Essentials of Diagnosis
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.
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.
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.
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.
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).
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.
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.
BOX 27-1 Enterovirus Infection
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.
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.
ACUTE BENIGN PERICARDITIS
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.
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.
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
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
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.
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.