eMedicine Specialties > Pulmonology > Infectious Lung Diseases

Pneumonia, Viral: Differential Diagnoses & Workup

Author: Sat Sharma, MD, FRCPC, Professor and Head, Division of Pulmonary Medicine, Department of Internal Medicine, University of Manitoba; Site Director, Respiratory Medicine, St. Boniface General Hospital
Contributor Information and Disclosures

Updated: Apr 28, 2009

Differential Diagnoses

Workup

Laboratory Studies

• Over the past decade, developments in diagnostic techniques have led to a significant improvement in the ability to detect viruses in the respiratory tract. However, the detection of viral pathogens does not always indicate active disease. For example, herpesviruses may become reactivated without causing significant active disease. Similarly, RSV and CMV can be detected in the presence of other known bacterial pathogens, making it difficult to decide whether these are the causative agents of pneumonia. Table 1. Diagnostic Techniques Used for Viral Pneumonia

  Open table in new window

  [ CLOSE WINDOW ]

  Table

  Virus	Viral Culture	Cytologic Evaluation	Rapid Antigen Detection	Gene Amplification
  Influenza virus	HA*, SV†		IF‡, ELISA§
  Adenovirus	CE¶, SV	Intranuclear inclusions	IF, ELISA	RT-PCR#
  Paramyxoviruses
  Respiratory syncytial virus	CE, SV	Eosinophilic cytoplasmic inclusions	IF, ELISA	RT-PCR
  Parainfluenza virus	HA, SV	Eosinophilic intranuclear inclusions	IF, ELISA	RT-PCR
  Measles virus	HA
  Herpes viruses
  Herpes simples virus	CE, SV	Cytoplasmic inclusions	IF, ELISA	PCR
  Varicella-zoster virus	CE	Cytoplasmic inclusions	IF	RT-PCR
  Cytomegalovirus	CE, SV	"Owl's eye" cells	IF, ELISA	RT-PCR
  Hantavirus			Antibodies against FCV**	FVC RNA by RT-PCR

  Virus	Viral Culture	Cytologic Evaluation	Rapid Antigen Detection	Gene Amplification
  Influenza virus	HA*, SV†		IF‡, ELISA§
  Adenovirus	CE¶, SV	Intranuclear inclusions	IF, ELISA	RT-PCR#
  Paramyxoviruses
  Respiratory syncytial virus	CE, SV	Eosinophilic cytoplasmic inclusions	IF, ELISA	RT-PCR
  Parainfluenza virus	HA, SV	Eosinophilic intranuclear inclusions	IF, ELISA	RT-PCR
  Measles virus	HA
  Herpes viruses
  Herpes simples virus	CE, SV	Cytoplasmic inclusions	IF, ELISA	PCR
  Varicella-zoster virus	CE	Cytoplasmic inclusions	IF	RT-PCR
  Cytomegalovirus	CE, SV	"Owl's eye" cells	IF, ELISA	RT-PCR
  Hantavirus			Antibodies against FCV**	FVC RNA by RT-PCR

  ^* HA - Hemaglutination
  ^† SV - Shell viral culture
  ^‡ IF - Immunofluorescence
  ^§ ELISA - Enzyme-linked immunosorbent assay
  ^¶ CE - Cytopathogenic effects
  ^# RT-PCR - Reverse transcriptase-polymerase chain reaction
  ^** FCV - Four corners' virus
• Cytologic evaluation
  • Respiratory secretions, bronchoalveolar lavage samples, and tissue specimens can be examined using cytologic and histologic techniques.
  • Intranuclear inclusions often exist in cells infected with DNA viruses; cytoplasmic inclusions usually are present in cells infected with RNA viruses.
  • CMV infection characteristically is associated with "owl's-eye" cells, which are large cells with basophilic intranuclear inclusions and a surrounding clear zone.
  • The presence of viral inclusions is diagnostic, although this method has low sensitivity. Therefore, absence of inclusions does not always exclude infection or active disease.
• Viral culture
  • Viral pneumonia can be diagnosed by isolation and identification of the pathogen through viral culture.
  • Tissue from the upper or lower respiratory tract, sputum samples, and samples obtained by nasopharyngeal washing, bronchoalveolar lavage, and biopsy may be submitted for viral culture. The use of an appropriate viral transport medium is required; this consists of enriched broth containing antibiotics and a protein substrate.
  • Viral cultures are performed on various cell lines (eg, monkey kidney cells, diploid fibroblasts). The cell cultures are incubated at 35°C and are examined microscopically on alternate days for an incubation period of 14 days.
  • The cultures are examined for cytopathogenic effects and for evidence of viral growth. The cytopathogenic effect is the formation of syncytial collections of multinucleated giant cells and rarely is virus specific. Viral growth is detected through hemadsorption testing by demonstrating adherence of red blood cells to the cultured cell monolayer of infected tissue.
  • Further identification of viruses is accomplished using immunofluorescence (direct or indirect) methods or nucleic acid probes. These techniques are used to identify the specific virus in cell cultures.
  • Modified cell culture methods called shell vial culture systems are able to detect slow-growing viruses such as CMV. CMV often requires 14-18 days to produce a cytopathogenic effect by fibroblast cell culture technique. The prepared clinical specimens are inoculated on to adherent cell monolayers grown on round coverslips in small vials. The vials are centrifuged at low speed for 1 hour, after which fresh culture medium is added. Next, the vials are incubated and examined serially to detect viral antigen or DNA expression. Shell vial culture systems are used widely for earlier detection of CMV, RSV, HSV, adenovirus, influenza viruses, PIV, and other viral pathogens.
• Rapid antigen detection
  • Rapid antigen detection tests provide faster results because the test is performed directly on specimens obtained from patients.
  • Immunofluorescence assay and enzyme-linked immunosorbent assay (ELISA) are available for the diagnosis of HSV, RSV, influenza virus A and B, PIV, CMV, and other respiratory viruses. ELISA can detect viral antigens, while an immunofluorescence assay requires the presence of prepared, intact, infected cells. The sensitivity and specificity of these methods varies depending on the virus being sought and the particular diagnostic assay being used.
  • The advantages of antigen detection tests are higher specificity for individual viruses. Furthermore, these assays remain positive for several days to weeks, long after the culture technique can detect viable virus.
  • The disadvantages of these methods are that the overall sensitivity is lower than that of viral cultures. Therefore, antigen detection methods should be used in conjunction with cell culture for optimal diagnosis of viral infections.¹⁵
• Gene amplification
  • Polymerase chain reaction (PCR) is a highly sensitive and specific technique for amplifying genes to detect the presence of a virus. PCR has become especially useful for the detection of CMV in various body fluids (eg, blood, urine, respiratory secretions). PCR detection should be used in combination with viral culture and immunocytologic and rapid antigen detection.
  • A newly developed molecular diagnostic technique, multiplex reverse transcriptase-polymerase chain reaction (MRT-PCR), permits rapid detection of RSV, adenoviruses, and PIV in appropriate respiratory tract secretions.¹⁶ The single-step MRT-PCR technique has high sensitivity and specificity. The sensitivity for diagnosis of CMV pneumonitis by PCR has been reported to be 85-100%.
• Cultures of nonrespiratory specimens also may be helpful in diagnosing the specific virus causing pneumonia. Adenoviruses are secreted into the stool for weeks. CMV may be detected in the urine and in buffy coat preparations from heparinized blood. Serologic studies have been helpful in diagnosing pneumonia caused by measles virus and CMV. The presence of immunoglobulin M antibodies indicates a recent infection.
• The diagnosis of Hantavirus infection is based on detection of Four Corners virus–specific antibody reactivities in serum and the detection of Four Corners virus (FCV) RNA in peripheral blood mononuclear cells by reverse transcriptase-polymerase chain reaction (RT-PCR). The serodiagnosis of acute FCV infection is confirmed by detecting FCV genetic material in peripheral blood mononuclear cell preparations with RT-PCR.
• VZV infection and pneumonia can be diagnosed mostly on clinical grounds. VZV can be isolated from vesicular fluid, respiratory secretions, blood, or cerebrospinal fluid. Rapid antigen detection tests such as direct immunofluorescence can be performed on cell scrapings. A Tzanck smear obtained by unroofing a cutaneous lesion for material may show multinucleated giant cells with eosinophilic intranuclear inclusions but cannot distinguish between HSV and VZV. Serologic assays include indirect immunofluorescence, complement fixation, neutralizing antibody test, and ELISA.
• Diagnosis of measles pneumonia is clinical, but laboratory diagnosis can be helpful. Measles virus can be grown in primary culture in monkey and human kidney cells. Cytopathic effects are observed in 6-10 days; eosinophilic inclusions are found in the cytoplasm and nucleus of the infected cells.
• Adenoviruses can be isolated from respiratory secretions and can be grown in human embryonic kidney cells, human laryngeal tumor cells (HEp-2), and HeLa cells. Cytopathic effects appear in 2-20 days and include eosinophilic and diffuse basophilic intranuclear inclusions.
• The diagnosis of HSV pneumonia can be confirmed using respiratory culture, immunofluorescence antigen assays, PCR, cytologic techniques, and serology.
• The diagnosis of CMV pneumonia is based on clinical presentation and positive culture and histopathologic findings. Positive CMV cultures should be interpreted in view of other evidence of disease because asymptomatic shedding can occur in saliva, urine, and other bodily fluids. Documentation of viremia by shell vial culture technique usually signifies CMV disease.

Imaging Studies

• Chest radiographic findings usually are nonspecific, but some features are characteristic of individual viruses.
  • HSV can produce focal lesions on chest x-ray that begin as small nodules in the periphery. As the disease progresses, the nodules coalesce to form extensive infiltrates.
  • In influenza pneumonia, radiographic findings are similar to those described for other respiratory viral infections. Perihilar and peribronchial infiltrates occur commonly, while progression to diffuse interstitial infiltrates is observed with severe disease. Other findings of influenza pneumonia include hyperexpansion of the lungs, subsegmental atelectasis of multiple lobes, and lobar atelectasis, particularly of the right-upper or right-middle lobe.
  • In CMV pneumonia, chest radiographs show interstitial infiltrates predominantly in the lower lobes. Advancement to diffuse interstitial infiltrates is observed in patients with organ transplant.
  • In RSV, chest radiographs show bilateral interstitial or patchy infiltrates. Lobar consolidation and pleural effusions are present in 25% and 5% of cases, respectively.
  • In PIV, chest radiographs may reveal findings ranging from focal infection to diffuse interstitial infiltrates or diffuse mixed alveolar-interstitial infiltrates consistent with acute lung injury.
  • In varicella pneumonia, radiographic findings are diffuse, fluffy, reticular or nodular infiltrates that can be rapidly progressive. Pleural effusion and peripheral adenopathy can occur. Radiographic abnormalities are more prominent during the peak of the rash and resolve rapidly with clinical improvement. Long-term respiratory sequelae are infrequent in survivors, although small, diffusely scattered, punctate lung calcifications may persist on chest films.

• Hantavirus infection may result in normal chest radiograph findings during early disease. This is followed by signs of interstitial edema, Kerley B lines, peribronchial cuffing, and indistinct hila. Progression to the pulmonary edema phase over the subsequent 48 hours is indicated by centrally located dense alveolar infiltrates unlike the more peripheral infiltrates of adult respiratory distress syndrome from other causes. With further progression, pleural effusions also may develop.

Procedures

• Infrequently, lung biopsy, ie, transbronchial via a bronchoscope, transthoracic via a thoracoscope, or open lung, is required to make a diagnosis in very ill patients, who often are immunocompromised.
• Bronchiolar lavage also may be useful to obtain material for cytopathologic analysis and microbiologic studies.

Histologic Findings

Histopathologic examination of lung tissue infected with influenza pneumonia reveals edema, focal hemorrhages, and cellular infiltration. Alveoli may be denuded of epithelium, and intraalveolar hemorrhage is common. The presence of an acellular hyaline membrane lining the alveoli is typical of influenza pneumonia.

Histopathology of varicella-zoster pneumonia shows focal necrosis, consolidation, a mononuclear infiltrate, and intranuclear inclusion bodies.

Measles pneumonia has been called Hecht giant cell pneumonia because a predominantly interstitial infiltrate with mononuclear cells and multinucleated giant cells is present on histology.

Histologically, HSV pneumonia shows parenchymal necrosis, hemorrhage, and mononuclear infiltrates. Upon bronchoscopy, one may observe trachitis, bronchitis, and typical punctate mucosal lesions. Pathology findings in HSV infection show multinucleated giant cells and intranuclear inclusions.

Cytopathologic findings in CMV demonstrate typical cytomegalic cells with intranuclear and cytoplasmic inclusions. Histopathologic examination of lung tissue shows mononuclear interstitial infiltrates, thickened alveolar walls, fibrinous exudates, and hemorrhage. The cells containing inclusion bodies can be difficult to detect in mild cases.

• Search for CME/CE on This Topic »

• Pneumonia, Viral (Radiology)
• Pneumonia, Community-Acquired (Infectious Diseases)
• Pneumonia, Bacterial (Pulmonology)
• Pneumonia, Viral (Emergency Medicine)

• Cold and Flu Center
• Flu in Adults Overview
• Viral Pneumonia Causes
• Pneumonia Center
• Viral Pneumonia Overview
• Viral Pneumonia Symptoms
• Viral Pneumonia Treatment

• Community-Acquired MRSA Emerging Cause of Pneumonia
• Harm From Inhaled Steroids May Outweigh Benefits in COPD
• Early vs Deferred Antiretroviral Therapy May Reduce AIDS Progression and Death

• Current Perspectives on Pneumonia in Clinical Practice
• Diagnostic Strategies for Healthcare-associated Pneumonia
• The Impact of Treatment With Tumour Necrosis Factor-alpha Antagonists on the Course of Chronic Viral Infections: A Review of the Literature

• Pneumonia Resource Center
• Pediatric Dermatology Resource Center
• Influenza Resource Center