Diagnostic Methods in Virology 1. Direct Examination 2. Indirect Examination (Virus Isolation) 3. Serology
Direct Examination 1. Antigen Detection immunofluorescence, ELISA etc. 2. Electron Microscopy morphology of virus particles immune electron microscopy 3. Light Microscopy histological appearance inclusion bodies 4. Viral Genome Detection hybridization with specific nucleic acid probes polymerase chain reaction (PCR)
Indirect Examination 1.Cell Culture cytopathic effect (CPE) haemabsorption immunofluorescence 2. Eggs pocks on CAM haemagglutination inclusion bodies 3. Animals disease or death
Serology Detection of rising titres of antibody between acute and convalescent stages of infection, or the detection of IgM in primary infection.
Virus Isolation Cell Cultures are most widely used for virus isolation, there are 3 types of cell cultures: 1. Primary cells - Monkey Kidney 2. Semi-continuous cells - Human embryonic kidney and skin fibroblasts 3. Continuous cells - HeLa, Vero, Hep2, LLC-MK2, MDCK Primary cell culture are widely acknowledged as the best cell culture systems available since they support the widest range of viruses. However, they are very expensive and it is often difficult to obtain a reliable supply. Continuous cells are the most easy to handle but the range of viruses supported is often limited.
Cell Cultures Growing virus may produce 1. Cytopathic Effect (CPE) - such as the ballooning of cells or syncytia formation, may be specific or non-specific. 2. Haemadsorption - cells acquire the ability to stick to mammalian red blood cells. Confirmation of the identity of the virus may be carried out using neutralization, haemadsorption-inhibition or immunofluorescence tests.
Cytopathic Effect (1) Cytopathic effect of enterovirus 71 and HSV in cell culture: note the ballooning of cells. (Virology Laboratory, Yale-New Haven Hospital, Linda Stannard, University of Cape Town)
Cytopathic Effect (2) Syncytium formation in cell culture caused by RSV (top), and measles virus (bottom). (courtesy of Linda Stannard, University of Cape Town, S.A.)
Haemadsorption Syncytial formation caused by mumps virus and haemadsorption of erythrocytes onto the surface of the cell sheet. (courtesy of Linda Stannard, University of Cape Town, S.A.)
Problems with cell culture Long period (up to 4 weeks) required for result. Often very poor sensitivity, sensitivity depends on a large extent on the condition of the specimen. Susceptible to bacterial contamination. Susceptible to toxic substances which may be present in the specimen. Many viruses will not grow in cell culture e.g. Hepatitis B, Diarrhoeal viruses, parvovirus, papillomavirus.
Rapid Culture Techniques Rapid culture techniques are available whereby viral antigens are detected 2 to 4 days after inoculation. The CMV DEAFF test is the best example, whereby The cell sheet is grown on individual cover slips in a plastic bottle. Following inoculation, the bottle then is spun at a low speed for one hour (to speed up the adsorption of the virus) and then incubated for 2 to 4 days. The cover slip is then taken out and examined for the presence of CMV early antigens by immunofluorescence.
DEAFF test for CMV (Virology Laboratory, Yale-New Haven Hospital)
Electron Microscopy 10 6 virus particles per ml required for visualization, 50, ,000 magnification normally used. Viruses may be detected in the following specimens. FaecesRotavirus, Adenovirus Norwalk like viruses Astrovirus, Calicivirus Vesicle FluidHSV VZV Skin scrapingspapillomavirus, orf molluscum contagiosum
Electronmicrographs Adenovirus Rotavirus (courtesy of Linda Stannard, University of Cape Town, S.A.)
Immune Electron Microscopy The sensitivity and specificity of EM may be enhanced by immune electron microscopy. There are two variants:- Classical Immune electron microscopy (IEM) - the sample is treated with specific anti-sera before being put up for EM. Viral particles present will be agglutinated and thus congregate together by the antibody. Solid phase immune electron microscopy (SPIEM) - the grid is coated with specific anti-sera. Virus particles present in the sample will be absorbed onto the grid by the antibody.
Problems with Electron Microscopy Expensive equipment Expensive maintenance Require experienced observer Sensitivity often low
Serology Criteria for diagnosing Primary Infection 4 fold or more increase in titre of IgG or total antibody between acute and convalescent sera Presence of IgM Seroconversion A single high titre of IgG (or total antibody) - very unreliable Criteria for diagnosing Reinfection fold or more increase in titre of IgG or total antibody between acute and convalescent sera Absence or slight increase in IgM
Typical Serological Profile After Acute Infection Note that during reinfection, IgM may be absent or present at a low level transiently
Complement Fixation Test Complement Fixation Test in Microtiter Plate. Rows 1 and 2 exhibit complement fixation obtained with acute and convalescent phase serum specimens, respectively. (2-fold serum dilutions were used) The observed 4-fold increase is significant and indicates recent infection.
ELISA for HIV antibody Microplate ELISA for HIV antibody: coloured wells indicate reactivity
Western Blot HIV-1 Western Blot Lane1: Positive Control Lane 2: Negative Control Sample A: Negative Sample B: Indeterminate Sample C: Positive
Usefulness of Serological Results How useful a serological result is depends on the individual virus. For example, for viruses such as rubella and hepatitis A, the onset of clinical symptoms coincide with the development of antibodies. The detection of IgM or rising titres of IgG in the serum of the patient would indicate active disease. However, many viruses often produce clinical disease before the appearance of antibodies such as respiratory and diarrhoeal viruses. So in this case, any serological diagnosis would be retrospective and therefore will not be that useful. There are also viruses which produce clinical disease months or years after seroconversion e.g. HIV and rabies. In the case of these viruses, the mere presence of antibody is sufficient to make a definitive diagnosis.
Problems with Serology Long period of time required for diagnosis for paired acute and convalescent sera. Mild local infections such as HSV genitalis may not produce a detectable humoral immune response. Extensive antigenic cross-reactivity between related viruses e.g. HSV and VZV, Japanese B encephalitis and Dengue, may lead to false positive results. immunocompromised patients often give a reduced or absent humoral immune response. Patients with infectious mononucleosis and those with connective tissue diseases such as SLE may react non-specifically giving a false positive result. Patients given blood or blood products may give a false positive result due to the transfer of antibody.
CSF antibodies Used mainly for the diagnosis of herpes simplex and VZV encephalitis CSF normally contain little or no antibodies presence of antibodies suggest meningitis or meningoencephalitis CSF antibody titre > _1_ is indicative of meningitis Serum antibody titre 100 Diagnosis depends on the presence of an intact blood-brain barrier
Rapid Diagnosis Based on the Detection of Viral Antigens Nasopharyngeal AspirateRSV Influenza A and B Parainfluenza Adenovirus FaecesRotaviruses Adenoviruses Astrovirus SkinHSV VZV BloodCMV (pp65 antigenaemia test)
Immunofluorescense Positive immunofluorescence test for rabies virus antigen. (Source: CDC) (Virology Laboratory, Yale-New Haven Hospital)
CMV pp65 antigenaemia test (Virology Laboratory, Yale-New Haven Hospital)
Advantages and Disadvantages Advantages Result available quickly, usually within a few hours. Potential Problems Often very much reduced sensitivity compared to cell culture, can be as low as 20%. Specificity often poor as well. Requires good specimens. The procedures involved are often tedious and time- consuming and thus expensive in terms of laboratory time.
Molecular Methods Methods based on the detection of viral genome are also commonly known as molecular methods. It is often said that molecular methods is the future direction of viral diagnosis. However in practice, although the use of these methods is indeed increasing, the role played by molecular methods in a routine diagnostic virus laboratory is still small compared to conventional methods. It is certain though that the role of molecular methods will increase rapidly in the near future.
Classical Molecular Techniques Dot-blot, Southern blot, in-situ hydridization are examples of classical techniques. They depend on the use of specific DNA/RNA probes for hybridization. The specificity of the reaction depends on the conditions used for hybridization. However, the sensitivity of these techniques is not better than conventional viral diagnostic methods. However, since they are usually more tedious and expensive than conventional techniques, they never found widespread acceptance.
Polymerase Chain Reaction (1) PCR allows the in vitro amplification of specific target DNA sequences by a factor of 10 6 and is thus an extremely sensitive technique. It is based on an enzymatic reaction involving the use of synthetic oligonucleotides flanking the target nucleic sequence of interest. These oligonucleotides act as primers for the thermostable Taq polymerase. Repeated cycles (usually 25 to 40) of denaturation of the template DNA (at 94 o C), annealing of primers to their complementary sequences (50 o C), and primer extension (72 o C) result in the exponential production of the specific target fragment. Further sensitivity and specificity may be obtained by the nested PCR. Detection and identification of the PCR product is usually carried out by agarose gel electrophoresis, hybridization with a specific oligonucleotide probe, restriction enzyme analysis, or DNA sequencing.
Polymerase Chain Reaction (2) Advantages of PCR: Extremely high sensitivity, may detect down to one viral genome per sample volume Easy to set up Fast turnaround time Disadvantages of PCR Extremely liable to contamination High degree of operator skill required Not easy to set up a quantitative assay. A positive result may be difficult to interpret, especially with latent viruses such as CMV, where any seropositive person will have virus present in their blood irrespective whether they have disease or not. These problems are being addressed by the arrival of commercial closed systems such as the Roche Cobas Amplicor which requires minimum handling. The use of synthetic internal competitive targets in these commercial assays has facilitated the accurate quantification of results. However, these assays are very expensive.
Schematic of PCR Each cycle doubles the copy number of the target
Other Newer Molecular Techniques Branched DNA is essentially a sensitive hydridization technique which involves linear amplification. Whereas exponential amplification occurs in PCR. Therefore, the sensitivity of bDNA lies between classical amplification techniques and PCR. Other Newer molecular techniques depend on some form of amplification. Commercial proprietary techniques such as LCR, NASBA, TMA depend on exponential amplification of the signal or the target. Therefore, these techniques are as susceptible to contamination as PCR and share the same advantages and disadvantages. PCR and related techniques are bound to play an increasingly important role in the diagnosis of viral infections. DNA chip is another promising technology where it would be possible to detect a large number of viruses, their pathogenic potential, and their drug sensitivity at the same time.
Comparison between PCR and other nucleic acid Amplification Techniques