|Human respiratory syncytial |
|An x ray of a child with HRSV showing the typical bilateral perihilar fullness|
The virus, human respiratory syncytial virus (HRSV), a member of the species Human orthopneumovirus (formerly Human respiratory syncytial virus), is a syncytial virus that causes respiratory tract infections. It is a major cause of lower respiratory tract infections and hospital visits during infancy and childhood. A prophylactic medication, palivizumab, can be employed to prevent HRSV in preterm (under 35 weeks gestation) infants, infants with certain congenital heart defects (CHD) or bronchopulmonary dysplasia (BPD), and infants with congenital malformations of the airway. Treatment is limited to supportive care, including oxygen therapy and more advanced breathing support with CPAP or nasal high flow oxygen, as required.
In the United States, 60% of infants are infected during their first HRSV season, and nearly all children will have been infected with the virus by 2–3 years of age. Of those infected with RSV, 2–3% will develop bronchiolitis, necessitating hospitalization. Natural infection with HRSV induces protective immunity which wanes over time—possibly more so than other respiratory viral infections—and thus people can be infected multiple times. Sometimes an infant can become symptomatically infected more than once, even within a single HRSV season. Severe HRSV infections have increasingly been found among elderly patients. Young adults can be re-infected every five to seven years, with symptoms looking like a sinus infection or a cold (infections can also be asymptomatic).
Signs and symptoms
The incubation time (from infection until symptoms arrive) is 4–5 days. For adults, HRSV produces mainly mild symptoms, often indistinguishable from common colds and minor illnesses. The Centers for Disease Control consider HRSV to be the "most common cause of bronchiolitis (inflammation of the small airways in the lung) and pneumonia in children under 1 year of age in the United States". For some children, RSV can cause bronchiolitis, leading to severe respiratory illness requiring hospitalization and, rarely, causing death. This is more likely to occur in patients that are immunocompromised or infants born prematurely. Other HRSV symptoms common among infants include listlessness, poor or diminished appetite, and a possible fever.
Recurrent wheezing and asthma are more common among individuals who suffered severe HRSV infection during the first few months of life than among controls; whether HRSV infection sets up a process that leads to recurrent wheezing or whether those already predisposed to asthma are more likely to become severely ill with HRSV has yet to be determined.
Symptoms of pneumonia in immuno-compromised patients such as in transplant patients and especially bone marrow transplant patients should be evaluated to rule out HRSV infection. This can be done by means of polymerase chain reaction (PCR) testing for HRSV nucleic acids in peripheral blood samples if all other infectious processes have been ruled out or if it is highly suspicious for RSV such as a recent exposure to a known source of HRSV infection. The incubation period is 2–8 days, but is usually 4–6 days.
- bronchiolitis or pneumonia
- recurring infections
- acute otitis media
|Transmission electron micrograph of RSV|
Human respiratory syncytial virus is a medium-sized (120–200 nm) enveloped virus that contains a linear negative-sense RNA genome (must be converted to a positive RNA prior to translation). The former contains virally encoded F, G, and SH lipoproteins. The F and G lipoproteins are the only two that target the cell membrane, and are highly conserved among RSV isolates. HRSV is divided into two antigenic subgroups, A and B, on the basis of the reactivity of the virus with monoclonal antibodies against the attachment (G) and fusion (F) glycoproteins. Subtype B is characterized as the asymptomatic strains of the virus that the majority of the population experiences. The more severe clinical illnesses involve subtype A strains, which tend to predominate in most outbreaks.
Four of the viral genes code for intracellular proteins that are involved in genome transcription, replication, and particle budding, namely N (nucleoprotein), P (phosphoprotein), M (matrix protein), and L (“large” protein, containing the RNA polymerase catalytic motifs). The RSV genomic RNA forms a helical ribonucleoprotein (RNP) complex with the N protein, termed nucleocapsid, which is used as template for RNA synthesis by the viral polymerase complex. The three-dimensional crystal structure of a decameric, annular ribonucleoprotein complex of the RSV nucleoprotein (N) bound to RNA has been determined at 3.3 Å resolution. This complex mimics one turn of the viral helical nucleocapsid complex. Its crystal structure was combined with electron microscopy data to provide a detailed model for the RSV nucleocapsid.
To date, 10 HRSV-A genotypes have been designated, GA1 to GA7, SAA1, NA1, and NA2. The HRSV-B genotypes include GB1 to GB4, SAB1 to SAB3, and BA1 to BA6.
The genome of HRSV was completely sequenced in 1997.
Bayesian estimates of the mutation rates in the subtype A genomes give a mutation rate of 6.47×10−4 (credible interval: 5.56×10−4 – 7.38×10−4) substitutions/site/year. This is similar to other RNA viruses. The population size has remained constant over the last 70 years and the G protein appears to be the main site of diversifying selection. The most recent common ancestor evolved ~1943 (credible interval: 1923–1954). The HRSV-B evolutionary rate (1.95×10−3 nucleotide substitutions/site/year) is similar to that previously estimated for HRSV-A (1.83×10−3 nucleotide substitutions/site/year). However, natural HRSV-B isolates appear to accommodate more drastic changes in their attachment G proteins. The most recent common ancestor of the currently circulating subgroup B strains was estimated to date back to around the year 1949. The divergence between the two major subgroups was calculated to have occurred approximately 350 years ago (mid-17th century).
HRSV is a negative-sense, single-stranded RNA virus of the family Pneumoviridae. Its name comes from the fact that F proteins on the surface of the virus cause the cell membranes on nearby cells to merge, forming syncytia.
HRSV spreads easily by direct contact, and can remain viable for a half an hour or more on hands or for up to 5 hours on countertops. Childcare facilities allow for rapid child-to-child transmission in a short period of time.
The HRSV is virtually the same as chimpanzee coryza virus and can be transmitted from apes to humans, although transmission from humans to apes is more common. The virus has also been recovered from cattle, goats and sheep, but these are not regarded as major vectors of transmission and there is no animal reservoir of the virus.
HRSV can last 2–8 days, but symptoms may persist for up to three weeks.
Human respiratory syncytial virus may be suspected based on the time of year of the infection; prevalence usually coincides with the winter flu season.
- Physical exam: wheezing and other abnormal sounds in the chest
- Chest X-rays to check for typical bilateral perihilar fullness of bronchiolitis induced by the virus
- Skin monitoring to check for hypoxemia, a lower than usual level of oxygen in the bloodstream
- Blood tests to check white cell counts or to look for the presence of viruses, bacteria or other organisms
- Lab test of respiratory secretions
Several different types of laboratory tests are commercially available for diagnosis of RSV infection. Rapid diagnostic assays performed on respiratory specimens are available commercially. Most clinical laboratories currently use antigen detection tests. Compared with culture, the sensitivity of antigen detection tests generally ranges from 80% to 90%. Antigen detection tests and culture are generally reliable in young children but less useful in older children and adults.
Sensitivity of virus isolation from respiratory secretions in cell culture varies among laboratories. RT-PCR assays are now commercially available. The sensitivity of these assays is equal to or exceeds the sensitivity of virus isolation and antigen detection methods. Highly sensitive RT-PCR assays should be considered when testing adults, because they may have low viral loads in their respiratory specimens.
Serologic tests are less frequently used for diagnosis. Although useful for research, a diagnosis using a collection of paired acute and convalescent sera to demonstrate a significant rise in antibody titre to HRSV can not be made in time to guide care of the patient. On top of that, the antibody level does not always correlate with the acuteness or activity level of the infection.
RSV infection can be confirmed using tests for antigens or antibodies, or viral RNA by reverse transcription PCR. Quantification of viral load can be determined by various assay tests.
As the virus is ubiquitous in all parts of the world, avoidance of infection is not possible.
However, palivizumab (brand name Synagis manufactured by MedImmune), a moderately effective prophylactic drug, is available for infants at high risk. Palivizumab is a monoclonal antibody directed against RSV surface fusion protein. It is given by monthly injections, which are begun just prior to the RSV season and are usually continued for five months. HRSV prophylaxis is indicated for infants that are premature or have either cardiac or lung disease, but the cost of prevention limits use in many parts of the world.
Studies of nebulized hypertonic saline have shown that the "use of nebulized 3% HS is a safe, inexpensive, and effective treatment for infants hospitalized with moderately severe viral bronchiolitis" where "respiratory syncytial virus (RSV) accounts for the majority of viral bronchiolitis cases". One study noted a 26% reduction in length of stay: 2.6 ± 1.9 days, compared with 3.5 ± 2.9 days in the normal-saline treated group (p=0.05).
Supportive care includes fluids and supplemental oxygen until the illness runs its course. Salbutamol may be used in an attempt to relieve any bronchospasm if present. Increased airflow, humidified and delivered via nasal cannula, may be supplied in order to reduce the effort required for respiration.[medical citation needed]
As of 2017 there was no vaccine for RSV despite many years of preclinical and clinical work. A formalin-inactivated candidate developed in the 1960s surprisingly increased infections, which came to be known as "immunopotentiation" or vaccine-enhanced disease.
- Robert M. Chanock, discoverer of the human respiratory syncytial virus
- Metapneumovirus, another genus of closely related viruses
- Orthopneumovirus, the genus of related syncytial viruses
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- Synagis (registered to MedImmune, manufacturer of Synagis)
- Virazole (registered to Valeant Pharmaceuticals, manufacturer of Virazole)
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- Biotrin providers of RSV kits
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