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 condition where pathogenic microbes penetrate host defenses enter 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 Opportunistic 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 factors Factors prevent phagocytosis by the host’s phagocytic cells Bacterial capsule Composed of chemicals not recognized as foreign Slippery; difficult for phagocytes to engulf bacteria Antiphagocytic chemicals Prevent fusion of lysosome and phagocytic vesicles Leukocidins 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 Streptokinase Hyaluronidase Leukocidins 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 Toxemias Intoxication Enterotoxins Antitoxins Toxoids 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 focal 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 Infection Mixed infection Primary infection several microbes grow simultaneously at the infection site Primary infection initial infection Secondary infection another infection by a different microbe

Warning Signals of Disease 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 Lesion General 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

Infections That Go Unnoticed Asymptomatic subclinical infections although infected, the host doesn’t show any signs of disease Inapparent infection person 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 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 Infections Health care-associated infections (HAIs) occur as a result of receiving treatment for another condition Nosocomial infections are HAIs associated with hospitals They occur as a result of chains of transmission

Nosocomial Infections From 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

Epidemiology study 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, GA principal 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 Morbidity rate total number of deaths in a population due to a certain disease Morbidity rate number of people afflicted with a certain disease

Epidemiology Endemic 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