Principles of Disease and Epidemiology

1. Principles of Disease and Epidemiology

2. Pathology • Branch of medicine to study disease • Cause oretiologyof disease • Manner which disease develops • Structural and functional changes by disease and effects on host

3. Infection and Disease • Exposure – left unprotected to contact with MO • Infection - invasion or colonization of body by pathogenic MO • Disease - any change from normal state of health or abnormal state, in which body not properly adjusted or not capable of carrying on normal function

4. Infection and Disease • Infection not necessarily indicate disease • Normal flora - may colonize or infect specific areas of body without causing disease; may however cause disease given right circumstances • Person infected with MO (i.e., HIV) but show no symptoms of disease

5. Normal Flora: Opportunist • E. coli NF in stool, but #1 cause of urinary tract infection • Viridans group Streptococcus NF in oropharyngeal area, but cause subacute bacterial endocarditis when gains entrance into bloodstream • Considered opportunistic pathogen

6. Normal Flora: Carriers • In addition to usual NF, some persons colonized by MO generally considered pathogenic, but without disease • These individuals are carriersand transmit pathogenic MO to others who develop disease • Neisseria meningitidis – URT • Salmonella typhi – GI tract, gall bladder (reservoir site)

7. Types of Infectious Disease • Infectious disease – by disease producing MO • Congenital disease – present at birth and result of condition in utero (maternal infection, use of drug or alcohol, etc.); may result in birth defects • Iatrogenic disease – caused by health care personnel during delivery of care; could be due to use of contaminated equipment, administration of drug, etc. • Nosocomical disease - hospital acquired disease • Idiopathic disease – undetermined cause

8. Nosocomial Infections • Hospitals provide perfect environment for MOs, in part because patient with weakened condition and more susceptible to infection and disease • Up to 15% of hospitalized patients get a nosocomial infection • Most nosocomial infections caused by opportunistic pathogens typically considered NF

9. Types of Nosocomial Infections Septicemia

10. Koch’s Postulates: Etiology of Infectious Disease • 1 – Pathogen present in every case of disease • 2 - Pathogen isolated from diseased host and grown in pure culture • 3 - Pathogen must cause same disease when inoculated into healthy, susceptible host • 4 - Pathogen must again be isolated from inoculated animal, shown to be same as originally isolated MO • Not possible to identify etiology of all infectious diseases following Koch’s postulates: • MO not isolated on culture media • MO not inoculated into healthy human host to cause disease

11. Epidemiology • Study of when and where diseases occur and how transmitted in a population • Also concerned with methods of controlling and preventing a disease; drugs, vaccines, and reservoirs • State and Federal Public Health Department • The Centers for Disease Control and Prevention, (CDC), Atlanta, Georgia • The World Health Organization (WHO), within the United Nations

12. Spread of Disease: Reservoirs • For a disease to perpetuate, must be a source of infection called a reservoir • Humans – Carriers or sick individuals during incubation, acute, or convalescent phases (when transmitted depends on disease) • Animals – diseases found in animals called zoonosis • Non-living entities - such as soil or water

13. Spread of Disease: Transmission • Causative agents transmitted from the reservoir of infection to a susceptible host via four main routes: • 1. Contact • 2. Common Vehicle • 3. Airborne • 4. Vector

14. 1. Contact Transmission • Direct contact – spread from person to person via touching, kissing, or sexual intercourse • Indirect contact – to a susceptible host by means of non-living object called fomite(i.e., contaminated needles) • Droplet transmission – spread by droplets in saliva and mucous; discharged by sneezing, coughing, laughing, or talking. The droplets travel only short distance (< 1 meter) and not considered airborne

15. 2. Common Vehicle Transmission • Refers to transmission of disease causing agents by a common inanimate reservoir to a large number of individuals • Food • Water • Blood • Drugs

16. 3. Airborne Transmission • Refers to spread of agents of infection by droplet nuclei or dust • In this instance the particles travel > 1 meter from the reservoir to the host • i.e., air condition unit

17. 4. Vector Transmission • An animal that carries pathogen from one host to another • Arthropods are most common type • Transmit disease either by: • Mechanical – pathogen on feet or other body parts to food ingested by humans • Biological – arthropod bites infected host, acquires the pathogen, MO reproduceinside the vector, pathogen transmitted to new host via salivary glands or feces

18. Infectious Disease: How Spread • Communicable disease – spreads from one host to another either directly or indirectly • Noncommunicable disease – doesn’t spread from host to host but caused by MO: • Normally inhabits body and occasionally causes disease • Resides outside the body and produces disease when introduced into the body • Contagious disease – a disease that spreads easily from one person to another

19. Infectious Disease: Severity/Duration • Acute disease – develops rapidly, lasts relatively short time i.e. “cold” • Chronic disease – develops slowly, body’s reaction usually less severe, but disease is continuous or recurrent for long period i.e. TB • Subacute disease – intermediate between acute and chronic i.e. “infectious” hepatitis • Latent disease – MO remains inactive for long period, then becomes active and produce symptoms i.e. chickenpox/shingles

20. Infectious Disease: Frequency of Occurrence • Sporadic– occurs occasionally i.e. measles in USA • Endemic – constantly present in population i.e. TB inner city LA • Epidemic – many people in a given area acquire disease in short period of time i.e. pertussis in USA 2010 • Pandemic – worldwide epidemic i.e. flu

21. Infectious Disease: Health of Body • Primary infection – acute infection causes initial illness • Secondary infection – by opportunistic pathogen after primary infection has weakened the body’s defenses • Inapparent or subclinical infection – doesn’t cause any noticeable illness (i.e., healthy carriers)

22. Infectious Disease: Signals • Symptoms – subjective changes in body function such as pain or malaise (not measurable) • Signs – objective changes that can be observed and measured; such as fever, swelling, or rash • Syndrome – a group of symptoms and signs that always accompany a particular disease

23. Infectious Disease: Stages • Period of incubation – time between acquiring infection and appearance of first symptoms/signs • Prodromal period – first signs/symptoms appear • Period of illness – disease most acute, overt signs/symptoms occur (increase or decrease in WBCs may occur here)

24. Infectious Disease: Stages • Period of decline – where signs/symptoms subside • If the decline occurs quickly, it is said to occur by crisis • If the decline occurs over longer period of time, it is said to occur by lysis • Period of convalescence - where person regains strength and body returns to pre-diseased state

25. Class Assignment • Textbook Reading: • Chapter 2 Host- Pathogen Interaction • B. Pathogenesis of Infection • Routes of Transmission • Key Terms • Learning Assessment Questions • Review, Review, Review!