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Microbiology · Other viruses and prions

Prions !

  • Virus
  • Non-enveloped (naked)
  • Exam priority: medium

Not viruses: misfolded proteins that bend normal ones out of shape and destroy the brain; they survive routine sterilisation.

Updated 2026-11-01

Also called: agents of the spongiform encephalopathies

At a glance

Shape
Protein aggregates and fibrils
Genome
None: a prion has no nucleic acid, it is protein only (PrPSc)
Envelope
Non-enveloped (naked)
Type
Misfolded prion protein rich in beta sheets; not a virus
Stain
Immunohistochemistry for PrP in brain tissue

Morphology

  • Normal prion protein (PrPc) sits on the neuron membrane
  • The abnormal form (PrPSc) has the same sequence but another shape and forces the normal one to change
  • Forms aggregates resistant to proteases, heat and many disinfectants

Transmission

  • Sporadic, with no known cause: the commonest form of Creutzfeldt-Jakob disease (CJD)
  • Inherited, through mutations in the PRNP gene
  • Iatrogenic: dura mater grafts, corneal transplants, human-derived growth hormone and contaminated neurosurgical instruments
  • Dietary: beef from cattle with bovine spongiform encephalopathy (variant CJD); kuru from ritual cannibalism in Papua New Guinea
  • Not spread by casual contact

Reservoir: Humans themselves (sporadic and inherited cases); cattle in variant CJD

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Virulence factors

  • Chain conversion of the normal protein into the abnormal one
  • Neuron death and vacuoles in the brain (spongiform change), without inflammation
  • No immune response: no fever, no CSF white cells and no useful antibodies

Diseases it causes

Clinical picture

  • Dementia that progresses very fast, over weeks or months
  • Myoclonus (muscle jerks), ataxia and visual disturbances
  • The variant affects young people, with early psychiatric symptoms and pain on touch
  • Always fatal

Diagnosis

  • MRI: diffusion hyperintensity in cortex, caudate and putamen
  • EEG: periodic sharp-wave complexes
  • Cerebrospinal fluid: RT-QuIC and 14-3-3 protein
  • Confirmation by examination of brain tissue

Treatment

No treatment cures or slows the disease; care is supportive and palliative.

Drug class only; doses and regimens are set by the clinician from the current guideline.

Prevention

  • High-infectivity tissues are brain, spinal cord and eye
  • Instruments that touch them need a special protocol (chemical treatment with sodium hydroxide or hypochlorite plus autoclaving) or are discarded and incinerated; routine sterilisation is not enough
  • Control of bovine spongiform encephalopathy in the meat chain
  • Deferral of at-risk blood and organ donors

Test yourself

Think of the answer, then tap the question to see it.

Why are prions not viruses?

They have no nucleic acid, capsid or envelope: they are proteins with an abnormal fold.

Which clinical triad suggests Creutzfeldt-Jakob disease?

Very rapidly progressive dementia, myoclonus and typical MRI or EEG changes.

Why do neurosurgical instruments used on a patient with CJD need a special protocol?

Because prions survive routine sterilisation; chemical treatment plus autoclaving is used, or disposable material is incinerated.

Which isolation precautions does a hospitalised patient with CJD need?

Standard precautions only, because it does not spread by casual contact or by the respiratory route.

Which exposure is linked to variant Creutzfeldt-Jakob disease?

Eating beef from cattle with bovine spongiform encephalopathy.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: About Prion Diseases | Prions | CDC (CDC, 2024); Infection Control for CJD | Classic CJD | CDC (CDC, 2024).

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Representative model, for educational purposes only.