Adenoviruses are members of a large group of enteroviruses. Other members include the the rotavirus, calicivirus, and coronavirus families. Adenoviruses are noted for causing diseases of the upper respiratory tract and conjunctiva, as well as some diarrheal diseases in infants. “Adeno” is Latin for ‘gland,’ referring to the fact that these viruses were isolated from the tonsils and adenoid glands of humans. They can be present in normal healthy people as a latent infection, and may induce malignancies in certain species. Only military personnel take measures to prevent adenovirus infections. A live attenuated vaccine against types 4, 7, and 21 are given orally to control epidemics of acute respiratory disease caused by these viruses. The parvovirus is also a member of this group, and is known to cause such diseases as the Fifth’s Disease.
The adenoviruses are widespread in nature. About forty-seven distinct serotypes have been isolated in humans, each designated by a number. The adenoviruses have the ability to remain sequestered in lymphatic tissue,s as well as in the intestinal tract long after primary exposure. Adenoviruses have a cubical symmetry with 252 capsomers and no envelope. These viruses contain ds-DNA and survive a long time once expelled from the host.
Some infections adenoviruses are responsible for include:
- Pharyngoconjunctival fever is a disease characterized by fever, conjunctivitis (inflammation of the eyelids), and a sore throat. Outbreaks are associated with swimming pools, summer camps, and small lakes, and usually involve children and young adults. The incubation period is six to nine days.
- Keratoconjunctivitis is an infection transmitted by ophthalmological instruments that have not been disinfected properly or through contaminated medications used during eye exams. The inflammation starts in the eyelid and spreads to the cornea, leading to impaired vision. The incubation period is eight to ten days.
- Besides their extensive multiplication in the respiratory tract, adenoviruses are also able to grow in and be shed from the intestinal tract during bouts of gastroenteritis.
- Adenoviruses 40 and 41 are the major causes of diarrhea in children, and are a common cause of death of children in underdeveloped countries.
- Fifths Disease (Erythema infectiosum) was so named because in 1899, it was the fifth of six most common childhood diseases found to cause a rash. It is caused by Parvovirus B19, and is spread through mouth and nose secretions much like the common cold. Fifths Disease can also be transmitted through blood to blood contact. Symptoms begin with a headache, mild fever, tiredness, and minor aches and pains. In two or three days, the cheeks will turn red with a rash that lasts about three days. By the time the rash appears, the child is feeling better. In 1981, a connection was discovered between the B19 strain and an aplastic crisis, a serious condition that affects people with sickle cell anemia when they are exposed to this virus. People with sickle cell anemia do not get Fifths Disease, but rather a more serious infection from the virus called aplastic crisis, in which the bone marrow stops making red blood cells. This disorder causes high fever, weakness, and extreme pain that requires hospitalization and blood transfusions. For children, it is a relatively mild disease, but, for some adults, the symptoms are more debilitating, with severe arthritic-like joint pain. Pregnant women may be more prone to stillbirths or spontaneous abortions. There is no specific treatment for the disease. Only symptomatic complications are treated.
Caliciviruses have been isolated from humans and other animals. They are RNA viruses that do not have an envelope and have cup-shaped depressions on their surface. Caliciviruses cause gastroenteritis in persons of all ages, and account for about 3% of all diarrhea in children requiring hospitalization. The highest rates of infection occur between three months and six years of age, with most people having been infected by the age of twelve. These viruses have been associated with sharp outbreaks of gastroenteritis in institutional settings.
The Norwalk agent is thought to be a calicivirus not responsible for infant deaths. It was discovered about the same time as the rotavirus, in the 1970’s. The Norwalk group of viruses cannot be cultivated so are not well characterized. What is known is that they are round with no envelope but the type of nucleic acid is not known.
Coronaviruses were first discovered in the mid-1960’s and shown to be the frequent cause of common cold-type infections. The structure of these viruses resembles that of the paramyxoviruses, except that the projections (peplomers) are more prominent, having knoblike structures on the ends. When viewed through the electron microscope, these projections give the appearance reminiscent of the solar corona – hence the name Coronaviridae for the family name. Two different serotypes have been identified.
Reoviruses are of the family Reoviridae, made up of ds-RNA molecules and possessing a double-layered protein coat. The first of this family was isolated from the respiratory and intestinal tracts of humans in 1959. At that time, they were not associated with any specific disease, so were referred to as “orphan” viruses. The acronym Reo came from respiratory, enteric, orphan viruses. These viruses are now classified as the genus Reovirus, which has three serotypes that infect humans and other animals.
A second genus called Oribivirus is an arbovirus widespread in insects and transmitted to humans or animals by the bite of an infected tick. Colorado tick fever virus is the only known oribivirus to cause disease in humans. The symptoms are much like those of denge fever, except that no rash is produced.
A third genus is called Rotavirus, and was discovered in 1973. The double-layered protein coat of these viruses gives a characteristic appearance of a wheel when viewed with an electron microscope. The term rotavirus is derived from the Latin word ‘rota’ which means wheel. Seven serogroups of rotaviruses have been identified, but most human infections are caused by group A. Other groups are frequently associated with diarrheal diseases. Rotaviruses are now recognized as the most common cause of severe viral diarrhea in infants and young children in all countries, with an estimated 150 million cases of severe diarrhea in children and about one million deaths. Rotavirus illness is characterized by vomiting, diarrhea, and dehydration. Treatment is mainly replacing fluids and electrolytes, but hospitalization is often required. Rotaviruses are found in drinking water, food, swimming areas, improperly cooked and unwashed foods, and areas with poor sanitation. They are transmitted via the fecal/oral route. About 10% of the Gulf War troops came down with Norwalk infections after eating unwashed fruits and vegetables from the local markets.