Some viral illnesses present with typical syndromes (measles, mumps, and chickenpox). In others, the clinical picture suggests any of a number of viruses. For example, aseptic meningitis can be caused by the mumps virus, lymphocytic choriomeningitis virus, and several enteroviruses, among others. Symptoms of respiratory disease with many viruses are indistinguishable. Some viral diseases have a characteristic rash, though in most the rash associated with viral syndromes is not pathognomonic.
Identification of a virus is useful for confirmation of atypical cases, help with outbreak investigation, elucidation of confusing syndromes, and increasingly to support the need for specific antiviral therapy. The frequency with which certain pathogens cause certain diseases allows for educated guesses, eg, respiratory syncytial virus (RSV) for bronchiolitis or parainfluenza virus for croup. Instances in which rapid diagnosis assists in patient management are noted.
Several techniques are used for diagnosis, including stain (the nonspecific Tzanck smear for herpesviruses), cell culture (coxsackievirus in suckling mice), immunologic detection (seroconversion with arboviruses, rabies virus on skin biopsy, detection of secretory immunoglobulin A [IgA]), or molecular techniques such as polymerase chain reaction (for herpes simplex virus [HSV]) or antigen assays (for cytomegalovirus [CMV]). Isolation of virus from a normally sterile site (cerebrospinal fluid, lung) or from a lesion (vesicles) in an immunocompetent individual is diagnostically significant. Isolation from nonsterile sites (nasopharynx, stool) may represent a carrier state, and seroconversion and pathologic change are needed to establish a diagnosis.
Microscopic techniques are used to examine cells, body fluids, or biopsy material in search of virus or cytopathic changes specific for one virus or a group of viruses (eg, multinucleated giant cells at the base of herpes lesions, rotavirus structures on electron micrographs of diarrheal stools).
Immunofluorescent methods, often with monoclonal antibodies, can rapidly identify some antigens (rabies, varicella, herpes simplex, RSV) in desquamated or scraped cells.
IMMUNOLOGIC STUDIES OF SERA
Specific antibodies to viruses rise during the course of illness, though the rise and persistence of titer depend on both the virus and the host response. A fourfold or greater rise in antibody titer during illness is considered evidence of disease.
Single determinations are seldom helpful, and laboratories require paired sera (acute and convalescent, typically 2–3 weeks apart). Antigenic detection is used for certain viruses (hepatitis B surface antigen [HBsAg], CMV, human immunodeficiency virus [HIV]), with viral presence detected independently of disease duration or host response. As more is learned of the natural history of HSV-1 and HSV-2 infections, these viruses are increasingly managed clinically with support from serologic data. Quantification of viral antigen titer is useful in the management of HIV disease and is becoming a standard of care for other chronic viral infections (eg, hepatitis B and C virus [HBV, HCV]).
Molecular technology has provided techniques such as polymerase chain reaction (PCR) and nucleic acid probes that have proved useful in the identification of new pathogens (eg, HCV and Kaposi's sarcoma herpesvirus) as well as for the management of patients in whom quantification of viral activity, as with immunologic techniques, assists in following the course of clinical illness or the response to therapy. Results vary among laboratories.
The armamentarium of antiviral therapy has expanded greatly with the advent of the HIV outbreak, though for many viruses there remains no definitive antiviral therapy. The mainstay of controlling viral diseases is vaccination. Currently available live vaccines include those against measles, mumps, rubella, poliovirus (Sabin vaccine), yellow fever, and varicella. The inactivated vaccines protect against the agents implicated in poliovirus (Salk vaccine), hepatitis A, hepatitis B, influenza A and B, rabies, RSV, and Japanese B encephalitis. Passive immunoprophylaxis remains important in prevention of hepatitis A and B, RSV infection, and, among the immunosuppressed, varicella.
- Current Medical Diagnosis & Treatment, 45th Edition (2006)