13. Diagnosis, management and treatment of fungal diseases Flashcards
Why is fast correct diagnosis for fungal infections important?
- Because fungal infections progress quickly and kill quickly.
- Treatment between species in the same genra can vary massively
What needs to be used alongside diagnostic tools to give accurate dianosis?
- Symptoms
- Medical or travel history
Why are superficial fungal infections easier to manage?
- They are more visible so they are easy to identify
- Treatment length depends on the place of infections.
What can cause invasive fungal infections?
- A range of different fungi
- Often, pathogens like candida, aspergillus or cryptococcus.
What are the risk factors for fungal infections?
For fungi to cause an infection they normally need a predisposing factor.
1. HIV infection
2. neutropenia
3. Diabetes
4. Antibiotic treatment
5. transplant
What is the exception to fungi needing a predisposing risk factor?
Primary fungi pathogens
Why is antibiotic treatment a risk factor for fungal infections?
It can wipe out the microbiome that competes with the fungi
How is imaging used to help diagnose fungal infection?
- You can see what is going on
- See masses in the tissue but you don’t know what it is just from images
- Use the image along with history and symptoms to help confirm a diagnosis.
- Based on a lot of assumptions, it is not used for primary diagnosis very often.
Why is diagnosis at a species level important?
- There are important differences between infections from species in the same genus.
- Disease outcomes like mortality depend on the specific organism.
- Different fungal species have susceptibility to different drugs and intrinsic resistance mechanisms.
- The species cannot be distinguished by eye.
What does type of specimen for diagnosis depend on?
The type of infection like superficial vs invasive
What specimens are needed for diagnosis of invasive fungal infections?
- Invasive infections need invasive sampling.
- Aspergillus infections need lung tissue samples through a bronchoalveolar lavage.
- Cryptococcus infections require a lumbar puncture to get CSF.
- Candidemia requires blood samples
What are the downsides of invasive fungal infections diagnosis?
- Invasive samples
- Take time during which the patient gets worse.
- Uncomfortable
What specimens are used for diagnosis of superficial and subcutaneous fungal infections?
- Easier to access
- Scrapings, clippings, external biopsies.
How is light microscopy used to diagnose superficial fungal infections?
To look at cell structures like hyphae or yeast cells.
How is Fluorescence microscopy used to diagnose superficial fungal infections?
- Using potassium hydroxide
- Mostly used on dermatophytes
- Good for differentiation from bacteria
How is invasive candida diagnosed using microscopy?
- Blood smears
- Normally looks like budding yeast.
- C. albicans form germ tubes which is unique and used to differentiate them.
- You can also look for hyphae formation.
How is invasive aspergillus diagnosed using microscopy?
- Using a Diff-Quik stain
- Looking for hyphae formation
How is invasive Mucor diagnosed using microscopy?
- Looking for non-septate hyphae
- This is different from other hyphae
- from nasal discharge
How is invasive crytococcus diagnosed using microscopy?
- India ink stain of the CSF
- Normal CSF is quite boring so fungal presence is quite obvious
What does microscopy tell us about an infection?
- If it is fungal or not
- If it is a yeast or hyphae
- Any distinct characteristics
What is the disadvantage of using fungal cultures for diagnosis?
It takes a long time, and during this, the patient is suffering.
What is the advantage of using fungal cultures for diagnosis?
You get more information than from microscopy
Why are most fungal cultures done in tubes not on plates?
- There is less risk of contamination
- There is less aeration of spores
- Done on a slant to increase surface area and good for long-term storage
Why are there different types of agar used for fungal culture?
Different fungi need different media to differentiate between them
Different fungal culture media: Sabouraud’s dextrose agar
- Recommended for most fungi as it is generic.
- Sugar rich
- Lots of fungi grow quite easily on it.
- It doesn’t distinguish between species
- All yeast grow as white, waxy, colonies on this agar
- Contains chloramphenicol to inhibit bacterial growth
Different fungal culture media: Mycosel agar
- A generic rich media
- Gives lots of growth
- Contains chloramphenicol to inhibit bacterial growth
- Contains cycloheximide to inhibit the growth of Aspergillus and C. neoformans
Different fungal culture media: Brain-heart infusion
Similar to Sabouraud’s but more enriched
Different fungal culture media: potato dextrose agar
Used to induce spore formation to aid identification
Different fungal culture media: Birdseed agar
- Used to see more distinct morphology
- Used to isolate cryptococcus neoformans from contaminated samples.
- It Also contains drugs to inhibit the growth of other fungi you have ruled out
Why are growth conditions for fungal culturing important?
- This is critical as fungi grow and present differently at different temperatures.
- Some fungi won’t grow at certain temperatures like dermatophytes not growing above 28oC.
- Some fungi can take weeks to grow as some fungi are very slow growing which is not good for diagnosis.
What helps determine the growth conditions for fungal cultures?
- The type of sample
- Like a nail sample being done at room temp
Why is it hard to distinguish between yeasts in cutlure?
They all look the same:
1. White
2. Waxy
3. Domed
4. Raised
What is chromogenic agar?
- Contains different metabolites that produce different pigments when metabolised by different fungi.
- This produces different colours depending on species.
- It is very very expensive so not commonly use.
- Can be used in research or in complex cases
What is a liquid culture test?
- Used to diagnose different candida species.
- Used bovine or horse serum
- C. albicans will produce germ tubes and others don’t so you look for these.
What can you do with all these different culture tests?
Run them in parallel so you have lots of information to be able to make a diagnosis
How can mould be diagnosed through culturing?
- They all have distinct morphologies when grown on different agar.
- They are also cultured on different media
- They can then be microscopically identified by the spore morphology
How can yeast be characterised using biochemical tests?
- Using API strips
- These contain lots of different tests like what sugar they can use
- Standardised version of chemically defined techniques
- Tricky to do in the lab as they are not hugely consistent and you need good technique.
- There is a specific window you need to read the results in to be valid.
- Not used as a primary diagnostic tool
How can fungal infections be diagnosed using PCR?
- Cost is reducing, so it is becoming more common.
- Amplify a specific region of a gene
- These are hyper-conversed regions that contain some hypervariable regions that distinguish the species.
What gene region is normally used for fungal diagnosis?
The ITS region of the 18s or 28s of rRNA
Why are rRNA genes used for PCR diagnosis?
- There are 100s of copies of genes in the cell
- This means you don’t need a high fungal load to get a good sample.
- Better than virulence factors that will have only 1 or 2 copies per cell.
What are the different types of PCR used to diagnose fungal infections?
- Nested PCR
- PCR-ELISA
- PCR-blot
- Real-time PCR
Why is real time PCR used for fungal infection diagnosis?
- It is qualitative
- You need to start with the same concentration in the samples and have a control.
What are the disadvantages of PCR for fungal diagnosis?
- There are lots of different kits with different steps and reagents. This introduces variation.
- Human error in following testing kits
- There is no standardisation of testing.
- Equipment and expertise varies.
What needs to be considered when diagnosing a fungal infection?
If the patient is on antifungal prophylaxis which can effect results.
What is the process of molecules identification of yeast and moulds?
- Start with a pure fungal culture
- Extract the DNA and purify to remove proteins
- PCR of the region of interest using primers
- Use of good primers
- Validate results on a gel.
- Sequence using sanger sequencing
- Then use BLAST and select the most likely species for diagnosis.
Why does molecular diagnosis of fungal infections need to start with a pure culture?
Because it can’t distinguish between infection and commensal fungi