Enterovirus infections

Enteroviruses are a group of viruses that contain ribonucleic acid as their genetic material. They are members of the picornavirus family. The various types of enteroviruses that infect humans are referred to as serotypes, in recognition of their different antigenic patterns. The different immune response is important, as infection with one type of enterovirus does not necessarily confer protection to infection by a different type of enterovirus. There are 64 different enterovirus serotypes. The serotypes include polio viruses, coxsackie A and B viruses, echoviruses and a large number of what are referred to as non-polio enteroviruses.

The genetic material is enclosed in a shell that has 20 equilateral triangles (an icosahedral virus). The shell is made up of four proteins.

Despite the diversity in the antigenic types of enterovirus, the majority of enterovirus cases in the United States is due to echoviruses and Coxsackie B viruses. The infections that are caused by these viruses are varied. The paralytic debilitation of polio is one infection. The importance of polio on a global scale is diminished now, because of the advent and worldwide use of polio vaccines. Far more common are the cold -like or flu-like symptoms caused by various enteroviruses. Indeed, the non-polio enteroviruses rival the cause of the "common cold," the rhinovirus, as the most common infectious agent in humans. In the United States, estimates from the Centers for Disease Control are that at least ten to fifteen million people in the United States develop an enterovirus infection each year.

Enterovirus infection is spread easily, as the virus is found in saliva, sputum or nasal secretions, and also in the feces of those who are infected. Humans are the only known reservoir of enteroviruses. Following spread to water via feces, enteroviruses can persist in the environment. Thus, surface and ground water can be a source of enterovirus.

Spread of an enterovirus occurs by direct contact with the fluids from an infected person, by use of utensils that have been handled by an infected person, or by the ingestion of contaminated food or water. For example, coughing into someone's face is an easy way to spread enterovirus, just as the cold-causing rhinoviruses are spread from person to person. Fecal contact is most common in day care facilities or in households where there is a newborn, where diapers are changed and soiled babies cleaned up.

The spread of enterovirus infections is made even easier because some of those who are infected do not display any symptoms of illness. Yet such people are still able to transfer the infectious virus to someone else.

The common respiratory infection can strike anyone, from infants to the elderly. The young are infected more frequently, however, and may indeed be the most important transmitters of the virus. Common symptoms of infection include a runny nose, fever with chills, muscle aches and sometimes a rash. In addition, but more rarely, an infection of the heart (endocarditis), meninges (meningitis ) or the brain (encephalitis) can develop. In newborns, enterovirus infection may be related to the development of juvenile-onset diabetes, and, in rare instances, can lead to an overwhelming infection of the body that proves to be lethal.

Although enterovirus-induced meningitis is relatively rare, it afflicts between 30,000 and 50,000 people each year in the United States alone.

Evidence is accumulating that suggests that enterovirus infections may not only be short in duration (also referred as acute) but may also become chronic. Diseases such as chronic heart disease and chronic fatigue syndrome may well have an enterovirus origin. Moreover, juvenile diabetes may involve an autoimmune response.

The climate affects the prevalence of the infections. In tropical climates, where warm temperatures are experienced throughout the year, enterovirus infections occur with similar frequency year-round. But, in more temperate climates, where a shift in seasons is pronounced, enterovirus infections peak in the late summer and fall.

Another factor in the spread of enterovirus infections is the socio-economic conditions. Poor sanitation that is often coincident with lower economic standing is often associated with the spread of enterovirus infections.

Following inhalation or ingestion of enterovirus, viral replication is thought to occur mainly in lymphoid tissues of the respiratory and gastrointestinal tract that are in the immediate vicinity of the virus. Examples of tissues include the tonsils and the cells lining the respiratory and intestinal tracts. The virus may continue to replicate in these tissues, or can spread to secondary sites including the spinal cord and brain, heart or the skin.

As with other viruses, enteroviruses recognize a receptor molecule on the surface of host cells and attach to the receptor via a surface molecule on the virus particle. Several viral molecules have been shown to function in this way. The virus then enters the host cell and the genetic material is released into the cytoplasm (the interior gel-like region) of the host cell. The various steps in viral replication cause, initially, the host cell nucleus to shrink, followed by shrinkage of the entire. Other changes cause the host cell to lose its ability to function and finally to explode, which releases newly made virus.

Currently, no vaccine exists for the maladies other than polio. One key course of action to minimize the chances of infection is the observance of proper hygiene . Handwashing is a key factor in reducing the spread of many microbial infections, including those caused by enteroviruses. Spread of enteroviruses is also minimized by covering the mouth when coughing and the nose when sneezing.

See also Cold, common; Viruses and responses to viral infection