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Medical Microbiology > Lecture 13 > Flashcards

Lecture 13 Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

What are parasites?

A

Eukaryotic organisms which live in or on another organism and cause harm to host

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2
Q

Explain pathogenicity

A

The ability for an organism to cause disease

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3
Q

Explain virulence

A

Degree to which pathogenic organisms can cause disease

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4
Q

2 main groups of parasites

A

Protozoa and helminths (worms)

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5
Q

What are protozoa and helminths

A

Protozoa - Single-celled organisms

Helminths - Multicellular - very small to very large - often have small eggs

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6
Q

Important protozoan parasites

A

Plasmodium species - Cause malaria

Leishmania species - Leishmania infantum, L. donovani cause Leishmaniasis

Giardia

Cryptosporidium

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7
Q

Important helminth parasites

A

Schistosomes: Schistosoma species

Roundworms: Ascaris lumbricoides

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8
Q

Methods of diagnosis: direct and indirect

A

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)

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9
Q

What are the most important sample types for parasites?

A

Blood and faeces

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10
Q

Good diagnostic tests

A

Tests aren’t always perfect - can give false readings

Prevalence is total number of individuals with disease

Prevalence = total positives/total population

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11
Q

What is sensitivity?

A

Proportion of true positives identified

High sensitivity - Small number of false negatives

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12
Q

Specificity

A

Proportion of true negatives identified

High specificity - Small number of false negatives

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13
Q

What are sensitivity and specificity so important in measuring a test’s effectiveness?

A

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

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14
Q

What should specificity and sensitivity be at?

A

90%

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15
Q

Does the ideal test exist?

A

No

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16
Q

What other factors affect the choice of test

A

Seriousness of infection

Extent of treatment side effects

These are the reasons as to why different tests are used for epidemiology

17
Q

Malaria

A

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

18
Q

Diagnositc stages of malaria

A

Bloodstream form:
A. Rings
B. Trophozoites
C. Schizonts
D. Gametocytes

19
Q

Epidemiology of malaria

A

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

20
Q

Indirect diagnostic test for malaria

A

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
21
Q

Symptoms of malaria

A
  • 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
22
Q

Direct tests for malaria

A

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

23
Q

Parasitological diagnosis of malaria - Blood films

A

Thick film = sensitivity - Detects parasitaemia as low as 0.0001%

Thin film = specificity - Identifies species, counts parasites

Stained with Giemsa, Leishmans, Fields, or similar

24
Q

Positive result for quantitative buffy coat for P. falciparum

A
  • 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
25
Q

Comparison between diagnostic tests - Sensitivity

A

Giemsa - 84/ul

QBC - <5/ul

NOW Malaria - 50-100/ul

26
Q

Comparison between diagnostic tests - Specificity

A

Giemsa - 100%

QBC and NOW Malaria - ~100%

27
Q

Comparison between diagnostic tests - Species identity

A

Giemsa - Clear

QBC - Unreliable

NOW Malaria - Pf specific

28
Q

Comparison between diagnostic tests - Parasite count

A

Giemsa - Accurate

QBC - N/A

NOW Malaria - Not quant

29
Q

Comparison between diagnostic tests - Speed

A

Giemsa - Slow

QBC - Moderate

NOW Malaria - fast

30
Q

Comparison between diagnostic tests - Cost

A

Giemsa - 0.30-0.40 dollars

QBC - 5 dollars

NOW Malaria - 0.65 dollars

31
Q

Comparison between diagnostic tests - Equipment

A

Giemsa - +

QBC - +++

NOW Malaria - None

32
Q

Comparison between diagnostic tests - Training

A

Giemsa - High

QBC - Medium

NOW Malaria - Low

Medical Microbiology (18 decks)

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