Adenoviruses cause a variety of upper and lower respiratory tract diseases such as pharyngitis, conjunctivitis (“pink eye”), the common cold, and pneumonia. Keratoconjunctivitis, hemorrhagic cystitis, and gastroenteritis also occur. Some adenoviruses cause sarcomas in rodents. Table 38–1 describes some of the important clinical features of adenoviruses and compares them with features of the other two medically important viruses in this chapter, human papillomavirus (HPV) and parvovirus B19.
TABLE 38–1Clinical Features of DNA Nonenveloped Viruses |Favorite Table|Download (.pdf) TABLE 38–1 Clinical Features of DNA Nonenveloped Viruses
|Virus ||Mode of Transmission ||Types Cause Different Diseases ||Certain Types Cause Cancer ||Vaccine Available |
|Adenovirus ||Respiratory; fecal–oral ||Yes ||Yes, in animals but not humans ||Yes, but used only in military |
|Human papillomavirus ||Sexual; skin contact ||Yes ||Yes, in humans ||Yes |
|Parvovirus B19 ||Respiratory; transplacental ||No ||No ||No |
Adenoviruses are nonenveloped viruses with double-stranded linear DNA and an icosahedral nucleocapsid. They are the only viruses with a fiber protruding from each of the 12 vertices of the capsid. The fiber is the organ of attachment and is a hemagglutinin. When purified free of virions, the fiber is toxic to human cells.
There are 41 known antigenic types; the fiber protein is the main type-specific antigen. All adenoviruses have a common group-specific antigen located on the hexon protein.
Certain serotypes of human adenoviruses (especially 12, 18, and 31) cause sarcomas at the site of injection in laboratory rodents such as newborn hamsters. There is no evidence that adenoviruses cause tumors in humans.
Summary of Replicative Cycle
After attachment to the cell surface via its fiber, the virus penetrates and uncoats, and the viral DNA moves to the nucleus. Host cell DNA-dependent RNA polymerase transcribes the early genes, and splicing enzymes remove the RNA representing the introns, resulting in functional mRNA. (Note that introns and exons, which are common in eukaryotic DNA, were first described for adenovirus DNA.) Early mRNA is translated into nonstructural proteins in the cytoplasm. After viral DNA replication in the nucleus, late mRNA is transcribed and then translated into structural virion proteins. Viral assembly occurs in the nucleus, and the virus is released by lysis of the cell, not by budding.
Transmission & Epidemiology
Adenoviruses are transmitted by several mechanisms: aerosol droplet, fecal–oral route, and direct inoculation of conjunctivas by tonometers or fingers. The fecal–oral route is the most common mode of transmission among young children and their families. Many species of animals are infected by strains of adenovirus, but these strains are not pathogenic for humans.
Adenovirus infections are endemic worldwide, but outbreaks occur among military recruits, apparently as a result of the close living conditions that facilitate transmission. Certain serotypes are associated with specific syndromes ...