Infection and Disease

The Host-Microbe Relationship

Human body maintains a symbiosis with microbes Infection refers to:

relationship between the host and microbe competition for supremacy between them

If the host loses the competition, disease occurs

Contact, Colonization, Infection, Disease

Infectioncondition where pathogenic microbes

penetrate host defensesenter tissues and multiply

Disease any deviation from health, disruption of a

tissue or organ

infectious disease Caused by microbes or their products

**Not all contacts lead to infections…….**Not all infections lead to disease!!!!!

Resident Flora Microbiota, transient,

resident, indigenous

Microbes that engage in mutual or commensal associations

most areas of the body in contact with the outside environment

internal organs & tissues & fluids are microbe-free

bacterial flora benefit host by preventing overgrowth of harmful microbes

Initial Colonization of the Newborn

Uterus and contents normally sterile remain so until just before

birth

Breaking of fetal membrane exposes the infant all subsequent handling and

feeding continue to introduce normal flora

Pathogens….

True pathogens capable of causing disease

in healthy persons normal immune defenses

influenza virus, plague bacillus, malarial protozoan

Opportunistic pathogens cause disease when hosts

defenses are compromised Pseudomonas sp & Candida

albicans

Pathogenicity Ability of a microorganism to

cause disease

Severity of disease depends on the virulence of the pathogen Degree of pathogenicity characteristic or structure that

contributes to the ability of a microbe to cause disease

virulence factor Adhesion factors Biofilms Extracellular enzymes Toxins Antiphagocytic factors

Pathogens…..

In order to become established: Portal of entry Attaching Surviving host defenses

1. Portals of entry characteristic route a

microbe follows to enter the tissues of the body skin

nicks, abrasions, punctures, incisions

gastrointestinal tract food, drink, and other

ingested materials Mucous membranes urogenital tract

Sexual transplacental

Infectious Dose (ID)

minimum number of microbes required for infection to proceed

microbes with small IDs have greater virulence 1 rickettsial cell in Q fever 10 bacteria in TB, giardiasis 109 bacteria in cholera

2. Mechanisms of Adhesion

fimbrae

flagella

adhesive slimes or capsules

cilia

scolex

hooks

barbs

3. Surviving Host Defenses

Initial response of host defenses comes from phagocytes Ability to survive

intracellular phagocytosis

Antiphagocytic factors

Antiphagocytic factorsFactors prevent phagocytosis by the host’s

phagocytic cells Bacterial capsule

Composed of chemicals not recognized as foreignSlippery; difficult for phagocytes to engulf bacteria

Antiphagocytic chemicalsPrevent fusion of lysosome and phagocytic vesiclesLeukocidins directly destroy phagocytic white blood cells

Enzymatic Destruction

Extracellular enzymes (exoenzymes) Secreted by the pathogen Dissolve structural chemicals in the body Help pathogen maintain infection, invade, and avoid body

defenses

Invasiveness

Coagulase form a blood clot that protects them from phagocytosis

Streptokinase dissolves fibrin clots and allows dissemination of the bacteria

Hyaluronidase enhances pathogen penetration through tissues

Leukocidins disintegrate neutrophils and macrophages

Hemolysiins dissolve red blood cells

Virulence can be enhanced in biofilms immune cells cannot reach bacterial cells

Toxins

toxigenicity capacity to produce toxins at the site of multiplication endotoxins

lipid A of LPS of gram-negative bacteria exotoxins

proteins secreted by gram-positive and gram-negative bacteria

Toxins Toxinoses

Disease caused by toxin

Toxemias Spread by blood

Intoxication Ingested

Enterotoxins act on the gastrointestinal tract

Antitoxins produced by the host body and

neutralize toxins

Toxoids toxins whose toxicity has been

destroyed but still elicit an immune response

Portals of Exit

Respiratory, saliva

Skin

Fecal exit

Urogenital tract

Removal of blood

Establishment of Infection and Disease

incubation period time between entry of the

microbe and symptom appearance

prodromal phase I time of mild signs or

symptoms

acme period (climax) signs and symptoms are most

intense

period of decline As sign and symptoms subside

period of convalescence body systems return to normal

Patterns of Infection localized infection

remains confined to a specific tissue

focal infection when infectious agent breaks loose from a local infection carried to other tissues

systemic infection infection spreads to several sites and tissue fluids usually in the bloodstream

Patterns of InfectionMixed infection

several microbes grow simultaneously at the infection site

Primary infection initial infection

Secondary infectionanother infection by a

different microbe

Warning Signals of Disease

Sign objective evidence of

disease as noted by an observer

Symptom subjective evidence of

disease as sensed by the patient

Syndrome Disease is defined by

complex of signs and symptoms

Signs and Symptoms of Inflammation

Earliest symptoms of disease as a result of the activation of the body defenses fever, pain, soreness, swelling

Signs of inflammation: edema

accumulation of fluid granulomas and abscesses

walled-off collections of inflammatory cells and microbes

LesionGeneral term for site of infections or

disease

Signs of Infection in the Blood

Changes in the number of circulating white blood cells leukocytosis

increase in white blood cells

leukopenia decrease in white

blood cells

Signs of Infection in the Blood

septicemia microorganisms multiplying in the blood and

present in large numbers

bacteremia small numbers of bacteria present in blood not necessarily multiplying

viremia small number of viruses present not necessarily multiplying

Infections That Go Unnoticed

Asymptomatic subclinical infections although infected, the host doesn’t show any signs

of disease

Inapparent infectionperson doesn’t seek medical attention

Persistence of Microbes and Pathologic Conditions

Latency microbe can periodically become active

produce a recurrent disease person may or may not shed it during the latent stage

Sequelae long-term or permanent damage to tissues or organs

Acute diseases develop rapidly cause severe symptoms fade quickly

Chronic diseases linger for long periods of time slower to develop and recede

Reservoirs: Where Pathogens Persist

Reservoir primary habitat of pathogen in

the natural world human or animal carrier, soil,

water, plants

Source individual or object from which

an infection is actually acquired

Animals as Reservoirs and Sources

vector live animal that transmits an

infectious agent from one host to another

Arthropods fleas, mosquitoes, flies, and ticks

larger animals mammals, birds, lower vertebrates

Animals as Reservoirs and Sources

zoonosis infection indigenous to

animals but naturally transmissible to humans

humans do not transmit the disease to others

Acquisition and Transmission of Infectious Agents

Communicable disease an infected host can transmit the infectious agent to

another host

Highly communicable disease is contagious

Non-communicable infectious disease does not arise through transmission from host to host occurs primarily when a compromised person is

invaded by his or her own normal microflora contact with organism in natural, non-living reservoir

Patterns of Transmission

Direct contact physical contact, droplet

nuclei, aerosols

Indirect contact passes from infected host to

intermediate conveyor and then to another host

vehicle food, water, biological

products, fomites

Nosocomial InfectionsHealth care-associated

infections (HAIs) occur as a result of

receiving treatment for another condition

Nosocomial infections are HAIs associated with hospitalsThey occur as a result of

chains of transmission

Nosocomial InfectionsFrom surgical procedures,

equipment, personnel, and exposure to drug-resistant microorganisms

More than 1/3rd of nosocomial infections could be prevented

2 to 4 million cases/year in U.S. approximately 90,000 deaths

Most common organisms involved Gram-negative intestinal flora E. coli, Pseudomonas,

Staphylococcus

Epidemiologystudy of the frequency and distribution of

disease & health-related factors in human populations

Surveillance collecting, analyzing, & reporting data on

rates of occurrence, mortality, morbidity and transmission of infections

Reportable, notifiable diseases must be reported to authorities

Epidemiology

Centers for Disease Control and Prevention (CDC) in Atlanta, GAprincipal government agency responsible for

keeping track of infectious diseases nationwide

http://www.cdc.gov

Epidemiology

Prevalence total number of existing cases with respect to the

entire population usually represented by a percentage of the population

Prevalence= Total # of cases in pop X 100 Total # of persons in pop

Epidemiology

Incidence measures the number of new cases over a certain

time period, as compared with the general healthy population

Incidence = # of new cases = ratio # of healthy persons

The incidence and estimated prevalence of AIDS

Epidemiology

Mortality rate total number of deaths in a population due to a

certain disease

Morbidity rate number of people afflicted with a certain disease

EpidemiologyEndemic

disease that exhibits a relatively steady frequency over a long period of time in a particular geographic locale

Sporadic when occasional cases are

reported at irregular intervals

Epidemic when prevalence of a disease is

increasing beyond what is expected

Pandemic epidemic across continents