General Pathology of Infectious Diseases

Prions

Prions are composed of abnormal forms of a host protein termed prion protein (PrP). These agents cause transmissible spongiform encephalopathies, including kuru (associated with human cannibalism), hereditary or sporadic Creutzfeldt-Jakob disease (CJD), bovine spongiform encephalopathy (BSE) (better known as mad cow disease), and variant Creutzfeldt-Jakob disease (vCJD) (probably transmitted to humans through consumption of meat from BSE-infected cattle). PrP is found normally in neurons. Diseases occur when the PrP undergoes a conformational change that confers resistance to proteases. The protease-resistant PrP promotes conversion of the normal protease-sensitive PrP to the abnormal form, explaining the transmissable nature of these diseases. CJD can be transmitted from person to person iatrogenically, by surgery, organ transplantation, or blood transfusion. These diseases are discussed in detail in Chapter 23.

Viruses

Viruses are obligate intracellular parasites that depend on the host cell's metabolic machinery for their replication. They consist of a nucleic acid genome surrounded by a protein coat (called a capsid) that is sometimes encased in a lipid membrane. Viruses are classified by their nucleic acid genome (DNA or RNA, but not both), the shape of the capsid (icosahedral or helical), the presence or absence of a lipid envelope, their mode of replication, their preferred cell type for replication (called tropism), or the type of pathology they cause. Some viral components and particles aggregate within infected cells and form characteristic inclusion bodies, which may be seen with the light microscope and are useful for diagnosis (Fig. 9.1). For example, cytomegalovirus (CMV)-infected cells are enlarged and show a large eosinophilic nuclear inclusion and smaller basophilic cytoplasmic inclusions; herpes­viruses form a large nuclear inclusion surrounded by a clear halo; and both smallpox and rabies viruses form characteristic cytoplasmic inclusions. However, many viruses (e.g., poliovirus) do not produce inclusions.

Accounting for a large share of human infections, viruses can cause disease in several ways (Table 9.2). Many viruses cause transient illnesses (e.g., colds, influenza). Other viruses are not eliminated from the body and persist within cells of the host for years, either continuing to multiply (e.g., chronic infection with hepatitis B virus [HBV]) or survive in some latent nonreplicating form, with the potential to be reactivated later. For example, herpes zoster virus, the cause of chickenpox, can enter dorsal root ganglia and establish latency at the site, with periodic reactivation at later times to cause shingles, a painful skin condition. Some viruses are involved in transformation of a host cell into a benign or malignant tumor (e.g., human papillomavirus [HPV]-induced benign warts and cervical carcinoma). Different species of viruses can produce the same clinical picture (e.g., adenovirus and rhinovirus causing upper respiratory infection); conversely, a single virus can cause different clinical manifestations depending on the age or immune status of the host (e.g., CMV causing congenital neurologic damage or gastroenteritis in the immunocompromised).

Bacteria

Bacteria are prokaryotes, meaning that they have a cell membrane but lack membrane-bound nuclei and other membrane-enclosed organelles. Most bacteria are bounded by a cell wall consisting of peptidoglycan, a polymer of long sugar chains linked by peptide bridges surrounding the cell membrane. There are two common forms of cell wall structure: a thick wall that retains crystal-violet stain (gram-positive bacteria) and a thin cell wall surrounded by an outer membrane (gram-negative bacteria) (Fig. 9.2). Bacteria are classified by Gram staining (positive or negative), shape (spherical, called cocci, or rod-shaped, called bacilli) (Fig. 9.3), and their requirement for oxygen (aerobic or anaerobic). Motile bacteria have flagella, long helical filaments extending from the cell surface that rotate and move the bacteria. Some bacteria possess pili, another kind of surface projection that can attach bacteria to host cells or extracellular matrix. Bacteria synthesize their own DNA, RNA, and proteins, but they depend on the host for favorable growth conditions. Many bacteria remain extracellular when they grow in the host, while others can survive and replicate both outside and inside of host cells (facultative intracellular bacteria such as mycobacteria), and some grow only inside host cells (obligate intracellular bacteria, such as rickettsia).

Bacteria cause a range of infections from common pharyngitis and urinary tract infections to rare diseases such as leprosy (Table 9.3). Chlamydia and Rickettsia are obligate intracellular bacteria that replicate inside membrane-bound vacuoles in epithelial and endothelial cells, respectively. These bacteria get most or all of their energy source, ATP, from the host cell. Chlamydia trachomatis is a frequent infectious cause of female sterility (by scarring and narrowing of the fallopian tubes) and blindness (by chronic inflammation of the conjunctiva that eventually causes scarring and opacification of the cornea). Rickettsiae injure the endothelial cells in which they grow, causing a hemorrhagic vasculitis, often visible as a rash, but they also may injure the central nervous system (CNS), with potentially fatal outcome, as in Rocky Mountain spotted fever and epidemic typhus. Rickettsiae are transmitted by arthropod vectors, including lice (in epidemic typhus), ticks (in Rocky Mountain spotted fever and ehrlichiosis), and mites (in scrub typhus).

Mycoplasma and the related genus Ureaplasma are unique among extracellular bacterial pathogens in that they do not have a cell wall. These are the tiniest free-living organisms known (125 to 300 nm).

Fungi

Fungi are eukaryotes with thick cell walls composed of complex carbohydrates such as beta-glucans, chitin, and mannosylated glycoproteins. Calcofluor-white, a fluorescent stain that binds chitin, provides a useful way to identify fungi in patient specimens. Assays for beta-glucans in blood are used to diagnose disseminated fungal infections. Fungi can grow either as rounded yeast cells or as slender, filamentous hyphae. An important distinguishing characteristic is whether hyphae are septate (with cell walls separating individual cells) or aseptate. Some of the most important pathogenic fungi exhibit thermal dimorphism; that is, they grow as hyphal forms at room temperature but as yeast forms at body temperature. Fungi may produce sexual spores or, more commonly, asexual spores called conidia. The latter are produced on specialized structures or fruiting bodies arising along the hyphal filament.

Fungi may cause superficial or deep infections.

Fungi are divided into endemic and opportunistic species.

• Endemic fungi are invasive species that are usually limited to particular geographic regions (e.g., Coccidioides in the southwestern United States, Histoplasma in the Ohio River Valley).

• Opportunistic fungi (e.g., Candida, Aspergillus, Mucor, Cryptococcus), by contrast, are ubiquitous organisms that either colonize individuals or are encountered from environmental sources but do not cause severe disease in healthy individuals. In immunodeficient individuals, opportunistic fungi give rise to life-threatening invasive infections characterized by vascular occlusion, hemorrhage, and tissue necrosis, with little or no inflammatory response (Fig. 9.4). Patients with AIDS are very susceptible to infection with the opportunistic fungus Pneumocystis jiroveci (previously called Pneumocystis carinii).

Protozoa

Protozoa are single-celled eukaryotes that are major causes of disease and death in developing countries. Protozoa can replicate intracellularly within a variety of cells (e.g., Plasmodium in red cells, Leishmania in macrophages) or extracellularly in the urogenital system, intestine, or blood. Trichomonas vaginalis organisms are sexually transmitted flagellated protozoal parasites that often colonize the vagina and male urethra. The most prevalent pathogenic intestinal protozoans, Entamoeba histolytica and Giardia lamblia, are ingested as nonmotile cysts in contaminated food or water and become motile trophozoites that attach to intestinal epithelial cells. Bloodborne protozoa (e.g., Plasmodium, Trypanosoma, Leishmania) are transmitted by insect vectors, in which they replicate before being passed to new human hosts. Toxoplasma gondii is acquired either through contact with oocyst-shedding cats or by eating cyst-ridden, undercooked meat.

Helminths

Parasitic worms are highly differentiated multicellular organisms. Their life cycles are complex; most alternate between sexual reproduction in the definitive host and asexual multiplication in an intermediate host or vector. Thus, depending on the species, humans may harbor adult worms (e.g., Ascaris lumbricoides), immature stages (e.g., Toxocara canis), or asexual larval forms (e.g., Echinococcus spp.). Once adult worms take up residence in humans, they usually do not multiply but they produce eggs or larvae that typically are passed in stool. Often, the severity of disease is proportional to the number of infecting organisms. For example, a burden of 10 hookworms is associated with mild or no clinical disease, whereas 1000 hookworms consume enough blood to cause severe anemia. In some helminthic infections, such as schistosomiasis, disease is caused by inflammatory responses to the eggs or larvae rather than the adult worms.

Helminths comprise three groups:

• Roundworms (nematodes) are circular in cross-section and nonsegmented. Intestinal nematodes include A. lumbricoides, Strongyloides stercoralis, and hookworms. Nematodes that invade tissues include the filariae, such as Wuchereria bancrofti and Trichinella spiralis (Fig. 9.5).

• Tapeworms (cestodes) have a head (scolex) and a ribbon of multiple flat segments (proglottids). They adsorb nutrition through their tegument and do not have a digestive tract. They include the fish, beef, and pork tapeworms that make their home in the human intestine. The larvae that develop after ingestion of eggs of certain tapeworms can cause cystic disease within tissues (Echinoccus granulosus larvae cause hydatid cysts; pork tapeworm larvae produce cysts called cysticerci in many organs).

• Flukes (trematodes) are leaf-shaped flatworms with prominent suckers that are used to attach to the host. They include liver and lung flukes and schistosomes.

Ectoparasites

Ectoparasites are insects (e.g., lice, bedbugs, fleas) or arachnids (e.g., mites, ticks, spiders) that cause disease by biting or by attaching to and living on or in the skin. Infestation of the skin by arthropods is characterized by itching and excoriations, such as pediculosis caused by lice attached to hairs, or scabies caused by mites burrowing into the stratum corneum. At the site of bites, mouth parts may be found associated with a mixed infiltrate of lymphocytes, macrophages, and eosinophils. Arthropods also can serve as vectors for other pathogens, such as Borrelia burgdorferi, the agent of Lyme disease, which is transmitted by deer ticks.

Skin

The dense, keratinized outer layer of skin is a natural barrier to infection; furthermore, the low pH of the skin (less than 5.5) combined with the presence of fatty acids inhibit growth of microorganisms other than the normal bacteria and fungi. Included among these normal flora are potential opportunists, such as S. aureus and Candida albicans.

Cutaneous infections are typically acquired by entry of microbes through breaks in the skin, including wounds or surgical incisions (Staphylococci), burns (Pseudomonas aeruginosa), and diabetic and pressure-related foot sores (multibacterial infections). Intravenous catheters in hospitalized patients provide portals for local or systemic infection. Needle sticks can expose the recipient to infected blood and transmit HBV, HCV, or HIV. Some pathogens penetrate the skin via an insect or animal bite. Bites by fleas, ticks, mosquitoes, mites, and lice break the skin and transmit arboviruses (causes of yellow fever and encephalitis), bacteria (plague, Lyme disease, Rocky Mountain spotted fever), protozoa (malaria, leishmaniasis), and helminths (filariasis). Animal bites can lead to infections with bacteria, such as Pasteurella, or viruses, such as rabies. Only a few microorganisms are able to cross the skin barrier directly. For example, Schistosoma larvae released from freshwater snails penetrate swimmers' skin by releasing enzymes that dissolve the extracellular matrix, traversing unbroken skin. Similarly, certain fungi (dermatophytes) can infect intact stratum corneum of the skin, hair, and nails.

Gastrointestinal Tract

Gastrointestinal pathogens are transmitted by food or drink contaminated with fecal material. When hygiene fails, as may occur with natural disasters such as floods and earthquakes, diarrheal disease becomes rampant. Acidic gastric secretions are important defenses and are lethal for many gastrointestinal pathogens. Healthy volunteers do not become infected by Vibrio cholerae unless they are fed 10¹¹ organisms, but neutralizing the stomach acid reduces the infectious dose by 10,000-fold. By contrast, some ingested agents, such as Shigella and Giardia cysts, are relatively resistant to gastric acid, so fewer than 100 organisms can cause illness.

Other normal defenses within the gastrointestinal tract include (1) the layer of viscous mucus covering the intestinal epithelium, (2) lytic pancreatic enzymes and bile detergents, (3) mucosal anti-microbial peptides called defensins, (4) normal flora, and (5) secreted IgA antibodies. IgA antibodies are made by plasma cells located in mucosa-associated lymphoid tissue (MALT). These lymphoid aggregates are covered by a single layer of specialized epithelial cells called M cells, which are important for transport of antigens to MALT. Numerous gut pathogens use M cells to enter the host from the intestinal lumen, including poliovirus, enteropathic E. coli, V. cholerae, Salmonella enterica serotype Typhi, and Shigella flexneri.

Infection via the gastrointestinal tract occurs when local defenses are weakened or the organisms develop strategies to overcome these defenses. Host defenses are weakened by low gastric acidity, by antibiotics that alter the normal bacterial flora (e.g., in pseudomembranous colitis due to C. difficile), or when there is stalled peristalsis or mechanical obstruction. Viruses that can enter the body through the intestinal tract (e.g., hepatitis A, rotavirus) are those that lack envelopes, because enveloped viruses are inactivated by bile and digestive enzymes.

Enteropathogenic bacteria cause gastrointestinal disease in several ways:

Fungal infections of the gastrointestinal tract occur in immunologically compromised individuals. Candida, part of the normal gastrointestinal flora, shows a predilection for stratified squamous epithelium, causing oral thrush or membranous esophagitis, but also may spread to the stomach, lower gastrointestinal tract, and other organs.

Most intestinal parasites enter the body by being ingested as cysts, eggs, or meat-borne larvae, but a few enter by penetrating the skin and finding their way to the intestine. Once in the intestine, these pathogens cause damage in several ways:

Respiratory Tract

A large number of microorganisms, including viruses, bacteria, and fungi, are inhaled daily by every individual. In many cases, the microbes are inhaled in dust or aerosol particles. The distance these particles travel into the respiratory system is inversely proportional to their size. Large particles are trapped in the mucociliary blanket that lines the nose and the upper respiratory tract. Microorganisms trapped in the mucus secreted by goblet cells are transported by ciliary action to the back of the throat, where they are swallowed or coughed out. Particles smaller than 5 µm travel directly to the alveoli, where they are phagocytosed by alveolar macrophages or by neutrophils recruited to the lung by cytokines.

Microorganisms that invade the normal healthy respiratory tract have developed specific mechanisms to overcome mucociliary defenses or to avoid destruction by alveolar macrophages. Some successful respiratory viruses evade these defenses by attaching to and entering epithelial cells in the lower respiratory tract and pharynx. For example, influenza viruses possess hemagglutinin proteins that project from the surface of the virus and bind to sialic acid on the surface of epithelial cells. This attachment induces the host cell to engulf the virus, leading to viral entry and replication within the host cell.

Certain bacterial respiratory pathogens release toxins that paralyze cilia. Examples include Hae­mophilus influenzae, Mycoplasma pneumoniae, and Bordetella pertussis. Some bacteria lack the ability to overcome the defenses of the healthy lung and can cause respiratory infections only in compromised hosts. Streptococcus pneumoniae and S. aureus can cause pneumonia subsequent to influenza because the viral infection leads to the loss of the protective ciliated epithelium. Chronic damage to mucociliary defense mechanisms occurs in smokers and individuals with cystic fibrosis, while acute injury occurs in intubated patients and in those who aspirate gastric acid.

Some respiratory pathogens avoid phagocytosis or destruction after phagocytosis. M. tuberculosis, for example, gains a foothold in alveoli because it escapes killing within the phagolysosomes of macrophages. Opportunistic fungi infect the lungs when cellular immunity is depressed or when leukocytes are reduced in number (e.g., P. jiroveci in patients with AIDS, Aspergillus spp. after chemotherapy).

Urogenital Tract

The urinary tract is almost always invaded from the exterior by way of the urethra. The regular flushing of the urinary tract with urine serves as a defense against invading microorganisms. Urine in the bladder is normally sterile or contains only small numbers of fastidious bacteria; however, successful pathogens (e.g., N. gonorrhoeae, E. coli) adhere to the urinary epithelium, overcoming the host defense of regular flushing. Anatomy plays an important role in infection. Women have many more urinary tract infections than men because the distance between the urinary bladder and skin (i.e., the length of the urethra) is 5 cm in women, in contrast with 20 cm in men. Obstruction of urinary flow, as in benign prostatic hyperplasia, or reflux can compromise normal defenses and increase susceptibility to urinary tract infections. Urinary tract infections can spread further up from the bladder to the kidney and cause acute and chronic pyelonephritis.

From puberty until menopause, the vagina is protected from pathogens by a low pH resulting from catabolism of glycogen in the normal epithelium by lactobacilli. Antibiotics can kill the lactobacilli, allowing overgrowth of yeast, with resultant vaginal candidiasis.

Bacterial Adherence to Host Cells

Bacterial surface molecules that bind to host cells or extracellular matrix are called adhesins. Diverse surface structures are involved in adhesion of various bacteria (see Fig. 9.2). S. pyogenes has protein F and teichoic acid projecting from its cell wall that bind to fibronectin on the surface of host cells and in the extracellular matrix. Other bacteria have filamentous proteins called pili on their surfaces. Stalks of pili are structurally conserved, whereas amino acids on the tips of the pili vary and determine the binding specificity of the bacteria. Strains of E. coli that cause urinary tract infections uniquely express a specific P pilus that binds to a Gal(α1–4)Gal moiety expressed on uroepithelial cells. Pili on N. gonorrhoeae bacteria mediate adherence of the bacteria to host cells and also are targets of the host antibody response. Antigenic variation affecting the antigens expressed in the pili is an important mechanism by which N. gonorrhoeae escapes the immune response.

Bacterial Toxins

Any bacterial substance that contributes to illness can be considered a toxin. Toxins are subclassified as endotoxins, which are components of the bacterial cell, or exotoxins, which are proteins that are secreted by the bacterium.

Bacterial endotoxin is a lipopolysaccharide (LPS) that is a component of the outer membrane of gram-negative bacteria (see Fig. 9.2). LPS is composed of a long-chain fatty acid anchor, termed lipid A, connected to a core sugar chain, both of which are very similar in all gram-negative bacteria. Attached to the core sugar is a variable carbohydrate chain (O antigen), which is used to serotype strains of bacteria to aid in diagnosis. Lipid A binds to CD14 on the surface of host leukocytes, and the complex then binds to Toll-like receptor 4, a pattern recognition receptor of the innate immune system that transmits signals to promote cell activation and inflammatory responses. Responses to LPS can be both beneficial and harmful to the host. The response is beneficial in that LPS activates protective immunity through induction of important cytokines and chemoattractants (chemokines), as well as increased expression of costimulatory molecules, which enhance T-lymphocyte activation. However, high levels of LPS play an important role in septic shock, disseminated intravascular coagulation, and acute respiratory distress syndrome, mainly through induction of excessive levels of cytokines such as tumor necrosis factor (Chapter 4).

Exotoxins are secreted proteins that cause cellular injury and disease. They can be classified into broad categories by their mechanism and site of action.

• Enzymes. Bacteria secrete enzymes (proteases, hyaluronidases, coagulases, fibrinolysins) that act on their respective substrates in vitro, but their role in disease is understood in only a few cases. For example, exfoliative toxins are proteases produced by S. aureus that cleave proteins known to hold keratinocytes together, causing the epidermis to detach from the deeper skin.

• A-B toxins: Toxins that alter intracellular signaling or regulatory pathways. The two-component toxins have an active (A) component with enzymatic activity and a binding (B) component that binds cell surface receptors and delivers the A protein into the cell cytoplasm. The effect of these toxins depends on the binding specificity of the B domain and the cellular pathways affected by the A domain. A-B toxins are made by many bacteria including Bacillus anthracis, V. cholerae, and Corynebacterium diphtheriae. The mechanism of action of the A-B anthrax toxin is well understood (Fig. 9.8). Anthrax toxin has two alternate A components, edema factor (EF) and lethal factor (LF), which enter cells following binding to the B component, and each A component mediates specific pathologic effects.

• Superantigens stimulate very large numbers of T lymphocytes by binding to conserved portions of the T cell receptor, leading to massive T lymphocyte proliferation and cytokine release. The high levels of cytokines lead to capillary leak and the systemic inflammatory response syndrome (Chapter 4). Superantigens made by S. aureus and S. pyogenes cause toxic shock syndrome.

• Neurotoxins produced by Clostridium botulinum and Clostridium tetani inhibit release of neurotransmitters, resulting in paralysis. These toxins do not kill neurons; instead, the A domains cleave proteins involved in secretion of neurotransmitters at the synaptic junction. Tetanus and botulism can result in death from respiratory failure due to paralysis of the chest and diaphragm muscles.

• Enterotoxins affect the gastrointestinal tract causing varied effects, including nausea and vomiting (S. aureus), voluminous watery diarrhea (V. cholerae), and bloody diarrhea (C. difficile).