disease – epidemiology and control introduction to microbiology chapters 14 and 20

Disease – Epidemiology and Control

Introduction to MicrobiologyChapters 14 and 20

Epidemiology

• The study of where and when diseases occur• Centers for Disease Control and Prevention

(CDC)– Collects and analyzes epidemiological information in

the United States– Publishes Morbidity and Mortality Weekly Report

(MMWR) – www.cdc.gov

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

Ignaz Semmelweis 1846–1848 Showed that handwashing decreased the incidence of puerperal fever

Florence Nightingale

1858 Showed that improved sanitation decreased the incidence of epidemic typhus

Epidemiology

Epidemiology

• Descriptive: Collection and analysis of data– Snow

• Analytical: Comparison of a diseased group and a healthy group– Nightingale

• Experimental: 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

Epidemiology

The CDC

• Morbidity: Incidence of a specific notifiable disease

• Mortality: Deaths from notifiable diseases• Morbidity rate: Number of people affected in

relation to the total population in a given time period

• Mortality rate: Number of deaths from a disease in relation to the population in a given time

Pathology, Infection, and Disease

• Pathology: The study of disease• Etiology: The study of the cause of a disease• Pathogenesis: The development of disease• Infection: Colonization of the body by pathogens• Disease: An abnormal state in which the body is

not functioning normally

Normal Microbiota and the Host

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

• Normal microbiota permanently colonize the host

• Symbiosis is the relationship between normal microbiota and the host

Representative Normal Microbiota

Figure 14.1

Symbiosis

• In commensalism, one organism benefits, and the other is unaffected

• In mutualism, both organisms benefit• In parasitism, one organism benefits at the

expense of the other• Some normal microbiota are opportunistic

pathogens

Normal Microbiota and the Host

• Microbial antagonism is a competition between microbes.

• Normal microbiota protect the host by– Occupying niches that pathogens might occupy– Producing acids– Producing bacteriocins

• Probiotics: Live microbes applied to or ingested into the body, intended to exert a beneficial effect

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

• 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

Occurrence of a Disease

• 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

Occurrence of a Disease

• 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

Severity or Duration of a 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 causative agent is inactive

Extent of Host Involvement

• Local infection: Pathogens are 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

• Superinfection: occurs when a pathogen develops resistance to the drug being used for treatment or when normally resistant microbiota multiply excessively, adding to infection.


• Sepsis: Toxic inflammatory condition arising from the spread of microbes, especially bacteria or their toxins, from a focus of infection

• Bacteremia: Bacteria in the blood• Septicemia: Growth of bacteria in the blood


• 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• Subclinical disease: No noticeable signs or

symptoms (inapparent infection)

Predisposing Factors

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

The Stages of a Disease

Figure 14.5

The Spread of Infection

• Reservoirs of Infection– Continual sources of infection

• Human: AIDS, gonorrhea– Carriers may have inapparent infections

or latent diseases

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

• Nonliving: Botulism, tetanus– Soil

The Spread of Infection – Transmission Types

• Contact• Vehicle• Vector

Transmission of Disease

• Contact– Direct: Requires close association between infected

and susceptible host– Indirect: Spread by fomites

• Fomite – inanimate object that can spread disease.– Example: toys, clothing, utensils, etc.

– Droplet: Transmission via airborne droplets


• Vehicle– Contact with food, water, other liquids

• These are constantly taken into the body, so they serve as “vehicles” into the body.


• Vector– Transmission from an animal (insect)


Figure 14.6a, d

Vehicle Transmission

• Transmission by an inanimate reservoir (food, water, air)

Figure 14.7b

Nosocomial Infections

• Are acquired as a result of a hospital stay• Affect 5–15% of all hospital patients

Figure 14.6b


Figure 14.9


Table 14.5

Percentage of Total Infections

Percentage Resistant to Antibiotics

Coagulase-negative staphylococci

25% 89%

S. aureus 16% 80%

Enterococcus 10% 29%

Gram-negative rods 23% 5-32%

C. difficile 13% None

Common Causes of Nosocomial Infections

MRSA

• USA100: 92% of health care strains • USA300: 89% of community-acquired strains

Clinical Focus, p. 422

Which Procedure Increases the Likelihood of Infection Most?


Emerging Infectious Diseases

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


• Contributing factors– Genetic recombination

• E. coli O157, avian influenza (H5N1)

– Evolution of new strains• V. cholerae O139

– Inappropriate use of antibiotics and pesticides• Antibiotic-resistant strains

– Changes in weather patterns• Hantavirus


• Modern transportation– West Nile virus

• Ecological disaster, war, and expanding human settlement– Coccidioidomycosis

• Animal control measures– Lyme disease

• Public health failure– Diphtheria

Crossing the Species Barrier


Treatment – Antibiotics/Antimicrobials

• Chemotherapy: The use of drugs to treat a disease

• Antimicrobial drugs: Interfere with the growth of microbes within a host

• Antibiotic: A substance produced by a microbe that, in small amounts, inhibits another microbe

• Selective toxicity: A drug that kills harmful microbes without damaging the host

• 1928: Fleming discovered penicillin, produced by Penicillium

• 1940: Howard Florey and Ernst Chain performed first clinical trials of penicillin

Antimicrobial Drugs

Figure 1.5

Most Antibiotics come from other Microorganisms

The Spectrum of Antimicrobial Activity

The Spectrum of Antimicrobial Activity

• Broad spectrum – Treats a variety of microbes

• Narrow spectrum– Treats few microbes

• Bactericidal– Kill microbes directly

• Bacteriostatic

– Prevent microbes from growing

• Ideally, an antimicrobial should target the pathogen only and not disrupt normal microbiota

– Risk vs. Benefit Analysis

The Action of Antimicrobial Drugs

The Action of Antimicrobial Drugs

Commonly used Antimicrobials – Modes of Action

- Inhibitors of Cell Wall Synthesis- Antimycobacterial Antibiotics- Inhibitors of Protein Synthesis - Injury to the Plasma Membrane - Inhibitors of Nucleic Acid (DNA/RNA)

Synthesis- Competitive Inhibitors of the Synthesis of

Essential Metabolites

Commonly Used Antimicrobials

• Antibacterial Antibiotics: Inhibitors of Cell Wall Synthesis– All penicillins (natural and semisynthetic)– Carbapenems– Cephalosporins– Bacitracin – Vancomycin


• Antimycobacterial Antibiotics– Isoniazid (INH) and ethambutol inhibit cell wall

synthesis in mycobacteria.


• Inhibitors of Protein Synthesis – Chloramphenicol, aminoglycosides, tetracyclines,

macrolides, and streptogramins inhibit protein synthesis at 70S ribosomes.

– Oxazolidinones prevent formation of 70S ribosomes.


• Injury to the Plasma Membrane – A new class of antibiotics inhibits fatty-acid

synthesis, essential for plasma membranes.• Polymyxin B and bacitracin cause damage to plasma

membranes.


• Inhibitors of Nucleic Acid (DNA/RNA) Synthesis

• Rifamycin inhibits mRNA synthesis; it is used to treat tuberculosis.

• Quinolones and fluoroquinolones inhibit DNA gyrase for treating urinary tract infections.


• Competitive Inhibitors of the Synthesis of Essential Metabolites– Sulfonamides competitively inhibit folic acid

synthesis.

Antiviral Drugs

• Protease inhibitors– Indinavir: HIV

• Integrase inhibitors– HIV

• Inhibit attachment– Zanamivir: Influenza– Block CCR5: HIV

• Inhibit uncoating– Amantadine: Influenza

• Nucleoside and Nucleotide Analogs– Inhibit DNA/RNA synthesis

• Enzyme inhibitors– Inhibit fusion, Inhibit attachment, Inhibit uncoating

• Interferons– Prevent spread of viruses to new cells

• Metronidazole

– Damages DNA– Entamoeba, Trichomonas

• Nitazoxanide

– Interferes with metabolism of anaerobes

Antiprotozoan Drugs

Antihelminthic Drugs

• Niclosamide– Prevents ATP generation

• Tapeworms

• Praziquantel– Alters membrane

permeability• Flatworms

Figure 12.26

Antihelminthic Drugs

Figure 12.28a

• Mebendazole– Inhibits nutrient

absorption• Intestinal roundworms

• Ivermectin– Paralyzes worm

• Intestinal roundworms

Testing Effectiveness of Treatment

• Disk Diffusion Test (Kirby Bauer)• Broth Dilution Test• MIC (minimal inhibitory concentration) Test

– The E-test– Microtitre plates

The Disk-Diffusion Method

Figure 20.17

The E Test

Figure 20.18

Microtitre Plate

Figure 20.19

Antibiotic Resistance

• A variety of mutations can lead to antibiotic resistance

• Mechanisms of antibiotic resistance1. Enzymatic destruction of drug2. Prevention of penetration of drug3. Alteration of drug's target site4. Rapid ejection of the drug

• Resistance genes are often on plasmids or transposons that can be transferred between bacteria

Antibiotic Resistance

• Misuse of antibiotics selects for resistance mutants. Misuse includes– Using outdated or weakened antibiotics– Using antibiotics for the common cold and other

inappropriate conditions– Using antibiotics in animal feed– Failing complete the prescribed regimen– Using someone else's leftover prescription

Effects of Combinations of Drugs

• Synergism occurs when the effect of two drugs together is greater than the effect of either alone

• Antagonism occurs when the effect of two drugs together is less than the effect of either alone

Synergism between Two Different Antibiotics

Figure 20.23

Antagonism Between Antimicrobials – The D-test

The Future of Antimicrobial Treatment and Development

• Chemicals produced by plants and animals are providing new antimicrobial agents called antimicrobial peptides.

• Phage therapy is also something that is being investigated for new treatment

disease – epidemiology and control introduction to microbiology chapters 14 and 20

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