Viral Pneumonia

Background

Viruses anniversary for the better admeasurement of adolescence pneumonia. Viral pneumonia decreases in abundance in advantageous adolescent and middle-aged adults, but it again increases about amid the elderly. Studies on community-acquired pneumonias consistently authenticate bacilli to be the added best accepted etiologic anniversary (behind Streptococcus pneumoniae), alignment from 13-50% of diagnosed cases. 

The arise accident of viral pneumonia has added during the accomplished decade. In part, this credible access artlessly reflects bigger analytic techniques, but an absolute access has additionally occurred, because of the growing citizenry of patients who are immunocompromised. (See Epidemiology.)

Depending on the acerbity of the animal as able-bodied as the age and comorbidities of the patient, viral pneumonia can adapt from a balmy and self-limited affliction to a life-threatening disease. Abnormally in immunocompromised patients, viral pneumonia may aftereffect in respiratory failure, astringent hypoxemia, and added pulmonary pathology. (See Prognosis.)

The 4 best accepted etiologies of viral pneumonia in accouchement and immunocompetent adults are affliction virus, respiratory syncytial virus (RSV), adenovirus, and parainfluenza virus (PIV). Affliction virus types A and B are amenable for added than bisected of all community-acquired viral pneumonia cases, decidedly during affliction outbreaks. (See Etiology.)

The about accent of added bacilli (eg, parainfluenza, rhinoviruses, coronaviruses, animal metapneumovirus) will acceptable access as analytic tests such as reverse-transcription polymerase alternation acknowledgment (PCR), become added broadly available. (See Workup.)

Outbreaks of adenovirus of assorted serotypes frequently action in aggressive recruits. Adenovirus blazon 14 (Ad 14), a fresh alternative in the United States, has been apparent to anniversary astringent and sometimes baleful astute respiratory affliction in patients of all ages but abnormally the young, the old, patients with basal comorbid conditions, and those who are immunocompromised.

Viral pneumonia is a subset of the pneumonitides, which were at one time alleged aberant pneumonias. In the past, all pneumonias were labeled aberant if a bacterial antibiotic could not be articular with Gram staining and if the pneumonia did not acknowledge to antibiotics. 

Many viral pneumonias accept overlapping analytic presentations with anniversary added and with bacterial pneumonia—and may action calm with bacterial pneumonia—making analysis on absolutely analytic area difficult or impossible. A cardinal of accelerated tests to actuate viral etiologies accept now been developed, and their use in the emergency administering (ED) has accustomed bedside analysis of the analysis of viral pneumonia (see Analytic Presentation, as able-bodied as Diagnosis).

An authentic and aboriginal etiologic analysis is important because specific therapies are acclimated adjoin assertive bacilli (see Analysis and Management). Alike with currently accessible tests, however, in some alternation a adroit bacillus could not be articular in 50-80% of appropriate patients.

Agents acclimated in cases of viral pneumonia accommodate acyclovir, ganciclovir, and immunoglobulin. (See Medication.)

For added information, see Medscape’s Pneumonia Resource Center and Affliction Resource Center.

Pathophysiology

A abounding compassionate of the pathophysiology and pathogenesis of viral diseases does not anon exist.

After contamination, best respiratory bacilli tend to accumulate in the epithelium of the aerial airway and secondarily affect the lung by agency of airway secretions or hematogenous spread. Astringent pneumonias may aftereffect in all-encompassing alliance of the lungs with capricious degrees of hemorrhage. Some patients showed blood-soaked effusions and broadcast alveolar damage.

The apparatus of accident to tissues depends on the virus involved. Some bacilli are mainly cytopathic, anon affecting the pneumocytes or the bronchial cells. With others, overexuberant deepening from the accustomed acknowledgment is the mainstay of the pathogenic process.

Immune responses can be categorized according to patterns of cytokine production. Blazon 1 cytokines advance cell-mediated immunity, while blazon 2 cytokines arbitrate allergic responses. Accouchement adulterated with respiratory syncytial virus (RSV) who advance astute bronchiolitis, rather than balmy aerial respiratory infection symptoms, accept broken blazon 1 amnesty or aggrandized blazon 2 immunity.

In accession to humoral responses, cell-mediated amnesty appears to be important for accretion from assertive respiratory viral infections. Broken blazon 1 acknowledgment may explain why immunocompromised patients accept added astringent viral pneumonias.

Respiratory bacilli accident the respiratory amplitude and activate the host to absolution assorted humoral factors, including histamine, leukotriene C4, and virus-specific immunoglobulin E in RSV infection and bradykinin, interleukin 1, interleukin 6, and interleukin 8 in rhinovirus infections. RSV infections can additionally adapt bacterial colonization patterns, access bacterial adherence to respiratory epithelium, abate mucociliary clearance, and adapt bacterial phagocytosis by host cells.

Influenza virus

Infection by affliction virus leads to corpuscle death, abnormally in the aerial airway. Back absolute viral infection of lung parenchyma occurs, drain is apparent forth with a about abridgement of anarchic cells. Mucociliary approval is impaired, and bacterial adherence to respiratory epithelium occurs.

Infection with the affliction virus impairs T beef and neutrophil and macrophage function, which leads to crime of host defenses and may advance bacterial infection of frequently antiseptic areas, including the lower respiratory tract. This crime of host defenses may explain why as abounding as 53% of outpatients with bacterial pneumonia accept a circumstantial viral infection.

Adenoviruses

Little is accepted apropos mechanisms of pathogenicity of adenoviruses in general. Studies in accouchement accept articular added assembly cytokines, decidedly bump afterlife factor-alpha (TNF-α), interleukin 6 (IL-6), and interleukin 8 (IL-8). Age, bloom of the patient, and added alien host factors are believed to comedy key roles.

Viral pneumonia in the elderly

Elderly bodies are at added accident of infection and complications in viral pneumonia because of comorbidities. Waning cellular, humoral, and complete accustomed action may blemish viral clearance, which allows advance of the virus to the lower respiratory amplitude consistent in added inflammation. Age-old bodies additionally accept decreased respiratory beef backbone and aegis of the respiratory amplitude by fungus levels.

Viral transmission

The apparatus of viral manual varies with the blazon of virus. Routes accommodate large-droplet advance over abbreviate distances (< 1 m), duke acquaintance with attenuated bark and fomites and consecutive bang assimilate the nasal film or conjunctiva (eg, rhinovirus, RSV), and small-particle aerosol advance (eg, influenza, adenovirus). Some bacilli are acutely fastidious, admitting others accept the adequacy of actual on ecology surfaces for as connected as 7 hours, on gloves for 2 hours, and on calmly for 30 minutes.

Transmission routes for alleged viral pneumonias are as follows:

Ecology factors (adenovirus, enterovirus, rhinovirus)

Absolute acquaintance with attenuated altar (VZV)

Transplantation of attenuated organs (cytomegalovirus [CMV]) or claret articles (CMV)

Lower-respiratory aspiration of virus asymptomatically afford in the saliva (CMV, canker canker virus [HSV])

Reactivation of a abeyant infection (HSV, CMV)

Hematogenous advance (CMV)

Advance by healthcare cadre (SARS, measles, adenovirus, parainfluenza virus, RSV).

Hantavirus manual is anticipation to action primarily through assimilation of adulterated excreta from afflicted rodents. The virus is additionally present in rodent saliva, so manual can additionally action from bites.

A cardinal of viruses, including adenoviruses, affliction virus, measles virus, PIV, RSV, rhinoviruses, and VZV, are calmly transmitted during hospital stays and anniversary nosocomial pneumonia. Adenoviruses, affliction viruses, PIV, and RSV anniversary for 70% of nosocomial pneumonias due to viruses.

Pulmonary host defense

The pulmonary host aegis is circuitous and includes the afterward components:

Mechanical barriers

Humoral immunity

Phagocytic cells

Cell-mediated immunity

Mechanical barriers are hairs from the adenoids that clarify particles beyond than 10 microns, mucociliary clearance, and sharp-angle aberration of the axial airways that helps the 5- to 10-micron particles to become impacted in the mucosa.

Humoral amnesty is represented by mucosal immunoglobulin A (IgA), alveolar immunoglobulin M (IgM), and immunoglobulin G (IgG) present in transudates from the blood.

Phagocytic beef abide of polymorphonuclear (PMN) cells; alveolar, interstitial, and intravascular macrophages; and respiratory blooming cells. Alveolar macrophages accommodate the aboriginal aegis circuitous in internalizing and aspersing the viral pathogens. They act as antigen-presenting and opsonin-producing cells.

Respiratory blooming beef abide maturation, activation, and aboriginal clearing into the bounded lymph nodes afterwards the viral exposure. They act as antigen-presenting beef and are circuitous in the activation and adverse of CD8+ T cells.

Cell-mediated amnesty is the best important aegis apparatus adjoin the intracellular viral pathogens. This amnesty is circuitous in antibiotic production, cytotoxic activity, and cytokine production. CD8+ anamnesis or effector T beef tend to boss the lymphocyte basic of the virus-induced anarchic component.

Experimental models showed that 30-90% of CD8+ T beef recovered from bronchoalveolar lavage (BAL) are virus specific at the aiguille of the primary response. Studies in transgenic mice adulterated with affliction bacilli showed that the CD8+ T beef are not recruited in the lung during the viral infection. They are comatose anamnesis beef formed afterwards a antecedent appointment with the antigen, or they are acutely activated T beef afterwards a nonrespiratory infection that abide aboriginal clearing in the lung and that are maintained there by specific ligands.

A abundant cardinal of borderline CD8+ anamnesis T beef abide in the lung afterwards a viral infection.

A accessory infection induces all-encompassing face-lifting of CD8+ T beef in both lymphoid nodes and lungs. This backup takes abode in the absence of abundant deepening or a abundant effector-cell citizenry in the lungs. Respiratory infection allows abundant T beef to access the airways and may assuredly adapt the permeability of the lung and mediastinal lymph nodes to lymphocytes.

Etiology

Both DNA and RNA bacilli are circuitous in the analysis of viral pneumonia. Some are acclaimed lung bacilli that aftermath accepted analytic and radiologic manifestations. Others are not about circuitous as lung pathogens.

Etiologic bacilli accommodate assorted families, as follows:

Adenoviridae (adenoviruses)

Coronaviridae (coronaviruses)

Bunyaviridae (arboviruses) -Hantavirus

Orthomyxoviridae (orthomyxoviruses) - Affliction virus

Papovaviridae (polyomavirus) – JC virus, BK virus

Paramyxoviridae (paramyxoviruses) -Parainfluenza virus (PIV), respiratory syncytial virus (RSV), animal metapneumovirus (hMPV), measles virus

Picornaviridae (picornaviruses) – Enteroviruses, coxsackievirus, echovirus, enterovirus 71, rhinovirus

Reoviridae (rotavirus)

Retroviridae (retroviruses)- Animal immunodeficiency virus (HIV), animal lymphotropic virus blazon 1 (HTLV-1)

Most of the associates of Herpesviridae ancestors are authentic lung bacilli in hosts with compromised corpuscle amnesty and accommodate the following:

Canker canker virus 1 (HSV-1) and canker canker virus 2 (HSV-2), additionally alleged animal herpesvirus 1 (HHV-1) and animal herpesvirus 2 (HHV-2), respectively

Herpesvirus 6, herpesvirus 7, and herpesvirus 8

Varicella-zoster virus (VZV)

Cytomegalovirus (CMV)

Epstein-Barr virus (EBV)

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

Influenza virus

The affliction bacilli are enveloped, single-stranded, RNA bacilli of the ancestors Orthomyxoviridae and are the best accepted viral anniversary of pneumonia. Three serotypes of affliction virus exist: A, B, and C.

Influenza blazon A can adapt apparent antigens and affect livestock; perhaps, this appropriate accounts for its adeptness to actualize a backlog for infection and anniversary epidemics in humans. The virus is advance by agency of small-particle aerosol and targets the columnar epithelial beef forth the absolute respiratory tract.

Influenza blazon B causes affliction that usually is apparent in about bankrupt populations such as boarding schools. Affliction blazon C is beneath accepted and occurs as desultory cases.

Influenza blazon A is usually the best baneful pathogen. The affliction virus has 2 envelope glycoproteins, hemagglutinin (H) and neuraminidase (N), which are important for a cardinal of reasons. The hemagglutinin initiates infectivity by bounden to cellular sialic acerbic residues, admitting the N protein cleaves anew actinic virus from sialic acerbic on corpuscle surfaces, appropriately acceptance advance of the virus to added cells.

The affliction virus maintains its infectivity by adeptness antigenic alluvion (small cardinal of amino acerbic substitutions) and about-face (large cardinal of amino acerbic substitutions) due to changes in the protein anatomy of the apparent protein, hemagglutinin. Epidemics action back a viral alluvion occurs, and pandemics are apparent with viral about-face (2 affliction A bacilli barter H or N genes during infection of the aforementioned hosts) because best bodies accept no above-mentioned amnesty to the virus.

Two affliction types accept emerged of authentic importance: H5N1 aerial affliction ache and the atypical H1N1 barbarian affliction strain.

Respiratory syncytial virus

Respiratory syncytial virus (RSV) is the best accepted anniversary of lower respiratory amplitude infection amid breed and accouchement and the added best accepted viral anniversary of pneumonia in adults. It is a medium-sized virus of the Paramyxoviridae ancestors that consists of alone 1 serotype. Structurally, RSV has 10 altered viral polypeptides, 4 of which are associated with virus envelope, and 2 of these (F and G) are important for infectivity and pathogenicity. Archetypal RSV infection causes syncytia accumulation in corpuscle culture, giving the virus its name.

RSV is awful contagious, overextension via atom and fomite exposure. Best accouchement are adulterated afore age 5 years—the infection amount during an catching approaches 100% in assertive settings such as daycare centers—but the consistent amnesty is incomplete. Reinfection in beforehand accouchement and adolescent adults is accepted but mild. However, the likelihood of added astringent ache and pneumonia increases with advancing age.

Adenoviruses

Adenoviruses are belted DNA bacilli that anniversary a advanced spectrum of analytic illnesses depending on the serotype of the infecting agent. These accommodate asymptomatic illness, conjunctivitis, delirious aerial respiratory disease, pneumonia, gastrointestinal illness, hemorrhagic cystitis, rash, and neurologic disease. Pneumonia is beneath accepted in adults alfresco of aggressive recruit camps and agnate facilities, but atomic ache has been declared in breed and in the immunocompromised citizenry and can action in allegedly advantageous hosts.

Although 52 serotypes exist, classified into 7 subgroups or breed (A-G), pulmonary ache is predominantly acquired by serotypes 1, 2, 3, 4, 5, 7, 14, and 21. Blazon 7 bacilli can anniversary bronchiolitis and pneumonia in infants. Types 4 and 7 bacilli are amenable for outbreaks of respiratory ache in aggressive recruits.

Adenovirus serotype 14 (subgroup B) is a added baneful ache that has been arise to anniversary astringent respiratory affliction and pneumonia. Emergence of this ache was arise in 2005 amid noncombatant and aggressive populations, with outbreaks occurring afterwards at aggressive training centers throughout the United States.

In 2007, adenovirus serotype 14 acquired a large, abiding beginning of delirious respiratory affliction amid aggressive trainees in Texas and, added recently, in a residential affliction adeptness in Washington State. In a association beginning in Oregon, the boilerplate age was 52 years, and 76% appropriate hospitalization, 47% appropriate analytical care, 24% appropriate vasopressors, and 18% died. The majority of these patients were contrarily immunocompetent adults.

Spread of adenovirus is by respiratory secretions, communicable aerosols, feces, and fomites. Neonates may access infection from acknowledgment to cervical secretions at birth.

Contaminated ecology surfaces can anchorage virus able of causing infection for weeks. The virus is aggressive to lipid disinfectants but is inactivated by heat, formaldehyde, and bleach.

Adenoviruses are acutely contagious. Studies of fresh aggressive recruits accept apparent seroconversion ante of 34-97% over a 6-week period. The majority of accouchement accept serologic affirmation of above-mentioned adenovirus infection by the age of 10.

Parainfluenza virus

Parainfluenza virus (PIV) is a accepted virus that infects best bodies during childhood. PIV is added in accent to alone RSV in causing lower respiratory amplitude ache in accouchement and pneumonia and bronchiolitis in breed adolescent than 6 months. Manual is through absolute person-to-person acquaintance or large-droplet spread.

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

There are 4 subtypes of PIV, based on antigenic characteristics. PIV blazon 3 is ancient year-round, while types 1 and 2 aiguille during the abatement season. Amnesty is abbreviate term, and alternate aerial or lower respiratory amplitude infections action throughout life. The infections adapt from a balmy affliction to life-threatening croup, bronchiolitis, or pneumonia. Infection in immunocompromised hosts can aftereffect in life-threatening pneumonia with lung abrasion and respiratory failure. In one study, 44% of hematopoietic axis corpuscle displace (HSCT) patients with PIV progressed to advance pneumonia, of which 37% died.

Rhinovirus

According to some authors, rhinovirus accounts for up to 30% of cases of all virus-related pneumonia. Analytic studies appearance that rhinovirus is the added best frequently accustomed abettor associated with pneumonia and bronchiolitis in breed and adolescent children. Rhinovirus infection is affiliated to asthma hospitalizations in both adults and children.

A abstraction of 211 French accouchement with rhinovirus infection arise bronchiolitis or bronchitis in 25.6% and pneumonia in 6.2%, afterwards cases of bifold bacterial or viral infections were eliminated.

A abstraction from the Netherlands showed that rhinoviruses anniversary 32% of all lower respiratory amplitude infections with an articular antibiotic in the age-old (> 60 y) appropriate population. Rhinoviruses were articular added frequently than coronaviruses (17%) or affliction bacilli (7%).

Human metapneumovirus

Human metapneumovirus (hMPV) is a about anew apparent respiratory pathogen, initially declared in the Netherlands in 2001. hMPV is in the Paramyxoviridae ancestors (like RSV and PIV) and is a pleomorphic-shaped virus amidst by apparent protein projections. This virus is a all-over organism, and best surveys announce that by age 5 years, about all accouchement accept been apparent to it. However, reinfection occurs throughout life, including in adults. This virus is advance via atom and fomite exposure.

As a animal pathogen, hMPV may accept been underestimated for a connected time. In accouchement and infants, hMPV was arise to be a notable anniversary of lower respiratory amplitude infections such as bronchiolitis (59%), croup (18%), asthma (14%), and pneumonia (8%).

As with added viruses, the severity of infection increases with beforehand age and with comorbid (cardiopulmonary disease) or immunosuppressive conditions. The best accepted diagnoses associated with developed hospitalizations with hMPV infection are abiding adverse pulmonary ache (COPD) exacerbations, bronchitis, and pneumonia. In immunocompromised hosts (eg, hematologic malignancies), astringent pneumonitis astute accelerated affliction or consistent in afterlife has been reported.

Coronavirus

Coronaviruses are from the ancestors Coronaviridae and are single-stranded RNA viruses, the apparent of which is covered by crownlike projections, giving the virus its name. This virus is advance via atom and fomite exposure. Connected accepted to anniversary aerial respiratory infections, coronaviruses were not acquainted to decidedly anniversary pneumonia until about recently. However, the astringent astute respiratory affection (SARS) communicable in 2003 brought the adeptness of this virus to anniversary life-threatening pneumonia to accepted absorption (see Zoonotic Viral Pneumonia, below).

Four animal coronaviruses (HCoVs) accept now been identified: HCoV-229E, HCoV-OC43, HCoV-NL63, and HCoV-HKU1. These HCoVs arise to be accustomed animal bacilli with accepted distribution, causing aerial and lower respiratory amplitude infections, abnormally in children. Typically, HCoV infection follows a melancholia arrangement agnate to that of influenza, although Hong Kong advisers begin that HCoV-NL63 infections mainly occurred in aboriginal summertime and autumn.

Viral pneumonias that apparent as a adventurous accommodate varicella-zoster virus (chickenpox) and measles virus.

Varicella-zoster virus

Varicella-zoster virus (VZV) is a awful catching canker virus that is advance by the respiratory avenue or absolute acquaintance with bark lesions. Primary infection manifests as chickenpox; the reactivation after-effects in zoster (shingles).

Pneumonia is a cogent and life-threatening aggravation in contrarily advantageous adults and immunocompromised hosts (including abundant women). This pneumonia is attenuate in contrarily advantageous accouchement but does action in immunocompromised children.

Complications accommodate accessory bacterial infections, encephalitis, hepatitis, and, with accessory aspirin use, Reye syndrome. VZV pneumonia additionally tends be added astringent in individuals who smoke.

Measles virus

Measles virus is a affiliate of the Paramyxoviridae ancestors and the brand Morbillivirus. It is a single-stranded RNA virus independent aural a nucleocapsid and amidst by an envelope. Measles is a respiratory amplitude virus that causes a delirious affliction with adventurous in children; balmy pneumonia about occurs but is usually of no aftereffect in advantageous adults.

Measles may aftereffect in astringent lower respiratory amplitude infection and aerial anguish in hosts who are immunocompromised and malnourished. This virus is awful catching and is transmitted from actuality to actuality by droplets. The change aeon is 10-14 canicule and peaks in backward winter and aboriginal spring.

Viral pneumonias that action in immunodeficient patients accommodate cytomegalovirus and canker canker virus.

Cytomegalovirus

Cytomegalovirus (CMV) is a herpesvirus that is a accepted anniversary of infections, usually asymptomatic, in the accepted population. In hosts who are immunocompetent, astute CMV infection causes a mononucleosis-like syndrome; manual is primarily through anatomy aqueous contact. The virus has been begin in the cervix and in animal milk, semen, and claret products. The prevalence of antibodies to CMV in adults ranges from 40-100%, with college ante in lower socioeconomic areas.

Reactivation of abeyant infection is about accepted in displace recipients and individuals adulterated with the animal immunodeficiency virus. CMV pneumonia may action and is about baleful in immunocompromised individuals, primarily hematopoietic axis corpuscle displace (HSCT) and solid agency displace (SOT) recipients; the severity of pneumonia is accompanying to the acuteness of immunosuppression. Additionally, CMV infection is itself immunosuppressive, causing added immunocompromise in these patients.

In blight patients accepting allogeneic cartilage bottom transplants, CMV pneumonia has a prevalence of 15% and a bloodshed amount of 85%, authoritative it the best accepted anniversary of afterlife in this population. Astute graft-versus-host ache is the above accident agency for CMV pneumonia in these patients.

Interestingly, although CMV is a well-recognized antibiotic in patients with AIDS (manifesting as retinitis, colitis, encephalitis, polyradiculitis, and/or cholangiopathy), clinically accordant pneumonia is actual aberrant in this group, alike if CMV is able from alveolar aqueous and/or apparent on lung histology.

Herpes canker virus

Herpes canker virus (HSV) is a attenuate anniversary of lower respiratory amplitude infections and is apparent primarily in acutely immunocompromised patients, primarily HSCT and SOT recipients, patients who are adeptness chemotherapy or are neutropenic, or those who accept complete immunodeficiency. HSV pneumonia develops either accessory to aerial airway infection (because of absolute addendum of the virus from the aerial to the lower respiratory tract) or afterward viremia accessory to broadcasting of HSV from animal or articulate lesions.

Herpes canker virus is advance by acquaintance with alive lesions or viral afford by asymptomatic excreters. While not a archetypal respiratory virus, canker canker virus can anniversary pneumonia in compromised hosts, with a bloodshed amount of 80%. Pneumonia may advance from primary infection or reactivation.

Zoonotic viral pneumonias

Zoonotic viral pneumonias accommodate those acquired by the hantavirus, aerial influenza, astringent astute respiratory affection (SARS), and H1N1 (swine) influenza.

Hantavirus

Hantavirus is a brand of belted RNA bacilli in the ancestors Bunyaviridae, the majority of which are transmitted by arthropod vectors. Hantaviruses, however, are harbored by rodents, with anniversary viral breed accepting one above rodent host species. Rodents, which are chronically infected, defecate hantaviruses from urine, saliva, and feces. Infection occurs afterwards aerosols of communicable excreta are inhaled.

The hantavirus pulmonary affection (HPS) is apparent in the Americas and is an astute pneumonitis acquired by the North American hantavirus, best conspicuously the Sin Nombre Virus. Two added agents, abandoned in added genitalia of North America, can additionally anniversary HPS.

Hantaviruses originally were accustomed in the 4-corners arena of the southwestern United States (New Mexico, Arizona, Utah, and Colorado) in May 1993, area the deer abrasion (Peromyscus maniculatus) was begin to be the reservoir.

Cases of HPS accept connected to be arise in the United States. As of July 2010, 545 cases of HPS had been arise in the United States from 32 states.

Avian influenza

In Hong Kong in 1997, an affliction virus (H5N1 virus) ahead accepted to affect alone birds was begin to affect humans. Manifestations included pneumonia, which in some cases led to baleful astute respiratory ache affection (ARDS) or multisystem agency failure.

Prior to the animal outbreak, the H5N1 virus acquired boundless deaths in chickens on 3 farms in Hong Kong. Epidemiologic investigations of this beginning approved that individuals in abutting acquaintance with the basis case or with acknowledgment to banty were at accident of actuality infected.

Concern is growing that aerial influenza, which is a subtype of affliction A, may aftereffect in a accepted communicable in the abreast future. The aerial affliction virus A/H5N1 has several apocalyptic characteristics, including added acerbity and human-to-human manual in several cases, rather than bird-to-human transmission, as is usually necessary. The ache causes aerial bloodshed as a aftereffect of pneumonia and respiratory failure.

The 1997 beginning in Hong Kong was anticipation to be controlled by depopulating 1.5 actor chickens in bounded farms and markets. However, animal infections occurred in 2001 through 2003 in added genitalia of Asia, and the virus has been begin in banty and birds in Europe.

The ascent accident and boundless advertisement of ache from H5N1 affliction bacilli can apparently be attributed to the accretion advance of the virus from absolute reservoirs in calm waterfowl and alive bird markets, arch to greater ecology contamination. As of August 2010, 504 cases of H5N1 animal infections accept been arise from 15 countries back 2003, with 299 deaths (59% mortality).

Severe astute respiratory syndrome

Severe astute respiratory affection (SARS) was due to a atypical coronavirus (CoV) that beyond the breed barrier through abutting acquaintance amid bodies and adulterated animals. Viral abreast and genomic sequencing accept arise that the SARS virus originated in the masked approach civet cat (Paguma larvata), raccoon dog (Nyctereutes procyonoides), and possibly the Chinese ferret-badger (Melogale moschata), with consecutive interspecies jumping, during which a fractional accident of genome apparently led to added able human-to-human transmission.

Horseshoe bats (Rhinolophus sinicus) accept additionally been begin to anchorage SARS-like coronaviruses (more distantly accompanying to SARS-CoV than that of the approach civets), adopting the achievability of bats actuality a backlog for approaching SARS infections.

SARS was a decidedly arduous ache because its connected change aeon accustomed acutely advantageous travelers who were adulterated with the virus to advance it. The SARS coronavirus (SARS-CoV) bound advance from China to the blow of the apple over a aeon of 1 year, affecting added than 8000 patients in 29 countries and consistent in 774 deaths.

Global manual of SARS was apoplectic in June 2003 afterwards the Apple Bloom Organization instituted acceptable accessible bloom measures, including award and isolating case-patients, quarantining contacts, and application added infection control. No cases of SARS accept been arise back 2004.

H1N1 (swine) influenza

Initially arise as an beginning in Mexico and afterwards the United States, infection from a atypical swine-origin affliction A (H1N1) virus rapidly advance to become a accepted communicable in 2009. The Apple Bloom Organization declared an end to the communicable in August 2010.

Virus-associated hemophagocytic affection may comedy an important role in development of multiorgan abortion and afterwards afterlife in H1N1 infection.

For added advice on H1N1 influenza, see H1N1 Affliction (Swine Flu). Attenuate causes of viral pneumonia accommodate Epstein-Barr virus and rotavirus.

Epstein-Barr virus

Epstein-Barr virus (EBV) is transmitted through adulterated saliva. Pneumonia as a aggravation of mononucleosis is actual uncommon. The virus can anniversary pneumonia in the absence of mononucleosis.

Lung captivation accessory to EBV infections is added about arise in immunocompromised bodies than in others. In 25% of pediatric patients with HIV infection, EBV can anniversary lesions accompanying to lymphocytic interstitial pneumonia or pulmonary lymphoid hyperplasia.

Rotavirus

Although aerial respiratory amplitude infection accessory to rotavirus is common, rotavirus pneumonia is rare. Just a few cases accept been reported.

Epidemiology

Traditionally, bacilli were acquainted to anniversary about 8% of cases of community-acquired pneumonia for which patients are hospitalized. Added contempo investigations accept apparent bacilli to comedy a beyond role, causing 13-50% of pathogen-diagnosed community-acquired pneumonia cases as sole bacilli and 8-27% of cases as alloyed bacteria-virus infections.

Influenza virus types A and B anniversary for added than 50% of all community-acquired viral pneumonias in adults. Assorted studies accept arise differing frequencies of the added bacilli causing community-acquired pneumonias, with RSV alignment from 1-4%, adenovirus 1-4%, PIV 2-3 %, hMPV 0-4%, and coronavirus 1-14% of pathogen-diagnosed pneumonia cases.

The appulse of affliction is aerial in age-old bodies and greatest for those with abiding illnesses. It has been estimated that at atomic 63% of the 300,000 influenza-related hospitalizations and 85% of 36,000 influenza-related deaths action in patients age-old 65 years or older, admitting the actuality that this accumulation accounts for alone 10% of the population.

RSV is the best accepted analysis of viral pneumonia in breed and children. In addition, RSV has become an added important antibiotic in the age-old citizenry and is now the added best frequently articular anniversary of pneumonia in age-old persons, causing 2-9% of the anniversary 687,000 hospitalizations and 74,000 deaths from pneumonia in this population.

Some studies accept appropriate that RSV-related ache is as accepted as affliction in age-old persons. About 10% of nursing home patients advance RSV infection annually, while 10% of these patients will advance pneumonia.

Parainfluenza infection is the added best accepted viral illness, afterwards RSV, in infants.

Adenovirus accounts for 10% of pneumonias in children. Ache from adenovirus can action at any time of the year. Assorted adenovirus serotypes are amenable for about connected epidemics of astute respiratory ache at aggressive recruit training accessories in the United States and worldwide.

During the prevaccination era, up to 20% of recruits had to be removed from assignment due to illness. Unfortunately, the vaccine adjoin adenovirus is no best accessible for administering to aggressive personnel.

Viral pneumonia in immunocompromised hosts

Although immunocompromised patients are at college accident for viral pneumonia from CMV, VZV, HSV, measles, and adenoviruses, melancholia bacilli (influenza, RSV, PIV) abide a above anniversary of pneumonia. HSCT and SOT recipients are decidedly at accident for accepting lower respiratory amplitude infection due to CMV and RSV.

CMV pneumonia has been empiric in 10-30% of patients with HSCT and 15-55% of heart-lung displace recipients, authoritative this virus the best accepted anniversary of viral pneumonia in the above accommodating group. Afterwards CMV, the abundance of bacilli abandoned from HSCT patients vary, with affliction virus alignment from 14-52%, RSV 14-48%, adenovirus 2-21%, and PIV 11-49% of viral isolates.

Although HSV has been apparent to anniversary pneumonia in this accommodating population, it is about attenuate back compared with the added viral pathogens, with one abstraction assuming HSV to anniversary 5% of nonbacterial pneumonias in HSCT recipients, compared with 46% for CMV.

Viral pneumonia in pregnancy

Acute viral pneumonia is accepted and about underdiagnosed in pregnancy. Although the severity of bacterial pneumonia does not assume to be added in pregnancy, viral pneumonia can accept a austere analytic evolution.

Among the viral pathogens, affliction virus, VZV, and measles virus are arise as etiologic agents in astringent lower respiratory amplitude infection. The infection may aftereffect in astute respiratory decompensation; respiratory failure; and/or ARDS, which can advance to maternofetal hypoxia, preterm labor, multisystem agency failure, and alike death.

Pregnant women assume to be at added accident for affliction pneumonia. VZV pneumonia is attenuate but potentially lethal, with bloodshed ante of 35-40% in abundant women, compared with 10% in the accepted population.

Measles virus can be a ample anniversary of pneumonia in abundant women. Added bacterial superinfection can complicate the analytic and radiologic picture.

Despite letters of a aerial bloodshed amount during outbreaks of hantavirus pulmonary syndrome, no cases of affectionate fatalities accessory to this ataxia accept been arise to date.

Sex differences in viral pneumonia

Men who are adulterated advance viral pneumonia at a hardly college amount than women. Abundant women with viral pneumonia accept a college accident for astringent ache than added females. Abundant patients accept a asymmetric accident of astringent ache with 2009 H1N1 infection; analysis should be accomplished as anon as the analysis is suspected.

Age-related differences in viral pneumonia

Most bacilli that can anniversary pneumonia about affect accouchement and anniversary a balmy illness; advantageous adults additionally advance balmy disease. In contrast, age-old bodies and bodies who are immunosuppressed advance astringent viral pneumonia, consistent in aerial anguish and bloodshed rates.

The capital barring to this was apparent in the 2009-2010 H1N1 affliction pandemic, in which astringent infection was added accepted in the citizenry age-old 5-59 years than in the elderly. This was anticipation to be from abridgement of exposure, and appropriately immunity, to the 1957 (and earlier) H1N1 affliction strain(s).

Mortality and morbidity

The US demography for 2000-2001 listed pneumonia/influenza as the seventh arch anniversary of afterlife (down from sixth) admitting a 7.2% abatement in the bloodshed amount for these diseases during this period. Astringent affliction seasons can aftereffect in added than 40,000 balance deaths and added than 200,000 hospitalizations.

Patients age-old 65 years or beforehand are at authentic accident for afterlife from viral pneumonia as able-bodied as from affliction not complicated by pneumonia. Deaths in these patients anniversary for 89% of all pneumonia and/or affliction deaths.

Morbidity, abnormally in age-old persons, is additionally high. Up to 10-12% of patients beforehand than 65 years appropriate a college akin of abetment for activities of circadian active afterwards analysis for astute respiratory illnesses. In one nursing home outbreak, association with astute affliction affliction showed cogent anatomic decline.

Pneumonia from adenovirus serotypes added than Ad 14 has a low accident rate, and best serotypes accept a low anguish rate.

Influenza virus

Influenza virus represents a accepted anniversary of pneumonia in the developed population, affecting 4-8% of advantageous adults. Ante accept been 10-20% during outbreaks and as aerial as 50% during epidemics. Anguish and bloodshed ante accompanying to affliction pneumonia in both the accepted citizenry and in alleged groups (eg, patients with abiding diseases, the elderly) are substantial.

The accomplished ante of analysis for affliction action in preschool-aged accouchement and in the age-old population. During outbreaks, the analysis ante are 27.9 cases per 10,000 bodies adolescent than 5 years and 55 cases per 10,000 bodies beforehand than 65 years. Amid 1972 and 1992, 426,000 deaths accompanying to affliction pneumonia were arise in United States. Individuals 85 years or beforehand were 16 times added acceptable than those age-old 65-69 years to die from influenza.

Also in adverse with melancholia influenza, bloodshed was college in adolescent patients with H1N1 influenza, with 87% of deaths and 71% of astringent pneumonia in the age accumulation of 5-51 years. The college bloodshed in patients adolescent than 60 years may reflect this cohort’s abridgement of acknowledgment to the 1957 (and earlier) H1N1 affliction strains; acknowledgment to those strains may accept conferred some amnesty in the beforehand population. Additionally of absorption is a address that articular blubber as a accessible accident agency for added astringent disease/mortality.

As of February 2010, the CDC had estimated that 8,330 to 17,160 H1N1-related deaths occurred amid April 2009 and January 16, 2010 in the United States, and the Apple Bloom Organization (WHO) has estimated that, worldwide, at atomic 16,226 deaths accept been anon attributable to H1N1.

The H5N1 aerial affliction seems to be added baneful than melancholia influenza, with a 59% bloodshed amount in cases arise appropriately far and boilerplate time from ache access to afterlife of 9 days; the majority of these patients had no basal medical problems.

Respiratory syncytial virus

RSV pneumonia is amenable for an boilerplate of 80,000 pediatric hospitalizations and 500 deaths every year. The bloodshed amount depends on the patient's immunologic status. In advantageous children, the arise bloodshed amount is 0.5-1.7% and college in immunosuppressed patients (< 80-100% in basic HSCT recipients vs 22% in advised ascendancy subjects).

In adults, RSV pneumonia is associated with a bloodshed amount alignment from 11-78%, depending on the severity of basal accustomed suppression. In abiding affliction facilities, 5-27% of respiratory amplitude infections accept been estimated to be acquired by RSV, 10% of which will advance into pneumonia and 1-5% of which will be fatal. In immunocompromised patients, decidedly HSCT recipients, the bloodshed amount for RSV pneumonia is high, at 41%.

Adenovirus

Adenovirus infection has been associated with low bloodshed in advantageous adults, but afterlife from a 2009 association beginning of serotype 14 pneumonia was 18%. In immunocompromised patients, adenovirus can be acquired not alone by person-to-person manual but additionally from reactivation, to aftermath a advanced array of syndromes, including gastroenteritis, hepatitis, and hemorrhagic cystitis (in accession to pneumonia), with bloodshed ante alignment from 38-100% and with a accumulative bloodshed amount of 56% in HSCT patients.

Parainfluenza virus

PIV pneumonia causes 250,000 emergency visits annually, consistent in 70,000 admissions. Fully 18% of ailing accouchement with respiratory amplitude infections in the United States accept this disease.

Rates of PIV pneumonia are added in immunocompromised pediatric groups, such as recipients of BMT, HSCT, lung transplantation, and solid-organ transplantation. PIV has been associated with 10% of astute respiratory illnesses in advantageous adults and 10-50% in displace recipients; in the closing group, the bloodshed ante ambit from 15-73%. One abstraction showed that 56% of PIV isolates were associated with aerial respiratory affection in HSCT recipients but that 44% developed pneumonia, of whom 37% died

Human metapneumovirus

HMPV accounts for up to 10% of alien respiratory infections in children. Some authors address that HMPV can anniversary for 30% of unidentified, doubtable cases of viral pneumonia in displace patients.

The about contempo acceptance of hMPV in causing pneumonia and the adversity in its analysis has precluded authentic estimates of bloodshed rates, but case letters of deaths exist, primarily in patients with hematologic malignancies adeptness chemotherapy or HSCT. Bloodshed ante in displace recipients accept ranged from 50% in lung displace patients to 80% in HSCT recipients. A 2009 hMPV beginning at a psychiatric area in Taiwan, all in immunocompetent patients, resulted in 1 (of 13 diagnosed patients) afterlife from respiratory failure.

Varicella-zoster virus

In the United States, varicella pneumonia occurs with a abundance of 0.32-1.36 cases per 100,000 bodies per year. Amid Americans ailing because of varicella, 1.0-2.3 in 400 advance pneumonia.

Varicella pneumonia complicates about 2-10% of the cases of VZV infection in adults. At atomic 25% of the fatalities from varicella in adults action in bodies who advance varicella pneumonia. The severity of varicella pneumonia is accomplished in immunosuppressed persons, with bloodshed ante of 15-18%, and in abundant women in the second/third trimesters, with a bloodshed amount of 41%.

The all-embracing bloodshed in the accepted citizenry has decreased from 19% (range, 10-30%) in 1960-1970 to 6%. In renal displace patients, the bloodshed decreased from 53% in 1981-1990 to 22% in 1991-2000. The bloodshed amount is about 50% in intubated patients with astute respiratory failure. In HIV-infected patients, a bloodshed amount of 43% is reported, and in abundant women, the bloodshed amount is about 41%.

Measles virus

In the United States, pneumonia is amenable for 60% of the measles bloodshed in infants. Although deaths from measles in the United States decreased steadily throughout the 20th century—from about 12 per 100,000 citizenry in 1912 to about 0.2 per 100,000 citizenry in 1960—mortality ante beneath clearly afterwards a measles vaccine was accountant in 1963.

Measles is about eradicated in the Western Hemisphere, with alone 71 cases accepted in the United States in 2009. Nevertheless, measles-virus pneumonia is still a notable anniversary of bloodshed and anguish in nonvaccinated accouchement and immunocompromised adults.

In 1990, 6.5% of Americans with measles developed pneumonia. A abstraction of 3220 US aggressive recruits showed that 3.3% had measles-related pneumonia; best cases were accessory to bacterial superinfection. No deaths were arise amid these contrarily advantageous adults. However, in a altered abstraction application altered analytic criteria, pneumonia was begin in 50% of recruits with measles. A aftereffect abstraction of Afghani accouchement ailing for measles arise an 85.4% amount of bronchopneumonia.

The CDC arise 4 cases of measles pneumonia, with 2 fatalities, amid HIV-infected accouchement in 1986-1987. In children, bloodshed ante due to measles bronchopneumonia are aerial (28%). The amount is alike college in immunocompromised groups: 70% in those with blight and 40% in those with AIDS. Investigators arise 10 fatalities accessory to measles pneumonia in 12 accouchement with leukemia.

Cytomegalovirus

CMV pneumonia is advised the best accepted life-threatening aggravation of cartilage bottom transplantation (BMT) and solid-organ transplantation.

The amount of CMV pneumonia in BMT recipients is 10-50%, as arise in altered studies. In patients accepting solid-organ transplants, CMV reactivation is arise in as abounding 70%, but alone 20% advance clinically cogent infections. Studies of CMV pneumonia in BMT recipients appearance a 31% bloodshed amount in advised patients and a abatement from ahead arise ante of 56-100% in basic patients. The bloodshed amount is reportedly 75% in basic immunosuppressed persons.

Herpes canker virus

HSV pneumonia develops mainly in immunocompromised patients. The amount of HSV pneumonia can be as aerial as 70-80% in hematopoietic stem-cell transplantation (HSCT) recipients not accepting prophylaxis, and it can be decreased to 5% with acyclovir prophylaxis. As with CMV, the bloodshed amount is aerial if ache charcoal basic in immunosuppressed patients (>80% mortality).

Hantavirus

As of July 2010, 545 cases of HPS had been arise in the United States from 32 states, mostly Fresh Mexico, Colorado, Arizona, California, Washington, Texas, and Utah (in abbreviating adjustment of prevalence). HPS is additionally arise in South America and in Canada. The bloodshed amount for HPS is 35%. Of individuals with HPS, 61% are men and 39% are women, with a beggarly age of 37 years. Caucasian patients anniversary for 77%, bodies of Native American coast anniversary for about 20%, and those of Hispanic coast anniversary for 13%.

Prognosis

The cast is acceptable in the all-inclusive majority of patients with viral pneumonia. It is attentive in age-old or immunocompromised patients. Some adenovirus serotypes, abnormally 2, 3, 7, and 21 accept been the anniversary of austere abiding anguish afterwards astute respiratory illness, including irreversible atelectasis, bronchiectasis, bronchiolitis obliterans, and unilateral hyperlucent lung. An estimated 14-60% of these accouchement will ache some amount of abiding lung damage. Abounding of these patients presented with pharyngitis, tonsillitis, and bronchitis. Adenovirus 14 has a aerial accident and anguish amount in advantageous patients. Austere sequelae occurred in those who survived.

Viral pneumonia may leave patients with balance affliction from interstitial fibrosis. Breed ailing with lower lung infection due to RSV are abundant added acceptable to after advance asthma.