Microbiology

B.E Pruitt & Jane J. Stein

AN INTRODUCTIONEIGHTH EDITION

TORTORA • FUNKE • CASE

Chapter 14

Principles of Disease and Epidemiology

Principles of Disease and Epidemiology

• Pathology: Study of disease• Disease: in a state of not being healthy• Pathogens: disease causing organisms• Etiology: Study of the cause of a disease• Pathogenesis: Development of disease• Infection : Colonization or invasion of

pathogens, may be microbes in the wrong place - Example: (E. coli in the urinary tract)

• Normal Flora - the normal bacteria in you and on you– You have 1013 eucaryotic cells and 1014 prokaryotic cells– Within 8 - 12 hours of life you are colonized by normal flora

(microbiota). • Breast feeding versus bottle - different organisms

• Transient microbiota may be present for days, weeks, or months

• Microbial antagonism– Normal microbiota overwhelm pathogens - no place for them

to colonize• Intestines and vagina - excessive antibiotics disrupts balance

– Vagina normally pH ~ 4 with Lactobacillus spp without can lead to Candida infections

• Symbiosis is the relationship between normal microbiota and the host

Normal Microbiota (Flora) and the Host

• In commensalism, one organism is benefited and the other is unaffected.

• In mutualism, both organisms benefit.• Be able to give a few examples of mutualistic bacteria

• In parasitism, one organism is benefited at the expense of the other.

• Some normal microbiota are opportunistic pathogens.– E. coli and urinary tract– Pneumocystis carinii and respiratory system– Streptococcus pneumoniae and pneumonia

Normal Microbiota and the Host:

Figure 14.2

• Locations of normal microbiota on and in the human body

Normal Microbiota and the Host:

• Microbial antagonism is competition between microbes.

• Normal microbiota protect the host by: – occupying niches that pathogens might occupy– producing acids– producing bacteriocins

• Probiotics are live microbes applied to or ingested into the body, intended to exert a beneficial effect.– Lactobacillus spp

Normal Microbiota and the Host:

• Koch's Postulates are used to prove the cause of an infectious disease.

Koch’s Postulates

Figure 14.3.1

• Koch's Postulates are used to prove the cause of an infectious disease.

• Problems with Koch’s Postulates:• Not all diseases have bacterial

etiologies– Genetic– Degenerative– Congenital

• Exceptions– Not culturable

• Treponema / Rickettsia / Chlamydia / viruses

– Some pathogens cause many different diseases

Koch’s Postulates

Figure 14.3.2

Classifying Infectious Diseases

• Symptom: A change in body function that is felt by a patient as a result of disease

• Sign: A change in a body that can be measured or observed as a result of disease.

• Syndrome: A specific group of signs and symptoms that accompany a disease.

Classifying Infectious Diseases

Diseases may be grouped by how they spread

• Communicable disease A disease that is spread from one host to another.

• Contagious disease A disease that is easily spread from one host to another.

• Noncommunicable disease A disease that is not transmitted from one host

to another. Example:Clostridium tetani

• Incidence: Fraction of a population that contracts a disease during a specific time.

• Prevalence: Fraction of a population having a specific disease at a given time.

• Sporadic disease: Disease that occurs occasionally in a population.

• Endemic disease: Disease constantly present in a population.

• Epidemic disease: Disease acquired by many hosts in a given area in a short time.

• Pandemic disease: Worldwide epidemic.

• Herd immunity: Immunity in most of a population.

By occurrence of Disease

• Acute disease: Symptoms develop rapidly

• Chronic disease: Disease develops slowly

• Subacute disease: Symptoms between acute and chronic

• Latent disease: Disease with a period of no symptoms when the patient is inactive Shingles

By Severity or Duration of a Disease

• Local infection: Pathogens limited to a small area of the body

• Systemic infection: An infection throughout the body

• Focal infection: Systemic infection that began as a local infection

• Bacteremia: Bacteria in the blood

• Septicemia: Growth of bacteria in the blood

By Extent of Host Involvement

• Toxemia : Toxins in the blood• Viremia: Viruses in the blood• Primary infection: Acute infection that causes the

initial illness• Secondary infection: Opportunistic infection after a

primary (predisposing) infection – examples: Pneumocystis pneumonia and AIDS

• Subclinical disease: No noticeable signs or symptoms (inapparent infection) – Examples: Hepatitis/Typhoid mary / Polio

Extent of Host Involvement

• Make the body more susceptible to disease– Short urethra in females– Inherited traits such as the sickle-cell gene– Climate and weather– Fatigue and Stress– Age– Lifestyle– Chemotherapy– Gender

Predisposing Factors

Development of DiseaseStages of Disease

1)Incubation: infection up to first symptoms may or may not be variable

2)Prodromal: short period of early mild symptoms and malaise

3)Period of Illness: overt signs; ie: fever and chills, swollen lymph nodes, GI disturbance increase in WBC’s

4)Period of Decline: signs and symptoms subside – Susceptible to 2˚ infections

5)Period of Convalescence: regain strength and recovery BUT maybe reservoir

The Stages of a Disease

Figure 14.5

• Reservoirs of infection are continual sources of infection.– Human — AIDS, gonorrhea

• Carriers may have inapparent infections or latent diseases. • Carriers may be in pre-symptom stage or recovery of a disease -

no symptoms

– Animal — Rabies, Lyme disease• Some zoonoses may be transmitted to humans

– Plague / psittacosis / swine flu / bird flu

– Nonliving — Botulism, tetanus• Soil• Water - rivers, lakes, snow, oceans and laundry water

Reservoirs of Infection

• Three main routes– Contact - Direct or indirect– Vehicles - inanimate objects - e.g. food or drugs– Vectors - arthropods

Types of Contact– Direct: Requires close association between

infected and susceptible host– Indirect: Spread by fomites (inanimate objects)

glass, toothbrush or clothing– Droplet: Transmission via airborne droplets

Transmission of Disease

Transmission of Disease

Figure 14.6a & 8

• Vehicle: Transmission by an inanimate reservoir (food, water) Shigella, cholera, airborne on dust aerosol >3’, tapeworm Staphylococci, Streptococci, tuberculosis, fungal spores (histoplasmosis, coccidiodomycosis)

• Vectors: Arthropods, especially fleas, ticks, and mosquitoes– Mechanical: Arthropod carries pathogen on

feet– Biological: Pathogen reproduces in vector

and bites host. Dengue fever, Yellow fever, Malaria, encephalitis, plague, Lyme disease, and RMSF

Transmission of Disease

Transmission of Disease

Figure 14.6b, c

Figure 14.7, 9

• Are acquired as a result of a hospital stay• 5-15% of all hospital patients acquire nosocomial

infections

Nosocomial (Hospital-Acquired) Infections

Nosocomial Hospital acquired

• 5 - 15% acquire • >20,000 per year die ……. why?

a) microbes in environment -- (lots of sick people)

b) already sick or wounded -- compromised host

c) close to people - chain of transmission• Also resistant strains - E. coli, Pseudomonas, enterics

like Serratia• Control by aseptic techniques• Antibiotic abuse

Figure 14.10

Relative frequency of nosocomial infections

Common Causes of Nosocomial Infections

Percentage of nosocomial infections

Percentage resistant to antibiotics

Gram + cocciStreptoccous and Staphylococcus

34% 28%-87%

Gram – rodsEnterics and Pseduomonas

32% 3-34%

Clostridium difficile 17%

Fungi 10%

• Diseases that are new, increasing in incidence, or showing a potential to increase in the near future.

• Contributing factors:– Evolution of new strains

• V. cholerae O139

– Inappropriate use of antibiotics and pesticides• Antibiotic resistant strains

– Changes in weather patterns• Hantavirus

– Spread of human populations and travel

Emerging Infectious Diseases

• Contributing factors:– Modern transportation

• West Nile virus

– Ecological disaster, war, expanding human settlement

• Coccidioidomycosis

– Animal control measures• Lyme disease

– Public Health failure• Diphtheria

Emerging Infectious Diseases

• The study of where and when diseases occur

Epidemiology

Figure 14.11

Epidemiology

John Snow 1848-1849 Mapped the occurrence of cholera in London

Ignaz Semmelweis 1846-1848 Showed the hand washing decreased the incidence of puerperal fever

Florence Nightingale 1858 Showed that improved sanitation decreased the incidence of epidemic typhus

• Descriptive Collection and analysis of data regarding occurrence of disease

Snow

• Analytical Comparison of a diseased group and a healthy group

Nightingale

• Experimental Study of a disease using controlled experiments

Semmelweis

• Case reporting Health care workers report specified disease to local, state, and national offices

• Nationally Notifiable Diseases

Physicians are required to report occurrence

Table 14.7

• Collects and analyzes epidemiological information in the U.S.

• Publishes Morbidity and Mortality Weekly Report (MMWR) www.cdc.gov

Morbidity: incidence of a specific notifiable diseaseMortality: deaths from notifiable diseasesMorbidity rate = number of people affected/total

population in a given time periodMortality rate - number of deaths from a disease/total

population in a given time

Centers for Disease Control and Prevention (CDC)