Chapter 12 Epidemiology of Infectious Diseases Infectious Diseases Importance o Significant because of morbidity and mortality o Infectious agents are associated with some types of cancer o Cause disease outbreaks in institutions Epidemiologic Triangle Model used to explain the etiology of infectious diseases Recognizes 3 major factors in the pathogenesis of disease agent host environment Microbial Agents o Bacteria Were once leading killers Now controlled by antibiotics Remain significant causes of human illness Emergence of antibiotic resistant strains is a growing concern TB and salmonellosis are common bacterial diseases o Viruses Viral Hepatitis A herpes and influenza caused by viruses o Mycoses Fungal diseases Ringworm and athletes foot Opportunistic mycoses infect immunocompromised patients Candidiasis o Protozoa Cause malaria amebiasis and giardiasis o Helminthes very gross bugs Found in tropical areas Include intestinal parasites such as roundworms pinworms and tapeworms Are responsible for schistosomiasis o Arthropods Act as insect vectors Transmit diseases such as malaria Ex mosquitos ticks flies mites o Characteristics of agents Infectivity Pathogenicity Virulence Toxigenicity Resistance Antigenicity Capacity to enter multiply in a susceptible host and produce infection or disease Capacity of the agent to cause disease in the infected host Measured by the proportion of individuals with clinically apparent disease Severity of the disease Measured by the proportion of severe or fatal cases Capacity of the agent to produce a toxin or poison Ability of the agent to survive adverse environmental conditions Ability of the agent to induce antibody production in the host Host o Person or animal who permits lodgment of an infectious disease agent under natural conditions o Once an agent infects the host the degree and severity of the infection will depend on the host s ability to fight off the infectious agent o Two types of defense mechanisms are present in the host nonspecific and disease specific Nonspecific defense mechanisms Ex skin mucosal surfaces tears saliva gastric juices immune system May decrease as we age Disease specific defense mechanisms Immunity against a particular agent Active administration of microorganism to invoke an immunologic response that mimics the natural infection o Natural results from an infection by the agent o Artificial results from a vaccine that stimulates antibody production in the host Passive short term immunity provided by a preformed antibody o Natural preformed antibodies are passed to the fetus during pregnancy and provide short term immunity in the newborn o Artificial preformed antibodies are given to exposed individuals Environment o Domain external to the host in which the agent may exist survive or originate o The environment serve to bring the agent and host into contact o Reservoirs of infectious diseases The environment can act as a reservoir that fosters the survival of infectious agents Ex contaminated water supplies or food soils vertebrate animals Animal reservoirs Means of transmission Direct Zoonosis infectious diseases potentially transmittable to humans by vertebrate animals Ex rabies and the plague o Person to person contact o Portal of exit site where infectious agents leave the body Ex respiratory system skin lesions o For the chain of transmission to continue the portal of exit must be appropriate to the agent o Agent must exit in large quantities to survive in the environment overcome defenses at entry o Portal of entry locus of access to the human body Mouth and digestive system respiratory system o Inapparent function No symptoms of infection present Important because disease can be transmitted to unsuspecting hosts o Incubation period The time interval between exposure to an infectious disease and the appearance of the first signs and symptoms of disease Provides a clue to the time and circumstance of exposure Useful for determining the etiologic agent o Herd immunity o Generation time o Colonization o Infestation Immunity of a population group or community against an infectious disease when a large proportion of individuals are immune either through vaccinations or prior infection Time interval between lodgment of an infectious agent in a host and the maximal communicability of the host Before showing signs of the disease different than incubation period Agents multiply on the surface of the body without invoking tissue or immune response Presence of a living infectious agent on the body s exterior surface upon which a local reaction may be invoked Indirect o The spread of infection through an intermediary source Vehicles contaminated water infected blood food Vectors living insects or animals involved with transmission of the disease agent Fomites inanimate objects laden with disease causing agent