Pathogenesis.

Neisseria use antigenic variation as a strategy to escape the immune response. The existence of multiple serotypes of N. meningitidis results in meningitis in some people on exposure to a new strain, as discussed above. In addition, Neisseria species also generate antigenic variation by special genetic mechanisms, which permit a single bacterial clone to change its expressed antigens (see below) and escape immune defenses.¹⁹ Neisseria organisms adhere to and invade nonciliated epithelial cells at the site of entry (nasopharynx, urethra, or cervix). Two surface proteins of Neisseria, both of which bind the bacteria to host cells, undergo antigenic variation through different mechanisms. Although both N. meningitidis and N. gonorrhoeae use these mechanisms, they seem to be more important in N. gonorrhoeae.

Pathogenesis.

Bordetella pertussis colonizes the brush border of the bronchial epithelium and also invades macrophages. Coordinated expression of virulence factors is regulated by the Bordetella virulence gene locus (bvg).⁷⁷ BVGS is a transmembrane protein that “senses” signals that induce expression of virulence factors. On activation, BVGS phosphorylates the protein BVGA, which regulates transcription of mRNA for adhesins and toxins. The filamentous hemagglutinin adhesin binds to carbohydrates on the surface of respiratory epithelial cells, as well as to CR3 (Mac-1) integrins on macrophages. Pertussis toxin is an exotoxin composed of five distinct proteins, including a catalytic peptide S1 that shows homology with the catalytic peptides of cholera toxin and E. coli heat-labile toxin.⁷⁸ Like cholera toxin, pertussis toxin ADP-ribosylates and inactivates guanine nucleotide–binding proteins, so these G proteins no longer transduce signals from host plasma membrane receptors. The toxin produced by B. pertussis paralyzes the cilia, thus impairing an important pulmonary defense.

Morphology. Bordetella bacteria cause a laryngotracheobronchitis that in severe cases features bronchial mucosal erosion, hyperemia, and copious mucopurulent exudate (Fig. 8-25). Unless superinfected, the lung alveoli remain open and intact. In parallel with a striking peripheral lymphocytosis (up to 90%), there is hypercellularity and enlargement of the mucosal lymph follicles and peribronchial lymph nodes.

FIGURE 8-25 Whooping cough showing a haze of bacilli (arrows) entangled with the cilia of bronchial epithelial cells.

Pathogenesis.

P. aeruginosa has pili and adherence proteins that bind to epithelial cells and lung mucin, and expresses an endotoxin that causes the symptoms and signs of gram-negative sepsis. Pseudomonas also has a number of distinctive virulence factors. In the lungs of people with cystic fibrosis, these bacteria secrete a mucoid exopolysaccharide called alginate, forming a slimy biofilm that protects bacteria from antibodies, complement, phagocytes, and antibiotics. The organisms also secrete an exotoxin and several other virulence factors. Exotoxin A, like diphtheria toxin, inhibits protein synthesis by ADP-ribosylating the ribosomal protein EF-2.⁸⁰ P. aeruginosa also releases exoenzyme S, which ADPribosylates RAS and other G proteins that regulate cell growth and metabolism. The organisms also secrete a phospholipase C that lyses red cells and degrades pulmonary surfactant, and an elastase that degrades IgGs and extracellular matrix proteins. These enzymes may be important in tissue invasion and destruction of the cornea in keratitis. Finally, P. aeruginosa produces iron-containing compounds that are extremely toxic to endothelial cells and so may cause the vascular lesions that are characteristic of this infection.⁸¹

Morphology. Pseudomonas causes a necrotizing pneumonia that is distributed through the terminal airways in a fleur-de-lis pattern, with striking pale necrotic centers and red, hemorrhagic peripheral areas. On microscopic examination, masses of organisms cloud the tissue with a bluish haze, concentrating in the walls of blood vessels, where host cells undergo coagulative necrosis (Fig. 8-26). This picture of gram-negative vasculitis accompanied by thrombosis and hemorrhage, although not pathognomonic, is highly suggestive of P. aeruginosa infection.

FIGURE 8-26 Pseudomonas vasculitis in which masses of organisms form a perivascular blue haze.

Bronchial obstruction caused by mucus plugging and subsequent P. aeruginosa infection are frequent complications of cystic fibrosis. Despite antibiotic treatment and the host immune response, chronic P. aeruginosa infection may result in bronchiectasis and pulmonary fibrosis (Chapter 15).

In skin burns, P. aeruginosa proliferates widely, penetrating deeply into the veins and spreading hematogenously. Well-demarcated necrotic and hemorrhagic oval skin lesions, called ecthyma gangrenosum, often appear. Disseminated intravascular coagulation (DIC) is a frequent complication of bacteremia.

Epidemiology.

Tuberculosis is estimated to affect 1.7 billion individuals worldwide, with 8 to 10 million new cases and 1.6 million deaths each year, a toll second only to HIV disease. Infection with HIV makes people susceptible to rapidly progressive tuberculosis; over 10 million people are infected with both HIV and M. tuberculosis. From 1985 to 1992, the number of tuberculosis cases in the United States rose by 20% because of increase in the disease in people with HIV, immigrants, and those in jail or homeless shelters. Because of public health efforts, the number of cases of tuberculosis has declined since 1993. Currently, there are about 14,000 new cases of active tuberculosis in the United States annually, about half of which occur in foreign-born people.

Tuberculosis flourishes wherever there is poverty, crowding, and chronic debilitating illness. In the United States tuberculosis is mainly a disease of the elderly, the urban poor, and people with AIDS. Certain disease states also increase the risk: diabetes mellitus, Hodgkin lymphoma, chronic lung disease (particularly silicosis), chronic renal failure, malnutrition, alcoholism, and immunosuppression.

It is important that infection with M. tuberculosis be differentiated from disease. Infection is the presence of organisms, which may or may not cause clinically significant disease. Most infections are acquired by person-to-person transmission of airborne organisms from an active case to a susceptible host. In most people primary tuberculosis is asymptomatic, although it may cause fever and pleural effusion. Generally, the only evidence of infection, if any remains, is a tiny, fibrocalcific nodule at the site of the infection. Viable organisms may remain dormant in such lesions for decades. If immune defenses are lowered, the infection may reactivate to produce communicable and potentially life-threatening disease.

Infection typically leads to the development of delayed hypersensitivity to M. tuberculosis antigens, which can be detected by the tuberculin (Mantoux) skin test. About 2 to 4 weeks after infection, intracutaneous injection of purified protein derivative of M. tuberculosis induces a visible and palpable induration that peaks in 48 to 72 hours. A positive tuberculin test result signifies T cell–mediated immunity to mycobacterial antigens. It does not differentiate between infection and disease. False-negative reactions may occur in the setting of certain viral infections, sarcoidosis, malnutrition, Hodgkin lymphoma, immunosuppression, and (notably) overwhelming active tuberculous disease. False-positive reactions may result from infection by atypical mycobacteria or prior vaccination with BCG (Bacillus Calmette-Guerin), an attenuated strain of M. bovis that is used as a vaccine in some countries.

Pathogenesis.

The pathogenesis of tuberculosis in a previously unexposed, immunocompetent person depends on the development of anti-mycobacterial cell-mediated immunity, which confers resistance to the bacteria and also results in development of hypersensitivity to mycobacterial antigens. The pathologic manifestations of tuberculosis, such as caseating granulomas and cavitation, are the result of the hypersensitivity that develops in concert with the protective host immune response. Because the effector cells that mediate immune protection also mediate hypersensitivity and tissue destruction, the appearance of hypersensitivity also signals the acquisition of immunity to the organism. A summary of the pathogenesis of tuberculosis is shown in Figure 8-27.

FIGURE 8-27 The sequence of events in primary pulmonary tuberculosis, commencing with inhalation of virulent Mycobacterium tuberculosis organisms and culminating with the development of cell-mediated immunity to the organism. A, Events occurring in the first 3 weeks after exposure. B, Events thereafter. The development of resistance to the organism is accompanied by the appearance of a positive tuberculin test. γ-IFN, interferon-γ; iNOS, inducible nitric oxide synthase; MHC, major histocompatibility complex; MTB, M. tuberculosis; NRAMP1, natural resistance–associated macrophage protein; TNF, tumor necrosis factor.

Macrophages are the primary cells infected by M. tuberculosis. Early in infection, tuberculosis bacilli replicate essentially unchecked, while later in infection, the cell response stimulates macrophages to contain the proliferation of the bacteria.

In summary, immunity to M. tuberculosis is primarily mediated by T_H1 cells, which stimulate macrophages to kill the bacteria. This immune response, while largely effective, comes at the cost of hypersensitivity and accompanying tissue destruction. Reactivation of the infection or re-exposure to the bacilli in a previously sensitized host results in rapid mobilization of a defensive reaction but also increased tissue necrosis. Just as hypersensitivity and resistance are correlated, so, too, the loss of hypersensitivity (indicated by tuberculin negativity in a previously tuberculin-positive individual) may be an ominous sign that resistance to the organism has faded.

Clinical Features of Tuberculosis.

The many clinicalpathologic patterns of tuberculosis are shown in Figure 8-28. Primary tuberculosis is the form of disease that develops in a previously unexposed, and therefore unsensitized, person. About 5% of newly infected people develop clinically significant disease. The elderly and profoundly immunosuppressed persons may lose their immunity to M. tuberculosis and so may develop primary tuberculosis more than once. With primary tuberculosis the source of the organism is exogenous.

FIGURE 8-28 The natural history and spectrum of tuberculosis.

(Adapted from a sketch provided by Professor R.K. Kumar, The University of New South Wales, School of Pathology, Sydney, Australia.)

In most people, the primary infection is contained, but in others, primary tuberculosis is progressive. The diagnosis of progressive primary tuberculosis in adults can be difficult. In contrast to secondary tuberculosis (apical disease with cavitation; see below), progressive primary tuberculosis more often resembles an acute bacterial pneumonia, with lower and middle lobe consolidation, hilar adenopathy, and pleural effusion; cavitation is rare, especially in people with severe immunosuppression. Lymphohematogenous dissemination may result in the development of tuberculous meningitis and miliary tuberculosis (discussed below).

Secondary tuberculosis is the pattern of disease that arises in a previously sensitized host. It may follow shortly after primary tuberculosis, but more commonly it appears many years after the initial infection, usually when host resistance is weakened. It most commonly stems from reactivation of a latent infection, but may also result from exogenous reinfection in the face of waning host immunity or when a large inoculum of virulent bacilli overwhelms the host immune system. Reactivation is more common in low-prevalence areas, while reinfection plays an important role in regions of high contagion.

Secondary pulmonary tuberculosis classically involves the apex of the upper lobes of one or both lungs. Because of the pre-existence of hypersensitivity, the bacilli elicit a prompt and marked tissue response that tends to wall off the focus of infection. As a result, the regional lymph nodes are less prominently involved early in secondary disease than they are in primary tuberculosis. On the other hand, cavitation occurs readily in the secondary form. Indeed, cavitation is almost inevitable in neglected secondary tuberculosis, and erosion of the cavities into an airway is an important source of infection because the person now coughs sputum that contains bacteria.

Localized secondary tuberculosis may be asymptomatic. When manifestations appear, they are usually insidious in onset. Systemic symptoms, probably related to cytokines released by activated macrophages (e.g., TNF and IL-1), often appear early in the course and include malaise, anorexia, weight loss, and fever. Commonly, the fever is low grade and remittent (appearing late each afternoon and then subsiding), and night sweats occur. With progressive pulmonary involvement, increasing amounts of sputum, at first mucoid and later purulent, appear. Some degree of hemoptysis is present in about half of all cases of pulmonary tuberculosis. Pleuritic pain may result from extension of the infection to the pleural surfaces. Extrapulmonary manifestations of tuberculosis are legion and depend on the organ system involved.

The diagnosis of pulmonary disease is based in part on the history and on physical and radiographic findings of consolidation or cavitation in the apices of the lungs. Ultimately, however, tubercle bacilli must be identified. Acid-fast smears and cultures of the sputum of patients suspected of having tuberculosis should be performed. Conventional cultures require up to 10 weeks, but culture in liquid media can provide an answer within 2 weeks. PCR amplification of M. tuberculosis DNA allows for even more rapid diagnosis. PCR assays can detect as few as 10 organisms in clinical specimens, compared with more than 10,000 organisms required for smear positivity. However, culture remains the gold standard because it also allows testing of drug susceptibility. Multidrug resistance is now seen more commonly than it was in past years; hence, all newly diagnosed cases in the United States are assumed to be resistant and are treated with multiple drugs. The prognosis is generally good if infections are localized to the lungs, except when they are caused by drug-resistant strains or occur in aged, debilitated, or immunosuppressed individuals, who are at high risk for developing miliary tuberculosis (see below).

All stages of HIV infection are associated with an increased risk of tuberculosis. The use of highly active antiretroviral therapy (HAART) reduces the risk of tuberculosis in people with HIV infection, but even with HAART, people infected with HIV are more likely to get tuberculosis than the uninfected. A low CD4 count before starting HAART is an important risk factor for development of tuberculosis, which underscores the role of the immune response in keeping reactivation of M. tuberculosis in check. The manifestations of tuberculosis differ depending on the degree of immunosuppression. People with less severe immunosuppression (CD4+ T-cell counts >300 cells/mm³) present with usual secondary tuberculosis (apical disease with cavitation). People with more advanced immunosuppression (CD4+ T-cell counts <200 cells/mm³) present with a clinical picture that resembles progressive primary tuberculosis. The extent of immunodeficiency also determines the frequency of extrapulmonary involvement, rising from 10% to 15% in mildly immunosuppressed people to greater than 50% in those with severe immune deficiency. Other atypical features of tuberculosis in HIV-positive people include an increased frequency of false-negative sputum smears and tuberculin tests (the latter due to “anergy”), and the absence of characteristic granulomas in tissues, particularly in the late stages of HIV. The increased frequency of sputum smear-negativity is paradoxical because these immunosuppressed patients typically have higher bacterial loads. The likely explanation is that cavitation and bronchial damage are more in immunocompetent individuals, resulting in more bacilli in expelled sputum. In contrast, the absence of bronchial wall destruction due to reduced T-cell–mediated hypersensitivity results in the excretion of fewer bacilli in the sputum.

Morphology.

Primary Tuberculosis. In countries where infected milk has been eliminated, primary tuberculosis almost always begins in the lungs. Typically, the inhaled bacilli implant in the distal airspaces of the lower part of the upper lobe or the upper part of the lower lobe, usually close to the pleura. As sensitization develops, a 1- to 1.5-cm area of gray-white inflammation with consolidation emerges, known as the Ghon focus. In most cases, the center of this focus undergoes caseous necrosis. Tubercle bacilli, either free or within phagocytes, drain to the regional nodes, which also often caseate. This combination of parenchymal lung lesion and nodal involvement is referred to as the Ghon complex (Fig. 8-29). During the first few weeks there is also lymphatic and hematogenous dissemination to other parts of the body. In approximately 95% of cases, development of cell-mediated immunity controls the infection. Hence, the Ghon complex undergoes progressive fibrosis, often followed by radiologically detectable calcification (Ranke complex), and despite seeding of other organs, no lesions develop.

FIGURE 8-29 Primary pulmonary tuberculosis, Ghon complex. The gray-white parenchymal focus is under the pleura in the lower part of the upper lobe. Hilar lymph nodes with caseation are seen on the left.

Histologically, sites of active involvement are marked by a characteristic granulomatous inflammatory reaction that forms both caseating and noncaseating tubercles (Fig. 8-30A to C). Individual tubercles are microscopic; it is only when multiple granulomas coalesce that they become macroscopically visible. The granulomas are usually enclosed within a fibroblastic rim punctuated by lymphocytes. Multinucleate giant cells are present in the granulomas. Immunocompromised people do not form the characteristic granulomas (Fig. 8-30D).

FIGURE 8-30 The morphologic spectrum of tuberculosis. A characteristic tubercle at low magnification (A) and in detail (B) illustrates central caseation surrounded by epithelioid and multinucleated giant cells. This is the usual response seen in patients who have cell-mediated immunity to the organism. Not all tubercular granulomas might show central caseation (C); hence, irrespective of the presence or absence of caseous necrosis, special stains for acid-fast organisms must be performed when granulomas are present. In immunosuppressed individuals without cellular immunity sheets of foamy macrophages are seen that are packed with mycobacteria (demonstrable with acid-fast stains) (D).

(D, Courtesy of Dr. Dominick Cavuoti, Department of Pathology, University of Texas Southwestern Medical School, Dallas, TX.)

Secondary Tuberculosis. The initial lesion is usually a small focus of consolidation, less than 2 cm in diameter, within 1 to 2 cm of the apical pleura. Such foci are sharply circumscribed, firm, gray-white to yellow areas that have a variable amount of central caseation and peripheral fibrosis (Fig. 8-31). In immunocomptetent individuals, the initial parenchymal focus undergoes progressive fibrous encapsulation, leaving only fibrocalcific scars. Histologically, the active lesions show characteristic coalescent tubercles with central caseation. Tubercle bacilli can often be identified with acid-fast stains in early exudative and caseous phases of granuloma formation but are usually too few to be found in the late, fibrocalcific stages. Localized, apical, secondary pulmonary tuberculosis may heal with fibrosis either spontaneously or after therapy, or the disease may progress and extend along several different pathways.

FIGURE 8-31 Secondary pulmonary tuberculosis. The upper parts of both lungs are riddled with gray-white areas of caseation and multiple areas of softening and cavitation.

Progressive pulmonary tuberculosis may ensue in the elderly and immunosuppressed. The apical lesion expands into adjacent lung and eventually erodes into bronchi and vessels. This evacuates the caseous center, creating a ragged, irregular cavity that is poorly walled off by fibrous tissue. Erosion of blood vessels results in hemoptysis. With adequate treatment the process may be arrested, although healing by fibrosis often distorts the pulmonary architecture. The cavities, now free of inflammation, may persist or become fibrotic. If the treatment is inadequate or if host defenses are impaired, the infection may spread via airways, lymphatic channels, or the vascular system. Miliary pulmonary disease occurs when organisms draining through lymphatics enter the venous blood and circulate back to the lung. Individual lesions are either microscopic or small, visible (2-mm) foci of yellow-white consolidation scattered through the lung parenchyma (the adjective “miliary” is derived from the resemblance of these foci to millet seeds). Miliary lesions may expand and coalesce, resulting in consolidation of large regions or even whole lobes of the lung. With progressive pulmonary tuberculosis, the pleural cavity is invariably involved, and serous pleural effusions, tuberculous empyema, or obliterative fibrous pleuritis may develop.

Endobronchial, endotracheal, and laryngeal tuberculosis may develop by spread through lymphatic channels or from expectorated infectious material. The mucosal lining may be studded with minute granulomatous lesions that may only be apparent microscopically.

Systemic miliary tuberculosis occurs when bacteria disseminate through the systemic arterial system. Miliary tuberculosis is most prominent in the liver, bone marrow, spleen, adrenals, meninges, kidneys, fallopian tubes, and epididymis, but could involve any organ (Fig. 8-32).

FIGURE 8-32 Miliary tuberculosis of the spleen. The cut surface shows numerous gray-white tubercles.

Isolated tuberculosis may appear in any of the organs or tissues seeded hematogenously and may be the presenting manifestation. Organs that are commonly involved include the meninges (tuberculous meningitis), kidneys (renal tuberculosis), adrenals (formerly an important cause of Addison disease), bones (osteomyelitis), and fallopian tubes (salpingitis). When the vertebrae are affected, the disease is referred to as Pott disease. Paraspinal “cold” abscesses in these patients may track along tissue planes and present as an abdominal or pelvic mass.

Lymphadenitis is the most frequent presentation of extrapulmonary tuberculosis, usually occurring in the cervical region (“scrofula”). In HIV-negative individuals, lymphadenitis tends to be unifocal and localized. HIV-positive people, on the other hand, almost always have multifocal disease, systemic symptoms, and either pulmonary or other organ involvement by active tuberculosis.

In years past, intestinal tuberculosis contracted by the drinking of contaminated milk was a fairly common primary focus of disease. In countries where milk is pasteurized, intestinal tuberculosis is more often caused by the swallowing of coughed-up infective material in patients with advanced pulmonary disease. Typically the organisms are seed to mucosal lymphoid aggregates of the small and large bowel, which then undergo granulomatous inflammation that can lead to ulceration of the overlying mucosa, particularly in the ileum.

Pathogenesis.

M. leprae is an acid-fast obligate intracellular organism that grows very poorly in culture but can be propagated in the armadillo. It proliferates best at 32° to 34°C, the temperature of the human skin and the core temperature of armadillos. Like M. tuberculosis, M. leprae secretes no toxins, and its virulence is based on properties of its cell wall. The cell wall is similar enough to that of M. tuberculosis that immunization with BCG confers some protection against M. leprae infection. Cell-mediated immunity is reflected by delayed-type hypersensitivity reactions to dermal injections of a bacterial extract called lepromin.

M. leprae causes two strikingly different patterns of disease. People with the less severe form, tuberculoid leprosy, have dry, scaly skin lesions that lack sensation. They often have asymmetric involvement of large peripheral nerves. The more severe form, lepromatous leprosy, includes symmetric skin thickening and nodules. This is also called anergic leprosy, because of the unresponsiveness (anergy) of the host immune system. Cooler areas of skin, including the earlobes and feet, are more severely affected than warmer areas, such as the axilla and groin. In lepromatous leprosy, widespread invasion of the mycobacteria into Schwann cells and into endoneural and perineural macrophages damages the peripheral nervous system. In advanced cases of lepromatous leprosy, M. leprae is present in sputum and blood. People can also have intermediate forms of disease, called borderline leprosy.

The T-helper lymphocyte response to M. leprae determines whether an individual has tuberculoid or lepromatous leprosy.⁸⁸ People with tuberculoid leprosy have a T_H1 response associated with production of IL-2 and IFN-γ. As with M. tuberculosis, IFN-γ is critical to mobilizing an effective host macrophage response. Lepromatous leprosy is associated with a weak T_H1 response and, in some cases, a relative increase in the T_H2 response. The net result is weak cell-mediated immunity and an inability to control the bacteria. Occasionally, most often in the lepromatous form, antibodies are produced against M. leprae antigens. Paradoxically, these antibodies are usually not protective, but they may form immune complexes with free antigens that can lead to erythema nodosum, vasculitis, and glomerulonephritis.

Morphology. Tuberculoid leprosy begins with localized flat, red skin lesions that enlarge and develop irregular shapes with indurated, elevated, hyperpigmented margins and depressed pale centers (central healing). Neuronal involvement dominates tuberculoid leprosy. Nerves become enclosed within granulomatous inflammatory reactions and, if small (e.g., the peripheral twigs), are destroyed (Fig. 8-34). Nerve degeneration causes skin anesthesias and skin and muscle atrophy that render the person liable to trauma of the affected parts, leading to the development of chronic skin ulcers. Contractures, paralyses, and autoamputation of fingers or toes may ensue. Facial nerve involvement can lead to paralysis of the eyelids, with keratitis and corneal ulcerations. On microscopic examination, all sites of involvement have granulomatous lesions closely resembling those found in tuberculosis, and bacilli are almost never found, hence the name “paucibacillary” leprosy. The presence of granulomas and absence of bacteria reflect strong T-cell immunity. Because leprosy pursues an extremely slow course, spanning decades, most patients die with leprosy rather than of it.

FIGURE 8-34 Leprosy. A, Peripheral nerve. Note the inflammatory cell infiltrates in the endoneural and epineural compartments. B, Cells within the endoneurium contain acid-fast positive lepra bacilli.

(Courtesy of E.P. Richardson, Jr., and U. De Girolami, Harvard Medical School.)

Lepromatous leprosy involves the skin, peripheral nerves, anterior chamber of the eye, upper airways (down to the larynx), testes, hands, and feet. The vital organs and CNS are rarely affected, presumably because the core temperature is too high for growth of M. leprae. Lepromatous lesions contain large aggregates of lipid-laden macrophages (lepra cells), often filled with masses (“globi”) of acid-fast bacilli (Fig. 8-35). Because of the abundant bacteria, lepromatous leprosy is referred to as “multibacillary”. Macular, papular, or nodular lesions form on the face, ears, wrists, elbows, and knees. With progression, the nodular lesions coalesce to yield a distinctive leonine facies. Most skin lesions are hypoesthetic or anesthetic. Lesions in the nose may cause persistent inflammation and bacilli-laden discharge. The peripheral nerves, particularly the ulnar and peroneal nerves where they approach the skin surface, are symmetrically invaded with mycobacteria, with minimal inflammation. Loss of sensation and trophic changes in the hands and feet follow the nerve lesions. Lymph nodes contain aggregates of bacteria-filled foamy macrophages in the paracortical (T-cell) areas and reactive germinal centers. In advanced disease, aggregates of macrophages are also present in the splenic red pulp and the liver. The testes are usually extensively involved, leading to destruction of the seminiferous tubules and consequent sterility.

FIGURE 8-35 Lepromatous leprosy. Acid-fast bacilli (“red snappers”) within macrophages.

Primary Syphilis.

This stage, occurring approximately 3 weeks after contact with an infected individual, features a single firm, nontender, raised, red lesion (chancre) located at the site of treponemal invasion on the penis, cervix, vaginal wall, or anus. The chancre heals in 3 to 6 weeks with or without therapy. Spirochetes are plentiful within the chancre and can be seen by immunofluorescent stains of serous exudate. Treponemes spread throughout the body by hematologic and lymphatic dissemination even before the appearance of the chancre.

Secondary Syphilis.

This stage usually occurs 2 to 10 weeks after the primary chancre and is due to spread and proliferation of the spirochetes within the skin and mucocutaneous tissues. Secondary syphilis occurs in approximately 75% of untreated people. The skin lesions, which frequently occur on the palms or soles of the feet, may be maculopapular, scaly, or pustular. Moist areas of the skin, such as the anogenital region, inner thighs, and axillae, may have condylomata lata, which are broad-based, elevated plaques. Silvery-gray superficial erosions may form on any of the mucous membranes but are particularly common in the mouth, pharynx, and external genitalia. All these painless superficial lesions contain spirochetes and so are infectious. Lymphadenopathy, mild fever, malaise, and weight loss are also common in secondary syphilis. The symptoms of secondary syphilis last several weeks, after which the person enters the latent phase of the disease. Superficial lesions may recur during the early latent phase, although they are milder.

Tertiary Syphilis.

This stage is rare where adequate medical care is available, but it occurs in approximately one third of untreated patients, usually after a latent period of 5 years or more. Tertiary syphilis has three main manifestations: cardiovascular syphilis, neurosyphilis, and so-called benign tertiary syphilis. These may occur alone or in combination.

Cardiovascular syphilis, in the form of syphilitic aortitis, accounts for more than 80% of cases of tertiary disease. The aortitis leads to slowly progressive dilation of the aortic root and arch, which causes aortic valve insufficiency and aneurysms of the proximal aorta (see Chapter 11).

Neurosyphilis may be symptomatic or asymptomatic. Symptomatic disease manifests in several ways, including chronic meningovascular disease, tabes dorsalis, and a generalized brain parenchymal disease called general paresis. These are discussed in Chapter 28. Asymptomatic neurosyphilis, which accounts for about one third of neurosyphilis cases, is detected when a patient’s CSF exhibits abnormalities such as pleocytosis (increased numbers of inflammatory cells), elevated protein levels, or decreased glucose. Antibodies stimulated by the spirochetes, discussed below, can also be detected in the CSF, and this is the most specific test for neurosyphilis. Antibiotics are given for a longer time if the spirochetes have spread to the CNS, and so patients with tertiary syphilis should be tested for neurosyphilis even if they do not have neurologic symptoms.

So-called benign tertiary syphilis is characterized by the formation of gummas in various sites. Gummas are nodular lesions probably related to the development of delayed hypersensitivity to the bacteria. They occur most commonly in bone, skin, and the mucous membranes of the upper airway and mouth, although any organ may be affected. Skeletal involvement characteristically causes local pain, tenderness, swelling, and sometimes pathologic fractures. Involvement of skin and mucous membranes may produce nodular lesions or, rarely, destructive, ulcerative lesions that mimic malignant neoplasms. Gummas are now very rare because of the use of effective antibiotics and are seen mainly in individuals with AIDS.

Congenital Syphilis.

Congenital syphilis occurs when T. pallidum crosses the placenta from an infected mother to the fetus. Maternal transmission happens most frequently during primary or secondary syphilis, when the spirochetes are most numerous. Because the manifestations of maternal syphilis may be subtle, routine serologic testing for syphilis is mandatory in all pregnancies. Intrauterine death and perinatal death each occurs in approximately 25% of cases of untreated congenital syphilis.

Manifestations of congenital disease are divided into early (infantile) and late (tardive) syphilis, depending on whether they occur in the first 2 years of life or later. Early congenital syphilis is often manifested by nasal discharge and congestion (snuffles) in the first few months of life. A desquamating or bullous rash can lead to sloughing of the skin, particularly of the hands and feet and around the mouth and anus. Hepatomegaly and skeletal abnormalities are also common.

Nearly half of untreated children with neonatal syphilis will develop late manifestations, which are discussed below.

Serologic Tests for Syphilis.

Serology remains the mainstay of diagnosis, although microscopy and PCR are also useful. Serologic tests include nontreponemal antibody tests and antitreponemal antibody tests. Nontreponemal tests measure antibody to cardiolipin, a phospholipid present in both host tissues and T. pallidum. These antibodies are detected in the rapid plasma reagin and Venereal Disease Research Laboratory (VDRL) tests. Nontreponemal tests typically become positive 4 to 6 weeks after infection, and so immunofluorescence of exudate from the chancre is important for diagnosis early in infection. The nontreponemal tests are nearly always positive in secondary syphilis, but they usually become negative in tertiary syphilis. The VDRL and rapid plasma reagin tests are used as screening tests for syphilis and to monitor response to therapy, since these tests become negative after successful treatment of infection. False-positive VDRL test results are not uncommon and are associated with certain acute infections, collagen vascular diseases (e.g., systemic lupus erythematosus), drug addiction, pregnancy, hypergammaglobulinemia of any cause, and lepromatous leprosy.

Treponemal antibody tests measure antibodies that specifically react with T. pallidum. These include the fluorescent treponemal antibody absorption test and the microhemagglutination assay for T. pallidum antibodies. These tests also become positive 4 to 6 weeks after infection, but unlike nontreponemal antibody tests, they remain positive indefinitely, even after successful treatment. They are not recommended as primary screening tests because they are significantly more expensive than nontreponemal tests. While more specific than the nontreponemal tests, false-positive treponemal antibody tests can also occur.

Serologic response may be delayed, absent, or exaggerated (false-positive results) in people co-infected with syphilis and HIV. However, in most cases, these tests remain useful in the diagnosis and management of syphilis even in people infected with HIV.

Morphology. In primary syphilis a chancre occurs on the penis or scrotum of 70% of men and on the vulva or cervix of 50% of women. The chancre is a slightly elevated, firm, reddened papule, up to several centimeters in diameter, that erodes to create a clean-based shallow ulcer. The contiguous induration creates a button-like mass directly adjacent to the eroded skin, providing the basis for the designation hard chancre (Fig. 8-38). On histologic examination, treponemes are visible at the surface of the ulcer with silver stains (e.g., Warthin-Starry stain) or immunofluorescence techniques. The chancre contains an intense infiltrate of plasma cells, with scattered macrophages and lymphocytes and a proliferative endarteritis (see Fig. 8-8). The endarteritis, which is seen in all stages of syphilis, starts with endothelial cell activation and proliferation and progresses to intimal fibrosis. The regional nodes are usually enlarged due to nonspecific acute or chronic lymphadenitis, plasma cell–rich infiltrates, or granulomas.

FIGURE 8-38 Syphilitic chancre in the scrotum (see Figure 8-8 for the histopathology of syphilis).

(Courtesy of Dr. Richard Johnson, Beth Israel–Deaconess Hospital, Boston, MA.)

In secondary syphilis widespread mucocutaneous lesions involve the oral cavity, palms of the hands, and soles of the feet. The rash frequently consists of discrete red-brown macules less than 5 mm in diameter, but it may be follicular, pustular, annular, or scaling. Red lesions in the mouth or vagina contain the most organisms and are the most infectious. Histologically, the mucocutaneous lesions of secondary syphilis show the same plasma cell infiltrate and obliterative endarteritis as the primary chancre, although the inflammation is often less intense.

Tertiary syphilis most frequently involves the aorta; the CNS; and the liver, bones, and testes. The aortitis is caused by endarteritis of the vasa vasorum of the proximal aorta. Occlusion of the vasa vasorum results in scarring of the media of the proximal aortic wall, causing a loss of elasticity. There may be narrowing of the coronary artery ostia caused by subintimal scarring with resulting myocardial ischemia. The morphologic and clinical features of syphilitic aortitis are discussed in greater detail with diseases of the blood vessels (Chapter 11). Neurosyphilis takes one of several forms, designated meningovascular syphilis, tabes dorsalis, and general paresis (Chapter 28). Syphilitic gummas are white-gray and rubbery, occur singly or multiply, and vary in size from microscopic lesions resembling tubercles to large tumor-like masses. They occur in most organs but particularly in skin, subcutaneous tissue, bone, and joints. In the liver, scarring as a result of gummas may cause a distinctive hepatic lesion known as hepar lobatum (Fig. 8-39). On histologic examination, the gummas have centers of coagulated, necrotic material and margins composed of plump, palisading macrophages and fibroblasts surrounded by large numbers of mononuclear leukocytes, chiefly plasma cells. Treponemes are scant in gummas and are difficult to demonstrate.

FIGURE 8-39 Trichrome stain of liver shows a gumma (scar), stained blue, caused by tertiary syphilis (the hepatic lesion is also known as hepar lobatum).

The rash of congenital syphilis is more severe than that of adult secondary syphilis. It is a bullous eruption of the palms and soles of the feet associated with epidermal sloughing. Syphilitic osteochondritis and periostitis affect all bones, but lesions of the nose and lower legs are most distinctive. Destruction of the vomer causes collapse of the bridge of the nose and, later on, the characteristic saddle nose deformity. Periostitis of the tibia leads to excessive new bone growth on the anterior surfaces and anterior bowing, or saber shin. There is also widespread disturbance in endochondral bone formation. The epiphyses become widened as the cartilage overgrows, and cartilage is found in displaced islands within the metaphysis.

The liver is often severely affected in congenital syphilis. Diffuse fibrosis permeates lobules to isolate hepatic cells into small nests, accompanied by the characteristic lymphoplasmacytic infiltrate and vascular changes. Gummas are occasionally found in the liver, even in early cases. The lungs may be affected by a diffuse interstitial fibrosis. In the syphilitic stillborn, the lungs appear pale and airless (pneumonia alba). The generalized spirochetemia may lead to diffuse interstitial inflammatory reactions in virtually any other organ (e.g., the pancreas, kidneys, heart, spleen, thymus, endocrine organs, and CNS).

The late manifestations of congenital syphilis include a distinctive triad of interstitial keratitis, Hutchinson teeth, and eighth-nerve deafness. In addition to interstitial keratitis, the ocular changes include choroiditis and abnormal retinal pigmentation. Hutchinson teeth are small incisors shaped like a screwdriver or a peg, often with notches in the enamel. Eighth-nerve deafness and optic nerve atrophy develop secondary to meningovascular syphilis.

Pathogenesis.

There are no good animal models of syphilis, and T. pallidum has never been grown in culture (it lacks genes for making nucleotides, fatty acids, and most amino acids). As a result, our scant knowledge of T. pallidum pathogenesis comes mainly from observations of the disease in humans.

Proliferative endarteritis occurs in all stages of syphilis. The pathophysiology of the endarteritis is not known, although the scarcity of treponemes and the intense inflammatory infiltrate suggest that the immune response plays a role in the development of these lesions. Regardless of the mechanism, much of the pathology of the disease, such as syphilitic aortitis, can be ascribed to the vascular abnormalities.

The immune response to T. pallidum reduces the burden of bacteria, but it may also have a central role in the pathogenesis of the disease. The T cells that infiltrate the chancre are T_H1 cells, suggesting that activation of macrophages to kill bacteria may cause resolution of the local infection.⁸⁹ Although there are many plasma cells in the syphilitic lesions and treponeme-specific antibodies are readily detectable, the antibody response does not eliminate the infection. The outer membrane of T. pallidum seems to protect the bacteria from antibody binding. The mechanism of this effect is not well understood, but either the paucity of bacterial proteins in the membrane or absorption (coating) of the membrane by host proteins may play a role.⁹⁰ The immune response is ultimately inadequate, since the spirochetes disseminate, persist, and cause secondary and tertiary syphilis.

In passing, it should be noted that antibiotic treatment of syphilis, in patients with a high bacterial load, can cause a massive release of endotoxins, resulting in a cytokine storm that manifests with high fever, rigors, hypotension, and leukopenia. This syndrome, called the Jarisch-Herxheimer reaction, is seen not only in syphilis but in other spirochetal diseases, such as Lyme disease, and can be mistaken for drug allergy.

Pathogenesis.

Borrelia burgdorferi does not produce LPS or exotoxins that damage the host. Much of the pathology associated with B. burgdorferi is thought to be secondary to the immune response against the bacteria and the inflammation that accompanies it. The initial immune response is stimulated by binding of bacterial lipoproteins to TLR2 expressed by macrophages. In response, these cells release proinflammatory cytokines (IL-6 and TNF) and generate bactericidal nitric oxide, reducing but usually not eliminating the infection.

The adaptive immune response to Lyme disease is mediated by CD4+ helper T cells and B cells. Borrelia-specific anti-bodies, made 2 to 4 weeks after infection, drive complement-mediated killing of the bacteria; however, B. burgdorferi escapes the antibody response through antigenic variation. Similar to Borrelia hermsii, a cause of endemic relapsing fever, B. burgdorferi has a plasmid with a single promoter sequence and multiple coding sequences for an antigenic surface protein, VlsE, each of which can shuttle into position next to the promoter and be expressed. Thus, as the antibody response to one VlsE protein is mounted, bacteria expressing an alternate VlsE protein can escape immune recognition. Chronic manifestations of Lyme disease, such as the late arthritis, are probably caused by the immune response against persistent bacteria.

Pathogenesis.

Clostridium perfringens does not grow in the presence of oxygen, so tissue death is essential for growth of the bacteria in the host. These bacteria release collagenase and hyaluronidase that degrade extracellular matrix proteins and contribute to bacterial invasiveness, but their most powerful virulence factors are the many toxins they produce. C. perfringens secretes 14 toxins, the most important of which is α-toxin.⁹⁵ This toxin has multiple actions. It is a phospholipase C that degrades lecithin, a major component of cell membranes, and so destroys red cells, platelets, and muscle cells, causing myonecrosis. It also has a sphingomyelinase activity that contributes to nerve sheath damage.

Ingestion of food contaminated with C. perfringens causes a brief diarrhea. Spores, usually in contaminated meat, survive cooking, and the organism proliferates in cooling food. C. perfringens enterotoxin forms pores in the epithelial cell membranes, lysing the cells and disrupting tight junctions between epithelial cells.⁹⁶

The neurotoxins produced by C. botulinum and C. tetani both inhibit release of neurotransmitters, resulting in paralysis.²⁶ Botulism toxin, eaten in contaminated foods or absorbed from wounds infected with C. botulinum, binds gangliosides on motor neurons and is transported into the cell. In the cytoplasm, the A fragment of botulism toxin cleaves a protein, called synaptobrevin, that mediates fusion of neurotransmitter-containing vesicles with the neuron membrane. By blocking vesicle fusion, botulism toxin prevents the release of acetylcholine at the neuromuscular junction, resulting in flaccid paralysis. If the respiratory muscles are affected, botulism can lead to death. Indeed, the widespread use of botulism toxin (Botox) in cosmetic surgery is based on its ability to cause paralysis of strategically chosen muscles on the face. The mechanism of tetanus toxin is similar to that of botulism toxin, but tetanus toxin causes a violent spastic paralysis by blocking release of γ-aminobutyric acid, a neurotransmitter that inhibits motor neurons.

Clostridium difficile produces toxin A, an enterotoxin that stimulates chemokine production and thus attracts leukocytes, and toxin B, a cytotoxin, which causes distinctive cytopathic effects in cultured cells. Both toxins are glucosyl transferases and are part of a pathogenicity island that is absent from the chromosomes of nonpathogenic strains of C. difficile.⁹⁷

Morphology. Clostridial cellulitis, which originates in wounds, can be differentiated from infection caused by pyogenic cocci by its foul odor, its thin, discolored exudate, and the relatively quick and wide tissue destruction. On microscopic examination, the amount of tissue necrosis is disproportionate to the number of neutrophils and gram-positive bacteria present (Fig. 8-41). Clostridial cellulitis, which often has granulation tissue at its borders, is treatable by debridement and antibiotics.

FIGURE 8-41 Boxcar-shaped gram-positive Clostridium perfringens in gangrenous tissue.

In contrast, clostridial gas gangrene is life-threatening and is characterized by marked edema and enzymatic necrosis of involved muscle cells 1 to 3 days after injury. An extensive fluid exudate, which is lacking in inflammatory cells, causes swelling of the affected region and the overlying skin, forming large, bullous vesicles that rupture. Gas bubbles caused by bacterial fermentation appear within the gangrenous tissues. As the infection progresses, the inflamed muscles become soft, blue-black, friable, and semifluid as a result of the massive proteolytic action of the released bacterial enzymes. On microscopic examination there is severe myonecrosis, extensive hemolysis, and marked vascular injury, with thrombosis. C. perfringens is also associated with dusk-colored, wedge-shaped infarcts in the small bowel, particularly in neutropenic people. Regardless of the site of entry, when C. perfringens disseminates hematogenously there is widespread formation of gas bubbles.

Despite the severe neurologic damage caused by botulinum and tetanus toxins, the neuropathologic changes are subtle and nonspecific.

Pathogenesis.

Rickettsiae do not produce significant toxins. The rickettsiae that cause typhus and spotted fevers predominantly infect vascular endothelial cells, especially those in the lungs and brain. The bacteria enter the endothelial cells by endocytosis, but they escape from the endosome into the cytoplasm before formation of the acidic phagolysosome. The organisms proliferate in the endothelial cell cytoplasm and then either lyse the cell (typhus group) or spread from cell to cell through actin-mobilized motion (spotted fever group). The severe manifestations of rickettsial infection are primarily due to vascular leakage secondary to endothelial cell damage.⁶ This causes hypovolemic shock with peripheral edema, as well as pulmonary edema, renal failure, and a variety of CNS manifestations that can include coma.

The innate immune response to rickettsial infection is mounted by NK cells, which produce IFN-γ, reducing bacterial proliferation. Subsequent CTL responses are critical for elimination of rickettsial infections. IFN-γ and TNF, from activated NK cells and T cells, stimulate the production of bactericidal nitric oxide. CTLs lyse infected cells, reducing bacterial proliferation. Rickettsial infections are diagnosed by immunostaining of organisms or by detection of antirickettsial antibodies in the serum.

Morphology

Typhus Fever. In mild cases the gross changes are limited to a rash and small hemorrhages due to the vascular lesions. In more severe cases, there may be areas of necrosis of the skin and gangrene of the tips of the fingers, nose, earlobes, scrotum, penis, and vulva. In such cases, irregular ecchymotic hemorrhages may be found internally, principally in the brain, heart muscle, testes, serosal membrane, lungs, and kidneys.

The most prominent microscopic changes are small-vessel lesions and focal areas of hemorrhage and inflammation in various organs and tissues. Endothelial swelling in the capillaries, arterioles, and venules may narrow the lumens of these vessels. A cuff of mononuclear inflammatory cells usually surrounds the affected vessel. The vascular lumens are sometimes thrombosed. Necrosis of the vessel wall is unusual in typhus (as compared to RMSF). Vascular thromboses lead to gangrenous necrosis of the skin and other structures in a minority of cases. In the brain, characteristic typhus nodules are composed of focal microglial proliferations with an infiltrate of mixed T lymphocytes and macrophages (Fig. 8-43).

FIGURE 8-43 Typhus nodule in the brain.

Scrub typhus, or mite-borne infection, is usually a milder version of typhus fever. The rash is usually transitory or might not appear. Vascular necrosis or thrombosis is rare, but there may be a prominent inflammatory lymphadenopathy.

Rocky Mountain Spotted Fever. A hemorrhagic rash that extends over the entire body, including the palms of the hands and soles of the feet, is the hallmark of RMSF. An eschar at the site of the tick bite is uncommon with RMSF but is common with R. akari, R. africae, and R. conorii infection. The vascular lesions that underlie the rash often lead to acute necrosis, fibrin extravasation, and occasionally thrombosis of the small blood vessels, including arterioles (Fig. 8-44). In severe RMSF, foci of necrotic skin appear, particularly on the fingers, toes, elbows, ears, and scrotum. The perivascular inflammatory response, similar to that of typhus, is seen in the brain, skeletal muscle, lungs, kidneys, testes, and heart muscle. The vascular lesions in the brain may involve larger vessels and produce microinfarcts. A noncardiogenic pulmonary edema causing adult respiratory distress syndrome is the major cause of death in patients with RMSF.

FIGURE 8-44 Rocky Mountain spotted fever with a thrombosed vessel and vasculitis.