Human Parainfluenza Viruses (HPIV) and Other Parainfluenza Viruses

Background

Parainfluenza viruses (PIVs) are paramyxoviruses of the order Mononegavirales, the family Paramyxoviridae, and the subfamily Paramyxovirinae. Human PIVs (HPIVs) are currently divided into 5 serotypes—HPIV-1, HPIV-2, HPIV-3, HPIV-4a, and HPIV-4b—in 2 different genera: Respirovirus (HPIV-1 and HPIV-3) and Rubulavirus (HPIV-2 and HPIV-4).

HPIVs primarily affect young children, in whom the pathogenic spectrum includes upper and lower respiratory tract infections. They are responsible for 30%-40% of all acute respiratory tract infections in infants and children. These conditions include common cold with fever, laryngotracheobronchitis (croup), bronchiolitis, and pneumonia. HPIVs are also a cause of community-acquired respiratory tract infections of variable severity in adults.

HPIV-1 is most commonly associated with croup. HPIV-2 is also associated with croup. HPIV-3 is second only to respiratory syncytial virus (RSV) as a cause of pneumonia and bronchiolitis in infants and young children. HPIV-4 is detected in patients less often, perhaps because HPIV-4 causes less severe disease. Reinfection with HPIV can occur throughout life, with elderly and immunocompromised persons being at a greater risk for serious complications of infections.

The seasonal patterns of HPIV-1, HPIV-2, and HPIV-3 are curiously interactive. HPIV-1 causes the largest, most defined outbreaks, which are marked by sharp biennial rises in croup cases in the autumn of odd-numbered years. Outbreaks of infection with HPIV-2, although erratic, usually follow HPIV-1 outbreaks. Outbreaks of HPIV-3 infections occur yearly, mainly in spring and summer, and last longer than outbreaks of HPIV-1 and HPIV-2. Because HPIV-4 is infrequently isolated, infection with this pathogen is less well characterized.^[1] The HPIV-3 is the most virulent of the HPIVs and is associated with significant morbidity and mortality.^[2]

Patients with HPIV infection typically present with a history of coryza and low-grade fever; they then develop the classic barking cough associated with croup. On physical examination, HPIV infection is associated with a broad range of findings, which may include fever, nasal congestion, pharyngeal erythema, nonproductive to minimally productive cough, inspiratory stridor, rhonchi, rales, and wheezing. Systemic flulike symptoms are not common in HPIV-infected patients, but adult patients more frequently present with flulike symptoms compared with children.^[3]

Supportive care is mandatory. Oxygen mist is often helpful. Corticosteroids and nebulizers may be used to treat respiratory symptoms and to help reduce the inflammation and airway edema of croup. Antiviral agents are of uncertain benefit; antibiotics are used only if bacterial complications (eg, otitis and sinusitis) develop.

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Pathophysiology

HPIVs can infect many different animals both naturally and under experimental conditions. Asymptomatic infection can be induced in hamsters, guinea pigs, and adult ferrets by all 5 serotypes of HPIV (HPIV-1, HPIV-2, HPIV-3, HPIV-4a, and HPIV-4b). Chimpanzees, macaques, squirrels, owls, and rhesus monkeys have been asymptomatically infected with HPIV-3 or HPIV-4, and only marmosets have developed symptomatic upper respiratory tract infections (URTIs) with HPIV-3 and Sendai virus.

Newcastle disease virus is a rubulavirus that infects poultry, penguins, and other birds and has been responsible for conjunctivitis in bird handlers and laboratory workers. There have been reports of human infections by some of the other nonhuman PIVs, but these have not been well established.

HPIV transmission occurs via direct inoculation of contagious secretions from the hands or via large-particle aerosols into the eyes and nose. Prolonged survival of HPIV on skin, cloth, and other objects emphasizes the importance of fomites in nosocomial spread. Respiratory epithelium appears to be the major site of virus binding and subsequent infection.

The viruses attach to the host cells through hemagglutinin, which specifically combines with neuraminic acid receptors in the host cells. Subsequently, the viruses enter the cell via fusion with the cell membrane mediated by F1 and F2 receptors.

When HPIV infects a cell, the first observable morphologic changes may include focal rounding and growing of the cytoplasm and nucleus and decreased host-cell mitotic activity. Other observable changes include single or multilocular cytoplasmic vacuoles, basophilic or eosinophilic inclusions, and formation of multinucleated giant cells. These giant cells (fusion cells) usually develop late in the infection, and each giant cell contains between 2 and 7 nuclei.

Viral replication starts with the fusion of the virus and the host cell lipid membranes, followed by the expulsion of the HPIV nucleocapsid into the cytoplasm of the cell. In the cytoplasm, transcription takes place by the help of virus-specific RNA-dependent RNA polymerase. Viral mRNAs are then translated into viral proteins by the cellular ribosomes.

There is full-length replication of the virus genome, first into a positive-sense RNA strand and then into the appropriate negative strand. Once produced, these single negative-sense strands of RNA are then encapsidated with nucleoprotein and may be used in further rounds of transcription and replication or may be packaged for export as a new virion.

The pathogenicity also depends on the accessory proteins being present with the anti-interferon (IFN) properties.^[4] The dynamics of primary infection depend on the mode of transmission. In a recent study, it was observed that following contact transmission of HPIV, the virus grew in large numbers in the upper respiratory tract and later spread minimally to the lungs, while following airborne transmission, the virus predominated in the trachea with dissemination throughout the respiratory tract and extensive involvement of the lungs.^[5]

For cytoplasmic RNA viruses such as HPIVs, viral RNA synthesis is the most potent stimulus for innate immune response. C ORF of HPIVs encodes certain proteins that prevent IFN induction, IFN signaling, and apoptosis. In a similar manner but in varying intensities among the different PIVs, these mechanisms inhibit the innate immune response.^[6]

Young age and lack of prior exposure to the infecting HPIV type are two major factors associated with HPIV lower respiratory tract infection (LRTI). Young infants are at greater risk in part because of their smaller airways, which are more susceptible to obstruction, and in part because their immune response is muted owing to immunologic immaturity and pre-existing maternal antibodies, which may suppress the immune response.^[6]

HPIV is affected by the external environmental conditions (eg, temperature, humidity, and pH). Above 37°C, viral survival decreases significantly, and above 50°C, HPIV is inactivated within 15 minutes. The survival of myxoviruses at room temperature has shown considerable survival variability by decreasing titers by more than 50% in as little as 2 hours or as long as 1 week. HPIV have their greatest stability at 4°C or when frozen (eg, –70°C).

Mechanisms of airway inflammation

HPIV infection in the respiratory tract leads to secretion of high levels of inflammatory cytokines such as interferon alfa, interleukin (IL)–2, IL-6, and tumor necrosis factor (TNF)-α. The peak duration of secretion is 7-10 days after initial exposure.

Increasing levels of certain chemokines such as RANTES (regulated upon activation, normal T-cell expressed and secreted) and macrophage inflammatory protein (MIP)–K are detected in the nasal secretions of pediatric patients. These are responsible for pathologic changes in the respiratory tract and clinical manifestations of this condition.

Studies have shown a possible role of virus-specific immunoglobulin E (IgE) antibodies earlier and in larger amounts in patients with HPIV URTI than in age-matched controls. Faster and increased production of this virus-specific IgE mediates histamine release in the trachea and the subglottic region, in turn producing croup.^[7]

The chief pathologic features include airway inflammation, necrosis and sloughing of respiratory epithelium, edema, excessive mucus production, and interstitial infiltration of the lung. Edema of the mucus layer causes swelling in the vocal cords, larynx, trachea, and bronchi. These changes lead to obstruction of the airway inflow and subsequent stridor, which is characteristic of croup.

Microarray analysis of epithelial cells infected with HPIVs has indicated a role for NF-Kb, IRF3, and type 1 IFN pathways in the regulation of cellular antiviral and anti-inflammatory response. HPIV C protein plays an important role in the suppression of these immune responses. Elevated nasal wash concentrations of inflammatory cytokines IL-8/CXCL8, MIP1α+1β/CCL3+4, RANTES/CCL5, and CXCL9, have been described in children with parainfluenza virus disease. CXCR3 ligands such as IP-10 and I-TAC, which attract activated Th1 cells, are dominant chemokines observed during HPIV infection. High concentrations of IL-8 and IP-10 have been correlated with severe HPIV infection.^[6]

In animal models, increased levels of histamine and eosinophils are detected in bronchoalveolar lavage (BAL) samples after infection with HPIV, suggesting a state of hyperresponsiveness of the respiratory tract.

HPIV-2 and HPIV-3 infection in humans is known to induce expression of intercellular adhesion molecule-1 (ICAM-1) in tracheal and other cells of the respiratory tract. These molecules serve as receptors for rhinoviruses, thus paving the way for rhinoviral superinfection.

The virus continues to be excreted in respiratory exudates for 3-16 days after primary infection and 1-4 days after infection.

Immune response

Host defense against HPIVs is mediated largely by humoral immunity to both surface glycol proteins of the virus, which are most immunogenic: hemagglutinin-neuraminidase (HN) and fusion (F). Most children are born with neutralizing antibodies to all 5 of the HPIV serotypes, but these titers quickly fall during the first 6 months of life.

Most antibody response appears to involve serum immunoglobulin G1 (IgG1), but levels of serum immunoglobulin G3 (IgG3), immunoglobulin G4 (IgG4), serum immunoglobulin A (IgA), and immunoglobulin M (IgM) rise significantly in 30% of adults. Secretory IgA plays an important but not fully defined role in the protection against natural HPIV infections.

A cell-mediated immune response to HPIV antigens, in addition to an HPIV-specific IgE response, has been documented to be greater among infants with HPIV bronchiolitis than among infants who developed only upper respiratory tract illness.

After natural infection with HPIV, most children and adults develop measurable levels of these antibodies in the serum; these antibodies have been shown to be correlated with disease prevention and amelioration in adults. Local interferon production has been noted in about 30% of children with HPIV infection.

Although immunity to HPIV infection is long-lasting, reinfection may occur many times throughout life and at variable intervals, even in the presence of neutralizing antibodies. This cannot be explained solely by the relatively stable antigenic determinants of HPIVs; thus, more research is needed.

In a recent study involving mice models, the level of reinfection was noted to show an inverse correlation with the level of primary infection in the same tissue. In this study, it was observed that primary airborne transmission of the HPIV rendered protection from reinfection throughout the respiratory tract, while contact transmission of the HPIV resulted in protection from reinfection in the upper respiratory tract, with only partial protection in the lungs.^[5]

HPIV infections tend to be more severe in individuals with defective cell-mediated immunity, indicating that T cells may have a greater role in containing the disease.

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Etiology

HPIVs are pleomorphic viruses whose envelope is derived from the host cell they last infected. These viruses are 150-300 nm in diameter and possess a single-stranded, nonsegmented, negative-sense RNA genome with nucleoprotein P and L proteins. Noninfectious virions with positive RNA polarity have been reported.^[8] The HPIV genome contains approximately 15,000-16,000 nucleotides, which are organized to encode at least 6 common structural proteins. A “rule of six” has been coined for HPIV, with the advent of reverse genetics. This means that the most efficient replication and transcription of HPIV takes place when the genome is divisible by 6, although exceptions have been found.^[8]

A lipid bilayer covered with glycoprotein spikes surrounds a helical nucleocapsid that measures 12-17 nm in diameter (see the image below), and matrix protein resides between the core and the envelope. These glycoproteins are the HN and F proteins, which play a major role in the pathogenesis of the disease caused by HPIVs.

View Image

Transmission electron micrograph of parainfluenza virus. Two intact particles and free filamentous nucleocapsid.

HPIVs belong to the order Mononegavirales, the family Paramyxoviridae, and the subfamily Paramyxovirinae. They currently comprise 5 serotypes—HPIV-1, HPIV-2, HPIV-3, HPIV-4a, and HPIV-4b—which display substantial serologic cross-reactivity. (The 2 serotypes of HPIV-4 are differentiated on the basis of hemadsorption inhibition pattern and monoclonal antibody reactivity.^[9] ) Serologic and antigenic analysis of all of the species in the Paramyxovirinae subfamily demonstrates the following 4 basic genera, 2 of which include HPIVs:

• Respirovirus – HPIV-1, HPIV-3, Sendai virus, and bovine PIV-3
• Rubulavirus – HPIV-2, HPIV-4, mumps virus, and simian viruses 5 and 41
• Morbillivirus – Measles virus and distemper virus
• Megamyxovirus – Hendra virus and Nipah virus

The most common primary and secondary cell lines that support the growth of HPIV are LLC-MK2, Vero, HMV-II, HEp-2, MDCK, BHK, HeLa, primary human embryo, and HEF. Organ cultures from mouse, guinea pig, ferret, and human fetal respiratory epithelium can also be used.

The following clinical conditions are caused by the various HPIV types:

• Croup – HPIV-1, HPIV-2, HPIV-3
• Bronchitis – HPIV-1, HPIV-3
• Bronchopneumonia – HPIV-1, HPIV-3
• Minor URTI – HPIV-1, HPIV-3, HPIV-4
• Pneumonia and bronchiolitis – HPIV-1, HPIV-3

Respiratory secretions from infected humans are the source of infection. Transmission is via respiratory droplets or via directs person-to-person contact with infected secretions or fomites; the virus can survive in aerosols for over an hour. The inoculating dose is very small. The incubation period for HPIVs ranges from 1 to 7 days.

HPIVs are common community-acquired respiratory pathogens without ethnic, socioeconomic, gender, age, or geographic boundaries. Many factors have been found that predispose individuals to these infections, including the following:

• Malnutrition
• Overcrowding
• Vitamin A deficiency
• Lack of breastfeeding
• Environmental smoke or toxins

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Epidemiology

United States statistics

Infections with HPIV-1 and HPIV-2 occur during autumn months. Infections with HPIV-3 occur throughout the year but appear to peak in the spring. HPIV-3 is the second most common cause of lower respiratory tract infections (LRTIs) treated in the United States, second only to RSV. HPIV-4 infection patterns are not well defined.

HPIV-3 infections occur earliest and most frequently. According to seroepidemiologic studies, 50% of US children aged 1 year and almost all US children aged 6 years have been infected by HPIV-3. Antibodies against HPIV-1 and HPIV-2 develop less rapidly, but 80% of children have antibodies against these types by age 10 years. Although HPIV-4 induces few clinical illnesses, infections with this serotype are apparently common: 70-80% of children aged 10 years have antibodies against HPIV-4.

A 2016 study of surveillance data collected over 10 years concerning estimates of parainfluenza virus–associated hospitalizations in children younger than 5 years revealed the cyclic pattern of HPIV circulation in the United States. HPIV-1 circulation was observed every other year, whereas HPIV-2 and HPIV-3 circulated annually. The study also analyzed the percentage of HPIV-associated manifestations in hospitalized children younger than 5 years. Of the bronchiolitis-associated admissions, 3.2% were associated with HPIV, with the highest percentage seen in children aged 1-2 years. Most hospitalizations were associated with HPIV-3 infection. Among croup-associated hospitalizations, HPIV was isolated in 46.6% of cases; HPIV-1 accounted for most cases (26.3%). HPIV was associated with 5.5% of hospitalizations due to pneumonia, with HPIV-3 being the most common HPIV isolate. HPIV-1 followed a biennial pattern, with circulation beginning to rise between May and July, peaking in September or October, and decreasing between December and January of odd-numbered years. HPIV-3 circulated every year, with proportions of detections rising in March and peaking between April and July. Circulation of HPIV-2 was less defined than that of HPIV-1 or HPIV-3, but highest activity was seen between August and January.^[10]

International statistics

Internationally, HPIV-1, HPIV-2, HPIV-3, and HPIV-4 have a worldwide distribution, and epidemics are known to occur, particularly with HPIV-1.

Parainfluenza viruses are responsible for disease throughout the year, but winter outbreaks of respiratory tract infections, especially croup, in children throughout the temperate zones of the northern and southern hemispheres represent peak periods of prevalence. Most infections are endemic, but sharp small epidemics involving HPIV-1 and HPIV-2 occasionally occur.

The first reported outbreak of HPIV-4 infection occurred in Hong Kong in the autumn of 2004, involving 38 institutionalized children and 3 staff members during a 3-week period in a developmental disabilities unit.^[9] For the influenzalike illnesses reported, the main etiologic agents in the early epidemic period were noninfluenza viruses, and among these noninfluenza viruses, HPIV accounted for about 24% of the infections.^[11] In a study from Southern China, seasonal peaks due to HPIV-3 and HPIV-1 were observed during autumn, while the HPIV-2 and HPIV-4 were detected less frequently, with their incidence increasing with the decline in the frequency of HPIV-3 and HPIV-1.^[3]

A 15-year analysis of HPIV from England and Wales revealed that most HPIV isolates detected were HPIV-3; detection was seen throughout the year but peaked between March and June. HPIV-1 and HPIV-2 circulated in low numbers, with a relative peak in the last quarter of the year and a biennial cycle.^[12]

Sentinel Surveillance for Severe Acute Respiratory tract Infection (SARI) data from the Mediterranean region revealed HPIV isolations of 1.7% for HPIV-1, 0.9% for HPIV-2, and 3.9% for HPIV-3.^[13]

A study on community-acquired pneumonia from Italy using the novel Luminex technology revealed that HPIV-related pneumonia accounted for 11% of cases, with most (7%) caused by HPIV-4.^[14]

Age-, sex-, and race-related demographics

HPIVs are ubiquitous and infect most people during childhood. The highest rates of serious HPIV illnesses occur among young children.

HPIV-1 can cause LRTIs in young infants but is rare in those younger than 1 month. However, recently an outbreak of HPIV-3 infection among 6 preterm infants was reported in a neonatal nursery.^[15] The full burden of HPIV-1 in adults and elderly persons has not been determined, but studies have shown that this virus causes yearly hospitalizations in healthy adults and may play a role in bacterial pneumonias and death in nursing-home residents.

HPIV-2 accounts for 60% of all infections that develop in children younger than 5 years, with a peak incidence between ages 1 and 2 years. Young infants (< 6 months) are particularly vulnerable to infection with HPIV-3. Unlike other HPIV infections, 40% of HPIV-3 infections occur in the first year of life.

HPIVs have no predilection for either sex or for any race. However, a recent study has shown that HPIVs were more commonly isolated from male patients than females.^[3]

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Prognosis

Approximately 41,000 individuals are admitted to the hospital for parainfluenza virus infections each year. Precautions are necessary within hospitals to prevent further spread.^[16] Only 1-5% of patients admitted to the hospital need artificial airway support.

HPIV infections in older children and adults are generally mild. Occasionally, bronchiolitis or viral pneumonia in children and tracheobronchitis in adults has been reported. Generally, pediatric patients with parainfluenza infections do well, with symptoms typically resolving in 7-10 days.

On occasion, the infection spreads to the lower respiratory tract, causing bronchiolitis or viral pneumonia. Denudation of respiratory epithelium places patients at a slightly increased risk of bacterial superinfection. Evaluate any patient recovering from croup who deteriorates suddenly for possible bacterial tracheitis.

In developed countries, mortality induced by HPIV is unusual, occurring almost exclusively in young infants or immunocompromised or elderly people. In developing countries, however, the preschool population is at considerable risk for HPIV-induced death. Whether because of primary viral disease or because of the facilitation of secondary bacterial infections in malnourished children, LRTI causes 25-30% of the death in this age group, and HPIV causes at least 10% of LRTIs.

It was recently reported that detection of the HPIV in lungs of infected patients was associated with worse outcomes than viral detection in the upper respiratory tract samples alone. This suggests that viral detection in the lungs of infected patients can be used to predict poor outcome.^[17]

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History

Human parainfluenza viruses (HPIVs) have been associated with every type of upper and lower respiratory tract illness, including common cold with fever, laryngotracheobronchitis (croup), bronchiolitis, and pneumonia. HPIVs are also a cause of community-acquired respiratory tract infections of variable severity in adults. The incubation period of HPIV infection generally lasts 1-7 days. Weinberg et al found that HPIV accounted for 6.8% of all hospitalizations for fever, acute respiratory illnesses, or both in children younger than 5 years.^[18]

All HPIV types are strongly correlated with specific clinical syndromes, ages, and times of year, though the lack of epidemiologic data on HPIV-4a and HPIV-4b has so far prevented a clear understanding of the true clinical significance of these serotypes. HPIV-1 and HPIV-2 are the pathogens most commonly associated with croup, and HPIV-3 is the pathogen most commonly associated with bronchiolitis and pneumonia in infants and young children.^[19]

Patients with HPIV infection typically present with a history of coryza and low-grade fever; they then develop the classic barking cough associated with croup. Symptoms of croup include the following:

• Fever
• Barking cough
• Coryza
• Stridor
• Retractions
• Tachypnea (when lower airways become involved)
• Irritability

Children with croup are usually more symptomatic at night. Coughing often awakens them from sleep. The reasons why symptoms are worse at night are unknown.

HPIV infections can also present as bronchiolitis or pneumonia. The typical presentation includes the following^[20] :

• Fever
• Coryza
• Tachypnea
• Coughing
• Wheezing

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Physical Examination

HPIV infection is associated with a broad range of findings, which may include fever, nasal congestion, pharyngeal erythema, nonproductive to minimally productive cough, inspiratory stridor, rhonchi, rales, and wheezing.

Croup

Croup is a generic term that encompasses a heterogeneous group of illnesses affecting the larynx, trachea, and bronchi. It affects about 3% of children in a given year, usually between ages 6 months and 3 years.^[21] HPIV-1 is the most common cause of croup; between them, HPIV-1, HPIV-2, and HPIV-3 account for almost 75% of all cases. Symptoms of croup include fever, hoarse barking cough, laryngeal obstruction, and inspiratory stridor.

Croup scoring systems have been developed to aid in grading the severity of infection. Factors addressed in such systems include stridor, retractions, air entry, color, and level of consciousness. However, these croup scoring systems were developed before the advent of pulse oximetry. Pulse oximetry may be beneficial in grading severity of illness, response to management, and disposition.

Bronchiolitis

All 5 serotypes of HPIV can cause bronchiolitis, but the ones most commonly associated with this condition are HPIV-1 and HPIV-3, each of which appears to cause 10-15% of bronchiolitis cases in nonhospitalized children. The incidence of bronchiolitis peaks during the first year of life (with 81% of cases occurring during this period) and then declines dramatically until it virtually disappears by school age. Predominant features include fever, expiratory wheezing, tachypnea, retractions, rales, and air trapping.

Pneumonias

HPIV-1 and HPIV-3 each cause about 10% of outpatient pneumonia cases, but as with bronchiolitis, HPIV-3 causes a larger percentage of cases in hospitalized patients. HPIV-2 and HPIV-4 can both cause pneumonia, but the incidence of disease attributable to these serotypes is not well described. HPIV-1 infection has been associated with secondary bacterial pneumonias in elderly persons. Features of pneumonia include fever, rales, and evidence of pulmonary consolidation.

Tracheobronchitis

More than 25% of the agents identified as causing tracheobronchitis have been HPIVs. (HPIV-3 is more commonly associated with tracheobronchitis than HPIV-1 or HPIV-2 is.) Tracheobronchitis is the most common feature seen in persons with HPIV-4 infections.

Other infections

HPIVs routinely cause otitis media, pharyngitis, and conjunctivitis coryza, and these can occur either singly or in combination with a lower respiratory tract infection (LRTI). HPIV-3 is the most frequently reported HPIV associated with otitis media.

Infections in immunocompromised patients

The growing number of patients who receive intense immunosuppression after undergoing transplantation of bone marrow and solid organs has highlighted the role of HPIVs as potential opportunistic pathogens.

HPIV-2 causes giant cell pneumonia in persons with severe combined immunodeficiency diseases (SCIDs). HPIV-3 has been found in persons with SCIDs and acute myeloid leukemia (AML), as well as in patients who have undergone bone marrow transplantation (BMT). The natural history of HPIV in patients infected with HIV is generally less severe than that in transplant recipients.

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Complications

Complications of HPIV infection may include the following:

• Acute respiratory distress syndrome (ARDS) and exacerbation of nephritic syndrome
• Serious morbidity in immunocompromised hosts (eg, transplant recipients) – Posttransplant HPIV infection is a cause of serious lower respiratory tract involvement in both adults and children who undergo BMT
• Fatal pneumonia
• Prolonged viral shedding
• Rare complications, including Guillain-Barré syndrome and meningitis

Long-term ribavirin therapy has been helpful in case reports.^[22]

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Approach Considerations

In human parainfluenza virus (HPIV) infection, the complete blood count (CBC) is usually within the reference range. The white blood cell (WBC) count is usually normal; however, lymphocytosis may be noted.

The diagnosis of HPIV infection can be confirmed in either of the following 2 ways:

• Isolation and identification of the virus in cell culture or direct detection of the virus in respiratory secretions by means of immunofluorescent assay, enzyme-linked immunosorbent assay (ELISA), or polymerase chain reaction (PCR) assay
• Demonstration of a significant rise in specific immunoglobulin G (IgG) antibodies between appropriately collected paired serum specimens or in specific immunoglobulin M (IgM) antibodies in a single serum specimen

On histologic examination, the epithelium of the respiratory tract may show inflammation and necrosis. Subglottic tissues in particular may appear to be involved.

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Viral Testing

Collection and preparation of clinical specimens

Nasopharyngeal aspirations, nasal washings, and nasal aspirations are the optimal specimens, though throat and nasal swabs can also be used. Specimens should be collected and placed in viral transport media, preferably at 4°C, as the infectivity is lost at temperatures above 4°C; if a delay of more than 24 hours is anticipated, specimens should be frozen. In rare situations, nonrespiratory specimens (eg, cerebrospinal fluid [CSF], rectal swabs, or stool samples) may be used.

Paired sera (acute and convalescent phase) should be collected, separated quickly, and stored at either –20°C or –70°C; the 2 samples should be tested simultaneously.

Electron microscopy

Micro drops of secretions or garglings are placed directly on carbon-coated electron microscopy grids and stained with phosphotungstic acid. Virions typical of the Paramyxoviridae may be observed. However, the sensitivity of this study is poor.

Indirect immunofluorescence

Indirect immunofluorescent assay is normally used with antisera against each of the HPIV serotypes. However, the findings from this assay cannot be used as the sole diagnostic criterion, because its sensitivity, like that of electron microscopy, is poor.

Direct detection of viral antigens

Synthesized recombinant HPIV-1 and HPIV-3 nucleocapsid proteins in the yeast Saccharomyces cerevisiae are used as a source of viral antigens.^[23] HPIV antigen can be detected with ELISA, radioimmunoassay, fluoroimmunoassay, or immunofluorescent assay; the last two tests are both rapid and specific. Shell vial assay is another method for rapid identification of HPIV. This method yields an average sensitivity of 84% in testing against standard tissue culture-positive HPIV cases.

Isolation of virus

HPIV grows best in primary monkey kidney (PMK) cells (from rhesus, cynomolgus, and African green monkeys). LLC-MK2 cells are also excellent for continued passage and are almost as good as PMK cells for primary isolation. HPIV-2 induces host ADAM8 expression in human salivary parotid adenocarcinoma cell line (HSY) during cell fusion. ADAM8 is responsible for cell-to-cell fusion and formation of multinucleate giant cells, especially osteoclasts.^[24]

To recover all HPIV serotypes, trypsin (2-3 mg/mL) must be added to the maintenance medium of LLC-MK2 cells. Proteases are necessary for parainfluenza virus to replicate, and it is hypothesized that proteases present in primary cell cultures are absent in continuous cell lines.

Detection and typing

Cytopathic effects are rarely demonstrated during primary isolation of HPIV in tissue culture, except in the case of HPIV-2, which shows syncytial formation when cultured. All HPIVs demonstrate greater cytopathic effects upon adaptation to a particular cell line; HPIV-3 is the most aggressive, destroying more than 50% of tissue culture monolayer by day 3.

Viral growth is detected with hemadsorption inhibition using guinea pig erythrocytes within 3-10 days of incubation. However, the prolonged incubation time required severely restricts the usefulness of virus isolation in short-term management. The neuraminidase of HPIV-4 appears to be temperature sensitive because this virus hemadsorbs better at room temperature or 37°C, while all of the other serotypes react well at 4°C.

Serologic diagnosis

A 4-fold rise or drop in titer is generally thought to signify acute infection if the testing is performed at the same time on paired acute- and convalescent-phase serum samples. Hemagglutination inhibition, neutralization, complement fixation, ELISA, radioimmunoassay, and Western blotting are frequently used antibody-based serologic tests for diagnosis of HPIV infections.

Genomic detection

HPIV RNA can be detected directly by means of Northern hybridization or a dot blot analysis using virus-specific DNA probes. PCR assay is sensitive and specific in detecting HPIV. In one study, PCR assay yielded twice the sensitivity of cultures and 4 times the sensitivity of immunofluorescent antibody staining.^[25] A multiplex reverse transcriptase PCR (RT-PCR) assay for detecting HPIV-1, HPIV-2, and HPIV-3 has been developed.

An RT-PCR enzyme hybridization assay (RT-PCR-EHA) is available for detecting HPIV types 1-4, respiratory syncytial virus (RSV) types A and B, and influenza A and B virus, with a reported sensitivity of 95-100% and specificity of 97-100% in comparison with the results of tissue culture. The RT-PCR-EHA yields results in approximately 7 hours.

A multiplex RT-PCR assay kit using a dual priming oligonucleotide (DPO) system is now available; this kit is capable of detecting 12 common viruses that cause respiratory tract infections in children.^[26] A multiplex PCR assay capable of detecting 15 common viruses that cause respiratory infections has been reported.^[27]

Currently, multiple multiplex real-time PCRs based on Taqman chemistry are commercially available, which can be used to detect HPIV, along with other respiratory pathogens.^{[28, 29]}

Luminex xTAG assays have enabled multiplexing of multiple targets in a single assay. The commercially available Luminex xTAG Respiratory Viral panel is capable of detecting 19 different pathogens, including all four types of HPIV. Initial comparative studies of Luminex respiratory panel with conventional or multiplex real-time PCR assays have revealed a higher diagnostic yield for most of the respiratory pathogens, including all four types of HPIVs.

In a study by Ko et al,^[30] Luminex panel results correlated with real-time assay results for most respiratory pathogens (94.1%-100% of cases), but the correlation percentage was relatively low (94.1%-97.6%) for HPIV-3. The overall yield was higher for the Luminex panel, with 4 additional HPIV-3 detections. Although the technology is high throughput, the high cost of the assay is the limiting factor for wide-scale use.

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Neck and Chest Radiography

Radiographs of the neck or chest are important if epiglottitis, croup, or pneumonia is a possibility.

In patients with croup, anteroposterior views of the neck may demonstrate the classic steeple sign (ie, subglottic swelling with narrowing of the air shadow of the trachea.

Lateral soft tissue films of the neck are normal in most cases. However, such films can be useful if the diagnosis is unclear, especially if conditions such as foreign body aspiration, retropharyngeal abscess, or epiglottitis are in the differential diagnosis. In patients with epiglottitis, lateral views may demonstrate enlargement of the epiglottis and ballooning of the hypopharynx.

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Approach Considerations

Because it is often difficult to distinguish pneumonia caused by HPIV from pneumonia caused by bacteria, patients with viral pneumonia are sometimes inappropriately treated with antibacterial antibiotics. In the setting of HPIV infection, antibiotics are used only if bacterial complications (eg, otitis and sinusitis) develop. Antiviral agents are of uncertain benefit for treatment of HPIV infection.

It is important to document an examination at time of discharge—for example, “Patient is breathing comfortably and is alert and consolable, without tachypnea, stridor, or retractions.” A pulse oximetry reading can also be included if available. If there are any concerns about the patient’s stability for discharge, always err on the side of admission.

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Initial Management

Prehospital care includes fever control and attempts to alleviate respiratory symptoms and patient anxiety.

Respiratory symptoms commonly improve with benign measures such as sitting in a bathroom with a steaming shower and allowing vapor droplets to soothe inflamed airways. Another option includes exposing the child to the cool night air. Often, the patient’s symptoms resolve en route to the hospital. Attempts at calming or distracting the child can be beneficial.

Antipyretics may assist with fever control. Moderate or severe croup requires medical evaluation in the office or emergency department (ED). Consultations may include pulmonologists and infectious disease specialists.

Indications for hospitalization in patients with HPIV infection include the following:

• Respiratory distress
• Dehydration
• Stridor at rest, even after receiving therapy
• Need for repeated doses of racemic epinephrine
• Rebound laryngospasm in patients who receive racemic epinephrine

A common issue in pediatric ED charts concerns variations in patient assessments. All notes on the chart (eg, from triage, nurses, and residents) should be examined. If any of these assessments differ from the physician’s assessment, the physician should address the differences in his or her notes. At discharge, ensure that proper discharge instructions, both written and oral, are given to the patient.

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Supportive Care and Pharmacologic Therapy

Supportive care is mandatory. Oxygen mist is often helpful. Corticosteroids and nebulizers may be used to treat respiratory symptoms and to help reduce the inflammation and airway edema of croup.

Management of croup caused by HPIV infection depends on the severity of disease.

Mild croup

Management of mild croup consists of cool blow-by oxygen mist, fever control, and observation to determine whether the airway appears compromised.

Moderate croup

Cool oxygen mist and steroids are common therapies for moderate croup. Controlled trials of steroids for the palliation of croup symptoms have yielded conflicting results, and routine use of dexamethasone in this disease remains controversial. Traditionally, dexamethasone was administered intramuscularly (IM); however, some studies have documented the use of oral steroids.

In patients who fail to improve, administration of racemic epinephrine with a nebulizer has been beneficial. If racemic epinephrine alleviates symptoms, observe the patient for a minimum of 3 hours to ensure that his or her condition does not worsen (eg, because of possible rebound laryngospasm as the racemic epinephrine dose wears off). If the patient is asymptomatic at the end of the observation period, he or she can be discharged with proper follow-up care.

In moderate croup, oral intake may be lacking; therefore, it is essential to evaluate the patient’s hydration status. Intravenous (IV) fluids may be required.

Severe croup

The same measures are indicated for severe croup as for moderate croup. Observe the patient for signs of impending respiratory failure.

Repeat racemic epinephrine nebulization may be needed, in addition to intensive care monitoring. Racemic epinephrine nebulizations can be repeated at 1-hour to 2-hour intervals as needed. Endotracheal intubation followed by a tracheotomy may be required in patients with severe respiratory obstruction. Fortunately, fewer than 5% of patients who are admitted require artificial airway support.

Antiviral therapy

Ribavirin is a broad-spectrum antiviral agent that has been shown to be effective against HPIV-3 infection in vitro and possibly in vivo. Although results are mixed, ribavirin aerosol or systemic therapy has been used to treat HPIV infections in children and adults who are severely immunocompromised. Use at this time is of uncertain clinical benefit.

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Prevention

Passively acquired maternal antibodies may play a role in protection from HPIV-1 and HPIV-2 in the first few months of life, an observation that highlights the importance of breastfeeding.

Currently, there are no effective vaccines for prevention of infections by HPIVs. Among the HPIVs, the HPIV-3 is the most virulent with ability to cause bronchioloitis and pneumonia in infants. Therefore, attempts are being made for the development of human vaccine against the HPIV-3.

In the late 1960s, field trials of formalin-killed whole HPIV-1, HPIV-2, and HPIV-3 vaccines failed to protect children against natural infections. Subsequent approaches to the development of HPIV vaccines have included intranasal administration of live attenuated strains, subunit strategies using hemagglutinin-neuraminidase (HN) and fusion (F) proteins, recombinant bovine human viruses, and strains engineered by means of reverse genetics.

Antigenically and genetically stable attenuated strains of HPIV-3 have been developed with cold adaptation (CA); their stability is enhanced because of multiple markers of attenuation in tissue culture. CA strains of HPIV-1 and HPIV-2 have been developed, and attenuation in tissue culture and animal models has been demonstrated.

HPIV3cp45 vaccine candidate has undergone a phase 1 clinical trial and has been found to be highly infectious and well-tolerated. Different dosing schedules, 1 month apart and 3 months apart, have been tested, and viral shedding is less when vaccine doses are given 1 month apart.^[31]

BCX 2798 and BCX 2855 were found to be effective inhibitors of HPIV-3 HN in a mouse model, as well as potentially promising candidates for prophylaxis and treatment of HPIV-3 infection in humans.^[32] A live attenuated vaccine against respiratory syncytial virus (RSV) and HPIV-3 was found to be safe and immunogenic in a phase 1 study.^[33] Reverse genetics produced an attenuated chimeric HPIV-1 that contains type 3 internal proteins in conjunction with type 1 HN and F surface glycoproteins.^[34]

Some of the potential vaccine candidates are the HPIV-3 cp45 nasal vaccine, which is derived from the JS wild-type strain of HPIV-3 and the rB/HPIV3b vaccine, which is a cDNA-derived chimeric HPIV-3. An intranasal vaccine has been developed against HPIV-3. The investigators determined that the vaccine is appropriately attenuated and immunogenic in infants as young as 1 month. Further development of this vaccine is warranted.^{[35, 36]} .

Two vaccines against HPIV-3 were determined to be safe and immunogenic in phase I trials involving HPIV-3–seronegative infants and children; vaccines against HPIV1 and HPIV2 were found to be less advanced.^[35]

Bovine PIV (BPIV) has also been used as a live vaccine candidate against HPIV-3. Phase 1 clinical trials found the vaccine candidate to be highly infectious, safe, and immunogenic; however, the mean geometric titers for BPIV were lower against HPIV-3 than against BPIV-3. Phase 2 clinical trials of the same candidate demonstrated that the vaccine was well–tolerated, and fever was the only adverse effect observed.^[31]

Combined RSV and HPIV-3 vaccines have also been attempted. The aforementioned BPIV/HPIV-3 virus has been modified to express RSV F protein alone or both the F and G proteins. This bivalent virus contains proteins from both RSV and HPIV-3. Preclinical studies have indicated good tolerance and adequate seroconversion.^[31]

Murine PIV-1/Sendai virus vaccine has also been evaluated as a live attenuated xenotropic vaccine candidate. Intranasal formulation of the candidate strain was well-tolerated in a phase 1 trial.

HPIV-1 has also been evaluated as a vector for RSV. In animal studies (hamster model), vaccination with an attenuated RSV strain followed by HPIV-1–expressing RSV F protein showed better immunogenicity than two doses of attenuated RSV.^[31]

Development of an HPIV-2 vaccine is being attempted; in preclinical studies, L protein has been identified as the major target for mutagenesis to develop attenuated HPIV-2.^[31]

However, the major limitation of these vaccines is their potential to cause actual infection in immunocompromised hosts and children. In order to overcome this limitation, subunit vaccines that target the HPIV-3 HN and F proteins are being investigated. In a recent study, intranasal coadministration of oligomannose-coated liposome-encapsulated HN with the poly(I:C) adjuvant was found to induce adequate viral-specific immunity against HPIV-3 in a mouse model.^[2]

Strict attention to infection control should decrease or prevent spread of infection. Frequent handwashing and avoidance of sharing items such as cups, glasses, and utensils with an infected person should decrease the spread of viruses to others. In a hospital setting, the spread of HPIVs can and should be prevented by strict attention to contact precautions, such as handwashing and wearing of protective gowns and gloves.

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Long-Term Monitoring

If the patient has a pediatrician or other primary care provider, attempt to contact the provider to ensure proper follow-up care. The pediatrician can also be consulted on management issues if the treating physician has concerns or doubts.

Helpful long-term measures may include the following:

• Bed rest
• Use of vaporizers producing moist air
• Administration of acetaminophen or ibuprofen for fever
• Increased oral fluid intake

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Medication Summary

No specific antiviral agents have been established as beneficial for treating human parainfluenza virus (HPIV) infections; however, ribavirin is sometimes given. Medications are administered to treat the respiratory symptoms associated with croup (eg, airway inflammation and edema). Such medications include corticosteroids and nebulized epinephrine. Antibiotics are used only if bacterial complications (eg, otitis and sinusitis) develop.

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Dexamethasone (Baycadron)

• Dosing, Interactions, etc.

Clinical Context:  Dexamethasone decreases airway inflammation by inhibiting migration of phagocytes and reversing capillary permeability, thereby reducing the edema that occurs in croup. It is the preferred anti-inflammatory drug for reducing airway edema in this setting, though other glucocorticoids have been used, including prednisone and prednisolone.

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Budesonide inhaled (Pulmicort Respules, Pulmicort Flexhaler)

• Dosing, Interactions, etc.

Clinical Context:  When nebulized, budesonide is useful for reducing inflammation and edema in patients with croup. It alters the level of inflammation in airways by inhibiting multiple types of inflammatory cells and decreasing production of cytokines and other mediators. Turbuhaler is used for adults; Pulmicort Respules is used only for children aged 1-8 years.

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Prednisone

• Dosing, Interactions, etc.

Clinical Context:  Prednisone may decrease inflammation by reversing increased capillary permeability and suppressing the activity of polymorphonuclear leukocytes (PMNs).

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Prednisolone (Orapred, Pediapred, Millipred)

• Dosing, Interactions, etc.

Clinical Context:  Prednisolone decreases inflammation by suppressing migration of PMNs and reducing capillary permeability. Many practitioners administer liquid prednisolone to patients with croup in lieu of dexamethasone. Prednisolone has not been proved superior to dexamethasone.

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Methylprednisolone (Medrol, Solu-Medrol, A-Methapred)

• Dosing, Interactions, etc.

Clinical Context:  Methylprednisolone blocks release of inflammatory mediators by inhibiting phospholipase A2. It may be useful in patients who have either asthma or bronchiolitis with asthmatic qualities.

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Class Summary

Corticosteroids have anti-inflammatory properties and cause profound and varied metabolic effects. They modify the body’s immune response to diverse stimuli. Anti-inflammatory drugs (specifically, dexamethasone) help reduce the inflammation and subglottic edema of croup. Despite its delayed onset of action, the high potency and prolonged intramuscular half-life of dexamethasone make it the preferred corticosteroid for croup.

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Epinephrine racemic (Adrenalin, Twinject, EpiPen 2-Pak)

• Dosing, Interactions, etc.

Clinical Context:  Racemic epinephrine solution causes alpha-adrenergic receptor–mediated vasoconstriction of edematous tissues, thereby reversing upper airway edema. It provides short-term relief. In concentrations of 1:1000, L-epinephrine may be used in place of racemic epinephrine for nebulized administration.

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Class Summary

When delivered by air or oxygen-powered devices, epinephrine is directly delivered to respiratory mucosal surfaces and smooth muscle. Because nebulizers deliver the medication directly to the target organ, fewer systemic adverse effects are encountered than are seen with oral or parenteral administration.

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Ribavirin (Virazole)

• Dosing, Interactions, etc.

Clinical Context:  Ribavirin (1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamide) is a synthetic nucleoside analogue that resembles guanosine and inosine. It is believed to act by interfering with expression of messenger RNA and inhibiting viral protein synthesis. Ribavirin appears safe but is expensive. Its efficiency and effectiveness have not been clearly demonstrated in large, randomized, placebo-controlled trials. At present, routine use of ribavirin cannot be recommended.

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Class Summary

Ribavirin is licensed by the US Food and Drug Administration (FDA) for the management of RSV bronchiolitis and pneumonia. It has a broad spectrum of antiviral activity in vitro, inhibiting replication of RSV as well as influenza, parainfluenza, adenovirus, measles, Lassa fever, and Hantaan viruses. No antiviral agents have been established as beneficial for treating human parainfluenza virus (HPIV) infections; however, ribavirin is sometimes given.

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