Classification of Spongiform Encephalopathies
This group of infectious disorders affects both humans and various animal species. The pathological process leads to hallmark changes in brain tissue, resulting in a characteristic spongy (spongiform) architecture.
Major nosological forms occurring in humans include:
- Creutzfeldt-Jakob disease (CJD);
- Kuru;
- Gerstmann-Sträussler-Scheinker (GSS) syndrome;
- Fatal familial insomnia (FFI);
- Amyotrophic leukospongiosis.
Prion infections also affect animals. The most notable are scrapie in sheep and goats, and bovine spongiform encephalopathy (BSE, commonly known as "mad cow disease"). Other animal prion diseases include transmissible mink encephalopathy, feline spongiform encephalopathy, and chronic wasting disease (CWD) affecting captive and wild cervids (deer and elk).
Creutzfeldt-Jakob Disease (CJD)
CJD is the most common human prion disease, with an incubation period that can extend up to 20 years. The clinical presentation is dominated by rapidly progressive dementia, diverse movement disorders (such as myoclonus), and visual and cerebellar disturbances.
The disease occurs in two main forms:
- Sporadic/Classical CJD. Primarily affects older adults (mean onset around 68 years). It has an aggressive clinical course, with death typically occurring just 4–5 months after symptom onset.
- Variant CJD (vCJD). Affects younger individuals (mean age around 28 years). The disease course is somewhat more protracted, with death occurring within 13–14 months.
Routes of Transmission and Etiology:
- Alimentary route: Infection occurs through ingestion of brain or muscle tissue from cattle infected with bovine spongiform encephalopathy, particularly when products are inadequately cooked.
- Iatrogenic route: Associated with medical and surgical procedures. Transmission has occurred via blood transfusions, corneal transplants, neurosurgical instruments, or contaminated biological products (such as human growth hormone derived from cadaveric pituitaries).
- Sporadic and genetic mechanisms: Disease development is driven by spontaneous conformational conversion of normal PrP^c into the pathological PrP^Sc isoform, or by inherited mutations and insertions in the prion protein gene (PRNP).
Kuru
A historically significant prion disease whose epidemiology is closely tied to the island of New Guinea, where it affected indigenous Fore people. The infectious nature of Kuru was famously demonstrated by D. Carleton Gajdusek.
The primary transmission route was alimentary, occurring through ritual endocannibalism in which grieving relatives consumed the brains of deceased family members without adequate thermal processing.
The clinical presentation of Kuru includes:
- Pronounced cerebellar ataxia (severe gait and coordination disturbances);
- Psychiatric and autonomic symptoms: intense shivering and inappropriate euphoria, which led to the disease being known as the "laughing death."
The prognosis for Kuru is uniformly fatal, with death occurring within 1 year of symptom onset.
Inherited Prion Syndromes
In addition to acquired forms, there are hereditary prion diseases with a strong familial aggregation.
Gerstmann-Sträussler-Scheinker (GSS) syndrome is an inherited autosomal dominant disorder with an incubation period ranging from 5 to 30 years. Clinical features include ataxia, cognitive decline progressing to dementia, hyporeflexia, and bulbar symptoms (dysphagia, dysarthria). The disease inexorably progresses, resulting in death 4–5 years after onset.
Fatal familial insomnia (FFI) is an autosomal dominant inherited prion disease. The hallmark manifestation is progressive intractable insomnia leading to a total disruption of circadian rhythms. This state is accompanied by sympathetic hyperactivity: patients exhibit tachycardia, hypertension, hyperthermia, and excessive sweating (hyperhidrosis). Later in the disease course, severe neurological deficits emerge, including myoclonus, ataxia, tremors, and hallucinations. The direct cause of death is typically progressive cardiovascular collapse.