Bronchiolitis is an acute viral infection of the lungs, mainly affecting babies and young children, in which the bronchioles (the airways branching off the bronchi) become inflamed. A common cause is the respiratory syncytial virus (RSV).
Symptoms of bronchiolitis include rapid breathing, a cough, and fever. No treatment may be necessary but, in severe cases, hospital admission is necessary so that oxygen therapy can be given. If treatment is prompt, recovery is usually within a few days. Antibiotic drugs may be given to prevent a secondary bacterial infection.
Bronchiolitis in detail - technical
Bronchiolitis is a common respiratory tract infection usually affecting infants and young children during annual epidemics. It is characterized by wheeze, respiratory distress, and poor feeding. Respiratory syncytial virus (RSV) is the most common cause for bronchiolitis and is amongst the most important pathogens causing respiratory infection in infants worldwide.
The healthcare burden of bronchiolitis is large, due to large numbers of hospitalized infants and the high risk of nosocomial spread during epidemics. Most children will suffer only mild, short-lived symptoms. A small proportion will need admission to hospital, where treatment is generally supportive until the illness resolves. Some will require ventilatory support for which mortality can be up to 10%. Infants at high risk of severe disease include those born prematurely, those with chronic lung disease, and immunocompromised infants. RSV infects ciliated epithelial cells, causing sloughing of the epithelium, cytokine and inflammatory mediator release, increases in mucus production, and interstitial edema.
Clinical manifestations of bronchiolitis are a combined result of viral toxicity and the immune response to infection. Innate immune responses are important to the pathogenesis of bronchiolitis, as severe infection tends to occur after maternal antibody protection has waned and before the infant’s adaptive immune responses have matured. Immunoprophylaxis, in the form of intramuscular anti-RSV IgG1, is effective in reducing rates of hospitalization for high-risk infants.
Bronchiolitis, meaning inflammation of the bronchioles, is a clinical complex usually affecting children less than 2 years old. It is characterized by wheezing, dyspnea, tachypnea, and poor feeding. The clinical characteristics, originally termed ‘congestive catarrhal fever’, have been recognized for over 150 years. It was not until the late 1950s that the epidemiology and viral etiology of the illness were described.
Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis and is amongst the most important pathogens causing respiratory infection in infants worldwide. Epidemics occur during the winter months in temperate climates and during the rainy season in tropical climates. In the US, more than 120 000 infants are hospitalized annually with RSV infection, with more than 200 deaths attributed to RSV lower respiratory tract disease. Total hospital charges for RSV-coded primary diagnoses over the 4-year period from 1997 to 2000 have been estimated at US$2.6 billion. Within hospitals, there is a high risk of nosocomial spread of RSV during epidemics, which can put vulnerable infants at risk of severe disease. Infection in infancy also predisposes children to the development of recurrent wheeze, thus increasing the long-term healthcare burden of this disease.
Prophylaxis is available as a recombinant monoclonal antibody, but is expensive and currently limited to infants at highest risk of severe disease. Development of an effective vaccine would have a dramatic effect on morbidity and healthcare costs, but this is unlikely to occur in the near future. Treatment is generally supportive until the infection runs its natural course.
RSV is the primary cause of bronchiolitis. Seroepidemiological studies show that over 90% of children are infected with RSV by 2 years of age. Infection is spread by droplet exposure or direct contact with secretions. Fomites are infectious outside the body for up to 12 h. RSV was first isolated from chimpanzees in 1955 and was originally called chimpanzee coryza agent (CCA). Subsequently, RSV has been shown to cause 75–85% of cases of bronchiolitis. Other pathogens known to cause bronchiolitis are shown in bullet list 1 below.
Bullet list: Bronchiolitis pathogens
- Respiratory syncytial virus
- Parainfluenza viruses (1,2,3)
- Influenza viruses (A and B)
- Human metapneumovirus
- Coronavirus (NL-63)
- Bordatella pertussis
- Mycoplasma pneumoniae
Historically, in over 10% of cases of bronchiolitis, no pathogen is detected. More recently, polymerase chain reaction (PCR) techniques have identified other pathogens, such as human metapneumovirus (hMPV) and coronavirus, which may have gone undetected previously. Co-infection with hMPV and RSV may be associated with a higher risk of severe disease.
RSV is a negative-sense single-stranded RNA pneumovirus from the Paramyxoviridae family. The viral genome contains only 10 genes that encode for 11 viral proteins. Nine of these are structural proteins and glycoproteins that form the viral coat and bring about attachment to host cells, whilst the other two direct viral replication within the host cell.
Based on immunological techniques, two different strains of the virus have been identified: A and B. Subsequently, it has been demonstrated that the two strains are also genetically distinct. Most variability between strains is due to differences in the amino acid sequence of the G protein on the viral coat. Strain A is seen more commonly in the UK and North America. There is some evidence that strain A results in more severe infections. The F protein, responsible for fusion of infected cells with adjacent uninfected cells, facilitates cell-to-cell transmission of the virus and results in epithelial cell syncytia (appearance of apparently large multinucleate cells), which give the virus its name.
RSV initially causes inflammation of the bronchioles and destruction of ciliated epithelial cells. The submucosa becomes edematous, whilst cellular debris and mucus form plugs within the bronchioles. Neutrophils and alveolar macrophages are the predominant inflammatory cells in the small airways. There is a peribronchiolar infiltration with lymphocytes that is associated with edema fluid accumulating within the alveoli. Severe disease involves destruction of the respiratory epithelium, parenchymal necrosis, and formation of hyaline membranes. The bronchiolar epithelium regenerates after 3–4 days, but cilia do not reappear until up to 15 days after the illness has resolved.
The spectrum of severity for infected children is wide. After an incubation period of 1–2 days, most infants exhibit predominantly upper respiratory tract symptoms. These include rhinorrhea, cough, and low-grade fever that often persists for several weeks, before resolving, without any other symptoms. Thirty to fifty percent of infants will progress to develop lower respiratory tract signs and symptoms within 2–3 days of infection. Infants may develop a rapid respiratory rate, wheeze, and other signs of respiratory distress (subcostal and intercostal recession, tracheal tug, and nasal flaring). Apnea at presentation is common, especially in infants less than 6 weeks old, those with a history of apnea of prematurity, and those with congenital heart disease.
Approximately 2–3% of infected children require admission to hospital. Reasons for admission include hypoxia, inadequate fluid intake, apnea, and signs of imminent respiratory failure. A more severe cough, pharyngitis, conjunctivitis, and otitis media may also be present. Irritability, signs of cardiovascular compromise, lethargy, and exhaustion are late signs and may be followed by respiratory arrest if no clinical interventions are made. One to two percent of infants hospitalized with bronchiolitis require ventilatory support and mortality in this group may be up to 10%. Premature birth, chronic lung disease of prematurity, congenital heart disease, cystic fibrosis, and immunodeficiency all predispose infants to a high risk of severe disease. In addition to this, boys are more likely to be hospitalized than girls. Infants exposed to high levels of particulate air pollution or cigarette smoke are also more likely to suffer from severe bronchiolitis.
A chest radiograph will typically show hyperinflation, a flattened diaphragm, and patchy peribronchial infiltration. The chest radiograph can be normal even in severe cases. A reduction in oxygen saturations is associated with increased respiratory rate and is an accurate indicator for reduced gas exchange that is seen in severe disease. Arterial or capillary blood gas analysis is helpful in assessing changes in the clinical status, particularly if ventilatory support is being considered.
Most infants, given adequate supportive care, improve clinically in 3–4 days. Within 2 weeks of the height of their illness most infants will have a normal respiratory rate and any radiological abnormalities will have cleared. However, up to 20% of infants may suffer from persistent wheezing and airway obstruction for several months, especially those that required hospital admission. In the longer term, airway reactivity is increased in children after infection and persists for at least 5–8 years. It is still uncertain whether RSV causes asthma in later life. RSV infection does not confer lasting immunity to reinfection. However, subsequent infections are usually less severe and are more likely to be limited to the upper respiratory tract.
RSV first infects the ciliated epithelial cells of the respiratory tract. Subsequent disease manifestations are caused by a combination of viral cytotoxicity and the immune response to infection.
On contact with epithelial cells, the G protein attaches the virus to epithelial cells and allows viral RNA and enzymes to enter the cell and initiate production of new viral RNA and proteins. The F protein mediates the formation of syncytia that enable the virus to spread rapidly. Multiple new viruses are assembled within cells, which are ultimately destroyed, leading to sloughing of the epithelium. As the cells are destroyed they release proinflammatory substances and cytokines that increase capillary permeability and attract inflammatory cells such as neutrophils (interleukin-8 (IL-8) secretion), macrophages (IL-1b,MIP-1a), eosinophils (RANTES, MIP- 1a), and natural killer cells (interferon (IFN)-a/b). Some secreted mediators, such as IL-9 stimulate mucus production, whilst others, such as leukotrienes, cause bronchoconstriction. Pulmonary surfactant function is also inhibited. Increased production and impaired clearance of cellular debris and mucus results in plugging of the small airways, air trapping, and atelectasis. For infants with preexisting poor lung function, such as those with chronic lung disease, this airway obstruction is likely to have more clinical significance and cause severe disease.
The immune response to RSV includes innate and adaptive components. Innate immune responses are now recognized as more important than previously thought and are capable of influencing the subsequent adaptive immune response.
Innate immunity is important in defense against RSV and other viruses that cause bronchiolitis as the adaptive immune responses of infants are relatively immature. The innate immune response is rapid and recruits effector molecules and phagocytic cells to the site of infection through the release of cytokines. Pulmonary surfactant provides early defense from viral infection. Surfactant proteins A and D are members of the collectin family that can bind to the surface of a number of pathogens including RSV. The virus is thus opsonized and capable of binding to complement and receptors on phagocytic cells (neutrophils and macrophages), eosinophils, and natural killer cells. Toll-like receptors (TLRs) are the most important of these receptors. They are present on epithelial cells and phagocytic cells and recognize distinctive nonhuman pathogen-associated molecular patterns (PAMPs). Although their precise role is still being elucidated, TLRs are important receptors to the innate and adaptive immune responses to RSV. TLR2 and TLR4 are receptors for surfactant proteins A and D, both of which bind to RSV. TLR4 and CD14 have been shown to be co-receptors for the RSV F protein. Genetic polymorphisms in the TLR4 gene are associated with a risk of severe disease. TLR3 recognizes double-stranded RNA in endosomes of the cell and initiates an immune response to RSV.
Pulmonary dendritic cells also express TLRs and are the predominant antigen-presenting cells in RSV infection. Circulating dendritic cells are recruited to sites of viral replication in the lungs. They produce large amounts of type I interferon (IFN-a and -b) and stimulate subsequent T-lymphocyte responses to RSV infection. Regulatory T lymphocytes (Treg) are known to dampen inflammatory responses and could be stimulated by RSV either via TLRs expressed on their cell surface or via pulmonary dendritic cells. Therefore, they provide a way by which the T-lymphocyte response to RSV can be influenced by both innate and adaptive immunity.
Alveolar macrophages have phagocytic functions, but act mainly as antigen-presenting cells, interacting with helper and cytotoxic T lymphocytes. Along with respiratory epithelial cells they respond, via TLR and other signaling pathways, by secreting cytokines that increase tissue permeability as well as attracting and activating additional inflammatory cells.
Neutrophils are the predominant airway leukocytes in RSV bronchiolitis, representing about 90% of cells in the upper airway and 80% of cells in the lower airway. Neutrophil chemotaxis is dependent on the production of the potent chemotractant, IL-8, by airway epithelial cells and macrophages. IL-8 is secreted very soon after infection, leading to an almost immediate inflammatory response before the infection is established. A later peak in IL-8 secretion is dependent on viral replication. Genetic polymorphisms in the IL-8 gene are associated with a risk of severe disease. Neutrophil recruitment and adherence to epithelial cells is increased by persistence of RSV infection. Neutrophils, whose survival is prolonged, secrete products such as myeloperoxidase and neutrophil elastase, which amplify viral cytotoxicity. Neutrophils have also been shown to secrete IL-9 in large quantities in the lungs of severely infected infants. IL-9 is a potent proinflammatory cytokine that is known to cause eosinophilic inflammation, bronchial hyperresponsiveness, and increased mucus production. Neutrophil function therefore plays an important role in the pathological changes that occur in bronchiolitis.
Eosinophils might be expected to have an important role in the pathogenesis of bronchiolitis. They have antiviral activity and eosinophil chemoattractants are secreted by infected respiratory epithelial cells. Following the clinical trials of the formalininactivated vaccine, postmortem examinations revealed massive eosinophilic infiltrates. However, subsequent clinical studies have not reported significant numbers of eosinophils in the airways of infants with RSV bronchiolitis.
Adaptive immunity to RSV is generally composed of protective humoral immunity (B cells and antibodies) and viral clearance (T cells). The humoral response to RSV infection results in the production of IgG, IgM, and IgA antibodies in both blood and airway secretions. Protective benefits of these antibodies are demonstrated by the reduced likelihood of infection in the first month of life in term babies, due to the carriage of maternal IgG antibodies. Prophylaxis with monoclonal anti-RSV IgG (palivizumab), which has been shown to reduce the incidence and frequency of infections in high-risk infants, also demonstrates the role of the humoral response. Evidence from animal models confirms that antibody protects against RSV disease, but once infection is established, the T-cell response promotes viral clearance.
The role of cell-mediated immunity is demonstrated in children with deficient cellular immunity who shed the virus for many months after initial infection, compared to 2 weeks for immunocompetent infants. The cell-mediated response is composed of the CD8þ cytotoxic T lymphocytes and CD4þ T-helper lymphocytes. Cytotoxic lymphocytes are important in both recovery from and the pathogenesis of RSV bronchiolitis. They promote clearance of RSV from the lungs but also cause pulmonary injury. Functional studies in T-lymphocyte-depleted mice demonstrated prolonged RSV replication, yet no overt evidence of illness. CD4þ T-helper cells have traditionally been subdivided further according to the cytokine profiles they secrete: T-helper-1 (Th1) cells produce interferon gamma (IFN-g) and other antiviral cytokines. Th2 cells produce cytokines that induce eosinophil proliferation, and the release of leukotrienes and IgE antibodies leading to an enhanced inflammatory response. The idea that RSV bronchiolitis might be a Th2-type disease, and that this may explain airway obstruction and postbronchiolitic wheeze, has somewhat limited evidence to support it. Multiple factors are more likely to influence the T-lymphocyte response to RSV infection.
Several experimental animal models have been used to improve our understanding of the pathophysiology of RSV bronchiolitis. The variety of animal models used reflects the diversity of clinical manifestations of RSV disease, dependent on an individual’s age, genotype, phenotype, immune status, and concurrent disease. RSV has been shown to replicate in animal models: primates, cotton rats, mice, calves, guinea pigs, ferrets, and hamsters have been used.
The need to investigate immunological responses to RSV, using good animal models, was highlighted after the unsuccessful development of the first formalin- inactivated (FI) RSV vaccine in the 1960s. When natural RSV infection occurred in children given the vaccine, 80% of these children were hospitalized and two children died. Interpretation of the trial results was hampered by the lack of a small animal model, in which vaccine-enhanced disease could be studied. It is now apparent that other vaccine formulations are also capable of stimulating inappropriate immune responses to RSV. Therefore, comprehensive studies using animal models are essential before any further human vaccine trials are undertaken and are key to our understanding of the pathogenesis of bronchiolitis.
Cotton rats have proved to be useful animal models, as they are uniformly susceptible to pulmonary infection through adulthood and are more immunologically responsive than mice to RSV. Studies using the cotton rat model were largely responsible for the development of RSV-neutralizing IgG in preventing pulmonary infection and treatment with ribavarin. Despite this, cotton rats have no congenic, transgenic, or knockout strains and there is only limited availability of reagents used to characterize immunological responses to RSV.
Mice have been the most widely used model, due to a wide array of inbred, congenic, transgenic, and knockout strains. They also have the most reagents available, specific to RSV immunology, and have relatively low maintenance costs. Many insights have been gained into the immune response to RSV, which could not have been achieved using other models. The BALB/c mouse is particularly important, as it develops similar airway inflammatory responses to humans after intranasal RSV infection. Mice studies have been useful in investigating the immunology of RSV infection, especially the cytokine responses to the virus and for experimental vaccines. Management and Current Therapy After admission to hospital, infants should be monitored and assessed regularly. Pulse oximetry, fluid balance, and respiratory rate are useful in assessing changes in the infant’s condition. Barrier nursing and stringent hand-washing policies have been shown to reduce nosocomial spread of RSV. Rapid RSV antigen testing, by either immunofluorescence or enzyme immunoassay, of nasopharyngeal secretions is useful in determining RSV status in hospital.
Despite considerable efforts to develop effective treatments for RSV bronchiolitis, no effective measures exist other than supportive care. The cornerstones of supportive care are supplemental oxygen to correct hypoxia and adequate fluid administration. Infants with inadequate fluid intake may need nasogastric feeding or, if this is unsuccessful, intravenous fluids. Intravenous fluids should be given with care due to the possibility of worsening lung fluid accumulation and left ventricular overload.
A large number of trials and meta-analyses have investigated the efficacy of b2 agonists and ipratropium for patients with bronchiolitis. There is no compelling evidence that bronchodilators are useful in the treatment of bronchiolitis. There may be a subgroup of patients for which bronchodilators are safe and efficacious. However, no criteria exist to identify this subgroup and a significant number of studies found that patients deteriorated after receiving bronchodilators. The inconsistency of these results can be explained by our knowledge of the causes of wheeze in bronchiolitis. Bronchodilators will have no effect on increased mucus production, sloughed epithelial cells in the airway, or interstitial edema.
Ribavarin is an antiviral preparation administered by aerosol and designed to inhibit the synthesis of viral structural proteins. Although early trials of ribavarin supported its use, later trials found no significant positive effect. In addition, it is potentially teratogenic and labor intensive to administer. Other treatments that have failed to demonstrate consistent benefits in the treatment of bronchiolitis include epinephrine, inhaled or systemic corticosteroids, recombinant human DNase, interferon-a, vitamin A, and antibiotics.
Two immunoprophylactic agents are currently available that are recommended for infants at risk of severe infection. RSV immune globulin intravenous (RSV-IGIV) is a polyclonal hyperimmune globulin, prepared from donors selected for having high serum titers of RSV neutralizing antibody. Palivizumab is a humanized murine monoclonal IgG1 antibody against the RSV F-glycoprotein and is given as a monthly intramuscular injection during the RSV season.
RSV-IGIV was the first immunoprophylactic agent to be licensed for prevention of severe RSV bronchiolitis and clinical trials demonstrated a 41–63% reduction in hospital admissions attributable to RSV lower respiratory tract infections. However, RSV-IGIV has a number of disadvantages that have resulted in a decline in its use in favor of palivizumab. Early trials reported an increase in postoperative mortality in patients with cyanotic congenital heart disease who received RSV-IGIV. Administration required intravenous access and a 4-h infusion every month. There are concerns regarding volume overload, interference with live vaccines, and theoretical risks of infection from human donors.
Trials of palivizumab have also demonstrated benefits to high-risk infants, with early trials demonstrating a 45–55% reduction in hospital admissions attributable to RSV lower respiratory tract infections. It is free from potential risk of infection from human donors, does not interfere with immune response to vaccines, and can be administered easily, without the need for hospital admission. Clinical trials also demonstrated its safety for infants with hemodynamically significant congenital heart disease. However, no trial has demonstrated a significant reduction in mortality due to RSV infection.
Although palivizumab was shown to reduce rates of hospitalization and intensive care admissions in large double-blind randomized control trials, several studies have questioned its cost-effectiveness. The American Academy of Pediatrics recommends its use for certain high-risk groups: children under 2 years with chronic lung disease requiring medical therapy; infants born before 28 weeks’ gestation in the first year of life; and infants born at 29–32 weeks’ gestation up to 6 months of age. Infants born at 32–35 weeks’ gestation have a lower risk of hospitalization and palivizumab is only recommended for these infants if they have two other risk factors such as child care attendance, school age siblings, parental smoking, or congenital abnormalities of the airways. Infants with hemodynamically significant congenital heart disease are also likely to benefit and palivizumab is recommended in this group, particularly those with cyanotic heart disease, congestive heart failure, or pulmonary hypertension. There is little knowledge regarding its benefits for immunocompromised infants, children with cystic fibrosis, or prophylaxis in the second year of life.
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