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Introduction ⬇

Viral infections are the most common of all human infections. Once thought to be confined to the childhood years, viral infections in adults have increasingly been recognized as the cause of significant morbidity and mortality. They also affect immunosuppressed and older patients (Chart 7.3).

Viruses are responsible for certain respiratory illnesses, hepatitis, HIV/AIDS, and certain STIs. Viruses are submicroscopic, filterable, infectious organisms that exist as intracellular parasites. They are divided into two groups according to the type of nucleic acid they contain: RNA or DNA.

The mycoplasmas are Scotobacteria without cell walls that are surrounded by a single triple-layered membrane; they are also known as pleuropneumonia-like organisms. Physiologically, mycoplasmal infections are considered to be intermediate between those caused by bacteria and those caused by rickettsiae. One species, Mycoplasma pneumoniae, is recognized as the causative agent of primary atypical pneumonia and bronchitis. Other species are suspected as possible causal agents for urethritis, infertility, early-term spontaneous abortion, rheumatoid arthritis, myringitis, and erythema multiforme.

Viruses and mycoplasmas are infectious agents small enough to pass through bacteria-retaining filters. Although small size is the only property they have in common, viruses and mycoplasma cause illnesses that are often indistinguishable from each other in terms of clinical signs and symptoms; in addition, both frequently occur together as dual infections. Therefore, the serologic (antigen–antibody) procedures commonly used for diagnosing viral infection are also used for diagnosing mycoplasmal infections (Table 7.7).

  1. Isolation of the virus in tissue culture remains the gold standard for detection of many common viruses. Diagnostic modalities include the following:

    1. Tissue culture

    2. Direct detection in specimens

    3. Identification through specific cytopathic effect

    4. Use of immunofluorescence and immunoperoxidase, latex agglutination, or ELISA to identify

    5. Visualization through an electron microscope

    6. Direct nucleic acid hybridization probe and nucleic acid amplification assay

  2. Serologic studies for antigen–antibody detection are valuable in regard to viral disease. Epstein–Barr virus and human hepatitis viruses are routinely serodiagnosed. Classically, a fourfold rise in antibody titer is used to identify a particular infectious agent, provided that the pathogenesis of the agent agrees with the symptoms of the infected patient. An acute-phase serum is collected within the first several days after symptom onset. A convalescent-phase serum is collected 2–4 weeks later. A fourfold difference in antibody titer between the two sera is statistically significant. Alternatively, detection of specific IgM suggests acute infection. IgG antibody without IgM suggests infection sometime in the past.

  3. Available cell cultures vary greatly in their sensitivity to different viruses. One cell type or species may be more sensitive than another for detecting the virus in low titers. For example, human embryonic kidney (HEK) can be used for adenovirus, enterovirus, herpes simplex, measles, influenza, parainfluenza, and rubella; however, HEK cannot be used for CMV or influenza.

  4. The critical first step in successful viral diagnosis is the timely and proper collection of specimens. The choice of which type of specimen to collect depends on typical signs and symptoms and the suspected virus. Improper specimen choice and collection is one of the biggest factors in diagnostic delays.

  1. Collect specimens for viruses as early as possible during the course of the illness, preferably within the first 4 days after symptom onset. If specimen collection is delayed for 7 or more days after symptoms appear, diagnosis will be compromised. Virus titers are highest in the early part of the illness when the host has not yet mounted a robust immune response. Little neutralizing antibody is present. Detection of a virus by culture, direct detection, or serology is greatly enhanced when the virus titers are high.

  2. Sampling procedure:

    1. For localized infection:

      1. Direct sampling of affected site (e.g., nasal swab, throat swab, skin scraping)

      2. Indirect sampling: For example, if CSF is the target sample in a central nervous system infection, the indirect approach would involve obtaining throat or rectal swabs for culture.

    2. Sampling from more than one site, for example, with disseminated infection or with nonspecific clinical findings.

    3. The type of applicator used to obtain specimens may affect accuracy of results. Do not use wooden applicators or random cotton swabs because they are toxic to viruses. A self-contained transport system is recommended to ensure that the specimen remains moist.

  3. When transporting specimens:

    1. Viral specimens are unstable and rapidly lose infectivity outside of living cells. Prompt delivery to the laboratory is essential. Samples must be refrigerated or placed on ice or cold packs while in transit.

    2. Freezing and thawing of specimens diminishes the quantity of available viable virus.

  4. The specimen should be labeled with the patient’s name, medical record number, healthcare provided, and date and time of collection. Additional patient information should be documented in the patient’s medical record and should include:

    1. Pertinent information that would influence processing of the specimen (e.g., patient is immunocompromised owing to kidney transplantation)

    2. Patient demographics

    3. Contact person or clinic so as to expedite notification of abnormal results

  5. Specimens of small volume (e.g., vesicular fluid, fine-needle aspiration, biopsy samples) should be transported in a liquid medium. Suggested viral transport media are Hank balanced salt solution or 0.2 mol/L sucrose phosphate. Several commercially available viral transport media can be used.

Often, a complete microbiologic workup of a specimen (tissue, bronchoscopy) is requested along with a viral workup. Because viral transport media contain antibiotic drugs, sterile saline is recommended. Personnel in the laboratory can then divide the specimen for workup within the microbiology subsections.

Specimens of a liquid nature (urine, CSF, sputum, body fluids) are collected in a sterile container. For patients with suspected viremia, a viral culture of the buffy coat of peripheral blood is submitted. Blood specimens are collected in evacuated tubes containing heparin or ethylenediaminetetraacetic acid (EDTA).

Clinical Implications ⬆

  1. Herpes simplex is the virus most frequently isolated and diagnosed virus in the laboratory.

  2. Viral culture results are normally available within 3–5 days, although rapid test results (24 hours or less) are accurate and available for certain viruses, such as CMV.

  3. Significance of viral cultures:

    1. Positive viral culture results from the following sources are diagnostically accurate:

      1. Autopsy specimens

      2. Blood (buffy coat)

      3. Biopsy

      4. CSF

      5. Other body fluids

      6. Cervix

      7. Eye

      8. Skin lesions

      9. Fine-needle aspirates

      10. Bronchial alveolar wash brushing

    2. Probably diagnostically accurate are:

      1. Throat

      2. Urine

      3. Sputum

      4. Genital (cervical, penile)

      5. Nasal aspirates or washes

      6. Vesicular

      7. Skin (mouth, lip)

    3. Possibly diagnostically accurate is stool or rectal swab.

    4. Viruses do not comprise normal flora in the body. However, bacterial or fungal contamination of specimens can occur.