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Unformatted text preview: Chapter 12 Chapter 12
Epidemiology of Infectious Diseases A model used to explain the etiology of infectious diseases. Epidemiologic Triangle Epidemiologic Triangle Recognizes three major factors in the pathogenesis of disease: agent, host, and environment. Microbial Agents of Infectious Disease Microbial Agents of Infectious Disease Bacteria Viruses Protozoa Helminths Arthropods Mycoses (fungal diseases) Bacteria Bacteria Once were the leading killers, but now most are controlled by antibiotics. Remain significant causes of human illness. Tuberculosis and salmonellosis are common diseases caused by bacteria. Viral hepatitis A, herpes, and influenza are caused by viruses. Rickettsial agents produce Q fever and Rocky Mountain spotted fever. Viruses and Rickettsia Viruses and Mycoses (Fungal Diseases) Sporotrichosis Caused by the dimorphic fungus Valley fever (coccidioidomycosis), ringworm and athlete’s foot. Opportunistic mycoses infect immunocompromised patients. Candidiasis, cryptococcosis, and aspergillosis. Protozoa Protozoa Cause malaria, amebiasis, babesiosis, cryptospordiosis, and giardiasis. Found in tropical areas. Include intestinal parasites such as roundworms, pinworms, and tapeworms. Are responsible for trichinellosis and schistosomiasis. Helminths Helminths Act as insect vectors. Arthropods Arthropods Examples: mosquitos, ticks, flies, mites. Transmit diseases such as malaria and encephalitis. Characteristics of Infectious Disease Agents Characteristics of Infectious Disease Agents Infectivity The capacity of an agent to produce infection or disease. Measured by the secondary attack rate. The capacity of the agent to cause disease in the infected host. Measured by the proportion of individuals with clinically apparent disease. Pathogenicity Characteristics of Infectious Disease Characteristics of Infectious Disease Agents (cont’d) Virulence Refers to the severity of the disease. Measured by the proportion of severe or fatal cases. If fatal, use case fatality rate. The capacity of the agent to produce a toxin or poison. Toxigenicity Characteristics of Infectious Disease Characteristics of Infectious Disease Agents (cont’d) Resistance The ability of the agent to survive adverse environmental conditions. The ability of the agent to induce antibody production in the host. Related to immunogenicity. Antigenicity Host: Definition Host: Definition A person (or animal) who “permits” lodgment of an infectious disease agent under natural conditions. Host Host 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. Two types of defense mechanisms are present in the host: nonspecific and diseasespecific. Nonspecific Defense Mechanisms Nonspecific Defense Mechanisms Examples include skin, mucosal surfaces, tears, saliva, gastric juices, and the immune system. Nonspecific defense mechanisms such as immunity may decrease as we age. DiseaseSpecific Defense Mechanisms DiseaseSpecific Defense Mechanisms Immunity (resistance) against a particular agent. Types of immunity: Active: administration of a microorganism to invoke an immunologic response that mimics the natural infection. Passive: shortterm immunity provided by a preformed antibody. Active Immunity Active Immunity Natural, activeresults from an infection by the agent. Artificial, activeresults from an injection with a vaccine that stimulates antibody production in the host. Passive Immunity Passive Immunity
Natural, passivepreformed antibodies are passed to the fetus during pregnancy and provide shortterm immunity in the newborn. Artificial, passivepreformed antibodies are given to exposed individuals to prevent disease (tetanus injection of antitoxins ) Environment Environment The domain external to the host in which the agent may exist, survive, or originate. The environment consists of physical, climatologic, biologic, social, and economic components that affect the survival of the agents and serve to bring the agent and host into contact. Reservoirs of Infectious Diseases Reservoirs of Infectious Diseases The environment can act as a reservoir that fosters the survival of infectious agents. Examples: contaminated water supplies or food; soils; vertebrate animals. Animal Reservoirs Animal Reservoirs Animals can be reservoirs of infectious agents. Zoonosesinfectious diseases that are potentially transmittable to humans by vertebrate animals. Examples: rabies and the plague. Direct Transmission from Reservoir Direct Transmission from Reservoir Spread of infection through personto person contact. Portal of exitsite where infectious agents leave the body, e.g., respiratory system, skin lesions. Direct Transmission (cont’d) Direct Transmission Portal of entrylocus of access to the human body, e.g., mouth and digestive system. Agent must exit in large enough quantities to survive in the environment and overcome the defenses at the portal of entry into the host. Inapparent Infection Inapparent Infection No symptoms of infection present. Important because disease can be transmitted to unsuspecting hosts. In asymptomatic individuals, clinicians can look for serologic evidence of infection. Example: Increase in antibodies and enzymes in patients with hepatitis A virus. Incubation Period Incubation Period The time interval between exposure to an infectious agent and the appearance of the first signs and symptoms of disease. Provides a clue to the time and circumstance of exposure to the agent. Useful for determining the etiologic agent. Herd Immunity Herd Immunity 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. Generation Time Generation Time Time interval between lodgment of an infectious agent in a host and the maximal communicability of the host. Can precede the development of active symptoms. Useful for describing the spread of infectious agents that have large proportions of subclinical cases. Colonization and Infestation Colonization and Infestation
Colonizationagents multiply on the surface of the body without invoking tissue or immune response. Infestationthe presence of a living infectious agent on the body’s exterior surface, upon which a local reaction may be invoked. Iceberg Concept of Infection Iceberg Concept of Infection Active clinical disease accounts for only a small proportion of host’s infections and exposures to disease agents. Iceberg Concept (cont’d) Iceberg Concept Indirect Transmission Indirect Transmission The spread of infection through an intermediary source:
Vehiclese.g., contaminated water, infected blood, food. Fomitesinanimate objects laden with disease causing agents. Vectorsliving insects or animals involved with transmission of the disease agent. Measures of Disease Outbreaks Measures of Disease Outbreaks Attack rate Secondary attack rate Case fatality rate Attack Rate Attack Rate Incidence rate that is used when the nature of the disease or condition is such that a population is observed for a short period of time. Formula: ___Ill __ X 100 during a time period Ill + Well Secondary Attack Rate Secondary Attack Rate An index of the spread of disease in a family, household, dwelling unit, dormitory or similar circumscribed group. A measure of contagiousness. Useful in evaluating control measures. Secondary Attack Rate: Definition Secondary Attack Rate: Definition The number of cases of infection that occur among contacts within the incubation period following exposure to a primary case in relation to the total number of exposed contacts. Secondary Attack Rate (%) (Multiply fraction by 100.) Number of new cases in group initial case(s) Number of susceptible persons in the group initial case(s) Initial case(s) = Index case(s) + coprimaries Index case(s) = Case that first comes to the attention of public health authorities. Coprimaries = Cases related to index case so closely in time that they are considered to belong to the same generation of cases. Case Fatality Rate (CFR) Case Fatality Rate (CFR) The number of deaths caused by a disease among those who have the disease. Examples of diseases with a high CFR are rabies and AIDS. Formula For CFR Formula For CFR Number of deaths due to disease “X “ x 100 Number of cases of disease “X “ Sample calculation: Assume that an outbreak of plague occurs in an Asian country. Health authorities record 98 case of the disease, all of whom are untreated. Among these, 60 deaths are reported. CFR = (60/98) x 100 = 61.2% Investigation of Infectious Disease Investigation of Infectious Disease Outbreaks Define the problem. Appraise existing data. Case identification Clinical observations Tabulation and spot maps Identification of responsible agent Investigation (cont’d) Investigation Formulate a hypothesis. Test the hypothesis. Draw conclusions and formulate practical applications. Epidemiologically Significant Epidemiologically Significant Infectious Diseases Foodborne illness Water and foodborne diseases Sexually transmitted diseases Vaccinepreventable diseases Diseases spread by persontoperson contact Zoonotic diseases Arthropodborne diseases Fungal diseases Foodborne Illness Foodborne Illness One of the most common infectious disease problems in the community. Examples include: Staphylococcus aureuspresent in contaminated food. Produces a toxin. Trichinosisassociated with inadequately cooked pork products. Water and Foodborne Diseases Water and Foodborne Diseases Examples include: Amebiasisintestinal disease. Choleraacute enteric disease. Giardiasis Legionellosis Schistosomiasisinfection caused by adult worms in the bloodstream. The cycle involves alternate snail and human hosts. Sexually Transmitted Diseases: Sexually Transmitted Diseases: AIDS, 19812000 Characteristics of persons with AIDS: Greater increase in cases among females than males; still more males than females. Greatest impact among men who have sex with men and among racial/ethnic minorities. Number of persons living with AIDS has increased as deaths have declined. VaccinePreventable Diseases VaccinePreventable Diseases Vaccines are routinely given to children for the prevention of the following nine diseases: Diphtheria, Haemophilus influenzae type b infections, hepatitis B, measles, mumps, pertussis, paralytic poliomyelitis, rubella, and tetanus. Diseases Spread by PersontoPerson Contact Diseases Spread by PersontoPerson Contact One example is tuberculosis. Resurgence of TB due to: Increase in persons infected with HIV. Increase in homeless population. Importation of cases from endemic areas. U.S. TB Cases, 19801992 U.S. TB Cases, 19801992 Source: Reprinted from Centers for Disease Control and Prevention. Tuberculosis morbidity—United States, 1992. MMWR, vol 42, p 696, September 17, 1993. Zoonotic Diseases Zoonotic Diseases Zoonosisa disease that under natural conditions can be spread from vertebrate animals to humans. Example: Q fever. Zoonotic diseases may be either: Enzooticsimilar to endemic in human diseases. Epizooticsimilar to epidemic in human diseases. Mycoses Mycoses Coccidioidomycosis (San Joaquin Valley fever )caused by the fungus Coccidioides immitis.
Usually attacks the lungs. Cases of infection usually have had contact with contaminated soil. Other examples: blastomycosis, ringworm, athlete’s foot, candidiasis, cryptococcosis, and aspergillosis. Arthropodborne Diseases Arthropodborne Diseases Include arboviral diseases. Bloodfeeding arthropod vectors transmit disease agents to vertebrate hosts. Examples of vectors: sand flies, ticks, mosquitoes. Examples of diseases: encephalitis, Lyme disease Emerging Infections Emerging Infections Suddenly increase in incidence or geographic scope. Many infections appear when an existing pathogen gains access to new host populations. Examples: U.S. hantaviral pulmonary syndrome, Lyme disease, AIDS, hepatitis C, hemorrhagic fever. Environmental changes may contribute to their emergence. ...
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