Disease Progression: Incubation, Prodromal, and Convalescent Periods

Virulence Factors

• Physical attributes or properties of pathogens that enable them to escape host defense mechanisms and cause disease
• Examples: attachment, receptors and adhesins, bacterial fimbriae, coagulase, obligated intracellular pathogens

Attachment

• Type of cells to which the pathogen is able to attach
• Receptors and adhesins: molecules on the surface of host cells and pathogens that recognize and bind to each other
• Example: Streptococcus pyogenes cells have an adhesin called Protein F

Bacterial Fimbriae (Pili)

• Long, thin, hair-like, flexible projections composed of proteins called pilin
• Enable bacteria to attach to surfaces, including tissues within the human body
• Example: Fimbriated strains of Neisseria gonorrhoaea attach to inner walls of the urethra and cause urethritis

Obligated Intracellular Pathogens

• Must live within host cells to survive and multiply
• Examples: Rickettsia, Chlamydia, certain sporozoan protozoa (e.g. Plasmodium spp. that cause human malaria)

Toxins

• Exotoxins: produced by pathogens and released into the host environment
• Neurotoxins: affect the central nervous system (CNS)
• Enterotoxins: affect the gastrointestinal tract, causing diarrhea and vomiting
• Examples: Clostridium tetani and Clostridium botulinum produce neurotoxins, causing tetanus and botulism

Camouflage and Molecular Mimicry

• Adult schistosomes (trematodes) conceal their foreign nature by coating themselves with host proteins
• Molecular mimicry: pathogen's surface antigens closely resemble host antigens, making them unrecognizable as foreign

Destruction of Antibodies

• Some bacterial pathogens produce an enzyme (IgA protease) that destroys IgA antibodies

Host Defense Mechanisms

• Nonspecific host defense mechanisms:
  • 1st line of defense: intact skin and mucous membranes
  • 2nd line of defense: inflammation, phagocytosis
• Specific host defense mechanisms: the immune system (3rd line of defense)

Immune System

• Produces antibodies in response to foreign substances (antigens)
• Antibodies recognize and bind to specific antigens

Gastrointestinal Tract (GIT) Defense

• Digestive enzymes
• Acidity of the stomach (approx. pH 1.5)
• Bile, which lowers the surface tension and causes chemical changes in bacterial cell walls and membranes

Disease Progression

• Incubation period: time between arrival of the pathogen and onset of symptoms
• Prodromal period: early signs and nonspecific symptoms of a disease
• Period of illness: patient experiences typical symptoms associated with the disease
• Convalescent period: patient recovers

Infection Types

• Localized infection: affects only one body part or organ
• Systemic infection: affects multiple parts of the body
• Acute disease: rapid onset, usually followed by rapid recovery
• Subacute disease: comes on more suddenly than a chronic disease but less suddenly than an acute disease
• Chronic disease: long-lasting or recurring

Symptomatic and Asymptomatic Diseases

• Symptomatic disease: patient experiences symptoms
• Asymptomatic disease: patient is unaware of the disease because they are not experiencing symptoms

Syphilis

• Primary stage: patient has an open lesion called a chancre, which contains the spirochete Treponema pallidum
• Secondary stage: symptoms include rash, fever, and mucous membrane lesions
• Latent stage: patient has few or no symptoms
• Tertiary syphilis: spirochetes cause destruction of organs in which they have been hiding, leading to death