Diseases and Epidemiology

Epidemiologists—what do they do?

• They identify a problem, collect data, formulate and test hypothesis

• Epidemiology is as old as medicine

• Who gets sick, why and when

• John Snow (Broadway pump) can be considered as the father of Epidemiology

• Determine the problem (outbreak) and implement control measurements to prevent additional disease

• Often described as “disease detective”

Definitions

Disease: occurs when an infection results in an abnormal state of health

Pathology: concerned with the cause of disease

Infection: is the invasion or colonization of the body by pathogenic microorganism

Normal Microbiota

•Animals, including humans, are generally free of microbes in utero

•At birth, a normal/characteristic microbial population will begin to establish itself

•The newborn’s first contact with MO is usually lactobacilli, which multiply rapidly prior birth

•More MO are introduced as the newborn breathes and feeding begins

Normal Microbiota•Many usually harmless MO establish themselves inside other parts of the adult body & on its surface

•Typical human body contains 1x1013 body cells, yet harbors 1x1014 bacteria

•Normal flora does not produce disease

•Transient microbiota are present for days/weeks/or months and then disappear

•Microbial antagonists: once established the normal biota can benefit the host by preventing the overgrowth of harmful MO

•It involves competition among MO

Normal Microbiota

•Presence of normal microbiota in the adult human vagina maintains a local pH of 3.5-4.5 & prevents overgrowth of the yeast Candida albicans

•If the bacterial population is eliminated by antibiotics, excessive douching or deodorants, the pH of the vagina reverts to neutral and C. albicans flourishes

Relationship btw the normal microbiota and the host

Symbiosis: living together---relationship btw host & normal microbiota

Commensalism: one of the organism is benefiting –the other one is unaffected

Mutualism: is a type of symbiosis that benefits both organisms I.e. the large intestine has E.coli, that synthesizes vitamin K and some B vitamins

These vitamins are absorbed into the bloodstream

Definitions

•Opportunistic pathogens: they do not cause disease in their normal habitat in a healthy person, but will in a different environment

•MO that gain access through broken skin or mucous membranes can cause opportunistic infections

•Or if the host is already weakened or compromised by infection (AIDS)

Opportunists

The Etiology of Infectious Diseases

•Some disease—polio/Lyme disease or TB have a well known etiology

•The etiology of Alzheimer’s disease is unknown

•Hemophilia is an inherited disease

•Osteroarthritis are degenerative diseases

Koch’s postulates•In 1877 he published some early papers on anthrax, a disease of cattle that can occur in humans

•He demonstrated that Bacillus anthracis, were always present in the blood of animals that had the disease and were not present in healthy animals

•Koch showed that a specific infectious disease (anthrax) is caused by a specific MO that can be isolated and cultured on artificial media

Koch’s 4 postulates1. Same pathogen must be present in every

case of the disease

2. Pathogen must be isolated from the diseased host & grown in pure culture

3. Pathogen from pure culture must cause disease when it is inoculated in a healthy animal

4. Pathogen must be isolated from inoculated animal & must be shown to be the original MO

Classifying Infectious Diseases•Symptoms: changes in body function (pain/malaise)

•Signs: are objective changes the physician can observe & measure

•Syndrome: specific group of symptoms or signs

•Communicable disease: disease that spreads from one host to another, either directly or indirectly (chickenpox, measles, gentical herpes)

Sporadic Disease : typhoid fever in the US—disease occurs occasionally

Endemic: a disease constantly common in a poplulationcommon cold

Epidemic : influenza

Pandemic : an epidemic disease that occurs worldwide (influenza)

Patterns of Disease

Reservoir: for a disease to perpetuate itself there must be a continuous source either living or an inanimate object that provides the pathogen with adequate conditions for survival & multiplication

Development of Disease

Communicable disease: disease spreads from host to host (TB, typhoid fever, gential herpes)

Noncommunicable diseases: Clostridium tetani produces disease only when introduced into the body via abrasions or wounds

Classifying Infectious Diseases

Acute disease: develops rapidly but lasts only a short time (influenza)

Chronic disease: develops more slowly, and the body’s rx’s may be less severe (Mono, TB and Hepatitis B)

Primary infection: is an acute infection that causes initial illness

Secondary infection: is one caused by an opportunistic pathogen after primary infection has weakened the body’s defenses (Pneumocystic pneumonia –AIDS)

3 types of transmission categories

Direct contact: person-to-person (touching, kissing) (common cold/influenza, measles, scarlet fever)

Indirect contact (fomites):contaminated syringes, toys, thermometers (Hep B and tetanus)

Droplet transmission: mucus droplets that travel short distances (1m)—sneeze contains 20,000 droplets (influenza, pertussis (whooping cough)

5 types of vehicle transmission

Waterborne: pathogens are spread by contaminated H2O or poorly treated sewage

Food-borne: foods that are incompletely cooked, poorly refrigerated or prepared under unsanitary conditions

Foodborne: Mother was changing the diapers of her infant and returned to canning products for the church picnic and the individual purchasing the last batch of the canned items became sick;

                     

 

Foodborne infections: Didn't anyone know?

Airborne transmission: spread of agent by droplet nuclei in dust that travels >1m

Mechanical transmission: passive transport of the pathogen on the insect’s feet or other body parts (houseflies can transfer typhoid fever and bacillary dysentery (shigellosis) from the feces

Biological transmission: active process/ is more complex---arthropod (insect) bites an infected person or animal & ingests some of the infected blood—(Plague/Lyme disease)

Nosocomial infections: infection acquired while staying in the hospital or nursing homes

•Prevention: aseptic techniques such as Lister and Semmelweis & use of disposable materials

•2 Mio people per year contract nosocomial infections & nearly 90,000 die as a result

•MO in the hsp environment

•Compromised status of host

•Chain of transmission

Combo of all 3 poses significant risk

Four different stages in the development of disease

1. Compromised host (cancer, surgery, HIV+)

2. Microorganisms in the hospital (antibiotic resistant ones & higher prevalence due to environment)

3. Nosocomial infections (urinary tract, surgical site, lower respiratory, bacteremia & skin infections)

4. Chain of transmission (direct/indirect contact w/ medical personel, fomites & ventilation system)