Adenoviruses

Subjects: virology, microbiology · Systems: virology, microbiology · Tags: virology, microbiology

Structure and Basic Properties

Adenoviruses are non-enveloped, double-stranded **DNA viruses ** with an icosahedral capsid. The absence of an envelope makes them resistant to drying, detergents, and even mild gastrointestinal conditions, which helps explain why some serotypes cause enteric disease. The capsid is studded with characteristic fiber proteins projecting from the vertices, which serve both as attachement proteins to host cell receptors and as antigens recognized by the immune system.

Epidemiology and transmission

Adenoviruses are ubiquitous, with over 50 serotypes infection humans. Transmission depends on the serotype and can occur via respiratory droplets, fecal-oral spread, contamined fomites, and direct contact. Because of their environmental stability, adenoviruses are well know for outbreaks in crowded settings, especially military barracks, schools, and daycare centers. Certain serotypes (e.g 40 and 41) are specifcally adapted to gastrointestinal infection and can causa diarrhea in children.

Pathogenesis and Immune Evasion

The virus attaches to host cells via its fiber proteins, commonly using the CAR (coxsackievirus and adenovirus receptor), and enters through receptor-mediated endocytosis. After uncoating, the viral genome travels to the nucleus, where transcription and replication occur. Adenoviruses are lytic: release of progeny virions kills the host cell, explaining the necrosis and inflammation seen in infection.

These viruses have also evolved immune evasion strategies. They produce proteins that block MHC class I presentation, limiting cytotixic T-cell recognition, and can inhibit apoptosis pathways, allowing for prolonged replication. Despide this, most infections are cleared in immunocompetent hosts, but in immunocompromised patients, adenovirus can become sever or disseminated.

Clinical Syndromes

The diversity of serotypes translates into a broad clinical spectrum:

Respiratory infections

• Cause pharyngitis, pharyngoconjunctival fever, and bronchitis.
• In military recruits, certain strains cause outbreaks of acute respiratory disease, characterized by fever, cough, and sore throat.

Ocular infections:

• Pharyngoconjunctival fever: a triad of pharyngitis, conjunctivitis, and fever, often seen in children exposed to contamined swimming pools.
• Epidemic keratoconjunctivitis: severe, painful conjunctivitis with corneal involvement, sometimes seen in adults exposed to contamined medical equipment or close contact environments.

Gastroenteritis

• Serotypes 40 and 41 cause watery diarrhea in infants and young children.

Hemorrhagic cystitis

• Especially in boys, adenovirus can cause acute hematuria due to bladder inflammation

Disseminated disease

• In immunocompromised patients (bone marrow transplant recipients, AIDS), adenovirus can cause pneumonia, hepatitis, or generalized disease, which may be fatal.

Diagnosis

In most cases, diagnosis is clinical. However, laboratory confirmation can be done with PCR, antigen detection, or viral culture. Histopathology may reveal characteristic smudge cells - epithelial cells with enlarged nuclei containing basophilic viral inclusions.

Treatment and Prevention

There is no widely used specific antiviral therapy for adenovirus. Supportive care is the mainstay. In sever cases among immunocompromised patients, antivirals such as cidofovir have been used experimentally.

Prevention relies on hygene and, in special populations, vaccination. A live oral adenovirus vaccine (against types 4 and 7) is used in the US military to prevent outbreaks of acute respiratory disease but is not available for civilian use.

Summary

Adenoviruses are hardy, non-enveloped DNA viruses with diverse serotypes capable of causing respiratory infections, conjunctivitis, gastroenteritis, hermorrhagic cystitis, and disseminated disease in immunocompromised hosts. They enter via fiber proteins, replicate in the nucleus, and are released by cell lysis. Immunity is type-specific but not cross-protective, which explains the frequency of reinfections. While most disease is self-limited, severe outcomes occur in immunosuppressed patients, highlighting the need for vigilance in those populations.

Disclaimer: For education only. Not medical advice; always follow your institution's guidance.