Disease and Epidemiology

Introduction to Disease and Epidemiology

• Mammals possess intricate physical, chemical, and immunological defenses for protection against disease-causing microbes.
• Pathogenic microbes exploit weaknesses in mammalian defenses, resulting in disease.
• A core question in microbial pathogenesis explores how microscopic organisms can cause mortality in larger beings.

Pathology, Infection, and Disease Defined

• Pathogen: An infectious agent.
• Pathology: The study of disease.
• Etiology: The cause of a disease.
• Pathogenesis: The development of disease.
• Infection: Pathogenic microbes breach host defenses, enter tissues, and multiply.
• Disease: The body's abnormal state prevents normal function.
• Infectious disease: Infections causing damage or disruption to tissues and organs.

The Human Microbiome

• Dynamic equilibrium characterizes the human body.
• Many interactions between the human body and microorganisms involve development of biofilms.
• "Give and take" defines Colonization of the body.

Normal Flora

• Resident microbes, also known as normal flora, inhabit body areas exposed to the outside.
• Internal organs, tissues, and fluids, such as blood and CSF, are typically microbe-free.
• Normal flora benefits hosts by inhibiting the overgrowth of harmful microbes through microbial antagonism.
• Endogenous infections arise when normal flora is introduced to previously sterile sites.

Flora of the Skin

• The skin, being the largest and most accessible organ, hosts two cutaneous populations: transients and residents.
• Transients are influenced by hygiene practices, whereas residents are stable, predictable, and less affected by hygiene.

Ecological Associations Between Microorganisms

• Symbiotic relationships involve organisms living in close nutritional interaction with each other, a requirement for one or both.
• Mutualism: Both members benefit and are dependant on each other.
• Commensalism: One, the commensal, benefits while the other is neither harmed nor benefited.
• Parasitism: The parasite benefits at the expense of the host.
• Nonsymbiotic relationships involve organisms that are free-living and do not require relationships for survival.
• Synergism: Members cooperate to yield a result that they couldn't achieve unaided.
• Antagonism: Actions by one organism affect the success or survival of others in the same community.
• Antibiosis: An example of antagonism- antibiotics.

Parasites

• Facultative parasites infect a host as saprobes.
• They often are opportunistic pathogens and occur when the host is compromised.
• Pseudomonas aeruginosa is a problem in hospitals.
• Obligate parasites cause disease, even death, to the host.
• Ectoparasites live on the body.
• Endoparasites live in organs and tissues.
• Intracellular parasites live within the cells.

Interrelationships Between Microbes and Humans

• Normal microbial flora represents symbiotic bacteria, fungi, and protozoa hosted by the human body.
• Commensal, parasitic, and synergistic relationships are all established relationships the human body.
• The Human Microbiome Project (HMP) analyzes the genomes of microbes in the human body.

Maintaining Normal Resident Flora

• Normal flora is crucial for human health.
• The flora creates an environment that can prevent infections and improve host defenses.
• Antibiotics, diseases, and dietary changes may alter flora.
• Probiotics introduce known beneficial microbes back into the body.

Predisposing Factors to Disease

• Predisposing factors increase the body's susceptibility to disease.
• Factors include:
• Gender.
• Inherited traits.
• Climate.
• Fatigue.
• Age.
• Lifestyle.
• Nutrition.
• Chemotherapy.

Contact, Colonization, Infection and Disease

• Morbidity: A diseased state.
• Mortality: Death.

Initial Newborn Colonization

• The uterus and its contents are normally sterile until just before birth.
• The breaking of the fetal membrane exposes the infant.
• Normal flora is introduced with handling and feeding after birth.

Key Factors in Infection Development

• True pathogens can cause disease in healthy individuals with normal immune defenses.
• Examples: Influenza virus, plague bacillus, malarial protozoan.
• Opportunistic pathogens cause disease when the host's defenses are compromised, or when they grow in unnatural areas;
• Examples: Pseudomonas sp. & Candida albicans.
• The virulence of pathogens decides the severity of the disease.
• Virulence factors are characteristics or structures that let a microbe cause disease.

Infectious Dose

• Infectious dose is the minimum number of microbes needed for successful infection. This is an indication of a microbe's virulence.
• Microbes with low IDs exhibit increased virulence.

Pathogens that Affect Pregnancy

• STORCH pathogens include syphilis, toxoplasmosis, hepatitis B, HIV-1, Zika, chlamydia, rubella, cytomegalovirus, and herpes simplex virus.

Koch's Postulates

• Used to prove the cause of an infectious disease.
• Exceptions:
• Some pathogens cause multiple disease conditions.
• Some pathogens only cause disease in humans.
• Some cannot be cultured.

Classifying Infectious Diseases

• Symptoms: Subjective changes in body function felt by a patient.
• Signs: Objective changes in the body that can be measured or observed.
• Syndrome: A specific group of signs and symptoms accompanying a disease.
• Communicable disease: Spread from one host to another.
• Contagious disease: Easily and rapidly spread from one host to another.
• Noncommunicable disease: Not spread from one host to another.

The Process of Infection and Disease

• The 4 stages of infection are:
• Incubation period: Initial contact with the agent to the appearance of first symptoms; the agent is multiplying with insufficient damage to warrant symptoms.
• Prodromal stage: Vague feelings of discomfort and nonspecific complaints.
• Illness: Multiplication occurs, and more signs and symptoms are established.
• Decline: Signs and symptoms subside.
• Convalescent period: The person responds to the infection, and symptoms decline.

Patterns of Infection

• Localized infection: Microbes enter the body and remain confined to a specific tissue.
• Systemic infection: Infection spreads to multiple sites and tissue fluids thru the bloodstream.
• Focal infection: Occurs when an infectious agent breaks from a local infection and spreads to other tissues.
• Mixed infection: Multiple microbes grow simultaneously at the infection site, also known as polymicrobial.
• Primary infection: The initial infection.
• Secondary infection: Occurs when another microbe infects.
• Sepsis: A toxic, inflammatory condition from the spread of microbes or their toxins.
• Bacteremia: Bacteria in the blood.
• Septicemia: Blood poisoning and growth of bacteria in the blood.
• Toxemia: Toxins in the blood.
• Viremia: Viruses in the blood.
• Subclinical disease: Exhibits no noticeable signs or symptoms.

Severity and Duration of a Disease

• Acute disease: Symptoms develop rapidly, but the disease is short-lived.
• Chronic disease: Symptoms develop slowly but persist for an extended time.
• Subacute disease: Between acute and chronic.
• Latent disease: The causative agent is inactive for a time and then activates.
• Herd immunity: Immunity in most of a population.

Persistence

• Recovery does not always mean all microbes are removed.
• Latency: After initial symptoms, microbes can reactivate and cause recurrent disease; patients may not shed it during the latent stage.
• Chronic carrier: A person with a latent infection that sheds the infectious agent.
• Sequelae: Long-term or permanent damage to tissues or organs.

Reservoirs and Sources

• Reservoirs: continual sources of infection; primary habitat of the pathogen in the natural world.
• Source: Individual or object from which an infection is acquired.
• Human reservoirs: Carriers with inapparent infections or latent diseases.
• Animal reservoirs: Zoonoses transmitted from animals to humans.
• Nonliving reservoirs: Soil and water.

Human Reservoirs

• Carrier: An individual that is inconspicuously sheltering a pathogen and spreads it to others.
• Asymptomatic carrier: Shows no symptoms.
• Passive carrier: Contaminated healthcare provider who transfers pathogens.
• Incubation carriers: Spread the infectious agent during the incubation period.
• Convalescent carriers: Are recovering without symptoms.
• Chronic carrier: An individual shelters the infectious agent for a long period.

Animals As Reservoirs And Sources

• Vector: A live animal (other than human) that transmits an agent from one host to another.
• The majority of vectors are arthropods.
• Some larger animals such as mammals, birds, and lower vertebrates.
• Biological vectors: actively participate in a pathogen’s life cycle.
• Mechanical vector: not necessary for a life cycle, just transports without being infected.
• Zoonosis: Naturally transmitted infection indigenous to animals.
• Zoonotic organism: Organism that causes zoonotic infections.
• Humans cannot transmit zoonotic diseases to others.
• Around 150 zoonoses exists worldwide.
• Zoonoses makes up to 70% new emerging diseases worldwide.
• Eradicating the diseases would need to eradicating the animal reservoir.

Patterns of Transmission

• Direct contact: Physical contact or fine aerosol droplets.
• Indirect contact: Passes from infected host to an intermediate conveyor and then to another host.
• Vehicle: Transmission by way of inanimate material, food, water, or fomites.
• Fomites are porous and nonporous surfaces objects that can become contaminated with pathogenic microorganisms and serve as vehicles in transmission.
• Airborne: Droplet nuclei, aerosols

Nosocomial (HAI) Infections

• Acquired during a hospital stay.
• These infections result from surgical procedures, equipment, personnel, or drug-resistant microorganisms.
• 1 in 25 hospital patients are affected.
• There are 2 to 4 million cases per year in the U.S., resulting in 90,000 deaths.
• Preventative measures include:
• Reduce number of pathogens.
• Handwashing. -Disinfecting tubs used to bathe patients
• Cleaning instruments scrupulously
• Using disposable bandages and intubation
• Infection control committees.
• Universal precautions are stringent measures to prevent spreading infections from person to person.
• Treat all patient specimens with the same degree of care and could harbor infectious agents

Emerging Infectious Diseases (EIDs)

• Diseases are new, increasing in incidence, or can potentially increase.
• Most are zoonotic, of viral origin, and likely vector-borne.
• Contributing factors include:
• Genetic recombination, e.g., avian influenza (H5N1) and E. coli 0157
• Evolution of new strains, e.g., Vibrio cholerae 0139.
• Widespread use of antibiotics and pesticides, leading to antibiotic-resistant strains.
• Changes in weather patterns, e.g., Hantavirus.
• Modern transportation, e.g., Chikungunya and West Nile virus.
• Animal control measures, e.g., Lyme disease.
• Public health failure, e.g., Diphtheria.

Epidemiology

• The study of where and when diseases occur and how they are transmitted in populations
• Functions performed by epidemiologists:
• Determine etiology.
• Identify other important factors concerning the spread of disease.
• Develop methods for controlling the diseases.
• Use data and graphs to outline incidence of the disease
• Descriptive epidemiology: data collection and analysis, done by Snow.
• Analytical epidemiology: to analyze a particular disease to determine it's probable cause, by Nightingale.
• Experimental epidemiology: hypothesis based and controlled experiments, done by Semmelweis
• The Centers For Disease Control And Prevention (CDC): Collects and analyzes epidemiological information in the US
• Publishes Morbidity and Mortality Weekly Report (MMWR).
• Morbidity: Incidence of a specific notifiable disease.
• Nonifiable infectious diseases: diseases in which physicians are required to report occurrence.
• Mortality: deaths from notifiable diseases.
• Morbidity rate: Number of people affected in relation to the total population in a given time period.
• Mortality rate: Number of deaths from a disease in relation to the population in a given time.
• Incidence: Number of people who develop a disease during a particular time period.
• Prevalence: Number of people who develop a disease at a specified time
• Sporadic disease: Occurs only occasionally.
• Endemic disease: Constantly present in a population.
• Epidemic disease: Acquired by many people within a given area within a short time
• Pandemic disease: Worldwide epidemic.