Human Parainfluenza viruses (HPIVs) are a group of enveloped RNA viruses belonging to the Paramyxoviridae family, in the genus Paramyxovirus.¹ There are four types (1 through 4) and two subtypes (4a and 4b) of HPIVs.² They are significant human respiratory pathogens, causing mild upper and lower respiratory tract infections to more severe conditions such as croup (laryngotracheobronchitis), bronchiolitis, and pneumonia,³ affecting mainly infants and small children.⁴ Seasonal HPIV epidemics place a major burden of disease on children, accounting for 40% of pediatric hospitalisations for lower respiratory tract infections (LRTIs) and 75% of croup cases.⁵ Understanding how parainfluenza is transmitted within the community is important for implementing effective control measures and mitigating its impact and burden on public health. 

Parainfluenza serotypes and seasonality

Human parainfluenza viruses are classified within four serotypes according to genetic and antigenic variation.⁶ HPIV2 and HPIV4 (subtypes HPIV4A and HPIV4B) are classified within the genus Rubulavirus, whereas HPIV1 and HPIV3 are placed into the genus Respirovirus.^7,8

Human parainfluenza virus type 1 (HPIV-1)

HPIV-1 has been reported as a biennial fall epidemic.⁹ One of the main manifestations of HPIV-1 infection is croup, a viral infection characterised by inflammation of the larynx and upper airways, leading to a barking cough, hoarse voice and difficulty breathing. Additionally, symptoms such as fever, runny nose, nasal congestion, and sore throat may also occur. In severe cases, respiratory distress and difficulty breathing may develop.¹

Human parainfluenza virus type 2 (HPIV-2)

HPIV-2 outbreaks are typically smaller in scale and occur biennially in alternate years with HPIV-1, although they can also cause yearly outbreaks.⁹ HPIV-2 infection typically manifests as mild to moderate respiratory illness, with symptoms resembling those of a common cold.¹ Patients may experience nasal congestion, runny nose, cough, and mild fever. Unlike HPIV-1, severe manifestations such as croup are less commonly associated with HPIV-2 infection.¹⁰ Lower respiratory tract involvement leading to bronchiolitis or pneumonia may occur in some patients such as young children and immunocompromised individuals.

Human parainfluenza virus type 3 (HPIV-3)

HPIV-3 outbreaks are reported to happen annually, mainly from April to June. In years when HPIV-1 is not prevalent (even-numbered years), HPIV-3 often experiences an extended spring season that may extend into fall or a secondary increase in activity from November to December.⁹ Symptoms of HPIV-3 infection include fever, runny nose and cough, but in some cases, it can evolve to conditions such as bronchiolitis, bronchitis and pneumonia.^11,12 As with the other HPIV serotypes, infants and young children and more vulnerable to HPIV-3 infections, as well as older adults and individuals with compromised immune systems.¹³

Human parainfluenza virus type 4 (HPIV-4)

HPIV-4 is rare and displays a seasonality pattern similar to that of HPIV-3.⁹ It is known to cause mostly upper respiratory tract infections in children and adults.¹⁵ PIV-4 is further divided into two subtypes, PIV-4A and PIV-4B.¹⁶ PIV-4 infections are generally less common and less severe when compared to PIV-1, PIV-2, and PIV-3.¹⁴ They can cause mild respiratory symptoms similar to the common cold, such as nasal congestion, runny nose, sore throat, and cough. Severe cases have been reported in young children and immunocompromised patients.¹⁷

Modes of transmission

According to the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC), there are at least three transmission routes for respiratory viruses,¹⁸ These are:

Contact transmission

It occurs when virus-containing respiratory secretions from an infected individual come in contact with a susceptible person. This can happen directly through physical contact or indirectly through contact with contaminated surfaces or objects (fomites).¹⁶

Droplet transmission

It occurs when an infected person expels virus-containing respiratory droplets, which land on mucosal surfaces (such as the eyes, nose, and mouth) of a susceptible person.¹⁹ Respiratory droplets can travel relatively short distances, typically within a radius of about 6 feet, making close contact with infected individuals a significant risk factor.^20,21

Airborne or aerosol transmission

While respiratory droplets are the primary mode of transmission for parainfluenza viruses, there is also evidence to suggest the potential for aerosol transmission in certain circumstances.² Aerosols are smaller respiratory particles that can remain suspended in the air for longer periods and travel greater distances when compared to respiratory droplets.²² These aerosols can be produced by the exhalation of an infected person during talking, breathing, coughing, singing or sneezing,²³ or by aerosol-generating medical procedures. Susceptible individuals can inhale these droplets, resulting in infection.

Factors influencing community spread

Several factors contribute to the spread of parainfluenza within the community, such as:

• Seasonality: Parainfluenza infections often exhibit seasonal patterns,²⁴ with peaks occurring in the fall and winter months in temperate regions.²⁵ This seasonality may be influenced by factors such as changes in weather conditions, school openings, and increased indoor crowding during colder months²⁶
• Viral Shedding: Infected individuals can shed parainfluenza virus particles in respiratory secretions several days before the onset of symptoms and continue shedding for up to two weeks or longer after symptoms resolve.²⁷ Asymptomatic or pre-symptomatic shedding can contribute to the transmission of the virus in the community, making it challenging to identify and isolate cases
• Population Susceptibility: The susceptibility of the population to parainfluenza viruses can vary depending on factors such as age, underlying health conditions, and immune status.² Infants, young children, older adults, and individuals with weakened immune systems are at increased risk of severe disease following parainfluenza infection⁹
• Crowded Environments: Environments where people gather close by, such as schools, pubs, public transport, daycare centres, nursing homes, and healthcare facilities, can facilitate the transmission of parainfluenza viruses. Transmission through close contact and the airborne route is facilitated in these crowded settings, particularly among susceptible individuals²⁸

Prevention measures for parainfluenza viruses

To prevent the spread of parainfluenza viruses in the community, several control measures can be implemented,²⁹ such as:

• Hand Hygiene: Frequent handwashing with soap and water or the use of alcohol-based hand sanitisers can reduce the risk of transmitting and acquiring parainfluenza viruses during close personal contact such as shaking hands, as well as through contaminated surfaces as it also reduces the contamination of frequently touched surfaces and objects
• Respiratory Protection: Promoting respiratory protection habits such as covering coughs and sneezes with a tissue or the elbow, can help prevent the spread of respiratory droplets containing the virus from infected people. Wearing masks whenever you have respiratory disease symptoms or are around someone showing respiratory symptoms is also another effective prevention measure to avoid infection by not only influenza but other respiratory viruses as well
• Physical Distancing: Maintaining physical distance from others, especially in crowded settings, can reduce the risk of exposure to respiratory droplets and minimize the transmission of parainfluenza viruses
• Environmental Cleaning: Regular cleaning and disinfection of frequently touched surfaces and objects can help reduce the presence of viable virus particles and interrupt the transmission cycle
• Vaccination: While no specific vaccine targeting parainfluenza viruses is currently available for widespread use, efforts to develop and deploy vaccines against certain strains of parainfluenza are ongoing and may provide future opportunities for disease prevention²

Conclusion

In conclusion, parainfluenza viruses spread within the community primarily through respiratory droplets, contaminated surfaces, and close personal contact such as touching or holding hands. Factors such as seasonality, viral shedding, population susceptibility, and crowded settings contribute to the transmission dynamics of parainfluenza viruses. Prevention measures such as hand hygiene, respiratory etiquette, physical distancing, environmental cleaning and disinfection, and vaccination can help mitigate the spread of parainfluenza and reduce its impact on public health.

FAQ’s

What are the primary modes of transmission for parainfluenza?

Parainfluenza viruses are primarily transmitted through respiratory droplets produced by infected people coughing, sneezing, or talking. These droplets can be swallowed by close people, enhancing transmission. Furthermore, the virus can be transmitted by contact with contaminated surfaces or intimate physical contact with infected individuals.

Can parainfluenza spread through the air?

While respiratory droplets are the primary mechanism of transmission, there is evidence that aerosol transfer may occur in some conditions. Aerosols, which are smaller respiratory particles, can remain suspended in the air for longer periods and travel further than respiratory droplets.

Who is most at risk of parainfluenza infection?

Parainfluenza viruses can infect people of all ages and groups, such as young children, newborns, the elderly, and those with compromised immune systems, who are more likely to become seriously ill. Close contact with infected people, crowded environments, and poor respiratory hygiene habits enhance the chance of transmission.

How can parainfluenza transmission be prevented?

Preventive strategies include washing your hands frequently, covering coughs and sneezes with a tissue or elbow, avoiding close contact with ill people and frequent sanitization of touched surfaces. Vaccination programs may potentially offer prospects for disease prevention.

What should I do if I suspect I've been exposed to parainfluenza?

If you feel you have been exposed to parainfluenza or have symptoms such as fever, cough, sore throat, or difficulty breathing, get medical attention immediately. Healthcare professionals can provide advice on testing, treatment, and preventive measures to keep you and others safe from further spread.