ch14 lect principles of disease and epidemiology
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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)
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