Lecture 2: Dynamics of Disease Transmission Reading: Gordis - Chapter 2 Lilienfeld and Stolley - Chapter 1, pp ; Chapter 3

The epidemiologic triad Host – Agent – Environment for infectious disease Host AgentEnvironment VECTOR Not to be confused with person, place, time

West Nile virus transmission cycle West Nile virus West Nile virus Mosquito vector Incidental infections Bird reservoir hosts Incidental infections

Characteristics of host/agent/environment HostAgentsEnvironment AgeBiologicTemperature SexChemicalHumidity ReligionPhysicalAltitude CultureNutritionalCrowding OccupationHousing Genetic profileWater Previous diseasesAir pollution Immune StatusNoise

The infectious disease process Agents –what is causing the illness Reservoirs –where the agents live Transmission –how they get in Host immunity –what factors affect disease progression

Agents Biologic: –worms, fungi and yeast, protozoa, bacteria, viruses, prions Physical: –noise, repetitive motion, violence Chemical: –tobacco, air pollutants, water pollutants Nutritional: –obesity, nutritional deficiencies

Characteristics of host An agent must be present for an infectious disease to develop But this is not a sufficient cause Infection depends on agent factors and host factors Host immunity – the capacity of a person when exposed to an infectious agent to remain free of infection or clinical illness

Characteristics of environment Physical –habitat, climate Biological –population density –flora –fauna Socioeconomical –occupation,urbanization, culture

Modes of transmission Mode of transmission – any mechanism by which an agent is spread to the host Disease can be transmitted by –Horizontal transmission  Direct contact: person to person contact  Indirect contact 1.Vectors (animate objects) 2.Vehicles (inanimate objects) –Vertical transmission (genetic transmission, mother-to-child during pregnancy or birth)

Modes of transmission Common vehicle spread –Air, water, food, blood, … Serial transfer –Human to human, –human to animal to human, –human to environment to human in sequence

Modes of transmission Airborne –respiratory disease Food/Waterborne (fecal-oral ) –enteric disease, polio, hepatitis A. Bloodborne (parental, perinatal) – hepatitis, HIV

Reservoirs Reservoirs – the normal habitat in which the agent lives, multiplies, and grows. –Symptomatic cases –Carriers –Animals (zoonoses) –Inanimate objects: water, food, soil, air

Iceberg concept of infection Iceberg concept of Infection –Tip of the iceberg active clinical disease –Most people are subclinical –Substantial number of exposures may not lead to infection

DIED Host Response Hospitalized Classical disease Moderate - Mild disease Infection without clinical illness Asymptomatic disease (Self-reported) (Found only on Population Screening) Exposure without infection

Incubation Stage Prodrome Stage Clinical Stage Decline StageConvalescent stage Agent enters healthy body First symptoms of disease. Highly com- municable Character- istic symptoms (peak) First signs of recovery. Disease ends, becomes latent or intermittently reoccurs Return to full health. (Recovery) Latent period Period in which illness is apparent Period of communicabilityCarrier Time (total period of disease)

Endemic, Epidemic, Pandemic Endemic: –habitual presence of disease within a given geographic area Epidemic: –occurrence of disease clearly in excess of normal expectancy Pandemic –Worldwide epidemic

Determinants of disease outbreaks Amount of disease in a population –depends on: number of infected number not susceptible, or immune

Herd immunity The resistance of a group to an attack by a disease to which a large proportion of the members of the group are immune. Because disease spreads from person to person - the probability of reaching a susceptible person decreases as the proportion of immune increases.

Herd immunity We do not have to immunize 100% of the population to be successful For herd immunity to work –the disease agent must be restricted to a single host species –transmission must be relatively direct from one member to the other

Herd immunity If there is an outside reservoir –herd immunity will not operate because other means of transmission are available –infections must induce solid immunity Herd immunity will only work if an infected person is random

Incubation period The period between exposure to the agent and onset of infection (with symptoms and signs of infection). In the incubation, infected persons feel well and show no signs, but can transmit the disease to others.

Epidemic curve A graphic with x-axis: the time of onset y-axis: the number of cases developing at certain time point In a single-exposure common-vehicle epidemic, the epidemic curve represents the distribution of the incubation period, and the median point on the curve represents the median incubation period. One can estimate the time of exposure by using the median incubation period.

Epidemic curve

Attack rate (incidence rate) Attack rate: Number of people at risk who develop a certain illness Total number of people at risk Primary case: the first person in a family (or defined group) who acquires the disease from that exposure. Secondary attack rate: Number of exposed people developing the disease Total number of people exposed to the primary case Secondary attack rate can measure the degree of spread of a disease after the disease has been introduced to a population.

Outbreak investigation 1.Define the epidemic a.Define the “numerator” (cases): 1)Clinical features – is the disease known? 2)What are its serological or cultural aspects? 3)Are the causes partially understood? b.Define the “denominator” – What is the population at risk of developing disease? c.Calculate the attack rates

Outbreak investigation 2.Examine the distribution of cases by the following: a.Time b.Place – look for time-place interactions. c.Person – examine the risk in subgroups of the affected population according to persona characteristics: sex, age, residence, occupation, social group, etc.

Outbreak investigation 3.Look for combinations (interactions) of relevant variables 4.Develop hypotheses based on the following: a.Existing knowledge of the disease b.Analogy to diseases of known etiology 5.Test Hypotheses a.Further analyze existing data b.Collect additional data 6.Recommend control measures a.Control of present outbreak b.Prevention of further similar outbreaks