Classification and discovery
Adenoviruses were first isolated in 1953 by W. Rowe and colleagues from the tonsil and adenoid tissue of children, which determined their name. This was made possible by the tissue culture method.
The Adenoviridae family is divided into 5 genera, but only one is of medical importance — Mastadenovirus (mammalian viruses). The remaining genera infect birds (Aviadenovirus), fish (Ichtadenovirus), and are also distinguished by genetic characteristics (Atadenovirus with an A-T rich genome and Siadenovirus with a sialidase gene). Human adenoviruses are divided into 6 groups (A–F).
Virion structure
The adenovirus is a simple virus (lacks a supercapsid) with a diameter of about 90 nm.
- Shape: regular icosahedron.
- Genome: double-stranded linear DNA associated with internal proteins.
- Capsid: consists of two types of capsomeres — hexons and pentons.
- Hexons form the faces of the icosahedron.
- Pentons (12 units) are located at the vertices. Each penton consists of a base and a glycoprotein fiber extending from it — the fiber.
Fibers play a crucial role: they act as an attachment protein for cell adhesion and perform the function of a hemagglutinin.
Life cycle and stability
Upon entering the body, the virus attaches to cell receptors with its fibers. Then it is engulfed via clathrin-mediated endocytosis. Deproteinization ("uncoating") of the virion begins in the cytoplasm and ends in the nucleus.
Synthesis of new viral DNA occurs in the nucleus. The assembly of new virions also takes place there, while capsid proteins are synthesized in the cytoplasm and transported to the nucleus. Mature viruses leave the cell, causing its lysis and degeneration. Up to 1 million new virions can be formed in a single cell.
Adenoviruses are characterized by high environmental stability. They are stable at pH from 5.0 to 9.0, can survive at 36–37 °C for up to a week, withstand lyophilization well, but are inactivated at 56 °C in 30 minutes.
Epidemiology and pathogenesis
The source of infection is a sick person. Incidence increases in the autumn-winter period, taking the form of outbreaks (especially in closed communities) or sporadic cases. Children under 14 years of age are most commonly affected.
Modes of transmission:
- Airborne droplet (respiratory);
- Contact;
- Fecal-oral (typical for enteric serotypes).
Primary replication of the virus occurs in the epithelium of the respiratory tract, conjunctiva, and lymphoid tissue. After the onset of symptoms, short-term viremia occurs. In addition to acute (productive) infection leading to cell death, adenoviruses can cause persistent (chronic asymptomatic) infection in lymphoid cells, and in animal experiments — even transforming (oncogenic) infection.
Clinical manifestations and diagnostics
Adenovirus infection can be asymptomatic, but most commonly manifests as ARVI. Pharyngoconjunctivitis is typical for early-age children, and gastroenteritis for younger children. In closed communities, complications such as pneumonia and encephalitis are possible. Hemorrhagic cystitis and severe corneal lesions are rare.
The diagnosis is confirmed in the laboratory by examining smears, conjunctival discharge, or feces. Virological methods (isolation on cell culture with cytopathic effect determination), serology (detecting an increase in antibody titer in paired sera using CF test, HI test, or IHA), and PCR to detect viral DNA are used. Post-infection immunity is type-specific and cell-humoral.
Briefly: answers to topical questions
What forms of persistent and oncogenic infection do adenoviruses cause?
Adenoviruses cause persistent and transforming (oncogenic) forms of infection.
- Persistent infection has a chronic, asymptomatic course with a slowed rate of virus reproduction, during which the cell survives ("repairs damage"). Latent infection in lymphoid cells is also possible.
- Transforming infection causes oncogenic cellular transformation. In experiments on newborn mice, rats, and hamsters, human adenoviruses lead to tumor formation.
The mechanism of oncogenicity is related to the ability of a segment of viral DNA to directly integrate into the target cell genome. One of the viral genes can act as a promoter for a cellular proto-oncogene. In addition, viral transactivator proteins can trigger oncogenic cellular transformation.
Class and exam questions
Where do adenoviruses replicate within the cell?
Viral DNA replication and assembly of new virions occur in the nucleus of the infected cell.
How do adenoviruses structurally differ from most ARVI viruses?
These are simple DNA-containing viruses. They lack a supercapsid (external lipid envelope) and contain double-stranded linear DNA rather than RNA.
Is there a vaccine against adenovirus?
Yes, there is an oral live trivalent vaccine (against serotypes 3, 4, and 7), but it is used strictly according to epidemiological indications, for example, in military collectives.
What else to study on this topic
- Characteristics of cell cultures for adenovirus isolation
- Mechanism of clathrin-mediated endocytosis
- Specifics of the immune response (cellular and humoral immunity)
- Oncogenic potential of adenoviruses in animal experiments
- Scheme of ELISA, IFA, and CFT assays for virus identification
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