Principles of Disease and Epidemiology

Pathology Branch of medicine to study disease Cause or etiology of disease Manner which disease develops Structural and functional changes by disease and effects on host

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

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

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

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

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

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

Types of Nosocomial Infections Septicemia

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

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

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

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

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

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

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

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 reproduce inside the vector, pathogen transmitted to new host via salivary glands or feces

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

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

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

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)

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

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)

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

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