Epidemiological cycle of
Diseases
Epidemiology means?
The science that evaluates patterns of disease
occurrence, frequency, determinants, distribution,
and control of health and disease in a defined
human population.
Epidemiology leads to the identification of causal
and preventive factors in human disease.
The Cycle of Microbial Disease
Cycle of Microbial Disease
Reservoir
Portal of exit
Person to person
transmission
Susceptible Host
Portal of entry
Agent
Mode of transmission
Chain of transmission
Reservoir
Human
Person with symptomatic illness
Carriers:
Asymptomatic
Incubating
Convalescent
Chronic
Animal: zoonosis
Environmental: soil, plant, water
Chain of transmission
Portal of exit
Human/animal
Respiratory tract
Genito-Urinary tract
Faeces
Saliva
Skin (exanthema, cuts, needles, blood-sucking arthropods)
Conjunctival secretions
Placenta
Environmental
Cooling towers
Chain of transmission
Mode of Transmission
Direct
Direct contact
Secretions,
Blood,
Faeces/urine
Droplet spread
Indirect
Food/water
Aerosol
Animal vectors
Fomites
Medical devices
and treatments
Transmission
• Transmission of a disease from its reservoir to the
next susceptible host.
• Through contact, ingestion of food or water, or via a
living agent such as an insect is called horizontal
transmission.
• Vertical transmission, the transfer of a pathogen
from a pregnant woman to the fetus, or from a
mother to her infant during childbirth.
e.g. Congenital syphilis, Group B streptococci, can
infect the newborn as it passes through the birth
canal
Chain of transmission
Portal of entry
Human/animal
Respiratory tract
Genito-Urinary tract
Faeces
Saliva
Skin (exanthema, cuts, needles,
blood-sucking arthropods)
Conjunctival secretions
Placenta
Frequency and distribution based Classification
• Sporadic level: occasional cases occurring at irregular intervals
• Endemic level: persistent occurrence with a low to moderate level
• Hyper Endemic level: persistently high level of occurrence
• Epidemic : occurrence clearly in excess of the expected level for a
given time period
• Pandemic: epidemic spread over several countries or continents,
affecting a large number of people
Disease Progression
•
•
•
•
•
Infection
Incubation period
Prodromal stage
Period of illness
Convalescence
Factors that Influence the Epidemiology of Disease
• The Dose
• The Incubation Period
• Population Characteristics
- Immunity to the pathogen.
- General health.
- Age.
- Gender
- Religious and cultural practices.
- Genetic background.
Measuring Frequency
• Three important statistical measures of disease
frequency
– morbidity rate
– prevalence rate
– mortality rate
Morbidity rate :
Measures the number of individuals that become ill due to a specific
disease within a susceptible population during a specific time
interval.
Morbidity rate =
Number of new cases during a specific time
Total number of individuals in population
• E.g. 700 new cases of influenza per 100,000 individuals is 0.7%.
Prevalence Rate
The total number of individuals infected in a population at any one
time no matter when the disease began
• Mortality rate
Number of deaths from a disease per number of cases of the
disease
• Mortality rate =
Number of deaths due to given disease
size of total population with disease
• Recognition of an Epidemic
• Two types of epidemics
– common source epidemic
– propagated epidemic
• Common source epidemic
• Reaches a peak within a short period of time -1 to 2 weeks)
• A moderately rapid decline in the number of infected patients
• A single common contaminated source - food (food poisoning) or
water (Legionnaires’ disease).
• Propagated epidemic (Host to Host)
• A relatively slow and prolonged rise and then a gradual decline in the
number of individuals infected.
• A single infected individual into a susceptible population.
• E.g. Increase in mumps or chickenpox cases, Spread of HIV infection.
Seasonal Cycles of diseases
• The season of the year in which the epidemic
occurs may also be significant.
• Respiratory diseases including Influenza, Respiratory Syncytial Virus
infections, and the common cold are more easily transmitted in
crowded indoor conditions during the winter.
• Conversely, vector- and food-borne diseases are more often
transmitted in warm weather when people are more likely to be
exposed to mosquitoes and ticks, or eating picnic food that has not
been stored properly.
Seasonal Cycles of diseases
Seasonal Occurrence of
Respiratory Infections Caused by
Respiratory Syncytial Virus
Seasonal Occurrence of
Gastrointestinal Diseases
Herd immunity
• Resistance of a population to infection and to spread of an
infectious organism because of the immunity of a large
percentage of the population
• Level can be altered by introduction of new susceptible
individuals into population
• Level can be altered by changes in pathogen
– antigenic shift – major change in antigenic character of pathogen
(recombination in birds, pigs totally new antigen types)
– antigenic drift – smaller antigenic changes (point-mutational
changes)
Virulence and the Mode of Transmission
Evidence suggests correlation between mode of
transmission and degree of virulence
– direct contact  less virulent
– vector-borne  highly virulent in human host; relatively
benign in vector
– greater ability to survive outside host  more virulent
Emerging and Reemerging Infectious Diseases
and Pathogens
By the 1990s, the idea that infectious diseases no
longer posed a serious threat to human health was
obsolete.
It is now clear that globally, humans will continually
be faced with both new infectious diseases and the
reemergence of older diseases once thought to be
conquered (e.g., tuberculosis, dengue hemorrhagic
fever, yellow fever)
Emerging and Reemerging Infectious Diseases
and Pathogens
Infectious Disease Mortality in the United States Decreased Greatly during Most of
the Twentieth Century.
The insert is an enlargement of the right-hand portion of the graph and shows that the
death rate from infectious diseases increased between 1980 and 1994.
Systematic epidemiology
The increased importance of emerging and reemerging
infectious diseases has stimulated the establishment of a
field called
systematic epidemiology
Which focuses on the ecological and social factors that
influence the development of these diseases.
Factors characteristic of the modern world favoring the development
and spread of these microorganisms and their diseases
1. Unprecedented worldwide population growth, population shifts
(demographics), and urbanization
2. Increased international travel
3. Increased worldwide transport (commerce), migration, and relocation of
animals and food products
4. Changes in food processing, handling, and agricultural practices
5. Changes in human behavior, technology, and industry
6. Human encroachment on wilderness habitats that are reservoirs for insects
and animals that harbor infectious agents
7. Microbial evolution (e.g., selection pressure) and the development of
resistance to antibiotics and other antimicrobial drugs (e.g., penicillinresistant Streptococcus pneumoniae, methicillin-resistant Staphylococcus
aureus, and vancomycin-resistant enterococci)
Factors characteristic of the modern world favoring the development
and spread of these microorganisms and their diseases
8.
Changes in ecology and climate
9.
Modern medicine (e.g., immunosuppression)
10.
Inadequacy of public infrastructure and vaccination programs
11.
Social unrest and civil wars
12.
The possibility of bioterrorism
13.
Virulence-enhancing mechanisms of pathogens (e.g., the mobile
genetic elements—bacteriophages, plasmids, transposons)
Some Examples of Emerging and Reemerging Infectious Diseases.
Although diseases such as HIV are indicated in only one or two significant locations,
they are very widespread and a threat in many regions.
Public Health System
The public health system comprises of a
network of clinical microbiologists, nurses,
physicians, epidemiologists, and infection control
personnel who supply epidemiological
information to a network of local, state, national,
and international organizations.
The Role of the Public Health System
• Control of Epidemics (Three Types)
• First Type : Directed toward reducing or eliminating
the source or reservoir of infection
1. Quarantine and isolation of cases and/or carriers
2. Destruction of an animal reservoir of infection
3. Treatment of sewage to reduce water contamination
4. Therapy that reduces or eliminates infectivity of the
individual
Control of Epidemics (Three Types)
• The second type : designed to break the connection
between the source of the infection and susceptible
individuals. (general sanitation measures Transmission)
1. Chlorination of water supplies
2. Pasteurization of milk
3. Supervision and inspection of food and food handlers
4. Destruction of vectors by spraying with insecticides
Control of Epidemics (Three Types)
•
The Third type : reduces the number of
susceptible individuals and raises the general level
of herd immunity by immunization.
1. Passive immunization to give a temporary
immunity
following exposure to a pathogen or when a disease
threatens to take an epidemic form
2. Active immunization to protect the individual from the
pathogen and the host population from the epidemic
The most important is Surveillance – Observation,
recognition and reporting of Diseases as they occur.
Nosocomial Infections
• Nosocomial diseases - caused by bacteria, most of which are
noninvasive and part of the normal microbiota;
• Viruses, protozoa, and fungi are rarely involved.
• Source of Hospital Infection
• Endogenous sources - Patient’s own microbiota.
• Exogenous sources are other than the patient’s own microbiota .
• In either case the pathogen colonizing the patient may subsequently
cause a nosocomial disease.
The Hospital Epidemiologist
• The services provided by the hospital epidemiologist should include
1. Research in infection control
2. Evaluation of disinfectants, rapid test systems, and other products
3. Efforts to encourage appropriate legislation related to infection control, particularly
at the state level
4. Efforts to contain hospital operating costs, especially those related to fixed expenses
such as the DRGs (diagnosis related groups)
5. Surveillance and comparison of endemic and epidemic infection frequencies
6. Direct participation in a variety of hospital activities relating to infection control and
maintenance of employee health
7. Education of hospital personnel in communicable disease control and disinfection
and sterilization procedures
8. Establishment and maintenance of a system for identifying, reporting, investigating,
and controlling infections and communicable diseases of patients and hospital
personnel
9. Maintenance of a log of incidents related to infections and communicable diseases
10. Monitoring trends in the antimicrobial drug resistance of infectious agents
The Emerging Threat of Bioterrorism
• The Centers for Disease Control and Prevention
recently defined bioterrorism as
“The intentional or threatened use of viruses, bacteria,
fungi, or toxins from living organisms to produce death or
disease in humans, animals, and plants.”
• a few kilograms of anthrax can kill as many people as a
Hiroshima-size nuclear bomb
The goal of bioterrorism is to produce fear in the
population with subsequent disruption of society
Types of Epidemiological Studies Undertaken
• Descriptive Studies – The Person, The Place, The Time,
• Analytical Studies
- Cross-Sectional Studies : A cross-sectional study surveys a range of people to
determine the prevalence of any of a number of characteristics including disease,
risk factors associated with disease, or previous exposure to a disease-causing
agent.
- Retrospective Studies : A retrospective study is done following a disease
outbreak. This type of study compares the actions and events surrounding clinical
cases (individuals who developed the disease) against appropriate controls (those
who remained healthy).
- Prospective Studies : A prospective study is one that looks ahead to see if the risk
factors identified by the retrospective study predict a tendency to develop the
disease. Cohort groups, which are study groups that have a known exposure to the
risk factor, are selected and then followed over time.
• Experimental Studies
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