishmania species - Leishmania infantum, L. donovani cause Leishmaniasis
Giardia
Cryptosporidium
Important helminth parasites
Schistosomes: Schistosoma species
Roundworms: Ascaris lumbricoides
Methods of diagnosis: direct and indirect
Direct: Involve visualisation of parasite itself (classical parasitological diagnosis) or detection of specific component of parasite e.g. DNA by PCR or antigen detecting tests
Indirect: Detection of the effects of parasites - Clinical diagnosis (signs/symptoms), biochemical tests (change in blood metabolites), serological tests (Antibodies against parasites)
What are the most important sample types for parasites?
Blood and faeces
Good diagnostic tests
Tests aren’t always perfect - can give false readings
Prevalence is total number of individuals with disease
Prevalence = total positives/total population
What is sensitivity?
Proportion of true positives identified
High sensitivity - Small number of false negatives
Specificity
Proportion of true negatives identified
High specificity - Small number of false negatives
What are sensitivity and specificity so important in measuring a test’s effectiveness?
Sensitivity measures how often a test correctly identifies a person who has the condition that is being tested for
Specificity measures how often a test correctly identifies people who do not have the condition that’s being tested for
Both required to understand a test’s strengths and shortcomings
What should specificity and sensitivity be at?
90%
Does the ideal test exist?
No
What other factors affect the choice of test
Seriousness of infection
Extent of treatment side effects
These are the reasons as to why different tests are used for epidemiology
Malaria
5 species known to cause malaria:
- Plasmodium falciparum
- Plasmodium vivax
- Plasmodium malariae
- Plasmodium ovale
- Plasmodium knowlesi
- Transmission by bites of female mosquitoes (Anopheles spp.) - A. gambiae best known vector for P. falciparum
Reservoirs - No sigificant animal reservoirs except P. knowlesi in macaque monkeys
Diagnositc stages of malaria
Bloodstream form:
A. Rings
B. Trophozoites
C. Schizonts
D. Gametocytes
Epidemiology of malaria
3.3 billion at risk across 91 countries
~247 million cases a year
~620,000 deaths
- 90% infections in Africa
Frequency of infections goes Plasmodium falciparum> P. vivax»P. malariae>P. ovale»P. knowlesi
P. falciparum responsible for 50% of cases and most deaths
P. vivax responsible for 43% cases (fatality rate 0.12/1000)
Malaria is caused by people travelling in non-endemic regions
Indirect diagnostic test for malaria
Clinical diagnosis: fever, clinical algorithm
Antibody detection - not used for diagnosis
- ELISA
- IFAT
- IFA procedure used to determine plasmodium infection
- Serologic testing not practical due to time for development of antibody and antibody persistence
- Screening blood donors in cases of transfusion-induced malaria where donor’s parasitemia may be below delectable level of blood film examination
- Testing patients who had repeated or chronic malaria for tropical splenomegaly syndrome
- Testing patients treated for malaria but diagnosis questioned
Symptoms of malaria
- No unique set of symptoms
- Symptoms occur with blood stage infection 2 weeks after bite
- Fever is classic symptom - irregular or regular with following stages
- Cold stage - Patient shivers, temp rise
- Hot stage - Peripheral vasodilation, rapid pulse, high temp ~41 degrees
- Sweating stage - Copious sweating leading to fall in temp
- Other symptoms can include anaemia, splenomegaly, and jaundice
- Cerebral malaria in brain is a serious complication of P. falciparum infection - coma
Direct tests for malaria
Parasitological:
- Microscopy - thick and thin blood films = current gold standard
- Quantitative buffy coat (QBC) - acridine orange
- PCR
- Detection of malaria antigens using immunochromatographic malaria rapid diagnostic tests (mRDTs)
- Paracheck-Pf - histidine rich protein (HRP2)
- Binax (ICT) - HRP2 + aldolase (pan-malarial antigen common to all 4 malaria species)
- OptiMal - detects species-specific enzyme, parasite lactate dehydrogenase (pLDH), produced by live malaria parasites
Parasitological diagnosis of malaria - Blood films
Thick film = sensitivity - Detects parasitaemia as low as 0.0001%
Thin film = specificity - Identifies species, counts parasites
Stained with Giemsa, Leishmans, Fields, or similar
Positive result for quantitative buffy coat for P. falciparum
- Crescent-shaped gametocytes appear near interface of lymphocyte/monocyte and platelet layers
- Schizonts and mature trophozoites appear near granulocyte layer
- Rings appear through red blood cell layer, with a concentration near interface of granulocyte layer
Comparison between diagnostic tests - Sensitivity
Giemsa - 84/ul
QBC - <5/ul
NOW Malaria - 50-100/ul
Comparison between diagnostic tests - Specificity
Giemsa - 100%
QBC and NOW Malaria - ~100%
Comparison between diagnostic tests - Species identity
Giemsa - Clear
QBC - Unreliable
NOW Malaria - Pf specific
Comparison between diagnostic tests - Parasite count
Giemsa - Accurate
QBC - N/A
NOW Malaria - Not quant
Comparison between diagnostic tests - Speed
Giemsa - Slow
QBC - Moderate
NOW Malaria - fast
Comparison between diagnostic tests - Cost
Giemsa - 0.30-0.40 dollars
QBC - 5 dollars
NOW Malaria - 0.65 dollars
Comparison between diagnostic tests - Equipment
Giemsa - +
QBC - +++
NOW Malaria - None
Comparison between diagnostic tests - Training
Giemsa - High
QBC - Medium
NOW Malaria - Low
