Infectious Disease Epidemiology Principles of Epidemiology Lecture 7 Dona Schneider, PhD, MPH, FACE

Epidemiology (Schneider) Concepts in Infectious Epidemiology  Agent, host, environment  Classification of human infections by modes of transportation  Incubation period  Spectrum of disease  Herd immunity

Epidemiology (Schneider) Major Factors Contributing to the Emergence of Infectious Diseases  Human demographics and behavior  Technology and industry  Economic development and land use  International travel and commerce  Microbial adaptation and change  Breakdown of public health measures

Epidemiologic Triad Concepts Infectivity – ability to invade a host (# infected / # susceptible) X 100 Pathogenicity – ability to cause disease (# with clinical disease / # of infected) X 100 Virulence – ability to cause death (# of deaths / # with disease (cases)) X 100 All are dependent upon the condition of the host - Immunity (active, passive) - Nutrition - Sleep - Hygiene

Epidemiology (Schneider) Mode of Transmission Person-to-person (respiratory, orogenital, skin) Examples: HIV, measles Vector (animals, insects) Examples: rabies, yellow fever Common vehicle (food, water) Examples: salmonellosis Mechanical vectors (personal effects) such as doorknobs, or toothbrushes are called FOMITES

Epidemiology (Schneider) Classification by Mode of Transmission  Dynamics of Spread through Human Populations  Spread by a common vehicle  IngestionSalmonellosis  InhalationLegionellosis  InoculationHepatitis  Propagation by serial transfer from host to host  RespiratoryMeasles  Anal-oralShigellosis  GenitalSyphilis

Epidemiology (Schneider)  Principle Reservoir of Infection  Man Infectious hepatitis  Other vertebrates (zoonoses) Tularemia  Agent free-living Histoplasmosis  Portal of Entry/Exit in Human Host  Upper respiratory tract Diphtheria  Lower respiratory tract Tuberculosis  Gastrointestinal tract Typhoid fever  Genitourinary tract Gonorrhea  Conjunctiva Trachoma  Percutaneous Leptospirosis  Percutaneous (bite of arthropod) Yellow fever

Epidemiology (Schneider)  Cycles of Infectious Agent in Nature  Man-man Influenza  Man-arthropod-man Malaria  Vertebrate-vertebrate-manPsittacosis  Vertebrate-arthropod-vertebrate-man Viral encephalitis  Complex Cycles  Helminth infections River blindness

Epidemiology (Schneider) Incubation Period The interval between the time of contact and/or entry of the agent and onset of illness (latency period) The time required for the multiplication of microorganisms within the host up to a threshold where the parasitic population is large enough to produce symptoms

Epidemiology (Schneider) Each infectious disease has a characteristic incubation period, dependent upon the rate of growth of the organism in the host and  Dosage of the infectious agent  Portal of entry  Immune response of the host Because of the interplay of these factors, incubation period will vary among individuals  For groups of cases, the distribution will be a curve with cases with longer incubation periods creating a right skew

Epidemiology (Schneider) Spectrum of Disease Exposure Subclinical manifestations Pathological changes Symptoms Clinical illness Time of diagnosis Death Whether a person passes through all these stages will depend upon infection and prevention, detection and therapeutic measures

Iceberg Concept of Infection Lysis of cell CELL RESPONSE Cell transformation or Cell dysfunction Incomplete viral maturation Subclinical Disease Exposure without cell entry Clinical Disease Fatal Clinical and severe disease Moderate severity Mild Illness Infection without clinical illness Exposure without infection HOST RESPONSE Below visual change Discernable effect

Epidemiology (Schneider) Spectrum of Disease (cont.) Example 90% of measles cases exhibit clinical symptoms 66% of mumps cases exhibit clinical symptoms <10% of poliomyelitis cases exhibit clinical symptoms Inapparent infections play a role in transmission. These are distinguished from latent infections where the agent is not shed

Subclinical/Clinical Ratio for Viral Infections >>>>99% <1:10,000 Any age CNS symptoms Rabies >99%1:995 to 20 yearsRash, feverMeasles 60%1.5:1Young adultFever, coughInfluenza 50%2:15 to 20 yearsRashRubella 80% to 95%1.5:1Adult 14%7:1 10 to 15 years 10%11:15 to 9 years 5%20:1< 5 yearsIcterusHepatitis A 50% to 75% 2:1 to 3:116 to 25 years 1% to 10%10:1 to 100:16 to 15 years 1%> 100:1 1 to 5 yearsMononucleosis Epstein-Barr 0.1% to 1.0% :1 ChildParalysis Polio Clinical cases Estimated ratio Age at infection Clinical feature Virus

Epidemiology (Schneider) Herd Immunity The decreased probability that a group will develop an epidemic because the proportion of immune individuals reduces the chance of contact between infected and susceptible persons The entire population does not have to be immunized to prevent the occurrence of an epidemic Example: smallpox, measles

Investigating an Epidemic  Determine whether there is an outbreak – an excess number of cases from what would be expected There must be clarity in case definition and diagnostic verification for each case

Epidemiology (Schneider) Investigating an Epidemic (cont.)  Plot an epidemic curve (cases against time)  Calculate attack rates  If there is no obvious commonality for the outbreak, calculate attack rates based on demographic variables (hepatitis in a community)  If there is an obvious commonality for the outbreak, calculate attack rates based on exposure status (a church supper)

Epidemiology (Schneider) Investigating an Epidemic (cont.)  Determine the source of the epidemic  If there is no obvious commonality for the outbreak, plot the geographic distribution of cases by residence/work/school/location to reduce common exposures  If there is an obvious commonality for the outbreak, identify the most likely cause and investigate the source to prevent future outbreaks

Epidemiology (Schneider) Index Case Person that comes to the attention of public health authorities Primary Case Person who acquires the disease from an exposure Attack rate Secondary Case Person who acquires the disease from an exposure to the primary case Secondary attack rate

Epidemiology (Schneider) Calculation of Attack Rate for Food X 64%114776%13310 Attack Rate TotalWell Ill Attack Rate TotalWellIll Did not eat the food (not exposed)Ate the food (exposed) Attack Rate = Ill / (Ill + Well) x 100 during a time period Attack rate = (10/13) x 100 = 76% ( 7/11) x 100 = 64% RR = 75/64 = 1.2

Epidemiology (Schneider) Secondary Attack Rate Used to estimate to the spread of disease in a family, household, dorm or other group environment. Measures the infectivity of the agent and the effects of prophylactic agents (e.g. vaccine) Secondary attack rate (%) Total number of cases – initial case(s) Number of susceptible persons in the group – initial case(s) = x 100

Epidemiology (Schneider) Mumps experience of 390 families exposed to a primary case within the family SecondaryPrimary No. susceptible before primary cases occurred Total Age in years CasesPopulation Secondary attack rate 2-4 years old = ( )/( ) x 100 = 33%

Epidemiology (Schneider) Case Fatality Rate Reflects the fatal outcome (deadliness) of a disease, which is affected by efficacy of treatment Case fatality rate (%) Number of deaths due to disease X Number of cases of disease X = x 100

Epidemiology (Schneider) Assume a population of 1000 people. In one year, 20 are sick with cholera and 6 die from the disease. The cause-specific mortality rate in that year from cholera = The case-fatality rate from cholera = 6 20 =0.3=30% =0.006=0.6%