Respiratory Syncytial Virus (RSV): Structure, Pathogenesis, and Prevention

 

Table of Contents

• Introduction of Respiratory Syncytial Virus (RSV)
• Structure of Respiratory Syncytial Virus (RSV)
• Genome Structure of Respiratory Syncytial Virus (RSV)
• Epidemiology of Respiratory Syncytial Virus (RSV)
• Transmission of Respiratory Syncytial Virus (RSV)
• Replication of Respiratory Syncytial Virus (RSV)
• Pathogenesis of Respiratory Syncytial Virus (RSV)
• Clinical Manifestations of Respiratory Syncytial Virus (RSV)
• Diagnosis of Respiratory Syncytial Virus (RSV)
• Rapid Antigen Detection Tests
• Direct Fluorescent Antibody
• Polymerase Chain Reaction
• Viral Culture
• Treatment of RSV
• Supportive treatment
• Hospital treatment
• Prevention and Control of RSV

Introduction of Respiratory Syncytial Virus (RSV)

• Respiratory Syncytial Virus (RSV) belongs to the Paramyxoviridae family and is a significant cause of lower respiratory tract illness in infants under 1 year of age.
• Besides infants, individuals with compromised immune, pulmonary, or cardiac systems, and the elderly are also at high risk for RSV infection.
• RSV infection can cause upper respiratory tract infection and bronchiolitis, which can rarely lead to pneumonia, respiratory failure, apnea, and death.
• The virus was first isolated in 1956 from the nasal secretions of chimpanzees with rhinorrhea and coryza and was initially named “Chimpanzee coryza agent” (CCA).
• There are two major antigenic subgroups of RSV, A and B, which differ in the glycoproteins G and F and show epidemiological differences.
• Although both subtypes cocirculate, one subtype generally predominates over the other depending on the region and climate.
• Due to the overlap of clinical symptoms of RSV infection with other respiratory illnesses, laboratory tests are required for diagnosis.

Structure of Respiratory Syncytial Virus (RSV)

• The RSV virion consists of a nucleocapsid packaged in a bilipid-layer envelope derived from the host membrane.
• Virions cultured in cell lines consist of spherical particles measuring 100-350 nm in diameter and long filaments measuring 60-120 nm in diameter and up to 10 μm in length.
• The virus envelope contains a fusion protein (F), a small hydrophobic protein (SH), and the attachment protein (G).
• The viral glycoproteins form separate homo-oligomers presented as short surface spikes (11-16 nm).
• The matrix protein (M) lies beneath the lipid envelope.
• The genome consists of single-stranded, negative-sense RNA encapsulated by the nucleoprotein (N).
• The RNA polymerase protein (L), phosphoprotein (P), and the transcription processivity factor M2-1 also remain associated with the nucleocapsid.

Genome Structure of Respiratory Syncytial Virus (RSV)

• The RSV genome consists of single-stranded, negative-sense RNA with 10 open reading frames (ORFs).
• The RNA is approximately 15.2 kb in size.
• It encodes 11 structural and nonstructural proteins.
• A complementary copy of the genome, called the antigenome, is involved in RNA replication.
• Both the genome and antigenome lack 5’ caps and 3’ polyA tails.
• The genome has a leader region at the 3’ end and a trailer region at the 5’ end.
• The RNA consists of the leader region, followed by the genes NS1, NS2, N, P, M, SH, G, F, M2, L, and the trailer region, in order from the 3’ to the 5’ end.
• RNA encapsidation protects it from degradation and shields it from recognition by host cell receptors that initiate immune responses.

Epidemiology of Respiratory Syncytial Virus (RSV)

• RSV is a common human pathogen known for causing frequent reinfections.
• 90% of children contract the virus within their first 2 years of life, with frequent reinfections occurring in the elderly.
• Among infected children, 0.5% to 2.0% require hospitalization for lower respiratory tract infections, 50% to 90% develop bronchiolitis, and 5% to 40% develop pneumonia.
• Annually, an estimated 33 million cases in children are reported, leading to 3 million hospitalizations and 120,000 deaths due to complications from the infection.
• RSV infections exhibit seasonal variations across different geographical regions.
• Populations at risk for severe illness and death include premature infants, individuals with preexisting cardiac, pulmonary, neurologic, or immunosuppressive conditions, and the elderly.
• In Canada, RSV results in 5,800 to 12,000 hospitalizations each year, while in the United States, 77,700 hospitalizations due to bronchiolitis were recorded between 1997 and 2000.
• Higher mortality rates are observed in underdeveloped countries, with an estimated 66,000 to 199,000 deaths in children under 5 years of age occurring in 2005.

Transmission of Respiratory Syncytial Virus (RSV)

• RSV spreads from person to person via respiratory droplets through coughs or sneezes, droplets getting into the eyes, nose, or mouth, through fomites, and direct contact with an infected individual (e.g., kissing an infected person).
• The incubation period for RSV is 2 to 8 days, typically 4 to 6 days, depending on factors such as the patient's age and whether it is their primary RSV infection.
• Patients at high risk for developing severe diseases associated with RSV infection include:
• Infants younger than 6 months of age
• Infants and children with underlying lung diseases, such as bronchopulmonary dysplasia or congenital heart diseases
• Infants exposed to secondhand smoke
• Immunocompromised patients (e.g., those with immune disorders or who have recently undergone organ transplantation)
• Individuals with asthma
• Individuals with cardiopulmonary disease
• Elderly patients with chronic obstructive pulmonary disease (COPD)

Replication of Respiratory Syncytial Virus (RSV)

• Attachment

RSV attaches to its receptors, such as the chemokine receptor CX3CR1, on the apical surface of ciliated epithelial cells via the attachment glycoprotein (G protein).

• Fusion

The fusion protein (F) facilitates the fusion of the viral membrane with the host membrane, releasing the nucleocapsid into the host cell cytoplasm.

• Biosynthesis

The negative-sense RNA serves as a template for the RSV polymerase to synthesize mRNAs and genome progeny. Surface glycoproteins are produced, post-translationally modified, and transported through the endoplasmic reticulum to the host cell membrane. Structural and nonstructural proteins are synthesized from the mRNAs, and progeny genomes and antigenomes are also synthesized.

• Assembly

The matrix protein (M) associates with the replication complex and interacts with the cytoplasmic tail of the F protein, enabling the viral nucleocapsid to be packaged into viral filament particles.

• Release

RSV is released from the cells through budding, incorporating surface glycoproteins into the membrane to form the viral envelope.

Pathogenesis of Respiratory Syncytial Virus (RSV)

Figure: Early-life immune responses to RSV infection. Following RSV infection, neonatal innate responses are subdued: cytokine levels, such as interferons, are low, Toll-like receptor (TLR) signaling is diminished, antigen-presenting function is altered, and Treg activation is reduced. These observations skew the adaptive immune response towards Th2 dominance and result in limited cytotoxic T lymphocyte (CTL) activation via Th1 pathways. Tfh activation is weakened, leading to reduced B cell memory production and antibody generation. IFN-γ inhibits antibody production, resulting in antibodies with low titers and affinity. These suppressive immune responses contribute to bronchiolitis in susceptible infants.

• RSV pathogenesis is a complex and variable process influenced by numerous host and viral factors.
• The disease spectrum ranges from mild rhinitis to severe conditions affecting the upper and lower respiratory tract, such as bronchiolitis and pneumonia.
• While the virus can directly damage respiratory epithelium, in immunocompetent individuals, the immune response plays a pivotal role in disease progression.
• Following transmission into the host, RSV quickly spreads within the respiratory tract, primarily infecting apical ciliated epithelial cells.
• During primary RSV infection, the host mounts a response dominated by IFN-ϒ production from NK cells, CD4+, and CD8+ T cells.
• RSV infection post-immunization with FI-RSV or the RSV G glycoprotein triggers an immune response skewed towards type 2 cytokines, often leading to lung eosinophilia and increased airway mucus production.
• In cases of RSV infection accompanied by allergic inflammation or in the absence of STAT1-mediated signaling, airway epithelial mucus production is induced with IL-17 cytokine expression.
• Activation of both humoral and cytotoxic T-cells occurs, resulting in viral cytotoxicity and host immune response-mediated necrosis of respiratory epithelial cells.
• This can lead to obstruction of small airways by mucus and cellular debris, along with alveolar obstruction.
• Additional effects may include impaired ciliary function, reduced mucus clearance, airway swelling, and decreased lung compliance.

Clinical Manifestations of Respiratory Syncytial Virus (RSV)

Symptoms of RSV infection typically begin 4 to 6 days after infection and often progress in stages rather than all at once. Common symptoms include:

• Runny nose
• Decreased appetite
• Cough
• Sneezing
• Fever
• Wheezing

In more severe cases affecting the lower respiratory tract, RSV can lead to conditions such as pneumonia or bronchiolitis, with signs and symptoms that may include:

• Fever
• Severe cough
• Wheezing
• Rapid or difficult breathing
• Cyanosis (bluish skin coloration due to oxygen deficiency)
• Middle ear infection (otitis media)

Infants, particularly, may experience more severe symptoms such as:

• Short, shallow, and rapid breathing
• Difficulty breathing (visible chest muscle and skin retractions with each breath)
• Cough
• Poor feeding
• Unusual tiredness
• Irritability

While most children recover within one to two weeks, infants with chronic heart or lung problems are at risk of life-threatening infections.

Diagnosis of Respiratory Syncytial Virus (RSV)

Rapid Antigen Detection Tests

Rapid Antigen Detection Tests (RADTs) are popular for RSV testing due to their ease of use, quick results, and acceptable sensitivity and specificity. However, they may occasionally yield false-negative results, necessitating consideration of more sensitive methods like PCR in such cases.

Direct Fluorescent Antibody

Direct Fluorescent Antibody (DFA) testing is reliable for diagnosing RSV in infants and young children but shows lower sensitivity in adults due to reduced viral shedding compared to younger individuals.

Polymerase Chain Reaction

Polymerase Chain Reaction (PCR) is highly sensitive and specific for detecting RSV in nasopharyngeal swabs and aspirates. It is particularly beneficial for older children and adults, and it is preferred for hospitalized and immunocompromised patients. However, PCR can be more costly than DFA and may have a longer turnaround time in certain laboratory settings.

Viral Culture

Viral Culture, while not recommended for initial clinical management due to its slow turnaround time (about 3-5 days) and lower sensitivity, remains important for detecting coinfections. It offers high specificity, and isolated virus can be preserved for further diagnostic studies.

Other laboratory tests used to diagnose RSV complications include:

• Blood tests to examine white cell counts or detect viruses, bacteria, and other pathogens.
• Chest X-rays to assess lung inflammation.
• Pulse oximetry, a painless method using a skin monitor, to detect lower-than-normal oxygen levels in the blood.

Treatment of RSV

Mild RSV infections typically do not require specific treatment, and antibiotics or bronchodilators are not used.

Supportive treatment

• Acetaminophen (such as Tylenol) may be given to reduce fever. Aspirin should never be administered to children.
• Nasal saline drops and suctioning can help clear a stuffy nose.
• Antibiotics may be prescribed if a bacterial infection, like bacterial pneumonia, is suspected.
• It's important to ensure adequate fluid intake to prevent dehydration. Signs of dehydration include dry mouth, decreased urine output, sunken eyes, and extreme fussiness or sleepiness.

Hospital treatment

For further complications, treatments at the hospital may include:

• Intravenous (IV) fluids may be administered.
• Humidified oxygen might be provided.
• In severe cases, a breathing machine (mechanical ventilation) may be necessary.

Currently, there are no licensed vaccines for RSV infections, but several vaccine candidates are undergoing clinical trials to assess their safety and effectiveness. However, palivizumab can be administered to infants and young children at high risk for severe disease.

Prevention and Control of RSV

RSV is highly contagious, so it's crucial to implement effective preventive measures to limit its spread. Here are some key interventions:

• Avoid close contact with individuals known to be infected with RSV.
• Avoid sharing cups, bottles, towels, toys, utensils, and other items that may be contaminated with the virus.
• Wash hands thoroughly with soap and water after contact with an infected person.
• Cover the mouth when sneezing or coughing using a tissue or the upper sleeve, rather than the hand.
• Individuals experiencing flu-like symptoms should avoid contact with high-risk groups for severe RSV complications. This includes premature infants, children under 2 years old, those with chronic lung or heart conditions, and individuals with weakened immune systems.
• Educate parents, caregivers, and healthcare providers about RSV risks, transmission methods, preventive actions, and associated factors.
• Passive immunizations like RSV-IGIV and palivizumab have been developed for RSV infections. However, RSV-IGIV, a polyclonal immunoglobulin product derived from pooled blood donors, is no longer available.