Fungi and parasites
Introduit| English | Sens |
|---|---|
| yeasts | yeasts |
| moulds/məʊldz/ | moulds |
| dimorphic fungi | dimorphic fungi |
| protozoa | protozoa |
| helminths | helminths |
Fever every other day
- A traveller back from Africa has a fever that comes and goes in a rhythm. A farmer coughs after clearing a barn full of bat droppings.
- Both are caused by eukaryotes: a parasite with a two-host life cycle, and a fungus that changes form with temperature.
Three forms of fungi
- Yeasts 酵母 are single cells that bud; moulds 霉菌 grow as branching threads (hyphae).
- Dimorphic fungi 双相真菌 switch: a mould in the environment at 25 °C and a yeast in the body at 37 °C.
Match each fungal form to its description.
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Form and temperature behaviour help name the fungus.
Fungi follow the immune defect
- Most serious fungal infections are opportunists: T-cell failure lets in Pneumocystis and Candida; neutrophil failure lets in Aspergillus.
- Fungi are eukaryotes like us, so there are few safe drug targets. Many antifungals hit ergosterol, the sterol in fungal membranes that human cells do not use.
A patient with very low neutrophils after chemotherapy develops a lung infection with branching hyphae. Which fungus is most likely?
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Neutrophil failure lets moulds such as Aspergillus invade.
Protozoa and helminths
- Protozoa 原虫 are single-celled parasites, such as malaria and amoebae; helminths 蠕虫 are worms.
- Diagnosis depends on where the parasite lives: blood films for malaria, stool for most gut parasites, serology or tissue for others.
Which sample is used to find most gut parasites?
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Where the parasite lives decides the sample.
The malaria life cycle
- A mosquito injects sporozoites, which multiply in the liver. Merozoites then enter red cells, multiply and burst out to infect more red cells.
- Some become gametocytes, which a biting mosquito takes up to continue the cycle.
Put the stages of malaria in a person in order.
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Liver first, then blood; only gametocytes carry the parasite back to a mosquito.
The fever keeps the parasite's clock
- When red cells burst together, fever follows: every 48 hours for P. vivax and P. ovale, every 72 hours for P. malariae. P. vivax and P. ovale can also sleep in the liver and relapse months later.
- P. falciparum is less regular, but its infected red cells stick to small vessels in the brain and other organs, which makes it the deadliest.
P. falciparum is the deadliest malaria because infected red cells stick to small blood vessels.
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Sequestration in the brain and other organs blocks blood flow, causing cerebral malaria and organ failure.
Worked example: reading the film
- (Fictional) A returned traveller's thin film shows ring forms in 3% of red cells and banana-shaped gametocytes: P. falciparum.
- With about $5 \times 10^{12}$ red cells per litre, 3% is $1.5 \times 10^{11}$ infected cells in every litre of blood. The film names the species and grades the severity.
Blood has 5 × 10¹² red cells per litre and 3% are infected. How many infected cells are there per litre, in units of 10¹¹?
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5 × 10¹² × 0.03 = 1.5 × 10¹¹.
Worked example: a fungus with two forms
- (Fictional) A farmer develops pneumonia after clearing old bird and bat droppings. The culture grows a mould at 25 °C and a yeast at 37 °C.
- That switch identifies a dimorphic fungus such as Histoplasma: a mould in the soil, a yeast inside you.
A culture grows a mould at 25 °C and a yeast at 37 °C. What kind of fungus is it?
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Temperature-dependent switching is the mark of dimorphic fungi such as Histoplasma.
Three common slips
- Treating fungi like bacteria. Antibacterial drugs do not work on them, and safe antifungal targets are few.
- "Every parasite shows on a blood film." Most gut parasites are found in stool.
- "Falciparum gives neat 48-hour fevers." It is the least regular and the most dangerous.
Recap
- Yeasts bud, moulds form hyphae, dimorphic fungi switch: mould at 25 °C, yeast at 37 °C.
- Fungal opportunists follow the immune defect.
- Malaria: sporozoite → liver → red-cell cycles → gametocyte. Fevers every 48 h (vivax, ovale) or 72 h (malariae); falciparum sticks and kills.