Chapter 256 Coronaviruses
Coronaviruses are increasingly recognized as important pathogens of humans. They cause up to 15% of common colds and are implicated as causes of croup, asthma exacerbations, and lower respiratory tract infections, including bronchiolitis and pneumonia. In addition there is evidence that coronaviruses may be causes of enteritis or colitis in neonates and infants and may be underappreciated as agents of meningitis or encephalitis. The discovery that severe acute respiratory syndrome (SARS) is caused by a novel human coronavirus (SARS-CoV) has led to increased surveillance and the recognition of additional human coronaviruses, revealing that new coronaviruses enter human populations from zoonotic vectors such as bats.
Coronaviruses are enveloped viruses of medium to large size (80-220 nm) that possess the largest known single-stranded positive-sense RNA genomes. Coronaviruses derive their name from the characteristic surface projections of spike protein, which give a corona or crownlike appearance on negative-stain electron microscopy. Traditionally coronaviruses have been organized taxonomically into 3 groups (1-3) on the basis of antigenic relationships. This has been changed to a lettering system based on genomic phylogenetic relationships. Group 1 coronaviruses (alpha—α) includes human coronavirus 229E (HCoV-229E) and the recently identified HCoV-NL63; group 2 (beta—β) includes HCoV-OC43 and the recently discovered HCoV-HKU1; and group 3 (gamma—γ) includes avian coronaviruses but no known human viruses. The SARS epidemic resulted in discovery of a vast number of additional coronaviruses in bats and other mammals, resulting in new genus and species designations. Non-SARS human diseases are associated principally with HCoVs OC43, 229E, NL63, and HKU1.
Coronavirus infections have a worldwide distribution. Past seroprevalence studies demonstrated that that seropositivity to 229E and OC43 increases rapidly during early childhood so that by early adulthood, 90-100% of persons are seropositive. Although less information is available for HKU1 and NL63, available studies demonstrate similar patterns of seroconversion to these viruses during early childhood. Although some degree of strain-specific protection may be afforded by recent infection, re-infections are common and have been noted to occur despite the presence of strain-specific antibody. Attack rates are similar in different age groups. Although infections occur throughout the year, there is a peak during the winter and early spring for each of the HCoVs. In the USA, outbreaks of OC43 and 229E have occurred in 2- to 3-yr alternating cycles. Independent studies of viral etiologies of upper and lower respiratory infections (URIs and LRIs) during the same period but from different locations and countries have confirmed that all known human coronaviruses have a worldwide distribution. Both previous culture and PCR multiplex studies have demonstrated that coronaviruses often occur as co-infections with other respiratory viruses, including respiratory syncytial virus (RSV), adenovirus, rhinovirus (RV), or human metapneumovirus (HMPV).
Volunteer studies demonstrated that OC43 and 229E are transmitted predominantly through the respiratory route. Droplet spread appears to be most important, although aerosol transmission may also play a role. Coronaviruses have been reported to cause minimal cytopathology. Studies with SARS-CoV in human airway epithelial cell cultures indicate that ciliated cells are principal targets for infection and that infected ciliated cells may be directly extruded or lost from the infected monolayer. Thus it may be that cytopathology of other human respiratory coronaviruses is manifested as individual cell infection and loss. The amount of pathology due to direct cell damage is not well-defined, but infection with OC43 and 229E is associated with the elaboration of cytokines, including interleukin-8 (IL-8) and interferon-γ (IFN-γ), suggesting that symptoms may be at least partially due to the host immune response. In experimentally infected volunteers, serum-specific immunoglobulin (Ig) A and IgG antibody levels peak 12-14 days after infection but decline rapidly thereafter. At 1 yr following experimental infection there is only partial protection against re-infection with the homologous strain.
Human coronaviruses OC43 and 229E have been conclusively demonstrated in human volunteer studies to cause respiratory disease, with these viruses and newly identified NL63 and HKU1 strongly implicated in colds, bronchiolitis, pneumonia, and croup. The possible role of human coronaviruses in gastrointestinal and neurologic disease is less well-defined and remains to be proven.
Up to 50% of respiratory tract infections with OC43 and 229E may be asymptomatic. Coronaviruses account for up to 15% of common colds. Cold symptoms caused by human coronaviruses are indistinguishable from those caused by rhinoviruses and other respiratory viruses. The average incubation period is 2-4 days, with symptoms typically lasting 4-7 days. Rhinorrhea, cough, sore throat, malaise, and headache are the most common symptoms. Fever may be more common than previously thought, occurring in up to 60% of cases. Coronavirus NL63 is implicated as a cause of croup in children <3 yr of age. Coronavirus infections have been linked to episodes of wheezing in asthmatic children, albeit at a lower frequency and severity than observed with rhinovirus and respiratory syncytial virus infections. Lower respiratory tract infections, including bronchiolitis and pneumonia, have also been reported in immunocompetent as well as immunocompromised children and adults. HCoV-HKU1 was originally identified in two adults with pneumonia. As with RSV or RV, coronavirus detection in URIs frequently may be associated with acute otitis media and isolation from middle ear fluid.
Although the precise role of coronaviruses in human gastrointestinal disease remains controversial, there is some evidence to support such a role, particularly in young children. Coronavirus-like particles have been detected by electron microscopy in the stools of infants with nonbacterial gastroenteritis. In addition, several outbreaks in neonatal intensive care units of gastrointestinal disease characterized by diarrhea, bloody stools, abdominal distention, bilious gastric aspirates, and classic necrotizing enterocolitis have also been associated with the presence of coronavirus-like particles in stools. In older children and adults, coronavirus-like viruses have been observed with similar frequency in symptomatic and asymptomatic individuals.
Coronaviruses are well-known causes of neurologic disease in other animals, but the role of coronaviruses in causing neurologic disease in humans remains controversial. The viruses have been detected by culture, in situ hybridization, and reverse transcriptase polymerase chain reaction (RT-PCR) in brain tissue from a few patients with multiple sclerosis. However, coronavirus RNA has also been recovered from the spinal fluid and brain tissue of adults without neurologic disease. Human coronavirus OC43 has been detected by RT-PCR in the spinal fluid and nasopharynx of 1 child with acute disseminated encephalomyelitis.
In the past, specific diagnostic tests for coronavirus infections were not available in most clinical settings. The use of conserved PCR primers for coronaviruses in multiplex RT-PCR viral diagnostic panels will likely allow more sensitive detection of the viruses in a variety of clinical settings. Virus culture remains the major challenge for most coronaviruses, because all of the known non-SARS human coronaviruses grow poorly from primary isolates in culture. Even laboratory-adapted or recombinant human coronaviruses may demonstrate poor growth under defined conditions, limiting studies of pathogenesis and disease in defined model systems. Serodiagnosis with complement fixation, neutralization, hemagglutination inhibition, enzyme immunoassay, or the Western blot test has been used in the research setting.
The vast majority of coronavirus infections are self-limited. There are no available antiviral agents for clinical use against coronaviruses, although strategies targeting conserved coronavirus proteases have been shown to block replication of the virus in vitro. A role for vaccines for OC43, 229E, HKU1, and NL63 is unlikely, because infections are rarely life-threatening and re-infection is the rule, even in the presence of natural immunity from infection with the homologous strain.
256.1 Severe Acute Respiratory Syndrome–Associated Coronavirus
The SARS outbreak of 2003 was contained and ultimately halted through a remarkable cooperative effort among countries around the world; the occurrence of several laboratory-acquired cases as well as sporadic cases likely associated with animal-to-human transmission in 2004 demonstrates the potential threat posed by trans-species transmission of coronaviruses. The identification of bats as likely reservoirs of SARS-like coronaviruses as well as large numbers of coronaviruses related to all other mammalian groups suggests mechanisms for ongoing introduction into human populations.
The causative agent of SARS is a novel coronavirus, referred to as the SARS-associated coronavirus. SARS-CoV was initially the sole member of a new subgroup (2b) of coronaviruses, but studies in bats worldwide have identified many new coronaviruses populating known and new groups 1a, 1b, and 2a-d. The detection of SARS-like coronaviruses in a live animal market in Guangdong province in Southern China, along with the finding of serologic evidence of exposure in food handlers and other persons whose occupation increased their exposure to these and other exotic animals held in the same market, suggests that these markets may have facilitated the spread of SARS-CoV to humans. Subsequent studies identified SARS-like coronaviruses in fecal specimens from asymptomatic Chinese horseshoe bats that are very closely related, but not direct precursors, to, SARS-CoV. Thus, although bats are thought to be probable reservoir hosts for SARS-like precursors, the precise antecedent to SARS-CoV remains to be identified.
There have been no identified natural or laboratory-acquired cases of SARS-CoV since 2004, but the mechanisms of introduction, spread, and disease remain important for potential new animal-to-human transmission and disease. The primary mode of SARS-CoV transmission occurred through direct or indirect contact of mucous membranes with infectious droplets or fomites. Aerosol transmission was less common, occurring primarily in the setting of endotracheal intubation, bronchoscopy, or treatment with aerosolized medications. Fecal-oral transmission did not appear to be an efficient mode of transmission but may have occurred because of the profuse diarrhea observed in some patients. The seasonality of SARS-CoV remains unknown. The SARS-CoV is not highly infectious, with on average only 2-4 secondary cases resulting from a single infected adult. However, a small number of infected individuals, the so-called super-spreaders, transmitted infection to a much larger number of persons. In contrast, persons with mild disease, such as children <12 yr of age, rarely transmitted the infection to others. Infectivity correlated with disease stage; transmission occurred only during symptomatic disease. During the 2003 outbreak, most individuals with SARS-CoV infection were hospitalized within 3-4 days of symptom onset. Consequently, most subsequent infections occurred within hospitals and involved either health care workers or other hospitalized patients.
A viral replication phase and an immunologic phase were the hallmarks of SARS-CoV infection in teenagers and adults. During the viral replication phase there was a progressive increase in viral load that reached its peak during the 2nd wk of illness. The appearance of specific antibody coincided with peak viral replication. The clinical deterioration that typifies the 2nd and 3rd wk of illness was characterized by a decline in the viral load and tissue injury proposed to have resulted from cytokine-mediated immune response. The explanation for milder clinical disease in children <12 yr of age has not been determined.
Seroepidemiologic studies suggest that asymptomatic SARS-CoV infections were uncommon. The incubation period ranged from 1 to 14 days, with a median of 4-6 days. The clinical manifestations were nonspecific, most commonly consisting of fever, cough, malaise, coryza, chills or rigors, headache, and myalgia. Coryza was more common in children <12 yr of age, whereas systemic symptoms such as headache, myalgia, chills, and malaise were seen more often in teenagers. Some young children had no respiratory symptoms. Gastrointestinal symptoms, including diarrhea and nausea or vomiting, occurred in up to a third of cases. The clinical course of SARS-CoV infection varied with age. Adults were most severely affected, with initial onset of fever, cough, chills, myalgia, malaise, and headache. Following an initial improvement toward the end of the 1st week, fever recurred and respiratory distress developed, with dyspnea, hypoxemia, and diarrhea. These symptoms progressed in 20% of patients to acute respiratory distress syndrome (ARDS) and respiratory failure. In contrast, children < 12 yr of age had a relatively mild nonspecific illness, with only a minority experiencing significant lower respiratory tract disease and illness typically lasting < 5 days. There were no deaths or ARDS in children < 12 yr of age from SARS-CoV infection. Adolescents (12-18 yr) manifested worsening severity in direct correlation to increasing age; respiratory distress and hypoxemia were observed in 10-20% of patients, one third of whom required ventilatory support.
The laboratory abnormalities and radiographic findings observed in SARS-CoV–-infected children cannot be differentiated from those associated with other commonly encountered viral illnesses. These abnormalities include lymphopenia in 70-90% of patients as well as leukopenia, neutropenia, and thrombocytopenia. As with clinical illness, laboratory abnormalities were more common and persisted longer in teenagers than in young children. Early radiographic changes in children ranged from normal to ground-glass opacities or consolidation, and characteristic viral interstitial infiltrates were not common. Progression to an ARDS-like pattern with widespread ground-glass opacities and patchy consolidation was characteristic of SARS-CoV in adolescents and young adults requiring ventilatory support.
The diagnosis of SARS-CoV infection can be confirmed by serologic testing, through detection of viral RNA using RT-PCR, or through isolation of the virus in cell culture. Serology is the most reliable diagnostic method, with sensitivity and specificity approaching 100%; detection of IgM antibody, seroconversion from negative to positive result, or a fourfold or greater rise in IgG titer is indicative of recent infection. The disadvantage of serology is that antibody is not detectable until 10 days after the onset of symptoms, and IgG seroconversion may be delayed for up to 4 wk. The mainstay of early diagnosis is RT-PCR. Nasopharyngeal aspirates, plasma or serum, and stool are the preferred samples, although other body fluids and tissues may also contain the virus. Viral culture is not recommended as a 1st-line diagnostic test because of its poor sensitivity and the requirement for biosafety level 3 containment.
The case fatality rate from SARS-CoV infection during the 2003 outbreak was 10-17%. No pediatric deaths were reported. The estimated case fatality rate according to age varied from <1% for those <20 yr of age to >50% for those >65 yr of age. The long-term prognosis of children recovering from SARS-CoV infection appears to be favorable. Persistent respiratory or exercise intolerance has not been reported, although, as a group, SARS-CoV–infected children have been noted to have lower peak oxygen consumption than healthy controls.
Treatment of SARS-CoV infection is primarily supportive. The role of antiviral and immune-modulating agents remains inconclusive, largely because none of these therapies has been evaluated in properly conducted randomized controlled trials. Ribavirin was extensively used during the 2003 outbreak but is of questionable benefit given its poor in vitro activity against SARS-CoV at clinically relevant concentrations. Systemic corticosteroid therapy was temporally associated with clinical improvement in some patients and should be considered in children with moderate to severe hypoxemia. In another small, open-label, nonrandomized pilot study, interferon-α was associated with more rapid resolution of oxygen requirements and radiographic abnormalities. Human monoclonal antibodies derived from SARS patients have shown broad neutralization effect against early and late epidemic strains of SARS-CoV.
An effective vaccine is highly desirable but is not yet available. For any future occurrence of SARS or a new novel zoonotic-human coronavirus, a likely approach to vaccines would be the use of spike protein delivered as recombinant protein or via viral or DNA vectors. This approach appears to be effective against closely related strains of SARS-CoV, but not necessarily early animal or human variants. Thus approaches for rapid development of stably attenuated live viruses or broadly immunogenic and cross-protective protein immunogens is a key area for future research. Although SARS-CoV demonstrated characteristics of reproduction number and symptomatic transmission that made it controllable by public health measures including quarantine, these characteristics cannot be assumed for any future novel human coronavirus. Thus, strategies for rapid recovery, testing, and development of vaccines and neutralizing human monoclonal antibodies may be essential.
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