Chapter 8 - Infectious Diseases

Question 1

Different routes of entry of microbes

Answer 1

Microbes can enter the host by breaching epithelial surfaces, inhalation, ingestion, or sexual transmission.

Question 2

TABLE: Routes of Microbial Infection

Answer 2

Question 3

Routes of entry of microbes:

GI tract:
Denfenses, mechanisms of microbial entry

Answer 3

Most gastrointestinal pathogens are transmitted by food or drink contaminated with fecal material. When hygiene fails, diarrheal disease becomes rampant.

GI tract has local defenses:

• Acidic secretions kill many organisms
• Viscous mucus layer covers the gut, protecting epithelium
• Pancreatic enzymes and bile detergents can kill enveloped viruses
• Defensins are produced by GI epithelium
• IgA antibodies are produced in MALT
• Peristalsis
• Normal gut flora

GI infections occur when local defenses are circumvented by a pathogen or when they are so weakened that even normal flora produce disease.

Norovirus is a non-enveloped virus resistant to many defenses

• *Enteropathogenic pathogens may establish symptomatic GI disease through these mechanisms:**
• Adhesion and local proliferation: binding intestinal epithelium and multiplying in mucous layer, elaborating potent exotoxins (e.g. Vibrio cholerae, enterotoxigenic E. coli)
• Adhesion and mucosal invasion: Invasion of intestinal mucosa / lamina propria, with ulceration, inflammation, hemorrhage –> dysentery (e.g. Shigella, Salmonella enterica, Campylobacter jejuni)
• ‘Hijacking’ of the host pathways of antigen uptake: M cells of Peyer’s patches, responsible for uptake and delivery of antigen to lymphoid cells, take up organisms through the same pathway (e.g. Poliovirus)

Some foodborne organisms (e.g. S. aureus) cause disease without causing infection of the host, just by elaborating exotoxins in the food.

Question 4

Routes of entry of microbes:

Respiratory tract:

Answer 4

Inhaled microorganisms of any kind, mainly in small (< 5 um) dust or aerosol particles, are carried into alveoli, phagocytized by alveolar macs and neuts, causing inflammation.

Microorganisms evade host defense in several ways:

• Viruses attach and enter epithelial cells in lower resp tract and pharynx (e.g. influenza virus has hemagglutinins that bind epithelial cells –> endocytosed virus –> replication –> promotes superinfection by bacteria (S. pneumoniae, S. aureus)
• Bacteria can release toxins that impair ciliary activity (e.g. Haemophilus influenzae, M. pneumoniae, Bordetella pertussis)
• Primary resistance to phagocytosis (e.g. Mycobacterium tuberculosis)

Mucociliary apparatus: Composed of mucus layer and ciliated columnar epithelium on mucosa; important defense mechanism

Question 5

Routes of entry of microbes:

Urogenital tract

Answer 5

Urine is sterile, and urinary tract is protected from infection by regular emptying during micturition. Urinary pathogens (e.g. E. coli) almost always gain access via urethra and must adhere urothelium to avoid being washed away.

Women have 10 times as many UTIs as men due to shorter distance between bladder and skin.

Obstruction of urinary flow or reflux of urine compromises normal defenses and increases susceptibility to UTIs.

Question 6

Routes of entry of microbes:

Skin

Answer 6

Keratinized epidermis provides a mechanical barrier against infection and produces antimicrobial fatty acids and defensins that are toxic to bacteria.

**Most skin infections are initiated by mechanical injury of the epidermis.** 
Some fungi (e.g. dermatophytes) can cause superficial infections of the stratum corneum, hair, and nails

Question 7

Routes of entry of microbes:

Vertical transmission

Answer 7

Common mode of transmission of certain pathogens, can occur through several routes:

1. Placental-fetal transmission: Most likely when mother infected with a pathogen during pregnancy; Resulting infections interfere with fetal development, and are based on age of the fetus (e.g. Rubella)
2. Transmission during birth: Causd by contact with infectious agents during birth (e.g. gonococcal and chlamydial conjunctivitis)
3. Postnatal transmission in maternal milk (e.g. cytomegalovirus, HIV, hepatitis B)

Question 8

Spread and dissemination of microbes within the body

Answer 8

While some disease-causing microorganisms remain localized to the initial site of infection, others have the capacity to invade tissues and spread to distant sites via the lymphatics, the blood, or the nerves.

Some pathogens secrete enzymes that break down tissues, allowing them to advance (e.g. S. aureus secretes hyaluronidase) to LNs and lymphatics

Certain viruses (e.g. Rabies, polio, varicella) spread to CNS by infecting peripheral nerves

Most common route of spread is through bloodstream. Many organisms are either transported free in plasma or within leukocytes

Consequences of blood-borne spread of pathogens vary widely depending on virulence of the organism, magnitude of infection, seeding pattern, and host faactors such as immune status.

Question 9

Seven mechanisms by which microbes cross mucosal barrier systems

Answer 9

1. Endosomes and transcytosis via M cells
2. Intercellular junctions
3. Endosomes and transcytosis via other types of epithelial cells
4. Mucosa-associated dendritic cells
5. Migrating mucosa-associated lymphocytes
6. Migrating mucosa-associated macrophages
7. Mucosa-associated nerve endings

Question 10

Answer 10

Routes of entry and dissemination of microbes. To enter the body, microbes penetrate the epithelial or mucosal barriers. Infection may remain localized at the site of entry or spread to other sites in the body. Most common microbes (selected examples are shown) spread through the lymphatics or bloodstream (either freely or within inflammatory cells). However, certain viruses and bacterial toxins may also travel through nerves.

Question 11

Release of microbes from the body and transmission of microbes

Answer 11

Microbes use various exit strategies to get from one host from the next.

Depending on location of infection, release may occur via skin shedding, coughing, sneezing, urine, feces, sex, insect vectors.

Shedding can be brief and during disease flares with some organisms.
Others (e.g. S. typhi) are shed for long periods by asymptomatic carriers.

Hardiness in environment varies by organism. Bacterial spores, protozoan cysts, helminth eggs can remain viable in a cool,d ry environment from months to years.

• *Most pathogens are transmitted from person to person by respiratory, fecal-oral, or sexual routes.**
• Respiratory viruses and bacteria are aerosolized in droplets when coughing. Pathogens in large droplets (e.g. influenza) can travel only 3 feet from source. Pathogens in small droplets (e.g. M. tuberculosis) can travel much longer distances.
• Enteric pathogens are spread usually by fecal-oral route; Foodborne (Vibrio, Shigella, Campy, Salmonella). Waterborne (Hepatitis A and E, polio, rotavirus). Helminths (e.g. hookworms) can shed eggs in stool that hatch as larvae that can penetrate the skin of the next host.
• STDs: Prolonged, intimate, mucosal contact

Question 12

KEY CONCEPTS: How microorgansims cause disease

Answer 12

Question 13

Host defenses against infection

Answer 13

The outcome of infection is determined by the virulence of the microbe and the nature of the host immune response, which may either eliminate the infection or, in some cases, exacerbate or even by the principal cause of tissue damage.

Host defenses include physical barriers, innate immunity, and adaptive immunity

Question 14

Immune evasion by microbes

Answer 14

Most pathogenic microbes have developed one or more strategies that allow them to evade host defenses.

Some examples:
1. Antigenic variation: Important to escape antibody-mediated defenses. Microbes can change their coats by expressing different surface antigens. Influenza viruses exhibit prominent antigenic drifts and shifts.

2. Resistance to antimicrobial peptides: Peptides (defensins) produced by epithelial cells and some leukocytes are toxic to microbes by forming pores in their membranes. These peptides also can augment anti-microbial immunity by inducing pro-inflammatory cytokines and chemokine production. Pathogens can become resistant to these peptides by changing net surface charge and membrane hydrophobicity, preventing peptide insertion.
3. Resistance to killing by phagocytosis: Many mechanisms developed to avoid phagocytosis (e.g. carb capsule on bacteria surface, special sialic acid-containing capsule of E. coli won’t bind C3b, S. aureus expresses protein A, which binds Fc of antibodies)
4. Evasion of apoptosis and manipulation of host cell metabolims: Some viruses produce proteins that interfere with apoptosis and/or autophagy, buying them time to replicate, enter latency, or transform host cells.
5. Resistance to cytokine-, chemokine-, and complement-mediated host defense: Viruses may express factors that interfere with JAK/STAT pathway, or inhibit dsRNA-dependent protein kinase (PKR), a mediator of IFN
6. Evasion of recognition by CD4+ helper T cells and CD8+ cytotoxic T cells: Viruses can alter MHC I proteins and/or MHC II proteins
7. Immunoregulatory mechanisms to downregulate anti-microbial T cell responses: Antigen-specific T cells lose potency during chronic viral infections (i.e. T cell exhaustion) - chronic feature of HIV, hep C, hep B
8. Latent infection: Ultimate means of avoiding immune system.

Question 15

Antigenic drift and antigenic shift definitions

Answer 15

Antigenic drift: Minor changes in DNA/RNA leading to different surface proteins, prompting immune responses and future immunity (e.g. hemagglutinin (HA) and neuraminidase (NA))

• *Antigenic shift:** More major change in virus DNA/RNA. This usually occurs from two strains of virus crossing and mutating to make a new subtype. Three ways this can happen:
  1. An animal (pig) is infected with a human flu and another (bird) flu. These mix and mutate to make a new flu that can infect humans
  2. A strain of bird flu passes to humans without any genetic change.
  3. Bird flu passes to another animal (pig) and then is passed to humans without genetic change.

These occur frequently in influenza viruses

Question 16

TABLE: Mechanisms of antigenic variation

Answer 16

Question 17

Answer 17

An overview of mechanisms used by viral and bacterial pathogens to evade innate and adaptive immunity.

Question 18

KEY CONCEPTS: Immune evasion by microbes

Answer 18

Question 19

Infectious agents establish infection and damage tissues by three mechanisms:

Answer 19

1. They can contact/enter host cells and directly cause cell death or changes in cellular metabolism
2. They may release toxins that kill cells at a distance, release enzymes that degrade tissue, or damage blood vessels and cause ischemic necrosis
3. They can induce host immune responses that cause additional tissue damage

Question 20

Mechanisms of viral injury

Answer 20

Viruses can enter host cells and replicate.

Viruses show a predilection (i.e. tropism) for certain cells and not others

A major determinant of tissue tropism is the presence of viral receptors on host cells.

Once viruses enter cells, they can damage and kill the cells by these three mechanisms:
1. Direct cytopathic effects: Some produce toxic enzymes and proteins, and some prevent host macromolecule synthesis. Some viruses initiate death receptor apoptosis pathway, and some lead to unfolded protein accumulation, some encode pro-apoptotic proteins.

2. Anti-viral immune responses: Cytotoxic T lymphocytes can cause tissue injury
3. Transformation of infected cells: Oncogenic viruses stimulate cell growth and survival

Question 21

Answer 21

Mechanisms by which viruses cause injury to cells.

Question 22

Mechanisms of bacterial injury:

Bacterial virulence

Answer 22

Bacterial damage to host tissues depends on the ability of the bacteria to adhere to host cells, to invade cells and tissues, or to deliver toxins.

Mobile genetic elements such as plasmids and bacteriophages can transmit functionally important genes to bacteria, including genes that influence pathogenicity and drug resistance.

• Quorum sensing*: Bacteria coordinately regulate gene expression in a large population, so they can acquire complex virulence properties
• Biofilms*: Communities of bacteria in which the organsims live in a viscous extracellular layer of polysaccharides that adhere to host tissues or objects (e.g. dog bowls). Biofilms enhance adherence to host tissues and increase virulence by protecting microbes from immune effecto rmechanisms

Question 23

Mechanisms of bacterial injury:

Bacterial adherence to host cells

Answer 23

Adhesins: bacterial surface proteins that bind bacteria to host cells or ECM

Pili: filamentous proteins on bacterial surface that act as adhesins

Question 24

Mechanisms of bacterial injury:

Virulence of intracellular bacteria

Answer 24

M. tuberculosis activates the alternative complement pathway, is opsonized by C3b, and then binds CR3 receptors on macrophages, enters them, and replicates inside.

Once inside a cell, Listeria monocytogenes modifies actin cytoskeleton to promote spreading of organism to neighboring cells

Question 25

Mechanisms of bacterial injury:

Bacterial toxins

Answer 25

• Endooxins:* Components of the bacterial cell
• Exotoxins*: Secreted by the bacterium

Bacterial endotoxin is a LPS in the outer membrane of gram negative bacteria that stimulates host immune responses and injures the host.
Lipid A anchors LPS to host cell membrane, inducing inflammatory response. High levels of endotoxin play a part in septic shock, DIC, ARDS through induction of TNF, IL-1, IL-12

• *Exotoxins are secreted bacterial proteins that cause cellular injury and disease.** Different categories include:
• Enzymes (e.g. proteases, hyaluronidases, coagulases, fibrinolysins)
• Toxins that alter intracellular signaling or regulatory pathways: Most have an active subunit with enzymatic activity and a subunit that binds receptors and delivers A subunit into cytoplasm
• Neurotoxins (e.g. by C. botulinum and C. tetani): Inhibit release of NTs, causing paralysis. A domains interact with proteins involved in NT secretion at synapses
• Superantigens: bacterial toxins that stimulate very large number of T lymphsby binding TCR, leading to massive T-lymph proliferation and cytokine release

Question 26

KEY CONCEPTS: Host damage

Answer 26

Question 27

Spectrum of inflammatory responses to infection

Answer 27

1. Suppurative (purulent) inflammation
   - Increased vascular permeability and leukocyte infiltration (mainly neuts)
   - ‘pyogenic’ bacteria release chemoattractants to pull neuts in
   - Mostly extracellular, gram-positive cocci and gram-negative rods
   - Tiny microabscesses to entire lung lobes of pneumonia can be involved
2. Mononuclear and granulomatous inflammation
   - Common feature of all chronic inflammatory processes
   - When they develop acutely, they are often in response to viruses, intracellular bacteria, or intracellular parasites
   - May include mostly lymphocytes, plasma cells, or macrophages
   - Granulomatous inflammation: usually evoked by bugs that resist eradication and stimulate T-cell immunity (e.g. M. tuberculosis, Histoplasma capsulatum) - epithelioid macs, multinucleated giant cells
3. Cytopathic-cytoproliferative reaction
   - Usually produced by viruses, cell necrosis and/or proliferation
   - Sparse inflammatory cells
   - Some viruses (e.g. herpes, adenovirus) cause make inclusion bodies
4. Tissue necrosis
   - C. perfringens and others secrete powerful toxins that cause rapid and severe gangrenous necrosis; tissue damage is the prominent feature.
5. Chronic inflammation and scarring
   - Chronic inflam can lead to scarring or complete healing
   - Dense fibrous septae seen with scarring

Question 28

TABLE: Spectrum of inflammatory responses to infection

Answer 28

Question 29

Special techniques for diagnosing infectious agents

Answer 29

The gold standards for diagnosis of infection are culture, biochemical/serologic identification, and in some cases, molecular diagnosis.

Some organisms can be seen on H&E (e.g. inclusion bodies from CMV and herpes simplex, bacterial clumps, Candida, protozoans, helminths). Others require various special stains.

Nucleic acid amplification tests (e.g. PCR) and transcription-mediated amplification, are increasingly being used for rapid microbe IDing.

Question 30

TABLE: Special techniques for diagnosing infectious agents

Answer 30

Question 31

Bacterial infections of the alimentary system:

Enteric colibacillosis (E. coli)

Answer 31

Disrupt ion and fluid transport systems, cause coagulative necrosis via toxins and inflammation

Enterotoxigenic E. coli (ETEC): K99 (F5) or F41 adhesins to bind receptors in mucus layer. Produce heat labile (LT) and heat stable (ST) enterotoxins which cause secretory diarrhea

Enteropathogenic E. coli (EPEC): Colonizes mucosa similar to ETEC. expresses P and S fimbriae adhesins, EPEC adherence factor, intimin. attaching and effacing injury. Causes osmotic (dec. digestive enzyme activity) and secretory (ion transport disruption) diarrhea. Some secrete verotoxin.

Enterohemorrhagic E. coli (EHEC): Colonizes mucosa in a similar way. Colonic enterocytes are the primary target. Secretes verotoxin, directly damaging cells and eliciting intense inflammation. Some produce shiga toxins. Exposes lamina propria to LPS and other toxins, enzymes that causes hemorrhagic colitis

Question 32

Bacterial infections of the alimentary system:

Salmonella spp.

Answer 32

Causes coagulative necrosis by toxins, inflammation, degradative enzymes.

Peracute, acute, and chronic forms.

Peracute - petechiation, cyanosis of skin, fibrinoous polyserositis, DIC
Acute - fibrinonecrotic ileotyphlocolitis, malodorous content
Chronic - discrete necrotic and ulcerative foci (button ulcers)

Colonizes the mucosae in two ways:

(1) Through M cells in intestinal crypts
(2) Penetrate mucus to gain access to apically positioned enterocyte membranes

Once phagocytosed or endocytosed, they survive and replicate within a salmonella-containing vacuole (SCV); cell then lyses and releases bacterium into Peyer’s patches, where more macrophages are killed via activation of caspase-1 and apoptosis

Clinical signs due to:

(1) exotoxin that produces secretory diarrhea
(2) cytotoxin that inhibits protein synthesis
(3) endotoxins, LPSs that kill cells

Question 33

Bacterial infections of the alimentary system:

Enterotoxemia (Clostridium perfringens)

Answer 33

Mechanism of injury: acute coagulative necrosis of cells and tissues caused by bacterial toxins –> dark red to purple-black segments of small intestine (hemorrhagic enteritis)

Produce cytotoxins (alpha, beta, epsilon, iota) which are membrane toxins and ECM toxins. Iota toxin disrupts cytoskeleton –> cell lysis

Abundance of toxins in lumen causes sloughing of enterocytes followed by further colonization and proliferation of bacteria and more toxins.

Toxins carried systemically, increasing vascular permeability causing edema and effusions
(e.g. focal symmetric encephalomalacia, pulpy kidney disease)

Question 34

Bacterial infections of the alimentary system:

Alimentary anthrax (Bacillus anthracis)

Answer 34

Mechanism of injury: acute coagulative necrosis of cells caused by bacterial toxins.

Dark red to purple-black segments of small intestine (hemorrhagic enteritis)

Animals encounter bacteria through ingestion of fomites, it exists in soil and water as an endospore that is resistant in environment. Animals ingest endospore –> vegetative forms in GI tract which cause disease. (Veg forms may persist in environment with heavy rain and be ingested).

Ingested bacterium evades gastric acidity and is peristalsis’d into the SI

Produces A-B toxin that diffuses into mucosa and LP, causing thrombosis in addition to the necrosis.
B part is protective antigen (PA) which gets it into cells via endocytosis, then creates a pore through which the A part (edema factor (EF) and lethal factor (LF)) enter the cell.

PA binds to tumor endothelial marker 8 (TEM8), and capillary morphogenesis protein 2 (CMG2) to enter cell. It then combines with EF or LF to cause edema or kill the cell

Question 35

Bacterial infections of the alimentary system:

Disorders of Horses

Answer 35

Rhodococcal Enteritis (Rhodococcus equi):
Gross lesions:
(1) ulcerative enteritis, discrete foci of ulceration/hemorrhage over PPs
(2) Chronic active pyogranulomatous lymphadenitis

Foals swallow organism in infected mucus, exudate, cell debris that were coughed up into oropharynx and is then peristalsis’d to where they bind to luminal surfaces of M cells and released into Peyer’s patches

Virulence factors block phagosome fusion with lysosome, block the actions of lysosomal enzymes, and block respiratory burst of macrophages. They replicate in macrophage phagosomes.

Tyzzer’s disease (Clostridium piliforme) - More info in hepatobiliary section

Question 36

Bacterial infections of the alimentary system:

Disorders of ruminants

Answer 36

Johne’s Disease (Mycobacterium avium subsp. paratuberculosis)
Mechanisms of injury:
(1) Dysfunction/lysis of epithelial cells and ECM proteins
(2) Dysfunction of drainage of afferent lymphatic vessels in LP of SI villi
(3) lysis of cells of MPS system and of all cells in LP of villi from chronic inflammation and degradative enzymes

Gross lesions: granulomatous enteritis, mesenteric granulomatous lymphadenitis, lymphangitis, lymphangiectasia

Mycobacteria infect macrophages and secrete iron-chelating virulence factors to get iron out of the macrophage ferritin. They can inhibit acidification of phagosome, phagoslysosome fusion, and lysosomal enzymes, block injury from toxic oxygen and nitrogen intermediates, and suppress macrophage ability to be activated

Young animals are more susceptible for unknown reasons.

Bovine intestinal tuberculosis (Mycobacterium bovis): Similar pathogenesis to Johne’s. Commonly begins as pneumonic form and spreads to intestine by coughing up and swalling sputum with macrophages or hematogenous/lymphatic spread. M cells and dendritic cells phagocytose it and release it into PPs where granulomatous inflammation follows. Ulceration results from vasculitis, thrombosis, ischemia

Wooden tongue (Actinobacillus lignieresii): Persistent pyogranulomatous inflammation and fibrotic reparative responses. Firm, large tongue protruding from oral cavity. Normal commensal bacterium of oral mucosa in cows and sheep.

Alimentary anthrax (Bacillus anthracis)

Question 37

Bacterial infections of the alimentary system:

Disorders of pigs

Answer 37

Porcine proliferative enteritis/hemorrhagic bowel syndrome (Lawsonia intracellularis):
Causes two distinct disease syndromes in a single continuum.
First syndrome - proliferative enteritis
Second syndrome - Cell lysis and hemorrhage (hemorrhagic bowel syndrome)

Proliferation of bacteria occurs intracellularly concurrently with crypt enterocyte proliferation.

It infects cells of crypts in the proliferative zone. These do not mature, but remain in proliferative state, causing massive thickening of mucosal surface. Proliferating cells migrate up to villiw here they die and dump bacteria in lumen, which is the source of bacteria for new crypt cells.

Swine dysentery (Brachyspira hyodysenteriae): Mechanism is lysis of mucosal epithelial cells of colon and cecum caused by hemolysins and proteases and inflammation. Goblet cell mucus is important as a physical matrix for colonization (this may be why it goes to colon and cecum - more goblet cells) - they replicate in goblet cell mucigen droplets.