Enteroviruses are so named because they reproduce initially in the gastrointestinal tract after infection occurs. Despite, this, they usually do not lead to intestinal symptoms; rather it is their spread to organs, such as the nervous system, heart, skin, and others that causes disease. Enteroviruses are part of a larger group of viruses known as Picornaviruses. The word comes from the combination of "pico" (Spanish, meaning "a little bit"), and RNA (ribonucleic acid, an important component of genetic material).
There are four groups of enteroviruses; 1) Coxsackievirus, 2) Echovirus, 3) ungrouped Enterovirus, 4) Y Poliovirus. This review will deal with the first three of these groups. Poliovirus is discussed elsewhere.
Viruses are generally divided into those that use DNA (desoxyribonucleic acid) or RNA as their genetic material; all enteroviruses are RNA viruses. They are found worldwide, but infection is more common in areas of poor hygiene and overcrowding.
Although most cases of enterovirus do not produce symptoms, some 5-10 million individuals in the United States each year suffer from one of the enteroviral diseases. Illness is more common in the very young. While there are close to seventy different strains of enteroviruses, over 70% of infections are caused by only 10 types.
The virus is most commonly transmitted by the fecal-oral route ((contamination of fingers or objects by human waste material); in some instances transmission is through contaminated food or water. Passage of some strains of virus by way of air droplets can lead to respiratory illness. Infection of fetuses by way of the placenta has also been documented. Breast milk contains antibodies which can protect newborns.
The incubation period for most enteroviruses ranges from 2-14 days. In areas of temperate climate, infections occur mainly in the summer and fall.
Causes & symptoms
Enteroviruses are believed to be the cause of at least 10 distinct illnesses. Once they enter the body, they multiply in the cells that line the gastrointestinal tract, and eventually reach sites of lymphatic tissue (such as the tonsils). While most of these diseases are of short duration and do not cause significant injury, some can produce severe illness.
The main syndromes caused by the various enteroviruses are the following:
- Summer grippe (nonspecific febrile illness). This is the commonest syndrome, and is characterized by flu-like symptoms of fever, headache, and weakness, which typically last three to four days. Many patients also develop upper respiratory symptoms and some nausea and vomiting. One of the major ways to distinguish this disease from influenza, is the fact that grippe most often occurs in the summer.
- Generalized disease of the newborn is a potentially serious infection in which infants from one week to three months of age develop a syndrome which can be difficult to distinguish from a severe bacterial infection. Fever, irritability and decreased responsiveness or excessive sleepiness are the major symptoms. Inflammation of heart muscle (myocarditis), low blood pressure, hepatitis, and meningitis sometimes complicate the illness.
- Aseptic meningitis encephalitis is a well known syndrome caused by this group of viruses. In fact, enteroviruses are responsible for over 90% of cases of aseptic meningitis, and most often hits children and young adults. Headache, fever, avoidance of light and eye pain are characteristic. Drowsiness may be prominent, and other symptoms include sore throat, cough, muscle pain, and rash. Occasionally, not only the meninges, which is the covering around the brain and spinal cord is infected, but also brain tissue itself, producing encephalitis. The illness resolves after about a week or so, and permanent damage is unusual. Enteroviruses can also produce the Guillian-Barré syndrome, which involves weakness and paralysis of the extremities and even the muscles of respiration.
- Pleurodynia (Bornholm's disease) is due to viral infection and inflammation of the chest and abdominal muscles used for breathing. Pain occurs as acute episodes, lasting 30 minutes or so. Coxsackie B virus is the usual cause of the illness.
- Myocarditis and/or pericarditis involves infection of the heart muscle (myocardium) and the covering around the heart (pericardium). Infants and young adults are the most susceptible, and for some reason, over two-thirds of cases occur in males. The disease usually begins as an upper respiratory tract infection with cough, shortness of breath and fever. Chest pain, increasing shortness of breath, irregularities of cardiac rhythm and heart failure sometimes develop. Some patients wind up with long term heart failure if the heart muscle is significantly affected.
- Exanthems is the medical term for rashes, and enterovirus is the number one cause of summer and fall rashes in children. They occur anywhere on the body, and often resemble diseases such as measles.
- Hand-foot-and-mouth disease occurs initially as a sore throat (often involving the tongue as well), and is followed by a rash on the hands, and sometimes the feet. The rash often forms small blisters, which lead to ulcers. Symptoms generally resolve within a week. A specific Coxsackievirus (A16) is the most frequent cause of this highly infectious disease.
- Herpangina is most often caused by one of the Coxsackie A viruses, and appears as the acute onset of fever and sore throat. This last symptom is particularly severe, as the virus produces multiple ulcers in the throat. Swallowing becomes very painful; symptoms can persist for several weeks.
- Acute hemorrhagic conjunctivitis involves viral infection of the conjunctiva, which is a covering around the eye. Pain, blurred vision, aversion to light, and a discharge from the eye are the main symptoms. Headache and fever occur in about one in five patients. The disease runs its course in about 10 days.
A number of other illnesses have been attributed to enteroviruses, including pneumonia and other respiratory infections, myositis or muscle inflammation, arthritis, and acute inflammation of the kidneys. It is clear then that these viruses produce a number of various illnesses, most often in younger age groups.
In the majority of cases, diagnosis is based on the characteristic symptoms that the virus produces (such as the chest pain in pleurodynia). Rarely is it necessary to identify a specific strain of virus causing the illness. It is more important to be certain that the infection is due to a virus which does not require treatment with antibiotics.
Culture, or growing the organism outside of the body, is helpful only when obtained from areas that tend to indicate recent infection, such as from swollen joints, cerebrospinal fluid or blood. Cultures from other areas, such as the throat, can be misleading. This is because the virus may remain for long periods of time in places with a large amount of lymphatic tissue. As a rule, cultures done early in the illness are more likely to identify the virus.
New techniques that involve identification of viral genetic material (PCR) are useful in certain cases, but are not indicated for routine testing.
As noted above, enterovirus is capable of attacking many different organs and producing a variety of symptoms. Most infections are mild and improve without complications, and require no specific therapy. When the virus attacks critical organs however, such as the heart, respiratory muscles, nervous system, etc., then specialized care is often needed.
As of 1998, no effective antiviral medication for enterovirus has undergone investigation in patients, though some drugs appear promising for the future. In some patients who are unable to produce antibodies (hypogammaglobunemia), administrating antibodies themselves is helpful.
The overall outlook for enterovirus infection depends on the organs involved, and the immune condition of the individual patient. Unless vital organs are involved or immunity is abnormal, infection causes few problems. On the other hand, patients who have diseases that affect antibody production can develop chronic infection of the brain or meninges.
In the hospital setting, the best means of avoiding transmission of infection is the use of good hand-washing practices and other appropriate precautions (gowns and gloves for hospital staff). The virus is found in feces for up to one week after infection; therefore precautions which isolate waste material (enteric precautions) will help decrease the chance of spreading the illness.
- Proteins that are formed by the body and play a role in defense against infection.
- A medication that is designed to kill or weaken bacteria.
- Outer covering of the spinal cord and brain. Infection is called meningitis, which can lead to damage to the brain or spinal cord and lead to death.
Gale Encyclopedia of Medicine. Gale Research, 1999.