Chapter 19 Lecture Outline See separate Power Point

Chapter 19 Lecture Outline See separate Power. Point slides for all figures and tables preinserted into Power. Point without notes. Copyright © Mc. Graw-Hill Education. Permission required for reproduction or display. 1

A Glimpse of History § Puerperal fever: bacterial infection of uterus • By mid 19 th century, one out of eight women died in Vienna hospitals following childbirth • Ignaz Semmelweis observed puerperal fever • Incidence in one section (run by medical students) was four times second section (run by midwives and midwifery students) • A friend sustained scalpel wound during an autopsy, died of symptoms similar to puerperal fever • Semmelweis reasoned “poison” that killed his friend was transferred to women in childbirth by medical students – Before Pasteur, Koch: germ theory of disease – “Poison” was probably Streptococcus pyogenes • washing hands with strong disinfectant: incidence of puerperal fever dropped to one-third previous level

Epidemiology § Epidemiology is study of distributions and causes of disease in populations • Epidemiologists collect, compile data about sources of disease and risk factors • Design infection control strategies, prevent or predict spread of disease • Expertise in many disciplines including ecology, microbiology, sociology, statistics, and psychology • Many of our daily habits (handwashing, waste disposal) based on epidemiology

19. 1. Basic Concepts of Epidemiology § Communicable (contagious) diseases • Transmitted from one host to another • For example, measles, colds, influenza • Transmission determined by interactions between environment, pathogen, and host • Control of any of these factors may break infection cycle – For example, improved sanitation (prevent infection); antimicrobial medications (kill or inhibit pathogens); vaccination (increase host resistance) § Non-communicable diseases • Do not spread from host to host • Microorganisms most often arise from individual’s normal microbiota or environment (for example, Legionella pneumophila in water systems of buildings)

19. 1. Basic Concepts of Epidemiology § Rates of Disease in a Population • Epidemiologists more concerned with rate than absolute number of cases (consider small vs. large city) • Attack rate is percentage of people who become ill in population after exposure • Reflects infectious dose, immune status of population • Incidence rate is number of new cases/time/population • Measure of risk of an individual contracting a disease • Prevalence is total number of cases at any time or for a specific period in a given population • Reflects overall impact of disease on society; includes old and new cases, as well as duration of disease • Both expressed as cases per 100, 000 people

19. 1. Basic Concepts of Epidemiology § Rates of Disease in a Population (continued…) • Morbidity is incidence of disease in a defined population • Contagious diseases (for example, influenza) often have high morbidity rate: infected individual may transmit to several • Mortality is overall death rate in population • Case-fatality rate is percentage of population that dies from a specific disease • Ebola feared because of very high rate

19. 1. Basic Concepts of Epidemiology § Rates of Disease in a Population (continued…) • Endemic diseases constantly present in population • For example, common cold in United States • Sporadic few cases from time to time • Epidemic is unusually large number of cases • Can be from introduced or endemic disease • Outbreak is group of cases at specific time and population • Pandemic is global • For example, AIDS

19. 2. Chain of Infection

19. 2. Chain of Infection § Reservoirs of Infection • Natural habitat in which pathogen lives • In or on animal, human, or in environment (soil, water) • Identification important in disease control – For example, control of rats, mice, prairie dogs (reservoirs of Yersinia pestis) prevents plague epidemics in U. S. • Human Reservoirs • Only in humans or exist in other animals, environment • Often easier to control if only in humans – For example, smallpox • Symptomatic or asymptomatic

19. 2. Chain of Infection § Reservoirs of Infection (continued…) • Human Reservoirs • Symptomatic infections: obvious source of pathogens • Asymptomatic infections: harder to identify, carriers may not realize, can spread to others – Up to 50% of women infected with Neisseria gonorrhoeae are asymptomatic, easily transmit – Many people carry Staphylococcus aureus • Non-Human Animal Reservoirs • Common (gastrointestinal pathogens, rabies virus) • Zoonoses (zoonotic diseases) exist in mostly in animals, can be transmitted to humans (examples, plague, rabies) • More severe in humans • Environmental Reservoirs • Difficult or impossible to eliminate (Clostridium)

19. 2. Chain of Infection § Portals of Exit and Portals of Entry • Body surface or orifice: entry, exit route for pathogen • Intestinal tract: shed in feces (example, Vibrio cholerae) • Respiratory tract: exit in droplets of saliva, mucus (example, Mycobacterium tuberculosis, respiratory viruses) • Skin: shed on skin cells (example, Staphylococcus aureus) • Genital pathogens: semen, vaginal secretions (example, Neisseria gonorrhoeae) • To cause disease, must be transmitted, also colonize surface or enter host

19. 1. Principles of Epidemiology § Disease Transmission • Vertical transmission is pregnant woman to fetus or mother to infant during childbirth, breast feeding • Horizontal transmission is person to person via air, physical contact, ingestion of food or water, or vector

19. 2. Chain of Infection § Disease Transmission (continued…) • Direct Contact: handshake, sexual intercourse • Infectious dose important (for example, for Shigella is 10– 100 cells) • From hands, can be ingested: fecal-oral transmission • Handwashing considered single most important measure for preventing spread of infectious disease • Some pathogens cannot survive in environment, require intimate sexual contact (Treponema pallidum, Neisseria gonorrhoeae) • Droplet Transmission: respiratory droplets generally fall to ground within a meter from release • Densely populated buildings (schools, military barracks) • Spread minimized by covering mouth when sneezing

19. 2. Chain of Infection § Disease Transmission (continued…) • Indirect Contact • Air: respiratory diseases commonly transmitted • Particles larger than 10 µm usually trapped by mucus • Smaller particles can enter lungs, carry pathogens • Talking, laughing, singing, sneezing, coughing generate – Droplet nuclei (microbes attached to dried material) remain suspended • Number of bacteria in air proportional to number of people • Difficult to control • Ventilation systems, negative pressure, HEPA filters

19. 2. Chain of Infection § Disease Transmission: Vehicle-borne § Fomites: inanimate objects • Clothing, table-tops, doorknobs, drinking glasses • Food and Water: can become contaminated • Animal products (meat, eggs) from animal’s intestines • Cross-contamination: transfer from one food to another • Municipal water systems can distribute to large numbers – For example, Cryptosporidium parvum outbreak in Milwaukee, Wisconsin (1993)

19. 2. Chain of Infection § Disease Transmission: Vector-borne • Vectors: living organisms that can carry pathogen • Most commonly arthropods: mosquitoes, flies, fleas, lice, ticks; can carry internally or externally • Can be mechanical or biological • Vector control important in preventing diseases

19. 3. Factors That Influence the Epidemiology of Disease § Characteristics of the Pathogen • Virulence: ability to cause disease • Factors that allow pathogen to adhere to or penetrate host cell, thwart immune defenses, damage host

19. 3. Factors That Influence the Epidemiology of Disease § Characteristics of the Pathogen (continued…) • The Dose: minimum number of pathogens required • Doses below minimum necessary may produce asymptomatic infection: immune system eliminates organism before symptoms appear • Very large dose (example, laboratory accident) may produce serious disease even in normally immune individual • The Incubation Period: influences extent of spread • Long incubation period can allow extensive spread – 10, 000 individuals drank water containing Salmonella enterica serotype Typhi; 10– 14 day incubation allowed spread of typhoid fever to at least 6 different countries (1963; ski resort in Switzerland)

19. 3. Factors That Influence the Epidemiology of Disease § Characteristics of the Host • Immunity to Pathogen: previous exposure, immunization • Herd immunity protects non-immune individuals in population; >90% immunity typically sufficient • Antigenic variation can overcome (for example, avian influenza) • General Health: malnutrition, overcrowding, fatigue • Developing world more susceptible: crowding, poor food, sanitation • Age: very young, elderly generally more susceptible • Immune system less developed in young; wanes in old • Elderly also less likely to update immunizations

19. 3. Factors That Influence the Epidemiology of Disease § Characteristics of the Host (continued…) • Gender: • Women more likely to develop urinary tract infections – Urethra is shorter; microbes more likely to ascend – Pregnant women more susceptible to listeriosis • Religious and Cultural Practices: • Breastfeeding provides protective antibodies to infant • Consumption of raw fish can increase exposure (for example, freshwater fish and tapeworm Diphyllobothrium latum) • Genetic Background: • Natural immunity varies widely • Specific receptors may differ (examples, lack of receptor on red blood cell yields immunity to Plasmodium vivax; lack of receptor on white blood cell reduces susceptibility to HIV)

19. 3. Factors That Influence the Epidemiology of Disease § Characteristics of the Environment • Temperature, water and nutrient supply, radiation, and the availability of light and O 2 • Crowding and poor sanitation • Endospores allow survival in damaging conditions • Exposure to antibiotics

19. 4. Epidemiological Studies • John Snow documented 1849– 1854 cholera epidemics in England due to contaminated water supplies • Demonstrated importance of epidemiologic studies long before relationship between microbes, disease accepted • Descriptive Studies: data collected following outbreaks • The Person: age, gender, ethnicity, occupation, personal habits, previous illnesses, socioeconomic class, marital status may all yield clues about risk • The Place: geographic location helps pinpoint source, yield clues about potential reservoirs, vectors, or boundaries that might affect transmission

19. 4. Epidemiological Studies • Descriptive Studies (continued…) • The Time: season important; also rate of spread – Propagated epidemic: slow rise in cases suggests contagious disease spreading in population; first case is called index case – Common-source epidemic: rapid rise in cases suggests exposure to single source of pathogen

19. 4. Epidemiological Studies § Analytical Studies • Determine relevancy of risk factors • Case-control study attempts to identify causative chain of events leading to disease • Cross-Sectional Studies: surveys range of people – May suggest associations between risk factors, disease • Cohort studies with known exposure to risk factor are selected and followed over time

19. 4. Epidemiological Studies § Experimental Studies • Judge cause/effect relationship of risk (or preventative) factors and development of disease • Treatment compared with known treatment or placebo • Double-blind studies conducted to avoid bias

19. 5. Infectious Disease Surveillance § National Disease Surveillance Network • Centers for Disease Control and Prevention publishes Morbidity and Mortality Weekly Report (MMWR)

19. 5. Infectious Disease Surveillance § National Disease Surveillance Network (cont…) • Public Health Departments in each state • Have authority to mandate diseases that must be reported • Washington State health authorities responded rapidly to Escherichia coli O 157: H 7 outbreak in 1993 – One of few states with surveillance and reporting for E. coli O 157: H 7; epidemic had started in other states • Fungus Cryptococcus gattii first appeared in WA in 2007, causes lung infections, meningitis; OR, CA now monitor • Other components of the Public Health Network • Public schools report absentee rates • Hospital laboratories report on isolation of pathogens with epidemiological significance • News media alert public to presence of infectious diseases

19. 5. Infectious Disease Surveillance § Worldwide Disease Surveillance • World Health Organization (WHO) has four main functions – Provide worldwide guidance in field of health – Set global standards for health – Cooperatively strengthen national health programs – Develop and transfer appropriate health technology • WHO provides education, technical assistance • Disseminates information via periodicals, books – For example, Weekly Epidemiological Record

19. 5. Infectious Disease Surveillance § Reduction and Eradication of Disease • Many diseases reduced through improved sanitation, reservoir and vector control, vaccination, and antibiotic treatment; smallpox has been eradicated globally – In U. S. , many formerly common diseases rare – Work underway to eradicate measles, polio, dracunculiasis • New pathogens emerging • Resurgence of infectious diseases

19. 6. Emerging Infectious Diseases § Include new or newly recognized Middle East respiratory syndrome (MERS), mad cow disease, avian flu; also reemerging malaria and tuberculosis • Many factors contribute to emergence and reemergence • Microbial evolution: example, Vibrio cholerae O 139 gained ability to produce capsule; antibiotic resistance • Complacency and public health efforts: successes can divert attention: example, tuberculosis • Changes in human society: example, day care centers relatively new in U. S. allow easy transfer of pathogens

19. 6. Emerging Infectious Diseases § Many factors contribute to emergence and reemergence • Advances in technology: example, contact lenses • Population expansion: increased contact with reservoirs of disease: Ebola virus, Lyme disease • Development: example, dams extending range of schistosomiasis by increasing habitat for host snail • Mass production, widespread distribution, and importation of food: foodborne illnesses easily spread • War and civil unrest: disruption of infrastructure and eradication efforts, refugee camps with poor conditions • Climate changes: warm temperatures favor reproduction of some arthropods that serve as vectors; heavy flooding from El Niño may have increased cholera cases in Africa

19. 7. Healthcare-Associated Infections § Healthcare-associated infections (HAIs) • Acquired while receiving treatment in healthcare setting • One of top 10 causes of death in United States • Hospital-acquired nosocomial infections problematic throughout history: hospitals are densely populated with unusually susceptible people, where resistant and virulent pathogens may exist • Estimated 5– 10% of patients admitted in U. S. acquire – About 2/3 are from patients’ own normal microbiota

19. 5. Healthcare-Associated Infections

19. 7. Healthcare-Associated Infections § Reservoirs of Infectious Agents in Healthcare Settings • Other Patients: pathogens discharged via skin cells, respiratory droplets, other bodily secretions and excretions • Healthcare Environment: some Gram-negative rods can thrive in sinks, respirators, toilets – Pseudomonas aeruginosa resistant to many disinfectants, antimicrobials; requires few nutrients – Many nosocomial infections traced to contaminated soaps, disinfectants, other aqueous solutions • Healthcare Workers: sick workers, carriers of pathogens (examples, Staphylococcus aureus, Streptococcus pyogenes) • Patient Microbiota: invasive procedures can transmit normal microbiota to sterile body sites; compromised immune system may allow infection to develop

19. 7. Healthcare-Associated Infections § Transmission of Infectious Agents in Healthcare Settings • Direct Transmission—Healthcare Personnel – Must be extremely vigilant with handwashing, disinfecting, wearing gloves • Fomite Transmission—Medical Devices – Often breach first-line barriers of defense catheters, IV, mechanical respirators, inadequately sterilized invasive instruments • Airborne Transmission – Reduce with air pressure, careful mopping, HEPA filters

19. 7. Healthcare-Associated Infections § Preventing Healthcare-Associated Infections • Important to detect, establish appropriate policies to stop • Hospitals have Infection Control Committee; implement policies based on Standard Precautions and the Transmission-Based Precautions – Includes guidelines on hand hygiene, personal protective equipment, respiratory hygiene/cough etiquette, patient placement, patient-care equipment and instruments/devices, care of the environment, textiles and laundry, safe injection practices, infection control practices for special lumbar puncture procedures, and worker safety • CDC has established Healthcare Infection Control Practices Advisory Committee (HICPAC)