Current Diagnosis & Treatment in Infectious Diseases

Section IV - Viral Infections

27. Enteroviruses

1. Lawrence Drew MD, PhD

Essentials of Diagnosis

• Isolation of enterovirus in tissue culture.
• Detection of antigen or antibody not practical for routine diagnosis.
• Detection of enterovirus in cerebrospinal fluid (CSF) by polymerase chain reaction.
• CSF usually has lymphocytic pleocytosis with normal glucose and protein.
• Typical illness is febrile rash, macular or vesicular (hand, foot, mouth syndromes).
• Most clinical illness occurs in patients < 20 years old.

General Considerations

Enteroviruses are one of three types of picornaviruses that cause disease in humans. As the name indicates, picornaviruses are small (pico) ribonucleic acid (RNA) viruses that have a naked capsid structure. The family includes > 230 members divided into five genera but only three—enteroviruses, rhinoviruses, and hepatoviruses (hepatitis A virus)—cause human disease. These genera can be distinguished by their stability at pH 3, optimum temperature for growth, mode of transmission, and the diseases they cause. Rhinoviruses are discussed in Chapter 28 and the hepatitis A virus is discussed in Chapter 39.

Nearly 70 serotypes of human enteroviruses exist, and they are divided into three subgroups: polio-, coxsackie-, and echoviruses. The capsids of these viruses are very stable in harsh environmental conditions (eg, sewage systems) and in the gastrointestinal tract, which facilitates their transmission by the fecal-oral route. Although they may initiate their infection in the gastrointestinal tract, the enteroviruses rarely cause enteric disease. Instead they cause myriad illnesses, especially diseases of the central nervous system and other systemic diseases. Several different disease syndromes may be caused by one enterovirus serotype, and several different serotypes may cause the same disease. The most well-known and well-studied picornavirus is poliovirus.

Coxsackieviruses are named after the town of Coxsackie, New York, where the viruses were first isolated. They are divided into two groups, A and B, on the basis of certain biologic and antigenic differences. These two groups are further subdivided into numeric serotypes by additional antigenic differences.

The name echovirus is derived from “Enteric Cytopathic Human Orphan,” because these agents were not thought to be associated with clinical disease. Thirty-one serotypes are now recognized. These viruses have a greater tendency than polioviruses to affect the meninges and cause meningitis, but a lesser tendency to infect anterior horn cells.

1. Epidemiology.The incubation period for enterovirus disease is usually 2–10 days but may be longer depending on the virus, the target tissue, and the age of the individual. Enteroviruses are highly contagious; poor sanitation and crowded living conditions foster transmission of enteroviruses, and sewage contamination of water supplies can result in enterovirus epidemics.

Humans are the major natural hosts of enteroviruses, and there is no evidence of spread from animals to humans. The enteroviruses are primarily spread person to person by enteric routes but may also be spread in droplets and cause respiratory infections. The most frequently isolated enteroviruses are coxsackieviruses A9, A16, and B1 to B5; and echoviruses 6, 9, 11, 16, and 30. These viruses are found worldwide, and each year there is a tendency for one of these to be the dominant circulating virus. Disease is most frequent in persons < 20 years of age but, as with poliovirus infection, it is generally less severe in children. However, coxsackie B virus and some of the echoviruses can cause severe disease. Secondary infections occur in ≤ 70% of susceptible individuals living in households where the viruses have caused infection. Summer and fall are the major seasons for contracting enterovirus disease.

7. Microbiology.As with all picornavirus RNA, the RNA of the enteroviruses is surrounded by a very small icosahedral capsid ~ 30 nm in diameter. The genome of these viruses resembles messenger RNA (mRNA). It is a single strand of (+)-sense RNA of ~ 7.2–8.5 kilobases. It encodes a polyprotein that is proteolytically cleaved to produce the enzymatic and structural proteins of the virus. In addition to the capsid proteins, these viruses encode at least one protease and an RNA-dependent RNA polymerase.

The specificity of enterovirus interaction with cellular receptors is the major determinant of their tissue tropism and the diseases they cause. The VP₁proteins at the vertices of the virion contain a canyon into which the receptor binds. The receptors for polioviruses have recently been identified as tissue-specific intercellular adhesion molecules (ICAMs), which are members of the immunoglobulin superfamily. Several serotypes of coxsackievirus recognize intercellular adhesion molecule 1 (ICAM-1). ICAM-1 is expressed on epithelial cells, fibroblasts, and endothelial cells. Poliovirus binds to a molecule of similar structure. The cells in which the poliovirus receptor is expressed correlate directly with the organs infected by poliovirus infection.

On binding to the receptor, the enteroviruses are internalized by receptor-mediated endocytosis, and the virions dissociate in the acidic environment of the endosome, releasing the genome into the cytoplasm. The genome then binds to ribosomes, and a polyprotein is synthesized within 10–15 min of infection. The polyprotein is initially cleaved by cellular proteases until a viral protease is generated to cleave the rest of the polyprotein.

The RNA-dependent RNA polymerase generates a negative-strand RNA template from which the new mRNA genome and templates can be synthesized. The amount of viral mRNA increases rapidly in the cell, with the number of viral RNA molecules reaching 400,000/cell.

Cellular RNA and protein synthesis are inhibited during infection by several enteroviruses; a viral protease blocks translation of cellular mRNA, and permeability changes induced by enteroviruses reduce the ability of cellular mRNA to bind to the ribosome. Viral mRNA also competes with cellular mRNA for the factors required in protein synthesis. These activities contribute to the cytopathic effect of the virus on the target cell.

As the viral genome is being replicated and translated, the structural proteins are cleaved from the polyproteins. After insertion of the viral genome, assembly of viral RNA into the viral capsid occurs in the cytoplasm, and the virion is released when lysis and destruction of the cell occur.

1. Pathogenesis.Differences in pathogenesis of the enteroviruses mainly result from differences in tissue tropism of the various subgroups. Poliovirus has been studied extensively and is the prototype for the pathogenesis of the enteroviruses.

The upper respiratory tract, the oropharynx, and the intestinal tract are the portals of entry for the enteroviruses. The virus initiates replication in the mucosa and lymphoid tissue of the tonsils and pharynx and later infects the gut. The virions are impervious to stomach acid, proteases, and bile and infect lymphoid cells of Peyer's patches and the intestinal mucosa. Primary viremia spreads the virus to receptor-bearing target tissues, where a second phase of viral replication may occur, resulting in symptoms and a secondary viremia. Virus shedding from the oropharynx can be detected for a short time before symptoms begin, whereas virus production and shedding into stool may last for ≤ 4 months, even in the presence of a humoral immune response.

The nature of the enterovirus disease is determined by the tissue tropism of the virus. Poliovirus has one of the narrowest tissue tropisms, recognizing a receptor expressed on anterior horn cells of the spinal cord, dorsal root ganglia, motor neurons, and few other cells. Coxsackieviruses and echoviruses recognize receptors expressed on more cell types and tissues and cause a broader repertoire of diseases. Coxsackievirus and echovirus receptors may be found in the central nervous system and on heart, lung, pancreatic, and other cells. Differences in the susceptibility to and severity of poliovirus and coxsackievirus infection with age may also be attributed to differences in distribution and amount of receptor expression. Adults are generally more susceptible to serious disease with poliovirus, whereas newborns experience the most serious symptoms from coxsackie B and echovirus infections.

Most enteroviruses are cytolytic, replicating rapidly and causing direct damage to the target cell. Histologic examination reveals cell necrosis and mononuclear cell infiltrates.

The production of antibody is the major protective immune response to enteroviruses. Secretory antibody can prevent the initial establishment of infection in the oropharynx and gut, and serum antibody prevents viremic spread to the target tissue. However, in the individual with poor immune response, antibody production may be too late to block infection of the target tissue, and patients with a deficiency of antibody production may have persistent enterovirus infections. Serum-neutralizing antibody generally develops 7–10 days after the initial onset of infection.

Cell-mediated immunity is not likely to be involved in protection but may play a role in pathogenesis. T cells appear to contribute to coxsackie B virus-induced myocarditis in mice. Indeed, certain late enterovirus syndromes (eg, myocarditis, myositis, and nephritis) may be immunologically mediated rather than resulting from direct viral invasion. Immune responses to the virus may cross-react with cellular antigens.

 

CLINICAL SYNDROMES

The clinical syndromes of the enteroviruses are determined by several factors, including the viral serotype, infecting dose, tissue tropism, portal of entry, age, sex, pregnancy status, and state of health (Box 27-1).

POLIOVIRUS INFECTION

Polio vaccines and global eradication efforts have eliminated poliomyelitis from the Western Hemisphere and are expected to eliminate “wild” polio infections from the world in the near future. However, vaccine-associated cases of polio do occur.

Clinical Findings

1. Signs and Symptoms.Poliovirus may cause one of four outcomes, depending on the progression of the infection:

• Asymptomatic illness results if the virus is limited to infection of the oropharynx and the gut. At least 90% of poliovirus infections are asymptomatic.
• Abortive poliomyelitis, the minor illness, is a nonspecific febrile illness occurring in ~5% of infected individuals. Symptoms of fever, headache, malaise, sore throat, and vomiting occur within 3–4 days of exposure.
• Nonparalytic poliomyelitis or aseptic meningitis occurs in 1–2% of patients with poliovirus infections. The virus progresses into the central nervous system and the meninges, causing stiff neck and back pain in addition to the symptoms of minor illness.
• Paralytic polio, the major illness, occurs in 0.1–2.0% of persons with poliovirus infections and is the most severe outcome. Major illness follows 3–4 days after minor illness has subsided, thereby producing a biphasic illness. In this disease the virus spreads from the blood to the anterior horn cells of the spinal cord and the motor cortex of the brain. The severity of the paralysis is determined by the extent of the neuronal infection and the neurons affected. Spinal paralysis may involve one or more limbs, whereas bulbar (cranial) paralysis may involve a combination of cranial nerves and even the medullary respiratory center.

Paralytic poliomyelitis is characterized by an asymmetric flaccid paralysis with no sensory loss. The degree of paralysis may vary from involving only a few muscle groups (eg, one leg) to complete flaccid paralysis of all four extremities. The paralysis may progress over the first few days and may result in complete recovery, residual paralysis, or death. Most recovery occurs within 6 months.

Bulbar poliomyelitis can be more severe and may involve the muscles of the pharynx, vocal cords, and respiratory system, resulting in death in 75% of patients.

1. Laboratory Findings.The peripheral blood white blood cell (WBC) level is only moderately elevated with a relative lymphopenia. The CSF WBC count is elevated with a lymphocytosis, and the protein level is > 35 mg/100 mL.
2. Differential Diagnosis.With the disappearance of poliomyelitis from the Western Hemisphere, it is more likely that a patient presenting with febrile flaccid paralysis is suffering from the Guillain-Barré syndrome, whose etiology is unknown. The CSF examination shows elevation of protein but no pleocytosis. Alternatively there are rare instances of paralytic illness associated with entero-, echo-, or coxsackieviruses. Rheumatic fever, cytomegalovirus (CMV) polyradiculopathy, bacterial meningitis, and infectious mononucleosis are other illnesses that may be mistaken for poliomyelitis.
3. Complications.Pneumonia, urinary tract infections, and decubiti are early complications, whereas the post-polio syndrome is a late complication. This entity is characterized by a recrudescence of increased fatigue and impaired motor function many years after the acute poliomyelitis illness.

BOX 27-1 Enterovirus Infection

Pediatric Adult More Common · Febrile rash · Meningitis · Meningitis Less Common · Hand-foot-mouth · Herpangina syndrome · Neonatal “sepsis” · Pleurodynia · Myopericarditis

COXSACKIEVIRUS & ECHOVIRUS INFECTIONS

Several clinical syndromes may be caused by either coxsackievirus or echovirus (eg, aseptic meningitis) (see Box 27-1), but certain illnesses are especially associated with coxsackieviruses. For example, coxsackie A viruses are highly associated with herpangina, whereas myocarditis and pleurodynia are more frequently caused by coxsackie B serotypes.

1. HERPANGINA

Clinical Findings

1. Signs and Symptoms.This syndrome is inappropriately named because it has no relation to herpesvirus.

Rather, it is caused by several types of coxsackie A virus. Fever, sore throat, pain on swallowing, anorexia, and vomiting characterize herpangina. The classic finding is vesicular, ulcerated lesions around the soft palate and uvula. Less typically the lesions may affect the hard palate.

1. Laboratory Findings.The coxsackie A virus can be recovered from throat, feces, or vesicular lesions. There are no consistent blood abnormalities.
2. Differential Diagnosis.Primary herpes simplex virus (HSV) stomatitis may resemble herpangina, but the latter is usually confined to the posterior pharynx, whereas HSV also affects the anterior mouth, gums, and lips.
3. Course.The disease is self-limited and requires only symptomatic management.
4. PLEURODYNIA (BORNHOLM DISEASE)

Clinical Findings

1. Signs and Symptoms.This syndrome, also known as the devil's grip, is an acute illness caused by coxsackie B virus. Patients have sudden onset of fever and unilateral low thoracic, pleuritic chest pain, which may be excruciating. Abdominal pain and even vomiting may also occur. Although a pleural friction rub may be heard, the physical findings of pneumonia are not present. Muscles on the involved side may be extremely tender. The pain tends to appear and disappear abruptly and repeatedly, for example, hourly, and can be very severe.
2. Laboratory Findings.Chest x-ray films are almost always normal, as are blood leukocyte counts. The virus can be recovered from throat samples, stool samples, or both.
3. Course.Pleurodynia lasts an average of 4 days and may relapse after the patient has been asymptomatic for several days. High fever and the waxing and waning of the pain help to distinguish pleurodynia from other pleuritic processes, for example, pulmonary embolism. The absence of lung abnormalities eliminates pneumonia as a diagnosis.
4. MENINGITIS

Clinical Findings

1. Signs and Symptoms.Viral, or aseptic, meningitis is an acute febrile illness accompanied by headache and signs of meningeal irritation, including nuchal rigidity, Kernig's or Brudzinski's sign, or both. Petechiae or skin rash may occur in patients with enteroviral meningitis. Both echo- and coxsackieviruses cause viral “aseptic” meningitis.
2. Laboratory Findings.Examination of the CSF reveals a predominantly lymphocytic pleocytosis, but very early in the disease, polymorphonuclear leukocytes (PMNs) may be more numerous. CSF glucose levels are usually normal but may be slightly low. CSF protein levels are normal to slightly elevated. Blood counts and chemistry examinations are normal. The causative virus can be recovered from throat, stool, and CSF, but culture of enteroviruses requires an average of 5 days. Polymerase chain reaction of CSF is becoming a most useful rapid diagnostic test.
3. Differential Diagnosis.The major differential diagnosis is bacterial meningitis, which is characterized by more severe illness and prostration. The CSF reveals elevated PMNs, low glucose, high protein, and a positive Gram stain. Other viruses, for example, mumps, Epstein-Barr (EB), and lymphocytic choriomeningitis (LCM) viruses, may cause an indistinguishable meningitis, but associated clinical features help to identify these etiologies.
4. Course.Unless associated encephalitis (meningoencephalitis) exists, recovery is uneventful. If encephalitis is present, permanent neurologic sequelae may ensue.
5. ENTEROVIRAL EXANTHEMS

Clinical Findings

1. Signs and Symptoms.This syndrome may occur in patients infected with either echo- or coxsackieviruses and is usually accompanied by fever. The rash is usually maculopapular but occasionally may appear as petechial or even vesicular.
2. Laboratory Findings.The virus can be recovered from throat samples, stool samples, or both.
3. Differential Diagnosis.The petechial type of eruption must be differentiated from that of meningococcemia. The child with enteroviral infection is not as ill and does not usually have a PMN leukocytosis in blood like the child with meningococcemia.
4. HAND-FOOT-AND-MOUTH DISEASE

Clinical Findings

16. Signs and Symptoms.This syndrome is a vesicular exanthem usually caused by coxsackievirus A16. The name is descriptive, since the main features of this infection are vesicular lesions of the hands, feet, mouth, and tongue. The oral lesions are identical to those of herpangina.
17. Laboratory Findings.There are no consistent laboratory abnormalities, but the virus can be recovered from throat and stool samples in an average of 5 days.
18. Course.The patient is mildly febrile, and the illness subsides in a few days.

 

ACUTE BENIGN PERICARDITIS

Clinical Findings

1. Signs and Symptoms.The chest pain, pericardial friction rub, and clinical illness often follow a preceding febrile illness, which may have been associated with a rash.
2. Laboratory Findings.EKG reveals changes characteristic of pericarditis, but there are no consistent blood abnormalities. Pericardial fluid cultures may be positive for virus, but throat and stool cultures are often negative due to the time lag between the viral illness and the development of this late complication.
3. Differential Diagnosis.Pericarditis is usually a disease of young adults but may be seen in older individuals, in whom the distinction from myocardial infarction may be difficult. Usually the symptoms are similar, but in pericarditis, fever may greater and more prolonged than in a patient with myocardial infarction. Other causes of pericarditis, for example, bacterial causes, must be considered, including complications of pneumonia and tuberculosis.
4. MYOCARDITIS

Clinical Findings

1. Signs and Symptoms.Myocarditis is caused by coxsackie B virus and occurs in older children and adults but is most threatening in newborns. Neonates with these infections have febrile illnesses and the sudden, unexplained onset of heart failure. Cyanosis, tachycardia, cardiomegaly, and hepatomegaly occur. In older children or adults the cardiac involvement is usually late, ie, days to weeks after initial symptoms.
2. Laboratory Findings.Electrocardiographic changes are those found in patients with myocarditis, but there are no characteristic blood abnormalities. Viral culture of pericardial fluid or myocardial biopsy may be positive but is often negative due to the late-onset nature of this complication.
3. Course.Myocarditis is usually self-limited but may be fatal due to arrhythmia or heart failure, especially in newborns.
4. Complications.Myocarditis may progress to chronic myocardiopathy.
5. OTHER SYNDROMES

Echoviruses may also produce severe disseminated infection in infants. Enterovirus 70 and a variant of coxsackie A24 have recently been associated with an extremely contagious ocular disease, acute hemorrhagic conjunctivitis. The infection causes subconjunctival hemorrhages and conjunctivitis. The disease has a 24-h incubation period and resolves within 1 or 2 weeks.

Respiratory disease, hepatitis, and diabetes are some additional syndromes attributed to enteroviruses. Coxsackieviruses A21 and A24 and echoviruses 11 and 20 can cause coldlike symptoms if the upper respiratory tract becomes infected. Enterovirus 72, or hepatitis A virus, causes hepatitis A (see Chapter 39). Coxsackie B infections of the pancreas have been suspected to cause insulin-dependent diabetes because of the destruction of the islets of Langerhans.

Diagnosis

Poliovirus. Poliovirus grows well in monkey kidney tissue culture, and the virus may be isolated from the pharynx during the first few days of illness and from the feces for ≤ 30 days. The CSF is rarely positive for the virus, although a pleocytosis of 25–300 leukocytes usually occurs. Neutrophils may predominate early, especially in aseptic meningitis. Protein and glucose levels in CSF are usually normal or only slightly abnormal. Serologic tests can document seroconversion to one of the three poliovirus serotypes.

Coxsackievirus and Echovirus. Coxsackievirus and echovirus can usually be isolated from the throat and stool during acute infection and often from CSF in patients with meningitis. Virus is rarely isolated in pericarditis or myocarditis, since the symptoms occur several weeks after the initial infection. The coxsackie B viruses can be grown on primary monkey or human embryo kidney cells. Many coxsackie A virus strains do not grow in tissue culture and must be grown in suckling mice.

Polymerase chain reaction analysis of CSF appears to be more sensitive than culturing and is becoming the diagnostic procedure of choice for documenting enteroviral meningitis.

Serologic confirmation of poliovirus infection can be made by detection of specific immunoglobulin M (IgM) or a fourfold increase in antibody titer between acute illness and convalescence; however, the many serotypes for echo- and coxsackieviruses make this approach impractical.

Treatment

No specific antiviral therapy is approved for enterovirus infections, but pleconaril, a drug active against picornaviruses in vitro, is being evaluated for the treatment of enteroviral meningitis and viremia. Immune globulin has been used in immunocompromised patients with chronic enteroviral infection of the central nervous system (Box 27-2) and can diminish viral titers in body fluids.

Prevention & Control

Poliovirus. The prevention of paralytic poliomyelitis is one of the triumphs of modern medicine. In the western world, complete control has been achieved by the use of vaccines, and worldwide eradication of poliomyelitis is expected soon.

BOX 27-2 Treatment of Enterovirus Infection

First Choice Immune globulin for chronic infection of CNS in incubation period Pediatric Considerations · See above

Two types of poliovirus vaccine exist: a formalin-inactivated product known as the inactivated, killed, or Salk vaccine, and an attenuated one known as the live, oral, or Sabin vaccine (Box 27-3). Both vaccines can induce a protective antibody response.

Oral vaccine is attenuated (ie, rendered less virulent) by passage in cell cultures. Attenuation yields a virus capable of replicating in the oropharynx and intestinal tract and of being shed in feces for weeks, but not of being invasive. The remote potential for reverting to virulence and causing paralytic disease is the major drawback of the live vaccine and is estimated to occur in 1 per 4 million doses administered (versus 1 in 100 of those infected with “wild” poliovirus).

BOX 27-3 Control of Enterovirus Infection

Prophylactic Measures · Poliovirus vaccine, which induces a protective antibody response · No vaccines exist for coxsackie-viruses or echoviruses. Transmission can presumably be reduced by improvements in hygiene and living conditions Isolation Precautions · Careful handwashing after patient contact · Stool precautions

The risk of vaccine-associated paralytic poliomyelitis is increased in immunocompromised individuals and is more likely to occur in susceptible adults than susceptible children. Since the live vaccine strain may spread to close, especially household, contacts (a virtue in achieving mass immunization), vaccine-associated poliomyelitis may occur in those contacted by the original recipient, rather than the actual vaccine recipient. Because of the above considerations, killed vaccine is now the recommended prophylaxis, and oral live polio vaccine is used only for those refusing injections or for those traveling to areas with endemic poliomyelitis.

Coxsackievirus and Echovirus. No vaccines exist for these viruses (see Box 27-3). Transmission can presumably be reduced by improvements in hygiene and living conditions.

REFERENCES

Expanded Program on Immunization, Pan American Health Organization: Certification of poliomyelitis eradication in the Americas. Morbid Mortal Weekly Rep 1994; 43:720.

Ramlow J et al: Epidemiology of the post-polio syndrome. Am J Epidemiol 1992;136:769.

Rotbart HA: Nucleic acid detection systems for enteroviruses. Clin Microbiol Rev 1991;4:156.