PATHOLOGY OF INFECTIOUS DISEASES PATHOLOGY OF INFECTIOUS DISEASES A Volume in the Series FOUNDATIONS IN DIAGNOSTIC PATHOLOGY EDITED BY Gary W. Procop, MD, MS Medical Director, Enterprise Test Utilization and Pathology Consultative Services Director, Molecular Microbiology, Parasitology, and Mycology Laboratories Professor of Pathology Cleveland Clinic Lerner College of Medicine Cleveland Clinic Cleveland, Ohio Bobbi S. Pritt, MD, MSc, (D)TMH Director, Clinical Parasitology and Initial Processing Laboratories Associate Professor of Pathology Mayo Clinic College of Medicine Mayo Clinic Rochester, Minnesota 1600 John F. Kennedy Blvd. Ste 1800 Philadelphia, PA 19103-2899 PATHOLOGY OF INFECTIOUS DISEASES ISBN: 978-1-4377-0762-5 Copyright © 2015 by Saunders, an imprint of Elsevier Inc. All rights reserved. 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Schmitt Content Development Specialist: Maria Holman Publishing Services Manager: Patricia Tannian Senior Project Manager: Sharon Corell Senior Book Designer: Lou Forgione Printed in China. Last digit is the print number: 9 8 7 6 5 4 3 2 1 To John Leon Procop, my father, who taught me what to do when the going gets tough and at the same time was always a fun dad. –Gary Procop To my parents, John and Sue Pritt, for their love and inspiration. –Bobbi Pritt 1 Principles of Infectious Disease Pathology: An Introduction ■■Michael L. Wilson ■■Gary W. Procop ■■Bobbi S. Pritt ■■DIAGNOSIS OF INFECTIOUS DISEASES: in Figure 1-1, where the starting point is gross ex- AN INTEGRATED APPROACH amination of specimens, frozen sections, aspirate smears, or hematoxylin and eosin (H&E)–stained permanent sections. In this approach, the evaluation The diagnosis of infectious diseases ranges from straight- is limited to the sequential use of histologic methods forward clinical diagnoses to those that are possible only with or without the accompanying use of microbio- with the use of advanced molecular methods. Between logic cultures. This is a cost-effective and adequate ap- these two extremes are the many infectious diseases for proach for many common infectious diseases. For more which an accurate and timely diagnosis requires the challenging cases, an integrated approach as shown in combined use of microbiologic cultures, histopathology/ Figure 1-2 is more appropriate. In such an approach, cytopathology, and molecular methods. It is this broad several methods are used simultaneously and in paral- group of infectious diseases where the histopathologist lel to obtain the most accurate and timely diagnosis, and medical microbiologist, either alone or together, including correlating histopathologic findings with the play a central role in diagnosis. results of clinical signs and symptoms, other laboratory As with all clinical or pathologic evaluations, pa- test results, radiographic findings, and, when appro- thologists need all available clinical, radiographic, and priate, consultation with infectious disease pathology laboratory findings in order to make accurate and specialists. Ideally, the result of this process is an inte- timely diagnoses. This is particularly important in in- grated, composite report that incorporates all of these fectious disease pathology: many infectious diseases findings into a summary and interpretation. are restricted geographically, present with distinctive (or at least highly suggestive) clinical signs and symp- toms, can present as localized or disseminated disease, ■■THE INFLAMMATORY RESPONSE IN THE may be associated with environmental or zoonotic ex- DIAGNOSIS OF INFECTIOUS DISEASES posure, and, because many are contagious, can pres- ent as part of an outbreak or be linked to transmission from another host. The immune status of the host also The inflammatory response to pathogenic microorgan- is important, as some infectious manifestations are isms is sufficiently consistent and predictable to usually seen primarily in the setting of immune compromise. allow pathologists to identify that an infection is present. Therefore, the amount of information necessary for the The type of inflammatory response—as well as its dis- diagnosis and treatment of infectious diseases can be tribution in tissues, fluids, and organs—also may allow substantially greater than that needed for many nonin- the pathologist to categorize the infection as one likely fectious diseases. It is imperative that clinicians provide caused by bacteria, fungi, mycobacteria, or parasites this information to pathologists, and that pathologists (Table 1-1). Some histopathologic clues, when combined make an active attempt to obtain it when it is not ini- with other findings in tissue, help narrow the differen- tially received. tial diagnosis. One example is the Splendor-Hoeppli The histopathologic and cytopathologic diagnoses of phenomenon, which is a radial aggregate of eosinophilic infectious diseases are progressive and sequential pro- material surrounding a nidus of infection. The pro- cesses that move from the general to the specific based tein was once believed to be composed of aggregates of on results from a combination of diagnostic meth- antigen-antibody complexes, but it is now known to be ods. The traditional approach is shown graphically composed of major basic protein. When it is identified 3 4 PATHOLOGY OF INFECTIOUS DISEASES Initial Examination of Tissue or Fluid TABLE 1-1 Gross Examination Frozen Section Patterns of inflammation associated with infection Impression Smears Quick Stains for Cytology Pattern of Inflammation Likely Pathogens Acute Inflammation Bacterial Early stages of mycobacterial, Evaluation of Pattern of ± Microbiological fungal, and parasitic Inflammation and Cultures infections Morphology of Visible Organisms Granulomatous inflammation Mycobacterial, fungal, rare bacterial, parasitic Endothelial damage, focal acute Viral (e.g., viral hemorrhagic hemorrhage fevers) Immunohistochemical Stains Rickettsial Immunoperoxidase Stains Pseudomonas aeruginosa in Situ Hybridization Focal necrosis Viral Bacterial Ulceration Bacterial Diagnosis and Report Mycobacterial FIGURE 1-1 Fungal Conceptual approaches to diagnosis: traditional approach. Viral Parasitic Initial Examination of Tissue or Fluid Gross Examination Frozen Section Bacteria Impression Smears Quick Stains for Cytology Most common bacterial pathogens elicit the acute in- flammatory immune response, a typical example be- ing acute bronchopneumonia caused by Streptococcus Evaluation of Pattern of pneumoniae. The acute inflammatory response consists NAA Microbiological Methods Inflammation and Cultures primarily of an infiltrate of segmented neutrophils, al- Morphology of Visible though in more severe or prolonged cases the infiltrate Organisms can also contain neutrophil precursors. The surround- ing tissue is often edematous, shows vascular conges- tion with margination of neutrophils along the vascular Immunohistochemical Stains endothelium, and may show focal necrosis with severe Immunoperoxidase Stains inflammation. As the segmented neutrophils lyse, they in Situ Hybridization release cellular debris that can mimic the appearance of extracellular bacterial cocci, but in general the cellular debris varies more in size and shape than do cocci. If a Diagnosis and Report tissue Gram stain is performed, histopathologists should remember to look for both free bacteria and intracellular FIGURE 1-2 bacteria. The location of bacteria is determined in part Conceptual approaches to diagnosis: integrated approach. by the nature of the infection (e.g., intracellular gram- positive cocci in pneumococcal pneumonia) and also in tissue sections it suggests that an infection is caused by the degree of cellular lysis, severity of infection, and by Sporothrix, some other fungi, Schistosoma eggs, and stage of infection. others. Although it is not a specific finding, it does help A small number of pathogenic bacteria are associated the pathologist narrow the differential diagnosis. Thus, with inflammatory responses other than typical acute although infections in tissues may show considerable inflammation. Rickettsia and other pathogens associated overlap in their histopathologic characteristics, the type with endothelial infections typically do not cause acute and distribution of inflammation—and often a combi- inflammation but rather cause vascular leakage with nation of otherwise nonspecific findings—provides suf- edema of affected tissues. Vascular thromboses and vas- ficient information for pathologists to order additional cular necrosis may also occur, depending on which rick- studies in a logical and sequential manner. ettsial pathogen is the cause of the infection. Affected CHAPTER 1 Principles of Infectious Disease Pathology: An Introduction 5 vessels may show a surrounding cuff of mononuclear rather than be filled with necrotic debris the centers of inflammatory cells, but in general these bacterial infec- the granulomas are filled with neutrophils. These gran- tions are not associated with an infiltrate of neutrophils. ulomas do not have a specific name, being referred to Similarly, Pseudomonas aeruginosa pneumonia is charac- as pyogenic granulomas or mixed inflammation, among terized by damage to the walls of blood vessels with the other terms. When identified in skin and subcutaneous subsequent development of acute hemorrhage with or tissues, they are highly suggestive of a cutaneous fungal without associated acute inflammation. Bacterial infec- infection such as phaeohyphomycosis or chromoblas- tions that are caused by toxin-producing bacteria, such tomycosis. As with mycobacteria and other causes of as Clostridium species, may show only extensive tissue granulomas, necrosis of the center of the granulomas is necrosis with minimal or no inflammation. Rapidly pro- more typical of infections that have persisted for some gressive infections caused by Streptococcus pyogenes in time. It is important for the diagnostician to remember skin and subcutaneous soft tissues may progress so rap- that granulomatous inflammation varies considerably in idly that extensive tissue necrosis occurs before acute its histopathologic appearance. inflammation can develop. One of the less intuitive in- One distinctive tissue reaction to fungal infection flammatory responses is that elicited by Brucella spp., occurs with the diverse group of fungi that cause what Yersinia pestis, Francisella tularensis, Bartonella hense- is variably termed zygomycosis, phycomycosis, or mu- lae (the causative agent of cat-scratch disease), and the cormycosis. Because the causative fungi all belong to Chlamydia trachomatis strains that cause lymphogranu- the class Zygomycetes, perhaps the best term is zygo- loma venereum. Unlike most other bacterial pathogens, mycosis. Infection with any of these agents, which are these bacteria are associated with the formation of stel- indistinguishable in tissue sections, results in invasion late necrotizing granulomas in infected tissues. of arterial walls with subsequent vascular occlusion and Some bacterial infections are associated with patterns thrombosis. Similar findings occur when members of of infection that are distinctive but may not be recog- the hyaline hyphomycetes (such as Aspergillus) invade nized due to their relative infrequency in routine clinical pulmonary arteries. Necrosis of infected tissues is the practice. The foamy macrophages observed with Whipple result of these fungal infections, with minimal acute in- disease in gastrointestinal biopsies is one example, as is flammation in the early stages of infection. the similar tissue reaction observed in the spleen with More comprehensive descriptions of the histopatho- infections caused by Mycobacterium avium complex in logic changes associated with fungal infections are pre- patients with severe immunosuppression caused by HIV sented in Chapters 23 to 26. infection. Infections caused by Rhodococcus are typically associated with malakoplakia. The finding of xantho- granulomatous inflammation, while nonspecific, strongly MycoBacteria suggests a chronic bacterial infection rather than an on- going fungal or mycobacterial infection. The histopathologic changes associated with bacte- As with fungi, mycobacterial infections are classically rial infections are discussed in detail in Chapters 12, 13, associated with granulomatous inflammation. As with 15, 16, 17, 18, and 19. bacteria and fungi, however, mycobacteria elicit a spec- trum of inflammatory reactions depending on the type of mycobacterium, the site of infection, and the stage of infection. Although the immediate tissue response Fungi to mycobacterial infection is that of acute inflamma- tion, this is rarely (if ever) seen in diagnostic speci- Fungal infections are typically associated with granu- mens: by the time clinical signs and symptoms develop, lomatous inflammation, which may be necrotizing or the inflammatory response has evolved to the stage of non-necrotizing depending on the stage of the infection. granulomatous inflammation. Early infections show Early stages of fungal infections are typically associated non-necrotizing granulomas; if infections persist, the with acute inflammation, with the evolution of granulo- granulomas will begin to show necrosis of the central matous inflammation occurring as cell-mediated immu- portions, which eventually become fully necrotic, even- nity develops. As occurs with mycobacterial infections, tually assuming the classic finding of caseation. At this the early stages of fungal infections are rarely seen: by stage of the infection, a rim of viable tissue, consisting the time the patient is symptomatic, or a biopsy is neces- mostly of epithelioid histiocytes, surrounds the central sary, the infection almost always has progressed to the area of necrosis. From a practical standpoint, the most stage at which granulomatous inflammation has devel- important feature for the histopathologist is that resid- oped. One notable exception occurs with cutaneous fun- ual mycobacteria are limited to the interface between gal infections, where mixed acute and granulomatous the necrotic and viable tissue; mycobacteria are almost inflammation occurs. In addition, many granulomas in never found in either the necrotic or the viable granu- cutaneous fungal infections have a necrotic center, but lomatous tissue. 6 PATHOLOGY OF INFECTIOUS DISEASES A number of unique patterns of inflammation with Parasites mycobacterial infections have also been described. Buruli ulcer, a tropical infection caused by Mycobacterium ul- cerans, is manifested by the development of chronic skin The classic description of the histopathologic changes asso- ulcers that show acute and chronic inflammation at the ciated with invasive parasitic infection is that of a chronic leading edge of ulcers, with minimal granulomatous in- inflammatory infiltrate with a marked eosinophilic com- flammation along the edge of ulcers. Tuberculoid lep- ponent. This description, however, is inadequate in many rosy shows a granulomatous inflammation, particularly ways. For example, some parasitic infections are associated in a perineural pattern, whereas lepromatous leprosy with minimal or no tissue reaction at all, such as the cysts shows aggregates of lipid-laden macrophages that may of Toxoplasma gondii observed in brain or skeletal mus- be filled with Mycobacterium leprae bacilli. The absence cle during the dormant state of infection. Other parasitic of granulomas in lepromatous leprosy reflects a lack of infections are associated with granulomatous inflamma- an effective T-cell immune response. Mycobacterium tion, such as cutaneous or mucocutaneous forms of leish- avium complex infections in patients with profound im- maniasis or the granulomatous pulmonary arteritis seen munosuppression also are characterized by foamy mac- when schistosome eggs circulate to the lungs. With many rophages distended by innumerable bacilli, and again parasitic infections, longstanding infection may result in the absence of granulomas reflects a lack of an effec- marked fibrosis of affected tissues, the best example being tive T-cell immune response. Mycobacterium marinum the so-called pipe-stem fibrosis seen in livers infected with causes small subcutaneous granulomas in the skin (usu- Schistosoma japonicum. Last, cutaneous or mucocutane- ally of the extremities) where the body temperature is ous forms of parasitic infections may be associated with sufficiently low to support the growth of the bacterium. secondary bacterial infections, which can result in acute The histopathologic changes associated with myco- inflammation of the affected site, thereby partially obscur- bacterial infections are discussed in detail in Chapters 20, ing the nature of the underlying infection. It is important 21, and 22. to remember that the various tissue reactions to parasitic infections are for the most part nonspecific: unless para- sites are directly visualized, it is not possible to make a definitive diagnosis in most cases. Viruses As with other types of infections, for many parasitic infections the only clues as to the causative agent are the Excluding viral cytopathic effects, most viral infections distribution of affected tissues or organs, epidemiologic are not associated with characteristic inflammatory or information, and a past medical history suggestive of a other tissue reactions in tissues or organs, although the given parasite. A careful clinical history is a mandatory distribution of changes within tissue or organs may sug- part of the evaluation of these patients: it is necessary gest certain viral infections. For example, changes lim- to obtain detailed information as to the patient’s birth- ited to the liver would suggest certain viral infections place, residence or travel to endemic regions, past clini- but not others due to the organ- and cell-specific tropism cal signs and symptoms, and previous evaluation and for many viruses. Other examples include the pattern(s) treatment. Serologic tests may be useful for document- of acute hemorrhage associated with viral hemorrhagic ing past exposure to parasites, but pathologists should fevers, changes associated with progressive multifocal remember that serologic tests may only document ex- leukoencephalopathy caused by JC virus infection, and posure, not disease, and they suffer from a number of viral myocarditis. That is not to say that a histopatho- pitfalls including lack of sensitivity or specificity as well logic diagnosis of viral infections is not possible, rather as cross-reactions with similar parasites. Moreover, ac- that the pattern of inflammation per se is not as charac- curate serologic tests are not widely available for many teristic as that of many bacterial, fungal, or mycobacte- parasitic infections and are typically limited to large ref- rial infections. erence laboratories and public health agencies (e.g., the It is important for pathologists to remember that U.S. Centers for Disease Control and Prevention). many viral infections can be accompanied by secondary The histopathologic changes associated with parasitic bacterial infections. Classic examples include bacterial infections are discussed in detail in Chapters 27 and 28. superinfection during and following viral pneumonias and bacterial infections following ulcers caused by vi- ruses (e.g., esophageal ulcers caused by the Herpes sim- ■■CLASSICAL HISTOPATHOLOGY plex virus or Cytomegalovirus). Depending on the timing AND CYTOPATHOLOGY of any biopsy or collection of fluid, the acute inflamma- tory infiltrate associated with the secondary bacterial in- fection can easily obscure the underlying viral infection. The first step in histopathologic or cytopathologic di- The histopathologic changes associated with viral in- agnosis is the recognition of findings that are consis- fections are discussed in detail in Chapters 2 to 9. tent with an infection. Although in many cases this