Human parainfluenza viruses (HPIVs) are the viruses that cause human parainfluenza. HPIVs are a paraphyletic group of four distinct single-stranded RNA viruses belonging to the Paramyxoviridae family. These viruses are closely associated with both human and veterinary disease. Virions are approximately 150–250 nm in size and contain negative sense RNA with a genome encompassing about 15,000 nucleotides.
thumb|300 px|Fusion glycoprotein trimer, Human parainfluenza virus 3 (HPIV3).
The viruses can be detected via cell culture, immunofluorescent microscopy, and PCR. HPIVs lack antiviral drug options and vaccine, remain a major contributor toward the annual burden of hospitalisations among children younger than age 5—being second only to respiratory syncytial virus (RSV) in this regard—and are the predominant cause of laryngotracheobronchitis (also known as croup).
Classification
The first HPIV was discovered in the late 1950s. The taxonomic division is broadly based on antigenic and genetic characteristics, forming four major serotypes or clades, which today are considered distinct viruses. These include:
{| class="wikitable"
|-
! Virus
! GenBank acronym
! NCBI taxonomy
! Notes
|-
|Human parainfluenza virus type 1
|HPIV-1
|12730
|Most common cause of croup
|-
|Human parainfluenza virus type 2
|HPIV-2
|11212
|Causes croup and other upper and lower respiratory tract illnesses
|-
|Human parainfluenza virus type 3
|HPIV-3
|11216
|Associated with bronchiolitis and pneumonia
|-
|Human parainfluenza virus type 4
|HPIV-4
|11203
|Includes subtypes 4a and 4b
|}
HPIVs belong to two genera: Respirovirus (HPIV-1 & HPIV-3) and Rubulavirus (HPIV-2 & HPIV-4).
{| class="wikitable"
|-
! Structural protein
! Location
! Function
|-
|Hemagglutinin-neuraminidase (HN)
|Envelope
|Attachment and cell entry
|-
|Fusion Protein (F)
|Envelope
|Fusion and cell entry
|-
|Matrix Protein (M)
|Within the envelope
|Assembly
|-
|Nucleoprotein (NP)
|Nucleocapsid
|Forms a complex with the RNA genome
|-
|Phosphoprotein (P)
|Nucleocapsid
|Forms as part of RNA polymerase complex
|-
|Large Protein (L)
|Nucleocapsid
|Forms as part of RNA polymerase complex
|}
With the advent of reverse genetics, it has been found that the most efficient human parainfluenza viruses (in terms of replication and transcription) have a genome nucleotide total that is divisible by the number 6. This has led to the "rule of six" being coined. Exceptions to the rule have been found, and its exact advantages are not fully understood.
Electrophoresis has shown that the molecular weight of the proteins for the four HPIVs are similar (with the exception of the phosphoprotein, which shows significant variation).
Viral entry and replication
Viral replication is initiated only after successful entry into a cell by attachment and fusion between the virus and the host cell lipid membrane. Viral RNA (vRNA) is initially associated with nucleoprotein (NP), phosphoprotein (P) and the large protein (L). The hemagglutinin–neuraminidase (HN) is involved with viral attachment and thus hemadsorption and hemagglutination. Furthermore, the fusion (F) protein is important in aiding the fusion of the host and viral cellular membranes, eventually forming syncytia.
Initially the F protein is in an inactive form (F<sub>0</sub>) but can be cleaved by proteolysis to form its active form, F<sub>1</sub> and F<sub>2</sub>, linked by di-sulphide bonds. Once complete, this is followed by the HPIV nucleocapsid entering the cytoplasm of the cell. Subsequently, genomic transcription occurs using the viruses own 'viral RNA-dependent RNA polymerase' (L protein). The cell's own ribosomes are then tasked with translation, forming the viral proteins from the viral mRNA.
The observable and morphological changes that can be seen in infected cells include the enlargement of the cytoplasm, decreased mitotic activity and 'focal rounding', with the potential formation of multi-nucleate cells (syncytia).
The pathogenicity of HPIVs is mutually dependent on the viruses having the correct accessory proteins that are able to elicit anti-interferon properties. This is a major factor in the clinical significance of disease. HPIV-1, HPIV-2 and HPIV-3 have been linked with up to a third of these infections. Upper respiratory infections (URI) are also important in the context of HPIV, however, they are caused to a lesser extent by the virus. The highest rates of serious HPIV illnesses occur among young children, and surveys have shown that about 75% of children aged 5 or older have antibodies to HPIV-1.
Symptoms include fever, cough (mild, barky or with mucus), a runny or stuffy nose, sore throat, wheezing, hoarseness and sneezing.
For infants and young children, it has been estimated that about 25% will develop "clinically significant disease".
Repeated infection throughout the life of the host is not uncommon and symptoms of later breakouts include upper respiratory tract illness, such as cold and a sore throat. In immunosuppressed people, parainfluenza virus infections can cause severe pneumonia, which can be fatal.
HPIV-1 and HPIV-2 have been demonstrated to be the principal causative agent behind croup (laryngotracheobronchitis), which is a viral disease of the upper airway and is mainly problematic in children aged 6–48 months of age. Biennial epidemics starting in autumn are associated with both HPIV-1 and -2; however, HPIV-2 can also have yearly outbreaks.
HPIV-4 remains infrequently detected. It is now believed to be more common than previously thought but less likely to cause severe disease. By the age of 10, the majority of children are seropositive for HPIV-4 infectionthis may be indicative of a large proportion of asymptomatic or mild infections. Associations between HPIVs and neurologic disease are known. For example, hospitalisation with certain HPIVs has a strong association with febrile seizures. HPIV-4b has the strongest association, up to 62% of HPIV-4b hospitalisations, followed by HPIV-3 (17%) and -1 (7%). and Guillain–Barré syndrome.
Airway inflammation
The inflammation of the airway is a common attribute of HPIV infection. It is believed to occur due to the large scale upregulation of inflammatory cytokines. Common cytokines observed to be upregulated include IFN–α, various interleukins (i.e., IL–2, IL-6), and TNF–α. Various chemokines and inflammatory proteins are also believed to be associated with the common symptoms of HPIV infection.
Immunology
The body's primary defense against HPIV infection is adaptive immunity involving both humoral and cellular immunity. With humoral immunity, antibodies that bind to the surface viral proteins HN and F protect against later infection. Patients with defective cell-mediated immunity also experience more severe infection, suggesting that T cells are important in clearing infection. The exact associations between HPIVs and diseases such as chronic obstructive pulmonary disease (COPD) are still being investigated.
In developing regions of the world, preschool children remain the highest mortality risk group. Mortality may be a consequence of primary viral infection or secondary problems, such as bacterial infection. Predispositions, such as malnutrition and other deficiencies, may further elevate the chances of mortality associated with infection.
Prevention
Despite decades of research, no vaccines currently exist.
Recombinant technology has however been used to target the formation of vaccines for HPIV-1, -2 and -3 and has taken the form of several live-attenuated intranasal vaccines. Two vaccines in particular were found to be immunogenic and well tolerated against HPIV-3 in phase I trials. HPIV-1 and -2 vaccine candidates remain less advanced.
Maternal antibodies may offer some degree of protection against HPIVs during the early stages of life via the colostrum in breast milk.
Medication
Ribavirin is one medication which has shown good potential for the treatment of HPIV-3 given recent in-vitro tests (in-vivo tests show mixed results).
The majority of transmission has been linked to close contact, especially in nosocomial infections. Chronic care facilities and doctors' surgeries are also known to be transmission 'hotspots' with transmission occurring via aerosols, large droplets and also fomites (contaminated surfaces).
The exact infectious dose remains unknown, but evidence exists that the infectious dose for hPIV1 is small (80 TCID50 of hPIV1) Influenza has a distinct seasonal pattern, with outbreaks occurring mainly in winter months. Parainfluenza viruses circulate year-round, with each type having its own seasonal patterns. The viruses have a tendency towards different complications: influenza is more likely to cause severe pneumonia in high-risk groups; parainfluenza is more likely to cause croup in children. Influenza has effective vaccines available and can be treated with antiviral medications like neuraminidase inhibitors. There are currently no vaccines or specific antiviral treatments for parainfluenza viruses. Parainfluenza tends to infect young children, with most children being infected by age 5. Influenza can affect all ages.
References
Further reading
- Human Parainfluenza Viruses (HPIVs)
External links
- Human Parainfluenza Viruses – information provided by the CDC