Adenoviruses
Adenoviruses are viruses that have twenty sides. As such they are called icosahedrons. The outer surface, the capsid, is made of particles of a protein. The protein is arranged in groups of six (hexagons) except at the twenty points where the sides meet (each is called an apex), where the particles are in a pentagon arrangement. A so-called penton fibre, which resembles a stick with a ball at the end, protrudes from each apex.
Adenoviruses contain DNA (deoxyribonucleic acid) as their genetic material. The DNA encodes 20 to 30 proteins, 15 of which are proteins that form the structure of the virus particle. Similar to other viruses, adenoviruses invade a host cell and use the host genetic machinery to manufacture new virus particles. The new viruses are released from the host cell.
Children suffer from adenovirus infections much more so than adults.
The viruses of this group infect the membranes that line the respiratory tract, the eyes, the intestines, and the urinary tract. The adenoviruses that infect humans usually cause mild maladies, including respiratory and intestinal illnesses and conjunctivitis (an inflammation of eye membrane, which is also commonly called "pink eye"). A more severe eye malady called keratoconjunctivitis can more widely infect the eye. The eye infections are very contagious and are typically a source of transmission of adenovirus from one person to another. Children can also develop a sore throat, runny nose, cough and flu-like illness. Bronchitis, an inflammation of the membranes lining the air passages in the lungs, can also result from adenovirus infection, as can an inflammation of the stomach called gastroenteritis. Urinary tract infections can cause pain and burning upon urination and blood in the urine. In dogs, adenovirus type 2 causes what is known as kennel cough. But curiously, the virus also protects dogs against hepatitis.
In the setting of the laboratory, some of the human strains of adenovirus can transform cells being grown in cell culture. Transformed cells are altered in their regulation of growth, such that the unrestricted growth characteristic of cancers occurs.
Adenoviruses have been known since the mid-1950s. They were first isolated from infected tonsils and adenoidal tissue in 1953. Within the next several years they had been obtained from cells involved in respiratory infections. In 1956, the multiple antigenic forms of the virus that had been discovered were classified as adenovirus. Then, in 1962, laboratory studies demonstrated that an adenovirus caused tumors in rodents. This was the first known human virus capable of inducing tumors in animals.
More recently, the basis of the tumor-inducing activity has been unraveled. Genes that are active early in the replication cycle of adenovirus produce proteins that interfere with host proteins that are known as anti-oncogenes. Normally, the anti-oncogen proteins are responsive to cell growth, and so act as a signal to the cell to halt growth. By disrupting the anti-oncogene proteins, this stop signal is eliminated, resulting in the continued and uncontrolled growth of the cell. A tumor is produced. Thus, adenoviruses have become important as one of the central triggers of cancer development.
Such cancers may be a by-product of adenovirus infections. These infections are not by themselves serious. Most tend to appear and run their course within a few weeks. The infections are fairly common. For example, most children will have antibodies to at least four types of adenovirus. Adenovirus gains entry through a break in the skin or are inhaled. The stick-and-ball appearing penton fibers may have a role in the attachment of the virus particle to a protein on the surface of the host epithelial cell.
Adenovirus infections have contributed to the spread of bacterial antibiotic resistance because of the overuse of antibiotics. The flu-like symptoms of some adenovirus infections can lead to the prescribing of antibiotics as a treatment. However, antibiotics are ineffective against viruses. But the circulating antibiotic can provide selective pressure on the development of resistant in bacterial populations.
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