Adenovirus

Structure of Adenovirus

Fig.1 Adenovirus structure. (Coughlan, 2020)

Adenovirus is a non-enveloped, double-stranded DNA virus. More than 100 human adenovirus types have been isolated which are classified into seven groups (A to G) based on their biochemical, immunological, and morphological criteria, though the true diversity may be much greater. Adenoviruses are about 90 to 100 nm in size and their structure consists of an icosahedral capsid which is made up of three major components: hexon located on the faces and edges of capsid; penton base located on the 12 fivefold apices; thin fibers attached on the penton base. Hexon assemblies are stabilized by minor capsid protein IIIa (IIIa), VI, and VIII (located in the capsid interior) and by IX (exposed on the capsid exterior). Components inside the capsid include the virion protease which plays a vital role in the assembly of the virion and the double-strand DNA genome associated with five polypeptides (terminal protein (TP), V, VII, Mu, and IVa2).

Pathogenesis

Adenoviruses are transmitted mainly by respiratory or fecal-oral contact from humans. They infect the conjunctiva or the nasal mucosa. They may multiply in respiratory transmission, pharynx, or small intestine, and then spread to preauricular, cervical, and mesenteric lymph nodes, where epithelial cells are infected. Most adenoviruses usually produce subclinical infection than overt symptoms. Some types are capable of establishing persistent asymptomatic infections in tonsils, adenoids, and intestines and shedding can occur for months or years in urine or stool. About 1/3 of human serotypes are commonly associated with human illnesses.

Epidemiology and Symptoms

Adenovirus is transmitted by direct contact and fecal-oral route, and occasionally waterborne transmission. Adenovirus primarily infects children and accounts less in adults. Adenovirus most commonly causes respiratory illness, but different subtypes can also cause cystitis, keratoconjunctivitis, primary pneumonia, and gastroenteritis. Recent research also suggests that subtypes Ad5, Ad36, and Ad37 may be associated with obesity in humans. These viruses are capable of infecting various organ systems, mostly the infections are asymptomatic or generally mild. However, in rare cases, severe diseases such as hepatitis, myocarditis, and nephritis can be observed. While in most cases, healthy people recover from adenovirus infection on their own, immunocompromised individuals can die as a result. If an ocular infection occurs, adenovirus may even cause epidemic keratoconjunctivitis, resulting in decreased visual acuity.

Prophylaxis and Treatment

Prevention of adenovirus infection relies on healthy habits applicable for the prevention of most viral infections. Like most other viral infections, Though adenoviruses have been identified for years, there is no FDA-approved drug for human adenovirus infection. The body’s immune system fights the infection and it resolves in three to five days in most individuals. Treatment is targeted towards relieving symptoms of the condition. Eye infections may benefit from topical corticosteroids to relieve symptoms and shorten the course of the disease. Hospitalization is usually required for severe pneumonia in infants and for keratoconjunctivitis (to prevent blindness).

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Reference

1. Coughlan, L. Factors Which Contribute to the Immunogenicity of Non-replicating Adenoviral Vectored Vaccines. Front Immunol. 2020, 11: 909.

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