Four Cardinal Features
Slow viral infections represent a special group of diseases distinguished by a specific complex of features. To diagnose and classify a condition into this category, four cardinal features must always be present in combination.
First, there is an extraordinarily prolonged latent stage. A prolonged incubation period can last from several months to several years. During this time, the patient experiences no signs of illness, yet the virus is already present in the tissues.
Second, there is a specific localization of the process. The viruses causing this group of diseases exhibit a marked tropism for neural structures. Damage predominantly involves the organs and tissues of the central nervous system (CNS), whereas the peripheral nervous system, gastrointestinal tract, or cardiovascular system are not primary targets.
Third, the course of the pathology is described as slow but steady progression of symptoms. As soon as clinical signs of damage appear, the patient's condition begins to steadily deteriorate.
Fourth, the outcome of such infections is strictly determined. Unfortunately, the final cardinal feature is an inevitable fatal outcome.
Etiology and Transformation of Infections
The etiology of these conditions has an important feature. Pathogens provoking slow infections are not always exotic agents. Often, the diseases can be caused by conventional viruses that are well-known as agents of typical acute infections.
Transformation of an acute form of infection into a slow one is regularly observed in humans. A striking example is the measles virus, which is capable of persisting and eventually causing subacute sclerosing panencephalitis (SSPE). Similarly, the rubella virus can lead to progressive congenital rubella, as well as act as the causative agent of progressive rubella panencephalitis.
Classical Animal Model
A deep understanding of such infections required animal studies. The classical model was developed during the study of the Maedi-visna virus. By nature, this pathogen belongs to the retrovirus group.
The host for the Maedi-visna virus is sheep, which develop clinical manifestations that have become the standard for describing this group of diseases. Infection leads to progressive pneumonia accompanied by the typical picture of a slow infection.
Spectrum of Human Pathogens
Medical science describes a wide range of human pathogens leading to the development of associated slow diseases. Different virus families are capable of causing specific pathologies:
- Paramyxoviruses: The measles virus provokes subacute sclerosing panencephalitis (SSPE).
- Togaviruses: The rubella virus causes progressive congenital rubella and progressive rubella panencephalitis.
- Flaviviruses: Tick-borne encephalitis virus is capable of transforming into a progressive form of tick-borne encephalitis (unlike the acute febrile, poliomyelitic, or meningeal forms).
- Herpesviruses: Herpes simplex virus in some cases causes subacute herpetic encephalitis.
- Retroviruses: HIV leads to the development of HIV/AIDS. Other representatives, HTLV-1 and HTLV-2, are associated with T-cell lymphoma.
- Polyomaviruses: The JC polyomavirus causes progressive multifocal leukoencephalopathy (PML) in humans.