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.  The HPIV-3 is the most virulent of the HPIVs and is associated with significant morbidity and mortality. 
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. 
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.
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 properties.  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. 
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. 
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.
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.
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. 
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.
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.  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. 
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.
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.  ) 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:
Vitamin A deficiency
Lack of breastfeeding
Environmental smoke or toxins
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.
Internationally, HPIV-1, HPIV-2, HPIV-3, and HPIV-4 have 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.  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.  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. 
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.  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. 
Approximately 41,000 individuals are admitted to the hospital for parainfluenza virus infections each year. Precautions are necessary within hospitals to prevent further spread.  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. 
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