Laboratory Approach to the Diagnosis of Smallpox: Module 6 Laboratory Methods Part II

Transcription

1 Slide 1 of 2 (musical lead-in) Slide 2 of 2: Welcome to Module 6 of this program on the laboratory approach to the diagnosis of smallpox. This module comprises Part 2 of Laboratory Methods. Module 6 of this series describes the utility of histopathology, immunohistochemistry and electron microscopy for the diagnosis of orthopoxvirus infections and other rash-like illnesses. We will learn about the recommended specimen types, biosafety considerations and the test procedures. The benefits and limitation of immunohistochemical staining and electron microscopy are reviewed. At the conclusion of this module, participants will be able to complete the following learning activities: Recognize negative stain electron microscopy images of orthopoxviruses. List the advantages of using electron microscopy for confirming poxvirus infections. Describe how immunohistochemical techniques can be used to detect orthopoxviruses in clinical samples. Introduction & Objectives 1

2 Slide 1 of 21 (musical lead-in) Slide 2 of 21: Unit 1 of this module will discuss the role of histopathology and immunohistochemistry in the diagnosis and study of smallpox and other clinical conditions that may be considered in the differential diagnosis. Slide 3 of 21: There are many other pathogens and diseases that can be mistaken for orthopoxvirus infections, as shown here and discussed in Module 2. Utilizing pathology and immunohistochemistry in the overall diagnostic scheme is often very useful for determining whether a rash-like illness is due to orthopoxvirus infection or one of these other illnesses. Lesion diagnoses that can be strongly suggested by histopathologic examination include: erythema multiforme, drug eruptions, insect bites, impetigo, viral infections (via their cytopathic effects), and bacterial infections. Various immunohistochemistry methods are available for poxviruses, herpes simplex and varicella zoster, spotted fever rickettsia, measles, enterovirus, syphilis, viral hemorrhagic fevers, anthrax, tularemia, neisseria, and others. Unit 1: Pathology 2

3 Slide 4 of 21: The laboratory algorithm (Laboratory Testing for Acute, Generalized Vesicular or Pustular Rash Illness in the United States) introduced and discussed in detail in Module 3 provides: 1) a standardized approach to rapidly triage and test specimens for the possible presence of variola virus, the infectious agent that causes smallpox; and 2) a logical progression of testing if the case patient is not considered to be at high risk for smallpox. According to the laboratory algorithm, histopathologic examination of clinical specimens with low to moderate risk is indicated when all diagnostic tests are negative. This includes negative results for the rule-out tests listed for common look-alike diseases as well as the orthopoxvirus PCR tests. In cases where all tests are negative, the patient should be reevaluated and the need for dermatologic and histologic testing should be assessed. In high-risk cases, histopathology and immunohistochemistry can provide additional confirmatory evidence of smallpox or point to an alternative diagnosis. Now, let s take a closer look at the various histopathology and immunohistochemistry methods used for orthopoxvirus testing of clinical samples. Slide 5 of 21: Immunohistochemistry or IHC can expand the diagnostic capability of the pathologist; provide rapid and specific identification of microorganisms; and support research applications. Immunohistochemistry involves the use of a primary antibody against the particular microbe, followed by various antibody links and colorimetric detection of the agent, which appears red, in tissue sections. The technique allows organism-specific diagnoses. Current Poxvirus IHC assays are specific for orthopoxviruses and parapoxviruses, but they are not species specific. Species-specific PCR assays analyzing formalin-fixed paraffin embedded (FFPE) tissues are also available. Unit 1: Pathology 3

4 Slide 6 of 21: For pathology specimens, an advantage of using formalin-fixed tissues is that formalin inactivates the virus in the tissues and the specimens are safe to handle in a routine pathology laboratory. However, if not fixed, skin biopsies and other specimens from suspected cases that are utilized in pathology testing could potentially contain viable virus. Therefore, for processing unfixed tissues, the biosafety conditions are the same as for virus culture and nucleic acid detection. As described in module 3, the biosafety regulations vary, depending on which orthopoxvirus is suspected or included in testing. Vaccinia requires biosafety level two (BSL-2) conditions, with all live virus manipulations conducted within a biosafety cabinet. The Advisory Committee on Immunization Practices (ACIP) recommends smallpox vaccination for anyone working with live virus. Remember that blood, sera, and plasma specimens being tested for orthopoxviruses may also contain other bloodborne pathogens, so it is important to take the proper precautions. Additionally, any laboratorian exposed to or working with bodily fluids, especially blood, should receive Hepatitis B vaccination. At the Centers for Disease Control and Prevention (CDC), current practices for working with possible monkeypox specimens include vaccinations of laboratory personnel and biosafety level 2 or 3 facilities with BSL-3 work practices. There is further guidance at the CDC website. Variola virus can only be studied at two World Health Organization (WHO) collaborating centers. One is at the CDC in Atlanta, Georgia. The other is at the State Research Center of Virology and Biotechnology, known as Vector, in the Novosibirsk Region of Russia. All research utilizing viable virus is conducted in the BSL-4 laboratory by vaccinated personnel. Currently no one outside of these two laboratories should attempt to grow any virus from a sample considered to be at high risk for containing variola virus. Slide 7 of 21: Several sample types are appropriate for histopathology and immunohistochemistry testing. These include skin biopsies obtained via a punch biopsy kit; vesicular fluid smears; scabs; and formalin-fixed tissue. A more complete description of collection methods and specimen types can be found in Module 4 and at the CDC website. Unit 1: Pathology 4

5 Slide 8 of 21: Skin biopsy material, collected using a punch biopsy method, is the preferred source of material for pathologic review. This evaluation is important if the patient is considered to be at moderate or high risk for smallpox infection, or when certain low risk patients are being evaluated. Evaluation of biopsy material is especially important for successful diagnosis of several of the orthopox look-alike syndromes. Processing of tissues for histopathology includes the following steps. Grossing, the initial visual evaluation of the submitted biopsy, is followed by processing, embedding and sectioning. Standard hematoxylin and eosin staining follows. These steps can take anywhere from 3 to 12 hours, depending on the automation of the laboratory. Slide 9 of 21: The basic method for immunohistochemical testing is similar to the methods used for other antibody-related diagnostic tests. First, a monoclonal mouse antibody or primary antibody binds to the antigen in infected tissue such as a skin biopsy. A secondary biotinylated antibody binds to the primary antibody. That secondary antibody is easily linked to a third complex. The third complex lights up and becomes red when it is bound to the final Fast Red developing solution. Specific immunohistochemical assays (IHC) for the Category A terrorism agents, such as smallpox, anthrax, and Ebola have been developed and are described in the June 11, 2004 issue of Morbidity and Mortality Weekly Report. After reviewing the link, click the Next button to continue. Slide 10 of 21: The following slides demonstrate some pathologic studies of experimentally infected nonhuman primates. These show some of the changes that may be visualized in different organs by standard stains, as well as immunohistochemistry. Shown here is a hematoxylin and eosin staining of a liver from a non-human primate infected with variola. The arrows indicate blue cytoplasmic inclusions in hepatocytes. Here is a similar slide, stained using antibodies against smallpox. The areas in red show the distribution of viral antigens in hepatocytes. Unit 1: Pathology 5

6 Slide 11 of 21: Shown here are immunohistochemical observations of variola virus from other non-human primates after smallpox infection and illness. Viral antigens are demonstrated in red in spleen, mucosa, and kidney. Slide 12 of 21: Here are images of the skin associated with human skin manifestations from a tanapox infection. Viral cytopathic changes in these skin biopsies include ballooning, indicated by arrows, and cytoplasmic inclusions, indicated by the arrowhead. Slide 13 of 21: The skin manifestations of varicella, a herpesvirus and the causative agent of chickenpox, can be distinguished from the skin manifestations of variola, the causative agent of smallpox, using hematoxylin and eosin staining as well as immunohistochemistry. Shown here are skin biopsies of a case of varicella. The arrow in this image indicates multinucleation. Both of these images show viral cytopathic effects the ground glass nuclear appearance in a multinucleated cell. Positive immunohistochemical staining is shown here. Slide 14 of 21: Another human disease with rash manifestations that could be confused with smallpox is rickettsialpox. Shown here are the rickettsialpox histopathology of eschar. This image shows acute inflammation and the necrotic surface indicated by the arrow. This image shows confirmation by specific immunohistochemical staining in red of bacteria in inflammatory cells. Slide 15 of 21: Manifestations of infection due to enteroviruses can also be confused with the smallpox rash. Here is an image is of the clinical EV71 rash. In this image of intracerebral changes in a fatal case of encephalitis, the arrow indicates classic Perivascular cuffing. Immunohistochemical staining of a neuron for enteroviral antigens indicates enterovirus as the cause of infection. Unit 1: Pathology 6

7 Slide 16 of 21: This is the typical maculo-papular rash of measles. It was historically confused with smallpox at the macular early rash stage of illness. Also shown here are pulmonary manifestations in lung tissue. This image shows giant cell pneumonia, with typical intracytoplasmic inclusions indicated by the arrow. In this slide, immunostaining is demonstrated in red. Slide 17 of 21: Syphilis is another disease historically confused with smallpox. Here are images from a skin rash associated with secondary syphilis: histopathology in skin on the left and immunohistochemical staining on the right, demonstrating the presence of spirochetes. Slide 18 of 21: Among the benefits of histopathologic and immunohistochemistry methods is the ability to perform multiple tests on a single sample; the results are rapid and sensitive due to automation. These techniques provide invaluable utility for the diagnosis of acute cases when serologic diagnosis of rash-like illness may not be helpful. Histopathology and immunohistochemistry can also provide independent confirmation (i.e., morphologic context along with specific microbial detection). Slide 19 of 21: These tests can be performed on formalin-fixed tissues. Oftentimes, formalin-fixed tissues are the only available specimens. Immunohistochemistry and histopathology testing of fixed-tissue specimens have three important features: 1) they minimize workers exposure to agents; 2) they allow for retrospective studies; and 3) they do not require cold chain for transport. Slide 20 of 21: Although there are significant advantages of these methods, interpretation of histopathology and IHC staining requires subject matter expertise by trained pathologists. Additionally, the quality of the biopsy material may impact the results. Unit 1: Pathology 7

8 Slide 21 of 21: This unit has provided a brief overview of some of the observations and techniques routinely used by the staff within the CDC Infectious Diseases Pathology Branch. The methods to look for smallpox infection in tissues have been validated on a number of specimen types and sources. It will be important for you to identify resources in your community that can assist with pathologic analysis of rash lesions deemed clinically suspicious for smallpox. Pathologists and electron microscopists may be an integral aspect of obtaining a definitive diagnosis. With the increased use of digital imagery and internet resources, it becomes easier to communicate in real time with other colleagues, including the CDC, allowing more rapid involvement of pathology expertise and consultation. Unit 1: Pathology 8

9 Slide 1 of 26 (musical lead-in) Slide 2 of 26: Unit 2 of this module will discuss the role of electron microscopy (EM) in the diagnosis and study of smallpox and other clinical conditions that may be considered in the differential diagnosis. Slide 3 of 26: There are many other pathogens and diseases that can be mistaken for orthopoxvirus infections, as shown here and discussed in Module 2. Using electron microscopy in the diagnostic approach is often very useful for determining whether the infection is due to an orthopoxvirus or one of the other rash-like illnesses listed here. Slide 4 of 26: During the smallpox eradication campaign, electron microscopic visualization of negatively stained poxvirus virions was a valuable technique for confirming poxvirus infections. Historically, negative-stain EM successfully detected orthopoxvirus particles in approximately 95% of clinical specimens from patients with smallpox and in approximately 65% of specimens from patients with vaccinia associated with smallpox vaccine-related adverse events. In the event of a deliberate release of smallpox virus and subsequent human disease, or in generalized vaccinia resulting from vaccination, negatively stained preparations derived from lesions or scab material would again provide a valuable method for assisting in poxvirus diagnosis and/or ruling out other causes of rash illness. Unit 2: Electron Microscopy 9

10 Slide 5 of 26: The laboratory algorithm (Laboratory Testing for Acute, Generalized Vesicular or Pustular Rash Illness in the United States) introduced and discussed in detail in Module 3 provides: 1) a standardized approach to rapidly triage and test specimens for the possible presence of variola virus, the infectious agent that causes smallpox; and 2) a logical progression of testing if the case patient is not considered to be at high risk for smallpox. Electron microscopic (EM) examination of clinical specimens is indicated at several steps in the laboratory algorithm. If the patient is determined to be at low to moderate risk for smallpox, and varicella zoster (chickenpox) diagnosis is questionable, then the laboratories within the laboratory response network (LRN) should begin testing the samples. Using biosafety level 2 conditions, EM for herpesviruses should be conducted if facilities are available. If the results are positive for herpesvirus, no further testing is required. However, if a negative result is found for all assays, the LRN laboratories may perform testing for orthopoxviruses, including vaccinia virus. Specimens can be collected for EM testing. If the patient is initially determined to be at high risk for smallpox based on the clinical algorithm, chain-of-custody documentation should be initiated and health care providers should immediately contact their state Public Health Lab as well as the CDC. Digital photos should be taken of all clinical presentations. For high-risk samples, DO NOT initiate sample preparation steps that include viral culture. If EM is available, it can be conducted under biosafety level 3 conditions at the local facility. Initial smallpox testing for patients with a high risk of clinical smallpox can occur simultaneously at the LRN variola (smallpox) surge laboratory and the CDC. However, results should NOT be released without CDC confirmation. Unit 2: Electron Microscopy 10

11 Slide 6 of 26: Specimens from suspected orthopoxvirus cases investigated via electron microscopy could potentially contain viable virus. Therefore, the biosafety conditions are the same as for virus culture and nucleic acid detection. As mentioned in module 3, the biosafety requirements vary, depending on which orthopoxvirus is suspected or being tested. Vaccinia requires biosafety level two (BSL-2) conditions, with all live virus manipulations conducted within a biosafety cabinet. The Advisory Committee on Immunization Practices (ACIP) recommends smallpox vaccination for anyone who works with live virus. Remember that blood, sera and plasma specimens being tested for orthopoxviruses may also contain other bloodborne pathogens, so it is important to take the proper precautions. Additionally, any laboratorian exposed to or working with bodily fluids, especially blood, should receive Hepatitis B vaccination. At the Centers for Disease Control and Prevention (CDC), current practices for working with possible monkeypox specimens include vaccinations of laboratory personnel and biosafety level 2 or 3 facilities with BSL-3 work practices. There is further guidance at the CDC website. Variola virus can only be studied at two World Health Organization (WHO) collaborating centers. One is at the CDC in Atlanta, Georgia. The other is at the State Research Center of Virology and Biotechnology, known as Vector, in the Novosibirsk Region of Russia. All research utilizing viable virus is conducted in the BSL-4 laboratory by vaccinated personnel. Currently no one outside of these two laboratories should attempt to grow any virus from samples considered to be at high risk for containing variola virus. Unit 2: Electron Microscopy 11

12 Slide 7 of 26: Negative-stain electron microscopy is a rapid technique. When available, it may provide the first laboratory result after the specimen reaches the laboratory. It is an excellent screening tool in that it can differentiate among several agents. Virions can be detected at a sensitivity of 5 logs of virus, or 100 virions per microliter. Once a virion is found, there is a definitive diagnosis to a virus family. Notably, orthopoxviruses can be differentiated from herpesviruses, such as varicella zoster virus, and from parapoxviruses, such as orf. However, visualizing virions with electron microscopy is not proof of smallpox infection. Variola, vaccinia, monkeypox, and molluscum viruses, for example, are morphologically indistinguishable. Electron microscopy also provides an archival specimen. If the diagnosis is uncertain, original clinical material can be shared with other electron microscopists. This unit will primarily discuss negative-stain electron microscopy preparations that are used to detect virus particles in liquid specimens. Thin-section electron microscopy would be used to examine tissue specimens. Both EM methods are excellent tools for evaluation of tissue culture isolates. Slide 8 of 26: Electron microscopy laboratories will be integral members of the smallpox biopreparedness team. But several issues should be considered before an EM laboratory agrees to participate in surveillance. Consideration should be given to the diagnostic capabilities of the lab personnel. They should have experience preparing negative-stain EM grids. More importantly, the microscopists must have experience analyzing negative-stain EM preparations to definitively identify virus morphology, and to differentiate these from other look-alikes agents and artifacts. The EM lab will need to have access to a BSL-2 or BSL-3 containment facility that uses BSL-3 practices. Additionally, EM and other laboratory personnel handling specimens from suspect smallpox patients require recent vaccination against smallpox, or no contra-indications to immediate or post-exposure vaccination. Unit 2: Electron Microscopy 12

13 Slide 9 of 26: These are the potential specimens that would be sent to an EM lab for diagnostics. The steps needed to process these will be discussed in this unit. Vesicular fluid can be collected on EM grids, or as smears on glass or plastic slides. Crusts from the lesions and tissue biopsies can be collected, as well as swabbing the rash area. Keep in mind that all laboratory manipulations of unfixed material must be carried out in a BSL-2 cabinet while using BSL-3 practices and safety equipment. Now let s discuss the collection of specimens for EM in more detail. Slide 10 of 26: EM grids with a plastic/carbon coating can be used to collect vesicular fluid from the patient by the direct-touch method. Once these grids have been inactivated via treatment to render specimen material non-infectious, they can be stained as outlined later in this unit. Slide 11 of 26: The technique for processing smears of vesicular fluid collected on slides is as follows: Add 1-2 drops of sterile water to the slide, scratch the dry material to resuspend, and make EM grids directly from this material. Slide 12 of 26: Use a grinder with a pestle to process crusts and tissue biopsies. Place the crust or tissue in the grinder tube and add one half milliliter of sterile water. Grind to produce an opalescent suspension, and short pulse spin in a microcentrifuge to sediment large particulates. The supernatant will be used as the liquid specimen for making the electron microscopy grid. Slide 13 of 26: Historically, swabs were the least likely to yield a good specimen for EM examination. However, if swabs are received, they can be processed as follows: Soak the swabs in a minimal volume of sterile deionized water for 15 minutes. Scrape any remaining specimen off the cotton swab and into the water. Unit 2: Electron Microscopy 13

14 Slide 14 of 26: Before the grid is applied to the reconstituted specimen, the hydrophilicity of the EM grids will need to be enhanced. There are a number of methods to attain this, including a glow discharge treatment (as seen here). Another method is to coat the grids with a 1% Alcian blue solution. This is done by placing a grid on a drop of 1% Alcian blue for 5 minutes, and then rinsing the grid with 3 drops of deionized water. Slide 15 of 26: Grids may also be prepared by the drop-to-drop method. This should be performed in a containment area such as a biosafety cabinet. Place a drop of the specimen on a sheet of Parafilm. Place a grid on top of the drop, plastic-side down. After 10 minutes, pick up the grid with tweezers and wick away the specimen drop with filter paper. Next, place the grid, specimen-side up, within a drop of 2% paraformaldehyde and let fix for approximately 15 minutes. Now wick away the excess fluid with filter paper, briefly place the grid on a drop of water, and wick away the excess water with filter paper. Next, place the grid, specimen-side down, on a drop of stain for seconds. Pick up the grid and wick away the excess stain with filter paper. To avoid cross-contamination between specimens, use different tweezers for each specimen. Alternatively, clean the tweezers with a gauze pad soaked in 70% alcohol between each specimen preparation. If the risk of the specimens containing variola virus has been assessed as low to moderate, place the grids into grid storage boxes and record which slot is used for each patient specimen. Unit 2: Electron Microscopy 14

15 Slide 16 of 26: Additional precautions are needed for specimens assessed to be at high risk for containing variola virus. While still in the biosafety cabinet, put the EM grids on filter paper that has been placed within the bottom of a small petri dish. Add enough of a 10% bleach solution to cover the bottom of a large petri dish. Place the small petri dish with the grids within the larger dish. Then place both dishes under a germicidal ultraviolet light to irradiate for 10 minutes. Turn the grids over and irradiate for an additional 10 minutes. This extra step using bleach and ultraviolet light will inactivate any stray virus particles that may have adhered to the specimen petri dish or to the filter paper. After the inactivation steps, using clean tweezers, place the grids into the grid storage boxes. Carefully record which slot is used for each patient specimen. Finally, inactivate the tweezers used for specimen preparation by placing them within a solution of 2% Amphyl, which is similar to Lysol, for at least 10 minutes. Then, rinse with 70% alcohol Slide 17 of 26: Examination under the electron microscope will show that almost all members of the family Poxviridae have a similar morphology. The only exception is the genus parapoxvirus, which will be described later. Shown here is a negative-stain image of monkeypox virus. It has the classic brick shape of most of the poxviruses, although sometimes the edges are rounded. The virions measure approximately 225 X 300 nanometers. Virus particles have characteristic tubular ridges with a random or whorled pattern. Depending on the experience of the microscopist, the specimen grid can be screened at a final magnification of 12,000 30,000. Screening each grid can take 15 minutes or longer. Slide 18 of 26: Here are more examples of poxviruses shown by negative-stain EM. This is an electron micrograph showing variola viruses. Here is an image of vaccinia virus from tissue culture. Shown here is vaccinia virus from a clinical specimen. This image demonstrates Monkeypox virus from a clinical specimen. Note that the morphology in clinical specimens may be less distinct than in tissue culture specimens. Unit 2: Electron Microscopy 15

16 Slide 19 of 26: Depending on the penetration of the stain, two forms of the orthopoxvirus particle may be seen. In the M (or mulberry ) form, the virion surface is covered with short, whorled filaments, and a circular depression is sometimes seen in the center of the virion. In particles penetrated by stain, the C (or capsular ) form, surface filaments are not visible; instead, the virion consists of a sharply defined, dense core surrounded by several laminated zones of differing densities. This image of fowlpox virus also illustrates the limitation of negative-stain EM. That is, while a poxvirus can be identified in the specimen, we are unable to specify whether it is fowlpox, variola, vaccinia, and other orthopoxviruses. Slide 20 of 26: As mentioned previously, parapoxvirus can be visually distinguished from the other poxviruses. The virus particles appear more ovoid than other poxviruses, and the surface filaments have a spiral arrangement in a criss-cross pattern. Particles are slightly smaller and measure approximately 150 X 200 nanometers. Parapoxviruses primarily infect sheep, cattle, and goats, and secondarily can be transmitted to humans. Slide 21 of 26: Electron microscopy is a useful tool to differentiate between poxviruses and more common herpesviruses. Historically, the diseases most commonly confused with smallpox were those caused by herpesviruses such as varicella zoster virus and herpes simplex viruses. They must be ruled out in any diagnostic specimen. By negative-stain EM, the herpesvirus naked nucleocapsid measures approximately 100 nanometers in diameter and is composed of an icosahedron formed by hollow capsomers. A stain-penetrated nucleocapsid may look like a hexagon rimmed by hollow capsomers. An enveloped virion may be identified when the stain penetrates the viral envelope and outlines the nucleocapsid. Note, again, that the morphology in clinical specimens may be less distinct than in tissue culture specimens. Unit 2: Electron Microscopy 16

17 Slide 22 of 26: These slides provide an example of the laboratory diagnosis of a rash illness. In 2002, a college student returned from an extended trip to Africa and presented with two firm, slightly umbilicated nodules on her skin. The histopathological examination of one of the lesions was suggestive of a poxvirus infection. The diagnosis of poxvirus infection was confirmed by negative-stain EM analysis. Testing by conventional polymerase chain reaction (PCR) established the specific diagnosis of tanapox. Slide 23 of 26: A portion of the patient s lesion was also examined by thin-section EM. It showed large accumulations of virions in keratinocytes, as seen here. This image shows a higher magnification of intracellular virus particles, illustrating the typical rectangular or oval shape seen in thin section. Virions have an internal core that is often dumbbell-shaped and is surrounded by two lateral bodies. Slide 24 of 26: In addition to using appropriate techniques, quality control measures are also required to achieve and maintain high diagnostic standards. This is the goal of the External Quality Assurance Program for Electron Microscopy Viral Diagnostics, abbreviated as EQA-EMV. Conducted by the Robert Koch Institut in Berlin, the program was established to evaluate, confirm, and improve the quality of diagnostic electron microscopy. EM laboratories involved in negative-stain viral diagnostics are encouraged to participate in this Quality Assessment program. Details are available by contacting Dr. Michael Laue at the Institut. Slide 25 of 26: EM visualization of virions compatible with a poxvirus would not constitute proof of a smallpox infection. Different poxviruses, such as variola, vaccinia and monkeypox, are morphologically indistinguishable. Parapoxviruses, such as orf, do have different morphology and can be distinguished from other poxviruses. Unit 2: Electron Microscopy 17

18 Slide 26 of 26: To summarize, negative-stain EM plays a very important role in the differential diagnosis of rash illnesses. This rapid technique is an excellent screening tool. However, the electron microscopist must be experienced with negative- stain virologic diagnostics. EM provides a visual, morphologic-based method for identifying pathogens in rash illnesses, which is independent of nucleic acid- and/or protein-based laboratory tests. Module 6 Review In this module we learned that histopathology, immunohistochemistry and electron microscopy can be very useful for the diagnosis of orthopoxvirus infections and other rash-like illnesses. Various immunohistochemistry methods are available for poxviruses, herpes simplex and varicella zoster viruses, spotted fever rickettsia, enterovirus, syphilis, and others. Negative-stain electron microscopy is a rapid technique that can be used as a screening tool and to differentiate among several agents. Notably, orthopoxviruses can be differentiated from herpesviruses, such as varicella zoster virus, and from parapoxviruses, such as orf. However, visualizing virions with electron microscopy is not proof of smallpox infection. Variola, vaccinia, monkeypox, and molluscum viruses, for example, are morphologically indistinguishable. As described in Module 5, virus isolation and nucleic acid detection methods would be used to confirm the identification of poxviruses detected by EM. In the event of a deliberate release of smallpox virus and subsequent human disease, or in generalized vaccinia resulting from vaccination, negatively stained preparations derived from lesions or scab material would provide a valuable method for assisting in poxvirus diagnosis and/or ruling out other causes of rash illness. Unit 2: Electron Microscopy 18