Infectious Disease Epidemiology

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

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

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

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

Classification by Mode of Transmission • Dynamics of Spread through Human Populations • Spread by a common vehicle • Ingestion Salmonellosis • Inhalation Legionellosis • Inoculation Hepatitis • Propagation by serial transfer from host to host • Respiratory Measles • Anal-oral Shigellosis • Genital Syphilis

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

Cycles of Infectious Agent in Nature • Man-man Influenza • Man-arthropod-man Malaria • Vertebrate-vertebrate-man Psittacosis • Vertebrate-arthropod-vertebrate-man Viral encephalitis • Complex Cycles • Helminth infections River blindness

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

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

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

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

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 Virus Clinical feature Age at infection Estimated ratio Clinical cases + 1000:1 Paralysis Child Polio 0.1% to 1.0% Mononucleosis 1 to 5 years Epstein-Barr > 100:1 1% 6 to 15 years 10:1 to 100:1 1% to 10% 16 to 25 years 2:1 to 3:1 50% to 75% Hepatitis A Icterus < 5 years 20:1 5% 5 to 9 years 11:1 10% 10 to 15 years 7:1 14% Adult 1.5:1 80% to 95% Rubella Rash 5 to 20 years 2:1 50% Influenza Fever, cough Young adult 1.5:1 60% Measles Rash, fever 5 to 20 years 1:99 >99% CNS symptoms <1:10,000 Rabies Any age >>>>99%

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

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)

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

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

Calculation of Attack Rate for Food X Ate the food (exposed) Did not eat the food (not exposed) Attack Rate Attack Rate Ill Well Total Well Total Ill 10 3 13 76% 7 4 11 64% 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

Secondary Attack Rate Secondary attack rate (%) Total number of cases – initial case(s) • 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) = x 100 Number of susceptible persons in the group – initial case(s)

Mumps experience of 390 families exposed to a primary case within the family Population Cases No. susceptible before primary cases occurred Age in years Total Primary Secondary 2-4 300 250 100 50 5-9 450 204 87 420 10-19 152 84 25 15 Secondary attack rate 2-4 years old = (150-100)/(250-100) x 100 = 33%

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

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 = 0.006 = 0.6% 1000 6 = 0.3 = 30% 20

Infectious Disease Epidemiology