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35 The Epidemiology of Infectious Disease

 

CHAPTER OVERVIEW

This chapter discusses the epidemiological parameters used to institute effective control, prevention, and eradication measures within an affected or potentially affected population. This chapter also discusses the epidemiology of hospital-acquired (nosocomial) infections, which have been of increasing concern in recent years.

 

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

! define epidemiology and explain how it relates to infectious diseases

! discuss the statistical parameters used to define and describe various infectious diseases

! discuss the need to identify the etiologic agent in order to trace the origin and manner of spread of an infectious disease outbreak

! describe the five epidemiological links (characteristics of the infectious organism, source and/or reservoir, mode of transmission, susceptibility of the host, and exit mechanisms) in the infectious disease cycle of any epidemic

! discuss the increase in nosocomial infections in recent years and the consequences of these infections

 

CHAPTER OUTLINE

I. Introduction

A. EpidemiologyCthe science that evaluates the occurrence, determinants, distribution, and control of health and disease in a defined human population

B. EpidemiologistCone who practices epidemiology (a disease detective)

C. DiseaseCan impairment of the normal state of an organism or any of its components, which hinders the performance of vital functions

II. Epidemiological Terminology

A. Sporadic diseaseCoccurs occasionally at irregular intervals in a human population

B. Endemic diseaseCmaintains a relatively steady low-level frequency at a moderately regular interval

C. HyperendemicCa gradual increase in frequency above the endemic level, but not to the epidemic level

D. EpidemicCsudden increase in frequency above the endemic level

E. Index caseCthe first case in an epidemic

F. OutbreakCan epidemic-like increase in frequency, but in a very limited (focal) segment of the population

G. PandemicCa long-term increase in frequency in a large (usually worldwide) population

H. ZoonosesCdiseases of animals that can be transmitted to humans

III. Measuring Frequency: The Tools of Epidemiologists

A. StatisticsCthe mathematics of collection, organization, and interpretation of numerical data

B. MorbidityCthe number of new cases in a specific time period per unit of population

C. PrevalenceCnumber of individuals infected at any one time per unit of population

D. MortalityCnumber of deaths from a disease per number of cases of the disease

IV. Infectious Disease EpidemiologyCConcerns of an Epidemiologist

A. What organism caused the disease?

B. What is the source and/or reservoir of the disease?

C. How is the disease transmitted?

D. What host and environmental factors facilitated development of the disease within a defined population?

E. How can the disease best be controlled or eliminated?

V. Recognition of an Infectious Disease in a Population

A. Recognition involves various surveillance methods to monitor the population for disease occurrence and for demographic analysis

B. Surveillance involves identification of the signs and/or symptoms of a disease

1. SignsCobjective changes in the body (e.g. fever, rash, etc.) that can be directly observed

2. SymptomsCsubjective changes (e.g. pain, appetite loss, etc.) experienced by the patient

3. Disease syndromeCa set of signs and symptoms that is characteristic of a disease

C. Characteristic Patterns of an Infectious Disease

1. Incubation periodCthe period after pathogen entry but before signs and symptoms appear

2. Prodromal stageConset of signs and symptoms but not yet clear enough for diagnosis

3. Period of illnessCdisease is most severe and has characteristic signs and symptoms

4. Period of decline (convalescence)Csigns and symptoms begin to disappear

D. Correlation with a single causative organism uses methods described in chapter 34 for the organism=s isolation and identification

VI. Recognition of an Epidemic

A. Common-source epidemicCcharacterized by a sharp rise to a peak, and then a rapid but not as pronounced decline in the number of cases; usually results from exposure of all infected individuals to a single common, contaminated source, such as food or water

B. Propagated epidemicCcharacterized by a gradual increase and then a gradual decline in the number of cases; usually results from the introduction of one infected individual into a population, who then infects others; these in turn infect more, until an unusually large number of individuals within the population are infected

C. Herd immunityCthe resistance of a population to infection and to the spread of an infectious organism because of the immunity of a large percentage of the population; this limits the effective contact between infective and susceptible individuals

D. Antigenic shiftsCgenetically determined changes in the antigenic character of a pathogen so that it is no longer recognized by the host=s immune system (e.g., new flu strains); smaller changes are called antigenic drift

VII. The Infectious Disease Cycle: Story of a DiseaseCLinks in the infectious disease chain

A. What pathogen caused the disease? Epidemiologists must determine the etiology (cause) of a disease

1. Koch=s postulates (or modifications of them) are used if possible

2. The clinical microbiology laboratory plays an important role in the isolation and identification of the pathogen

3. Communicable diseaseCone that can be transmitted from one host to another

B. What was the source and/or reservoir of the pathogen?

1. SourceClocation from which organisms are immediately transmitted to the host

2. Period of infectivityCthe time during which the source is infectious or is disseminating the organism

3. ReservoirCsite or natural environmental location where organism is normally found

4. CarrierCan infected individual who is a potential source of infection for others

a. Active carrierCa carrier with an overt clinical case of the disease

b. Convalescent carrierCan individual who has recovered from the disease but continues to harbor large numbers of the pathogen

c. Healthy carrierCan individual who harbors the pathogen but is not ill

d. Incubatory carrierCan individual who harbors the pathogen but is not yet ill

e. Casual (acute, transient) carriersCany of the above carriers who harbor the pathogen for a brief period (hours, days, or weeks)

f. Chronic carriersCany of the above carriers who harbor the pathogen for long periods (months, years, or life)

C. How was the pathogen transmitted?

1. AirborneCsuspended in air; travels a meter or more

a. Droplet nucleiCmay come from sneezing, coughing, or vocalization

b. Dust particlesCmay be important in airborne transmission because microorganisms adhere readily to dust

2. ContactCtouching between source and host

a. Direct (person-to-person)Cphysical interaction between infected person and host

b. IndirectCinvolves an intermediate, such as eating utensils, thermometers, dishes, glasses, and bedding

c. DropletsClarge particles that travel less than one meter through the air

3. Vehicle (fomite)Cfood and water, as well as those intermediates described for indirect contact

4. Vector-borneCliving transmitters, such as arthropods or vertebrates

a. External (mechanical) transmissionCpassive carriage of the pathogen on the body of the vector with no growth of the organism during transmission

b. Internal transmissionCcarried within the vector

(1) HarborageCorganism does not undergo morphological or physiological changes within the vector

(2) BiologicCorganism undergoes morphological or physiological changes within the vector

D. Why was the host susceptible to the pathogen? Depends on defense mechanisms of the host and the pathogenicity of the organism

E. How did the pathogen leave the host?

1. Active escapeCmovement of organism to portal of exit

2. Passive escapeCexcretion in feces, urine, droplets, saliva, or desquamated cells

VIII. Virulence and the Mode of Transmission

A. A virus that is spread by direct contact (e.g., rhinoviruses) cannot afford to make the host so ill it cannot be spread effectively

B. A virus that is vector-borne can afford to be highly virulent

C. Pathogens that do not survive well outside the host and that do not use a vector are likely to be less virulent while pathogens that can survive for long periods of time outside the host tend to be more virulent

IX. The Emergence of New Diseases

A. New diseases have emerged in the past few decades such as AIDS, Hepatitis C and E, hantavirus, Lyme disease, Legionnaire=s disease, toxic shock E. coli 0157:H7, cryptosporidiosis, hepatitis G and others

B. Systematic epidemiology focuses on the ecological and social factors that influence the development and emergence of disease

C. Rapid transportation systems aid in the spread of disease out of areas where they are endemic

D. Ecological disruption can favor the spread of disease (e.g., loss of deer predators lead to an increase in the deer and deer tick population, thereby spreading Lyme disease to human hosts)

E. Narcotic use and sexual promiscuity have stimulated the spread of disease

X. Control of Epidemics

A. Reduce or eliminate the source of infection through:

1. Quarantine and isolation of cases and carriers

2. Destruction of an animal reservoir, if one exists

3. Treatment of sewage to reduce water contamination

4. Therapy that reduces or eliminates infectivity of individuals

B. Break the connection between the source and susceptible individuals through sanitization, disinfection, vector control, etc.; examples include:

1. Chlorination of water supplies

2. Pasteurization of milk

3. Supervision and inspection of food and food handlers

4. Destruction of insect vectors with pesticides

C. Reduce the number of susceptible individualsCincrease herd immunity

D. Role of the public health system: epidemiological guardianCa network of health professionals involved in surveillance, diagnosis, and control of epidemics

XI. Nosocomial InfectionsCproduced by infectious agents that develop within a hospital or other clinical care facility and that are acquired by patients while they are in the facility; infections that are incubating within the patient at the time of admission are not considered nosocomial

A. Source

1. EndogenousCpatient=s own microbiota

2. ExogenousCmicrobiota other than the patient=s

3. AutogenousCcannot be determined to be endogenous or exogenous

B. Control, prevention, and surveillance should include proper handling of the patient and the materials provided to the patient, as well as monitoring of the patient for signs of infection

C. The hospital epidemiologist (other terms are also used) is an individual (usually a registered nurse) responsible for developing and implementing policies to monitor and control infections and communicable disease; usually reports to an infection control committee or other similar group

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