Viruses of the family Adenoviridae infect various species of animals, including humans. Adenoviruses were first isolated in human adenoids (tonsils), from which the name is derived, and are classified as group I under the Baltimore classification scheme. Adenoviruses represent the largest nonenveloped viruses, because they are the maximum size able to be transported through the endosome (i.e. envelope fusion is not necessary). The virion also has a unique "spike" or fiber associated with each penton base of the capsid (see picture below) that aids in attachment to the host cell via the coxsackie-adenovirus receptor on the surface of the host cell. There are 51 immunologically distinct human adenovirus serotypes (6 species: Human adenovirus A through F) that can cause human infections ranging from respiratory disease (mainly species HAdV-B and C), and conjunctivitis (HAdV-B and D), to gastroenteritis (HAdV-F serotypes 40 and 41). Adenoviruses are unusually stable to chemical or physical agents and adverse pH conditions, allowing for prolonged survival outside of the body and water. Adenoviruses are primarily spread via respiratory droplets, however they can also be spread by fecal routes as well.
Most infections with adenovirus result in infections of the upper respiratory tract. Adenovirus infections often show up as conjunctivitis, tonsilitis (which may look exactly like strep throat and cannot be distinguished from strep except by throat culture), an ear infection, or croup. Adenoviruses can also cause gastroenteritis (stomach flu). A combination of conjunctivitis and tonsilitis is particularly common with adenovirus infections. Some children (especially small ones) can develop adenovirus bronchiolitis or pneumonia, both of which can be severe. In babies, adenoviruses can also cause coughing fits that look almost exactly like whooping cough. Adenoviruses can also cause viral meningitis or encephalitis. Rarely, adenovirus can cause cystitis (inflammation of the urinary bladder—a form of urinary tract infection—with blood in the urine).
Most people recover from adenovirus infections by themselves, but people with immunodeficiency sometimes die of adenovirus infections, and—rarely—even previously healthy people can die of these infections.
Adenoviruses are often transmitted by coughed-out droplets, but can also be transmitted by contact with an infected person, or by virus particles left on objects such as towels and faucet handles. Some people with adenovirus gastroenteritis may shed the virus in their stools for months after getting over the symptoms. The virus can be passed from one person to another through some sexual practices, and through water in swimming pools that do not have enough chlorine in them. As with many other illnesses, good handwashing is one way to lessen the spread of adenoviruses from one person to another. Heat and bleach will kill adenoviruses on objects.
In the past, US military recruits were vaccinated against two serotypes of adenotypes, with a corresponding decrease in illnesses caused by those serotypes. The vaccine is no longer manufactured, and there are currently no vaccines available to protect against the adenovirus. Good hygiene, including handwashing, is still the best way to avoid picking up the adenovirus from an infected person.
The adenovirus genome is linear, non-segmented double stranded (ds) DNA which is around 30–38 Kbp. This allows the virus to theoretically carry 30 to 40 genes. Although this is significantly larger than other viruses in its Baltimore group it is still a very simple virus and is heavily reliant on the host cell for survival and replication. An interesting feature of this viral genome is that it has a terminal 55 kDa protein associated with each of the 5' ends of the linear dsDNA, these are used as primers in viral replication and ensure that the ends of the virus' linear genome are adequately replicated.
Once the virus has successfully gained entry into the host cell the endosome acidifies, which alters virus topology by causing capsid components to disassociate. These changes as well as the toxic nature of the pentons results in the release of the virion into the cytoplasm. With the help of cellular microtubules the virus is transported to the nuclear pore complex whereby the adenovirus particle disassembles. Viral DNA is subsequently released which can enter the nucleus via the nuclear pore. After this the DNA associates with histone molecules. Thus viral gene expression can occur and new virus particles can be generated.
The adenovirus life cycle is separated, by the DNA replication process, into two phases: an early and a late phase. In both phases a primary transcript is generated which is alternatively spliced to generate monocistronic mRNAs compatible with the host’s ribosome, allowing for the products to be translated.
The early genes are responsible for expressing mainly non-structural, regulatory proteins. The goal of these proteins is threefold: to alter the expression of host proteins that are necessary for DNA synthesis; to activate other virus genes (such as the virus-encoded DNA polymerase); and to avoid premature death of the infected cell by the host-immune defenses (blockage of apoptosis, blockage of interferon activity, and blockage of MHC class I translocation and expression).
Some adenoviruses under specialized conditions can transform cells using their early gene products. E1a (binds Retinoblastoma tumor suppressor protein) has been found to immortalize primary cells in vitro allowing E1b (binds p53 tumor suppressor) to assist and stably transform the cells. Nevertheless, they are reliant upon each other to successfully transform the host cell and form tumors.
DNA replication separates the early and late phases. Once the early genes have liberated adequate virus proteins, replication machinery and replication substrates, replication of the adenovirus genome can occur. A terminal protein that is covalently bound to the 5’ end of the adenovirus genome acts as a primer for replication. The viral DNA polymerase then uses a strand displacement mechanism, as opposed to the conventional Okazaki fragments used in mammalian DNA replication, to replicate the genome.
The late phase of the adenovirus life cycle is focused on producing sufficient quantities of structural protein to pack all the genetic material produced by DNA replication. Once the viral components have successfully been replicated the virus is assembled into its protein shells and released from the cell as a result of virally induced cell lysis.
Adenoviruses are also known to cause respiratory infections in horses, cattle, pigs, sheep, and goats. Equine adenovirus 1 can also cause fatal disease in immunocompromised Arabian foals, involving pneumonia and destruction of pancreatic and salivary gland tissue.