Upper respiratory illness predominates in most patients

Children commonly present with croup (laryngotracheobronchitis)

Barking cough + hoarseness are characteristic

Stridor indicates upper airway obstruction

Nasal congestion + rhinorrhoea frequent early features

Fever usually mild–moderate; high fever uncommon

Lower respiratory involvement → bronchiolitis or pneumonia

Wheezing and dyspnea in infants and elderly

Symptoms more severe in immunocompromised patients

Seasonal outbreaks — especially in pediatric populations

Recurrent infections occur; immunity is incomplete

Acute laryngotracheobronchitis (croup) — most common complication in children

Airway obstruction due to subglottic edema

Bronchiolitis in infants

Secondary bacterial infection — otitis media, sinusitis, pneumonia

Dehydration due to poor feeding and fever

Hypoxia in severe respiratory involvement

Respiratory failure (rare; severe pediatric or immunocompromised cases)

Recurrent wheezing in susceptible children after infection

Influenza (A/B/C) — higher fever, systemic toxicity more prominent

Respiratory Syncytial Virus (RSV) — bronchiolitis predominant in infants

Adenovirus infection — conjunctivitis + pharyngitis common

COVID-19 — systemic symptoms, anosmia, pneumonia patterns

Bacterial tracheitis — high fever, toxic child, purulent secretions

Epiglottitis — sudden onset, drooling, severe airway obstruction

Diphtheria — membranous pharyngitis, neck swelling

Allergic rhinitis — no fever, eosinophilic inflammation

Foreign body aspiration — sudden onset stridor without prodrome

Pertussis — paroxysmal cough with inspiratory whoop

Human parainfluenza viruses (HPIV) are the causative agents

• Family: Paramyxoviridae

• Enveloped, single-stranded negative-sense RNA viruses

• Spread mainly via respiratory droplets and direct contact

Four major human types

• HPIV-1 → most common cause of croup outbreaks

• HPIV-2 → croup and URTI

• HPIV-3 → bronchiolitis and pneumonia (infants)

• HPIV-4 → milder respiratory illness, less common

Primary mode of transmission

• Respiratory droplets (cough/sneeze)

• Contaminated surfaces → hand-to-nose/eye transmission

• Close contact settings (households, daycare, hospital)

High-risk groups

• Infants and young children

• Elderly

• Immunocompromised patients

• Chronic lung disease patients

Seasonal pattern

• HPIV-1 → autumn outbreaks

• HPIV-2 → late autumn

• HPIV-3 → spring and early summer

• HPIV-4 → variable, less defined seasonality

Not a bacterial disease

• Secondary bacterial infection may occur but virus is primary cause.

Diagnosis is primarily clinical in mild upper respiratory infections

RT-PCR from nasopharyngeal swab — most sensitive and specific test

Viral antigen detection (DFA/rapid tests) — supportive but less sensitive

Multiplex respiratory viral panel — identifies parainfluenza with other viruses

CBC usually normal or mild lymphocytosis

CRP/ESR mildly elevated in inflammatory cases

Chest X-ray if LRTI (lowerRespiratoryTractInfection) suspected — peribronchial thickening, hyperinflation

Neck X-ray (croup) — steeple sign due to subglottic narrowing

Pulse oximetry — assess hypoxia in moderate–severe disease

ABG in severe respiratory distress

Parainfluenza is the most common viral cause of croup in children

Croup age group: 6 months – 3 years (peak incidence)

Barking cough + inspiratory stridor strongly suggests laryngotracheitis

Severity depends on degree of subglottic edema rather than viral load

Symptoms often worse at night due to circadian airway reactivity

Cool mist and calm environment reduce airway spasm in children

Steroids are the mainstay of treatment in moderate–severe croup

Nebulized adrenaline provides rapid but temporary relief in airway obstruction

Antibiotics are NOT indicated unless secondary bacterial infection suspected

Recurrent croup may suggest airway hyperreactivity or structural anomaly

Adults usually develop mild URTI; severe airway disease uncommon

Immunity is incomplete — reinfections occur throughout life

Virus enters through upper respiratory mucosa via inhaled droplets

Initial replication occurs in nasal and nasopharyngeal epithelial cells

Spread to larynx, trachea, and bronchi leads to inflammatory airway edema

Croup results from subglottic mucosal edema and narrowing of pediatric airway

Bronchiolitis develops due to infection of small airway epithelium

Pneumonia occurs when infection extends to lower respiratory tract and alveoli

After mucosal entry, virus attaches to epithelial cells via hemagglutinin-neuraminidase proteins.

Local viral replication triggers epithelial cell injury and mucociliary dysfunction.

Host immune response activates cytokines → IL-6, interferons, TNF-α

Inflammatory cell infiltration leads to airway obstruction and wheeze

Severe disease occurs in infants due to narrow airway caliber

Immunocompromised hosts show prolonged viral replication and severe LRTI

Secondary bacterial infection may follow epithelial damage in severe cases.

Virus infects respiratory epithelial cells of upper and lower airway

Attachment via hemagglutinin-neuraminidase (HN) protein to host cells

Fusion protein (F) mediates viral entry and syncytium formation

Epithelial cell necrosis and desquamation occur in infected airway

Submucosal edema develops due to inflammatory response

Perivascular and peribronchial lymphocytic infiltration seen

In croup — marked subglottic mucosal edema causes airway narrowing

In bronchiolitis — small airway obstruction due to inflammation + mucus plugging

Goblet cell hypersecretion increases mucus production

Vascular congestion contributes to mucosal thickening

Severe cases may show diffuse lower respiratory tract involvement

No chronic structural lung damage in uncomplicated cases

No licensed vaccine currently available for routine prevention

Droplet transmission is the main mode → respiratory hygiene is key

Frequent hand washing reduces viral spread

Avoid close contact with symptomatic individuals

Isolation of infected children in hospital/ward settings

Mask use during outbreaks reduces transmission

Surface disinfection in daycare and pediatric wards

Avoid sharing utensils, towels, toys during illness

High-risk groups require protection:

Infants

Elderly

Immunocompromised patients

Breastfeeding provides partial passive immune protection in infants

Community awareness during seasonal outbreaks helps early containment

Neck X-ray (AP view) in croup shows “steeple sign” due to subglottic narrowing

Lateral neck film may show airway narrowing without epiglottic swelling

Chest X-ray usually normal in mild URTI cases

Perihilar infiltrates may be seen in lower respiratory involvement

Hyperinflation seen in bronchiolitis due to air trapping

Patchy atelectasis may occur from mucus plugging

No focal lobar consolidation unless secondary bacterial infection

Ultrasound rarely used — may show airway narrowing in pediatric settings

CT chest not routinely required — used only in severe/complicated cases

Human Parainfluenza Virus (HPIV) belongs to Paramyxoviridae family

Enveloped, single-stranded negative-sense RNA virus

HPIV-1 — most common cause of croup (autumn outbreaks)

HPIV-2 — also associated with croup but less frequent

HPIV-3 — more commonly causes bronchiolitis and pneumonia (infants)

HPIV-4 — milder respiratory illness; less commonly detected

HPIV-4 has two subtypes: 4A and 4B

Immunity is incomplete → reinfections occur throughout life

No true “serotype switching” like influenza — antigenic variation is limited compared to influenza viruses

Fever (low–moderate; higher in children)

Erythematous pharynx and nasal mucosa

Hoarseness of voice due to laryngeal inflammation

Inspiratory stridor (croup — upper airway narrowing)

Barking cough heard clinically

Wheezing (lower airway involvement)

Tachypnea in moderate–severe disease

Chest retractions in children (suprasternal/intercostal)

Reduced air entry in bronchiolitis/pneumonia cases

Cyanosis — late sign of severe respiratory compromise

Parainfluenza is a common viral respiratory infection affecting children and immunocompromised adults

• Major cause of croup (barking cough, stridor).

• Usually causes mild upper respiratory infection in older children/adults.

• Can cause bronchiolitis and pneumonia in infants.

• Spread by respiratory droplets and contact.

• No specific antiviral therapy in routine use.

• Most cases are self-limiting.

HPIV-1 and HPIV-2 commonly cause croup.

HPIV-3 is more associated with bronchiolitis and pneumonia.

References

PDF

https://www.ncbi.nlm.nih.gov/books/NBK8461/#A3128

2026-02-21 14:24:53

Parainfluenza virus type 1, Sendai strain

2026-02-21 14:23:36

Fever — usually low-grade to moderate

Rhinorrhoea — watery discharge due to nasal mucosal inflammation

Nasal congestion and blockage

Sore throat (pharyngitis)

Dry cough — early stage

Hoarseness of voice — laryngeal involvement

Barking cough — characteristic of croup (laryngotracheobronchitis)

Noisy breathing (stridor) — especially in children

Breathing difficulty — due to upper airway inflammation

General malaise and fatigue

Headache and body ache (mild)

Reduced appetite in children

Infants and young children may present primarily with:

Irritability

Feeding difficulty

Sleep disturbance due to cough and airway obstruction

Supportive management is the mainstay of therapy

No specific antiviral approved for routine use

Rest and adequate oral hydration

Paracetamol for fever and discomfort

Humidified air reduces airway irritation and helps mucus clearance

Nasal saline drops relieve congestion

For croup (laryngotracheobronchitis):

Single-dose oral/IM dexamethasone reduces airway edema

Nebulized adrenaline (epinephrine) for moderate–severe stridor

Oxygen therapy if hypoxic

For bronchiolitis or pneumonia:

Oxygen support as needed

Close monitoring of respiratory effort

IV fluids if poor oral intake

Ventilatory support in severe respiratory failure

Antibiotics only if secondary bacterial infection suspected

Hospital admission for severe airway obstruction, persistent stridor at rest, or respiratory distress