Viral Pneumonia: Practice Essentials, Background, Pathophysiology - Medscape Reference

Both DNA and RNA viruses are involved in the etiology of viral pneumonia. Some are well-known lung pathogens that produce common clinical and radiologic manifestations. Others are rarely involved as lung pathogens.

Etiologic viruses include various families, as follows:

• Orthomyxoviridae (orthomyxoviruses) - Influenza virus

• Papovaviridae (polyomavirus) - JC virus, BK virus

• Picornaviridae (picornaviruses) - Enteroviruses, coxsackievirus, echovirus, enterovirus 71, rhinovirus

Most of the members of Herpesviridae family are documented lung pathogens in hosts with compromised cell immunity and include the following:

Influenza virus, respiratory syncytial virus, adenovirus, parainfluenza virus, coronavirus, rhinovirus, and human metapneumovirus may cause community-acquired viral pneumonia.

Influenza virus

The influenza viruses are enveloped, single-stranded, RNA viruses of the family Orthomyxoviridae and are the most common viral cause of pneumonia. Three serotypes of influenza virus exist: A, B, and C.

Influenza type A can alter surface antigens and infect livestock. This characteristic may account for its ability to create a reservoir for infection and cause epidemics in humans. The virus is spread by means of small-particle aerosol and targets the columnar epithelial cells along the entire respiratory tract.

Influenza type B causes illness that usually is seen in relatively closed populations such as boarding schools. Influenza type C is less common and occurs as sporadic cases.

Influenza type A is usually the most virulent pathogen. The influenza virus has two envelope glycoproteins, hemagglutinin (H) and neuraminidase (N), which are important for a number of reasons. The hemagglutinin initiates infectivity by binding to cellular sialic acid residues, whereas the N protein cleaves newly synthesized virus from sialic acid on cell surfaces, thus allowing spread of the virus to other cells.

The influenza virus maintains its infectivity by undergoing antigenic drift (small number of amino acid substitutions) and shift (large number of amino acid substitutions) due to changes in the protein structure of the surface protein, hemagglutinin. Epidemics occur when a viral drift occurs, and pandemics are seen with viral shift (two influenza A viruses exchange H or N genes during infection of the same hosts) because most people have no prior immunity to the virus.

Two influenza types have emerged of particular importance: H5N1 avian influenza strain and the novel H1N1 swine influenza strain.

Respiratory syncytial virus

Respiratory syncytial virus (RSV) is the most frequent cause of lower respiratory tract infection among infants and children and the second most common viral cause of pneumonia in adults. It is a medium-sized virus of the Paramyxoviridae family that consists of only 1 serotype. Structurally, RSV has 10 unique viral polypeptides, 4 of which are associated with virus envelope, and 2 of these (F and G) are important for infectivity and pathogenicity. Classic RSV infection causes syncytia formation in cell culture, giving the virus its name.

RSV is highly contagious, spreading via droplet and fomite exposure. Most children are infected before age 5 years—the infection rate during an epidemic approaches 100% in certain settings such as daycare centers—but the resulting immunity is incomplete. Reinfection in older children and young adults is common but mild. However, the likelihood of more severe disease and pneumonia increases with advancing age.

Adenoviruses

Adenoviruses are enveloped DNA viruses that cause a wide spectrum of clinical illnesses depending on the serotype of the infecting agent. These include asymptomatic illness, conjunctivitis, febrile upper respiratory disease, pneumonia, gastrointestinal illness, hemorrhagic cystitis, rash, and neurologic disease. Pneumonia is less common in adults outside of military recruit camps and similar facilities, but fulminant disease has been described in infants and in the immunocompromised population and can occur in apparently healthy hosts. ^[15]

Although 52 serotypes exist, classified into 7 subgroups or species (A-G), pulmonary disease is predominantly caused by serotypes 1, 2, 3, 4, 5, 7, 14, and 21. Type 7 viruses can cause bronchiolitis and pneumonia in infants. Types 4 and 7 viruses are responsible for outbreaks of respiratory disease in military recruits.

Adenovirus serotype 14 (subgroup B) is a more virulent strain that has been reported to cause severe respiratory illness and pneumonia. Emergence of this strain was reported in 2005 among civilian and military populations, with outbreaks occurring subsequently at military training centers throughout the United States.

In 2007, adenovirus serotype 14 caused a large, sustained outbreak of febrile respiratory illness among military trainees in Texas and, more recently, in a residential care facility in Washington State. ^{[16, 17, 18]} In a community outbreak in Oregon, the median age was 52 years, and 76% required hospitalization, 47% required critical care, 24% required vasopressors, and 18% died. The majority of these patients were otherwise immunocompetent adults. ^[19]

Spread of adenovirus is by respiratory secretions, infectious aerosols, feces, and fomites. Neonates may acquire infection from exposure to cervical secretions at birth.

Contaminated environmental surfaces can harbor virus capable of causing infection for weeks. The virus is resistant to lipid disinfectants but is inactivated by heat, formaldehyde, and bleach.

Adenoviruses are extremely contagious. Studies of new military recruits have shown seroconversion rates of 34-97% over a 6-week period. ^[16] The majority of children have serologic evidence of prior adenovirus infection by the age of 10.

Parainfluenza virus

Parainfluenza virus (PIV) is a common virus that infects most persons during childhood. PIV is second in importance to only RSV in causing lower respiratory tract disease in children and pneumonia and bronchiolitis in infants younger than six months. Transmission is through direct person-to-person contact or large-droplet spread.

PIV is characterized by nucleocapsids, which develop in the cytoplasm of infected cells, with hemagglutinin present in the virion envelope.

There are four subtypes of PIV, based on antigenic characteristics. PIV type 3 is endemic year-round, while types 1 and 2 peak during the fall season. Immunity is short term, and recurrent upper or lower respiratory tract infections occur throughout life. The infections vary from a mild illness to life-threatening croup, bronchiolitis, or pneumonia. Infection in immunocompromised hosts can result in life-threatening pneumonia with lung injury and respiratory failure. In one study, 44% of hematopoietic stem cell transplant (HSCT) patients with PIV progressed to develop pneumonia, of which 37% died. ^[20]

Rhinovirus

Some authors report that rhinovirus accounts for up to 30% of cases of all virus-related pneumonia. Clinical studies show that rhinovirus is the second most frequently recognized agent associated with pneumonia and bronchiolitis in infants and young children. Rhinovirus infection is linked to asthma hospitalizations in both adults and children.

A study of 211 French children with rhinovirus infection revealed bronchiolitis or bronchitis in 25.6% and pneumonia in 6.2%, after cases of dual bacterial or viral infections were eliminated.

A study from the Netherlands demonstrated that rhinoviruses cause 32% of all lower respiratory tract infections with an identified pathogen in the elderly (> 60 y) symptomatic population. Rhinoviruses were identified more frequently than coronaviruses (17%) or influenza viruses (7%).

Human metapneumovirus

Human metapneumovirus (hMPV) is a relatively newly discovered respiratory pathogen, initially described in the Netherlands in 2001. ^[21] hMPV is in the Paramyxoviridae family (like RSV and PIV) and is a pleomorphic-shaped virus surrounded by surface protein projections. This virus is a ubiquitous organism, and most surveys indicate that by age five years, almost all children have been exposed to it. However, reinfection occurs throughout life, including in adults. This virus is spread via droplet and fomite exposure.

As a human pathogen, hMPV may have been underestimated. In children and infants, hMPV was reported to be a notable cause of lower respiratory tract infections such as bronchiolitis (59%), croup (18%), asthma (14%), and pneumonia (8%).

As with other viruses, the severity of infection increases with older age and with comorbid (cardiopulmonary disease) or immunosuppressive conditions. The most common diagnoses associated with adult hospitalizations with hMPV infection are chronic obstructive pulmonary disease (COPD) exacerbations, bronchitis, and pneumonia. ^[22] In immunocompromised hosts (eg, hematologic malignancies), severe pneumonitis requiring intensive care or resulting in death has been reported. ^{[23, 24]}

Coronavirus

Coronaviruses are from the family Coronaviridae and are single-stranded RNA viruses, the surface of which is covered by crownlike projections, giving the virus its name. This virus is spread via droplet and fomite exposure. Long known to cause upper respiratory infections, coronaviruses were not felt to significantly cause pneumonia until relatively recently. However, the severe acute respiratory syndrome (SARS) pandemic in 2003 brought the ability of this virus to cause life-threatening pneumonia to worldwide attention (see Zoonotic Viral Pneumonia, below).

Seven human coronaviruses (HCoVs) have now been identified: HCoV-229E, HCoV-OC43, HCoV-NL63, HCoV-HKU1, SARS-COV (which causes severe acute respiratory syndrome), MERS-COV (Middle East respiratory syndrome), and 2019-nCoV. All but 2019-nCoV appear to be established human pathogens with worldwide distribution, causing upper and lower respiratory tract infections, especially in children. Typically, HCoV infection follows a seasonal pattern similar to that of influenza, although Hong Kong researchers found that HCoV-NL63 infections mainly occurred in early summer and autumn. ^[25]

A novel coronavirus (2019-nCoV) was first reported in late 2019 and early 2020 in China. ^[14]

Varicella-zoster virus

Varicella-zoster virus (VZV) is a highly contagious herpes virus that is spread by the respiratory route or direct contact with skin lesions. Primary infection manifests as chickenpox. The reactivation results in zoster (shingles).

Pneumonia is a significant and life-threatening complication in otherwise healthy adults (including pregnant women) and immunocompromised hosts. This pneumonia is rare in otherwise healthy children but does occur in immunocompromised children.

Complications include secondary bacterial infections, encephalitis, hepatitis, and, with concomitant aspirin use, Reye syndrome. VZV pneumonia also tends be more severe in individuals who smoke.

Measles virus

Measles virus is a member of the Paramyxoviridae family and the genus Morbillivirus. It is a single-stranded RNA virus contained within a nucleocapsid and surrounded by an envelope. Measles is a respiratory tract virus that causes a febrile illness with rash in children. Mild pneumonia often occurs but is usually of no consequence in healthy adults.

Measles may result in severe lower respiratory tract infection and high morbidity in hosts who are immunocompromised and malnourished. This virus is highly contagious and is transmitted from person to person by droplets. The incubation period is 10-14 days and peaks in late winter and early spring.

Cytomegalovirus

Cytomegalovirus (CMV) is a herpesvirus that is a common cause of infections, usually asymptomatic, in the general population. In hosts who are immunocompetent, acute CMV infection causes a mononucleosis-like syndrome. Transmission is primarily through body fluid contact. The virus has been found in the cervix and in human milk, semen, and blood products. The prevalence of antibodies to CMV in adults ranges from 40-100%, with higher rates in lower socioeconomic areas.

Reactivation of latent infection is almost universal in transplant recipients and individuals infected with the human immunodeficiency virus. CMV pneumonia may occur and is often fatal in immunocompromised individuals, primarily hematopoietic stem cell transplant (HSCT) and solid organ transplant (SOT) recipients. The severity of pneumonia is related to the intensity of immunosuppression. Additionally, CMV infection is itself immunosuppressive, causing further immunocompromise in these patients.

In cancer patients receiving allogeneic bone marrow transplants, CMV pneumonia has a prevalence of 15% and a mortality rate of 85%, making it the most common cause of death in this population. Acute graft-versus-host disease is the major risk factor for CMV pneumonia in these patients.

Interestingly, although CMV is a well-recognized pathogen in patients with AIDS (manifesting as retinitis, colitis, encephalitis, polyradiculitis, and/or cholangiopathy), clinically relevant pneumonia is very uncommon in this group, even if CMV is cultured from alveolar fluid and/or seen on lung histology.

Herpes simplex virus

Herpes simplex virus (HSV) is a rare cause of lower respiratory tract infections and is seen primarily in severely immunocompromised patients, primarily HSCT and SOT recipients, patients who are undergoing chemotherapy or are neutropenic, or those who have congenital immunodeficiency. ^[26] HSV pneumonia develops either secondary to upper airway infection (because of direct extension of the virus from the upper to the lower respiratory tract) or following viremia secondary to dissemination of HSV from genital or oral lesions.

Herpes simplex virus is spread by contact with active lesions or viral shed by asymptomatic excreters. While not a classic respiratory virus, herpes simplex virus can cause pneumonia in compromised hosts, with a mortality rate of 80%. Pneumonia may develop from primary infection or reactivation.

Zoonotic viral pneumonias

Zoonotic viral pneumonias include those caused by the hantavirus, avian influenza, severe acute respiratory syndrome (SARS), and H1N1 (swine) influenza.

Hantavirus

Hantavirus is a genus of enveloped RNA viruses in the family Bunyaviridae. The majority are transmitted by arthropod vectors. Hantaviruses, however, are harbored by rodents, with each viral species having one major rodent host species. Rodents, which are chronically infected, excrete hantaviruses from urine, saliva, and feces. Infection occurs after aerosols of infectious excreta are inhaled.

The hantavirus pulmonary syndrome (HPS) is seen in the Americas and is an acute pneumonitis caused by the North American hantavirus, most notably the Sin Nombre Virus. ^{[27, 28]} Two other agents, isolated in other parts of North America, can also cause HPS.

Hantaviruses originally were recognized in the four-corners region of the southwestern United States (New Mexico, Arizona, Utah, and Colorado) in May 1993. The deer mouse (Peromyscus maniculatus) was identified to be the reservoir.

Cases of HPS have continued to be reported in the United States. As of July 2010, 545 cases of HPS had been reported in the United States from 32 states. ^[29]

Avian influenza

In Hong Kong in 1997, an influenza virus (H5N1 virus) previously known to infect only birds was found to infect humans. Manifestations included pneumonia, which in some cases led to fatal acute respiratory distress syndrome (ARDS) or multisystem organ failure.

Prior to the human outbreak, the H5N1 virus caused widespread deaths in chickens on three farms in Hong Kong. Epidemiologic investigations of this outbreak demonstrated that individuals in close contact with the index case or with exposure to poultry were at risk of being infected.

Concern is growing that avian influenza, which is a subtype of influenza A, may result in a worldwide pandemic in the near future. The avian influenza virus A/H5N1 has several ominous characteristics, including increased virulence and human-to-human transmission in several cases, rather than bird-to-human transmission, as is usually necessary. The disease causes high mortality as a result of pneumonia and respiratory failure.

The 1997 outbreak in Hong Kong was thought to be controlled by depopulating 1.5 million chickens in local farms and markets. However, human infections occurred in 2001 through 2003 in other parts of Asia, and the virus has been found in poultry and birds in Europe.

The rising incidence and widespread reporting of disease from H5N1 influenza viruses can probably be attributed to the increasing spread of the virus from existing reservoirs in domestic waterfowl and live bird markets, leading to greater environmental contamination. As of January 2014, 650 cases of H5N1 human infections have been reported from 16 countries since 2003, with 386 deaths (59% mortality). ^[30]

Severe acute respiratory syndrome

Severe acute respiratory syndrome (SARS) was due to a novel coronavirus (CoV) that crossed the species barrier through close contact between humans and infected animals. Viral isolation and genomic sequencing have revealed that the SARS virus originated in the masked palm civet cat (Paguma larvata), raccoon dog (Nyctereutes procyonoides), and possibly the Chinese ferret-badger (Melogale moschata), with subsequent interspecies jumping, during which a partial loss of genome probably led to more efficient human-to-human transmission.

Horseshoe bats (Rhinolophus sinicus) have also been found to harbor SARS-like coronaviruses (more distantly related to SARS-CoV than that of the palm civets), raising the possibility of bats being a reservoir for future SARS infections.

SARS was a particularly challenging disease because its long incubation period allowed seemingly healthy travelers who were infected with the virus to spread it. The SARS coronavirus (SARS-CoV) quickly spread from China to the rest of the world over a period of 1 year, affecting more than 8000 patients in 29 countries and resulting in 774 deaths.

Global transmission of SARS was halted in June 2003 after the World Health Organization instituted traditional public health measures, including finding and isolating case-patients, quarantining contacts, and using enhanced infection control. ^[31] No cases of SARS have been reported since 2004.

H1N1 (swine) influenza

Initially reported as an outbreak in Mexico and subsequently the United States, infection from a novel swine-origin influenza A (H1N1) virus rapidly spread to become a worldwide pandemic in 2009. The World Health Organization declared an end to the pandemic in August 2010.

Virus-associated hemophagocytic syndrome may play an important role in development of multiorgan failure and ensuing death in H1N1 infection. ^[32]

For more information on H1N1 influenza, see H1N1 Influenza (Swine Flu). Rare causes of viral pneumonia include Epstein-Barr virus and rotavirus.

Epstein-Barr virus

Epstein-Barr virus (EBV) is transmitted through infected saliva. Pneumonia as a complication of mononucleosis is very uncommon. The virus can cause pneumonia in the absence of mononucleosis.

Lung involvement secondary to EBV infections is more often reported in immunocompromised people than in others. In 25% of pediatric patients with HIV infection, EBV can cause lesions related to lymphocytic interstitial pneumonia or pulmonary lymphoid hyperplasia. ^[33]

Rotavirus

Although upper respiratory tract infection secondary to rotavirus is common, rotavirus pneumonia is rare. Just a few cases have been reported.

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