Infection, Infectious Diseases, and Epidemiology Chapter 14 Infection, Infectious Diseases, and Epidemiology

Symbiotic Relationships Between Microbes and Their Hosts Symbiosis means “to live together” We have symbiotic relationships with countless microorganisms Types of symbiosis Mutualism Commensalism Parasitism © 2012 Pearson Education Inc. 2

Table 14.1 The Three Types of Symbiotic Relationships

Symbiotic Relationships Between Microbes and Their Hosts Normal Microbiota in Hosts Also termed normal flora and indigenous microbiota Organisms that colonize the body’s surfaces without normally causing disease Two types Resident microbiota Throughout lifetime Most are commensal Transient microbiota Only remain in/on body for a short time period knocked out by competition or immune system or physiological changes © 2012 Pearson Education Inc. 4

Figure 14.2 Normal microbiota of the human nasal cavity

Table 14.2 Some Resident Microbiota

Symbiotic Relationships Between Microbes and Their Hosts Normal Microbiota in Hosts Acquisition of normal microbiota Development in womb free of microorganisms (axenic) Microbiota begin to develop during birthing process Much of one’s resident microbiota established during first months of life © 2012 Pearson Education Inc. 7

Symbiotic Relationships Between Microbes and Their Hosts How Normal Microbiota Become Opportunistic Pathogens Opportunistic pathogens Normal microbiota that cause disease under certain circumstances Conditions that provide opportunities for pathogens Introduction of normal microbiota into unusual site in body Immune suppression Changes in the normal microbiota © 2012 Pearson Education Inc. 8

Reservoirs of Infectious Diseases of Humans Most pathogens cannot survive for long outside their host Reservoirs of infection Sites where pathogens are maintained as a source of infection Three types of reservoirs Animal reservoirs Human carriers Nonliving reservoirs © 2012 Pearson Education Inc. 9

Animal Reservoirs Zoonoses Acquire zoonoses through various routes Diseases naturally spread from animal host to humans Acquire zoonoses through various routes Direct contact with animal or its waste Eating animals Bloodsucking arthropods Humans are usually dead-end host to zoonotic pathogens © 2012 Pearson Education Inc. 10

Human Carriers Infected individuals who are asymptomatic but infective to others Some individuals eventually develop illness while others never get sick Healthy carriers may have defensive systems that protect them © 2012 Pearson Education Inc. 11

Nonliving Reservoirs Soil, water, and food can be reservoirs of infection Presence of microorganisms often due to contamination by feces or urine © 2012 Pearson Education Inc. 12

The Movement of Microbes into Hosts: Infection Exposure to Microbes: Contamination and Infection Contamination The mere presence of microbes in or on the body Infection When organism evades body’s external defenses, multiplies, and becomes established in the body © 2012 Pearson Education Inc. 13

Portals of Entry Sites through which pathogens enter the body Four major pathways Skin Mucous membranes Placenta Parenteral route Punctures, surgery, etc. © 2012 Pearson Education Inc. 14

Portals of Entry Skin Outer layer of dead skin cells acts as a barrier to pathogens Some pathogens can enter through openings or cuts Others burrow into or digest outer layers of skin © 2012 Pearson Education Inc. 15

Portals of Entry Mucous membranes Line the body cavities that are open to the environment Provide a moist, warm environment hospitable to pathogens Respiratory tract is the most common site of entry Entry is through the nose, mouth, or eyes Gastrointestinal tract may be route of entry Must survive the acidic pH of the stomach © 2012 Pearson Education Inc. 16

Portals of Entry Placenta Typically forms effective barrier to pathogens Pathogens may cross the placenta and infect the fetus Can cause spontaneous abortion, birth defects, premature birth © 2012 Pearson Education Inc. 17

Portals of Entry Parenteral route Not a true portal of entry Means by which the portal of entry can be circumvented Pathogens deposited directly into tissues beneath the skin or mucous membranes Needle surgery © 2012 Pearson Education Inc. 18

The Role of Adhesion in Infection Process by which microorganisms attach themselves to cells Required to successfully establish colonies within the host Uses adhesion factors Specialized structures Attachment proteins © 2012 Pearson Education Inc. 19

The Role of Adhesion in Infection Attachment proteins help in adhesion Found on viruses and many bacteria Viral or bacterial ligands bind host cell receptors Interaction can determine host cell specificity Changing/blocking a ligand or its receptor can prevent infection Inability to make attachment proteins or adhesins renders microorganisms avirulent Some bacterial pathogens attach to each other to form a biofilm © 2012 Pearson Education Inc. 20

Figure 14.6 Dental plaque

The Nature of Infectious Disease Infection is the invasion of the host by a pathogen Disease results if the invading pathogen alters normal body functions Disease is also referred to as morbidity © 2012 Pearson Education Inc. 22

Manifestations of Disease: Symptoms, Signs, and Syndromes Subjective characteristics of disease felt only by the patient Pain, fatigue, nausea Signs Objective manifestations of disease observed or measured by others Swelling, rash, fever, vomiting Syndrome Symptoms and signs that characterize a disease or abnormal condition Asymptomatic, or subclinical, infections lack symptoms but may still have signs of infection © 2012 Pearson Education Inc. 23

The Nature of Infectious Disease Causation of Disease: Etiology Study of the cause of disease Germ theory of disease Disease caused by infections of pathogenic microorganisms Robert Koch developed a set of postulates to prove a particular pathogen causes a particular disease © 2012 Pearson Education Inc. 24

The Nature of Infectious Disease Causation of Disease: Etiology Exceptions to Koch’s postulates Some pathogens can’t be cultured in the laboratory Diseases caused by a combination of pathogens and other cofactors Pathogens that require a human host Difficulties in satisfying Koch’s postulates Diseases can be caused by more than one pathogen Pathogens that are ignored as potential causes of disease (example is Helicobacter and ulcers) © 2012 Pearson Education Inc. 25

The Nature of Infectious Disease Virulence Factors of Infectious Agents Pathogenicity Ability of a microorganism to cause disease Virulence Degree of pathogenicity Virulence factors contribute to virulence Adhesion factors Biofilms Extracellular enzymes Toxins Antiphagocytic factors © 2012 Pearson Education Inc. 26

The Nature of Infectious Disease Virulence Factors of Infectious Agents Extracellular enzymes Secreted by the pathogen Dissolve structural chemicals in the body Help pathogen maintain infection, invade, and avoid body defenses © 2012 Pearson Education Inc. 28

Virulence Factors of Infectious Agents Toxins Chemicals that harm tissues or trigger host immune responses that cause damage Toxemia refers to toxins in the bloodstream that are carried beyond the site of infection Two types Exotoxins Released by live bacteria Endotoxins Lipid A Released when bacteria die © 2012 Pearson Education Inc. 29

The Nature of Infectious Disease ANIMATION Virulence Factors: Exotoxins © 2012 Pearson Education Inc. 31

The Nature of Infectious Disease ANIMATION Virulence Factors: Endotoxins © 2012 Pearson Education Inc. 32

Virulence Factors of Infectious Agents Antiphagocytic factors Factors prevent phagocytosis by the host’s phagocytic cells Bacterial capsule Composed of chemicals not recognized as foreign Slippery Antiphagocytic chemicals Prevent fusion of lysosome and phagocytic vesicles Leukocidins directly destroy phagocytic white blood cells © 2012 Pearson Education Inc. 33

The Nature of Infectious Disease ANIMATION Virulence Factors: Hiding from Host Defenses © 2012 Pearson Education Inc. 34

The Stages of Infectious Disease The disease process occurs following infection Many infectious diseases have five stages following infection Incubation period Prodromal period – mild symptoms Illness Decline Convalescence - recovery © 2012 Pearson Education Inc. 35

The Movement of Pathogens Out of Hosts: Portals of Exit Pathogens leave host through portals of exit Many portals of exit are the same as portals of entry Pathogens often leave hosts in materials the body secretes or excretes © 2012 Pearson Education Inc. 37

Three groups of transmission Transmission is from a reservoir or a portal of exit to another host’s portal of entry Three groups of transmission Contact transmission Direct, indirect, or droplet Vehicle transmission Airborne, waterborne, or foodborne Vector transmission Biological or mechanical © 2012 Pearson Education Inc. 38

Modes of Infectious Disease Transmission ANIMATION Epidemiology: Transmission of Disease © 2012 Pearson Education Inc. 39

Classification of Infectious Diseases Diseases can be classified in number of ways The body system they affect Their longevity and severity How they are spread to their host The effects they have on populations (rather than on individuals) © 2012 Pearson Education Inc. 40

Terms used to classify infectious disease Acute disease Common cold Chronic disease Hep C, tuberculosis Subacute disease In between acute and chronic Bacterial endocarditis Latent disease herpes Communicable Spread from one infected host to another host Contagious Easily spread © 2012 Pearson Education Inc. 41

Epidemiology of Infectious Diseases ANIMATION Epidemiology: Overview © 2012 Pearson Education Inc. 42

Epidemiology of Infectious Diseases Frequency of Disease Track occurrence of diseases using two measures Incidence Number of new cases of a disease in a given area during a given period of time Prevalence Number of total cases of a disease in a given area during a given period of time Occurrence also evaluated in terms of frequency and geographic distribution © 2012 Pearson Education Inc. 43

Epidemiology of Infectious Diseases ANIMATION Epidemiology: Occurrence of Disease © 2012 Pearson Education Inc. 46

Epidemiological Studies Descriptive epidemiology Careful tabulation of data concerning a disease Record location and time of the cases of disease Collect patient information Try to identify the index case (or first case) of the disease © 2012 Pearson Education Inc. 47

Epidemiological Studies Analytical epidemiology Seeks to determine the probable cause, mode of transmission, and methods of prevention Useful in situations when Koch’s postulates can’t be applied Often retrospective Investigation occurs after an outbreak has occurred © 2012 Pearson Education Inc. 48

Epidemiological Studies Experimental epidemiology Involves testing a hypothesis concerning the cause of a disease Application of Koch’s postulates is experimental epidemiology © 2012 Pearson Education Inc. 49

Hospital Epidemiology: Nosocomial Infections Types of nosocomial infections Exogenous Pathogen acquired from the health care environment Endogenous Pathogen arises from normal microbiota due to factors within the health care setting Iatrogenic Results from modern medical procedures Use of catheters, surgery © 2012 Pearson Education Inc. 50

Presence of microorganisms in hospital environment Figure 14.20 The interplay of factors that result in nosocomial infections Presence of microorganisms in hospital environment Immunocompromised patients Nosocomial infection Transmission of pathogens between staff and patients and among patients

Hospital Epidemiology: Nosocomial Infections Control of nosocomial infections Precautions designed to reduce factors that result in disease Hand washing is the most effective way to reduce nosocomial infections © 2012 Pearson Education Inc. 52

Epidemiology and Public Health Local, state, national, and global agencies share information concerning disease The United States Public Health Service World Health Organization (WHO) Public health agencies work to limit disease transmission Monitor water and food safety Public health agencies campaign to educate the public on healthful choices to limit disease © 2012 Pearson Education Inc. 53