Pathogenesis and Clinical Findings of Adenoviral Diseases in Humans
Adenoviruses affect human epithelial cells, causing cell aggregation and enlargement usually without lysis. Moreover, they suppress immune defensive factors. For instance, adenoviruses inhibit interferon action by blocking interferon-induced genes transcription. Viral proteins also inhibit cytotoxic activity of TNF-alpha. Finally, they slow down HLA-Ag I class expression on the membranes of infected cell, preventing cell-mediated cytolysis.
Adenoviruses are highly versatile in their pathogenicity. Various human serotypes affect respiratory and gastrointestinal tracts, eyes, and urinary bladder. They propagate in the epithelial cells and spread to regional lymph nodes, where they persist. The leading sites of adenoviral persistence are adenoids and tonsils. Also adenoviruses replicate in the epithelium of intestinal tract.
Adenoviral infections are spread by airborne and fecal-oral transmission. Infants and young children are the most susceptible to the infection.
Infectious dose of adenoviruses is very low – dozens or even several viral particles.
The sources of infections are sick or convalescent individuals. Shedding of adenoviruses after the infection may be long (for 3-6 weeks).
About 50% of cases of human adenoviral infections are mild or asymptomatic. Nevertheless, they provoke a great number of acute and chronic human diseases. Among them are:
– acute febrile pharyngitis;
– pharyngoconjunctival fever;
– acute respiratory diseases in children and adults;
– adenoviral pneumonia;
– adenoviral gastroenteritis;
– eye infections;
– acute hemorrhagic cystitis;
– urethritis and cervicitis.
Acute febrile pharyngitis resembles other similar viral respiratory infections.
Pharyngoconjunctival fever manifests by the symptoms both of pharyngitis and conjunctivitis. It is often caused by adenoviruses of serotypes 3 and 7; outbreaks of pharyngoconjunctival fever are called “swimming pool conjunctivitis”.
Adenoviruses of types 3 and 7 as well as the limited number of other serotypes cause adenoviral pneumonia. It comprises about 10% of all pneumonia cases affecting children. Lethality of adenoviral pneumonia in early postnatal period may exceed 5%.
Adenoviruses of types 40 and 41 mainly affect gasrointestinal tract, being responsible for about 10% of cases of viral gastroenteritis in infants.
Eye adenoviral infections include follicular conjunctivitis and epidemic keratoconjunctivitis. The latter is predominantly caused by 8, 19, and 37 viral types.
Viruses of types 11 and 21 provoke acute hemorrhagic cystitis in children, while type 37 causes urethritis and cervicitis.
Systemic adenoviral infection in infants leads to serious complications affecting parenchymatous organs (hepatitis, nephritis), heart (myocarditis), CNS (meningoencephalitis).
After most adenoviral diseases strong long-lasting immunity is developed. It is maintained by type-specific neutralizing antibodies. Group-specific antibodies can’t prevent the relapse of infection.