BIO4503 APPLIED EPIDEMIOLOGY
EPIDEMIOLOGY OF
INFECTIOUS/
COMMUNICABLE
DISEASES
1
Dr. Carmen Aceijas, PhD
Lecture description
At the end of this lecture….
1. Students will have been introduced to main definitions and related concepts of
communicable diseases
2. A description on the basic epidemiological model for infectious diseases and main
strategies for disease control will have been provided
3. Students will be familiar with the role of communicable diseases in historical and
current mortality trends.
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DEFINITION AND RELATED CONCEPTS
Def: “Illness due to a specific infectious agent or its toxic products that arises
through transmission of that agent or its products from an infected person,
animal or inanimate reservoir to a susceptible host” (Heyman D.L. 2004)
Related concepts:
Control: reduction of incidence, prevalence, morbidity and mortality to a locally
acceptable levels....
Elimination of the disease: reduction to 0 of the incidence of a disease in a
specified geographical area as a result of deliberate efforts such as
continued intervention measures...
Elimination of the infection : reduction to 0 of the incidence of the infection
caused by an specific agent in a specified geographical area as a result of
deliberate efforts such as continued intervention measures
Eradication: The specific infectious agent no longer exists in nature or lab.
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THE AGENT: MICROBES
• Bacteria: Bacteria are single-celled organisms. Bacteria have
the tools to reproduce by themselves. They are larger than
viruses.
• Virus: Viruses have a core of genetic material, but can not
reproduce it on their own. Viruses infect cells and take over
their reproductive machinery to reproduce.
• Fungi: Fungi are like plants made up of many cells. They are
not plants because they cannot produce their own food from soil
and water.
• Protozoa: Protozoa are very small. Most live in water. They are
parasites, which means they live off other organisms, in some
cases humans. Malaria is a parasitic protozoan.
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INFECTIOUS DISEASES – CHRONIC INFECTIONS
• Infectious disease contribute to the
pathogenesis of chronic diseases too.
• Examples:
– HPV > cervical cancer
– Hep B* and C > hepatic cancer and cirrhosis
– Chlamydia pneumonial > CVD
*vaccine preventable
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THE EPIDEMIOLOGICAL TRIAD
• Traditional model of causation of infectious
diseases.
• Based in three components: external agent,
susceptible host and environment
host
agent
environment
Source: Bailey et al, 2005
NATURAL HISTORY OF DISEASES
• Def: Progress of the disease process in a individual
over time and in the absence of intervention
Key
events
Exposure to or
completion of a
sufficient cause
Onset of
symptoms
Diagnosis
time
Incubation/
latency
period
time
Disease
stages
Susceptib
ility
Subclinical
disease
Clinical
disease
Recovery,
disability or
death
Related concept: PROGNOSIS = estimate of an individual outcome taking into account
the natural history of a disease and other risk factors
CLASSIFICATION OF COMMUNICABLE DISESASES BY PRINCIPAL MODES OF
TRANSMISSION
mode
method
examples
DIRECT
Airborne
[droplet and
aerosols]
Measles, streptococcal diseases, upper and
lower respiratory tract diseases, TB, legionnaire’s
disease, influenza
DIRECT
Physical contact
Leprosy, impetigo, scabies, anthrax
DIRECT
Sexual contact
HIV, syphilis, gonorreha, genital herpes, hep B,
Chlamydia, HPV
INDIRECT
Blood
Blood products
HIV, hep B, Hep C
INDIRECT
Oral-faecal.
Hygiene. Foodborne. Water-borne
Cholera, salmonella, botulism, worms, hep A
INDIRECT
Transcutaneous
Vector-borne via insects [malaria, viral
hemorrhagic fevers]
Animal bite [zoonoses]: rabies
Health care: hospital infections, HIV, Hep B
Self injected: HIV, Hep B [among IDUs]
VERTICAL
Congenital.
Congenital rubella, congenital syphilis, HIV,
cases humans.
Malaria is agonorrhea,
parasiticChlamydia
protozoan, as is Giardia.
Maternal-foetal
INFECTIOUS DISEASES AND
THEIR ROLE IN WORLD MORTALITY
• Historically the most powerful cause of disease and death
• Last 50 years approx.: switch from infectious diseases to
chronic diseases as prevalent main causes of mortality.
• However, three still cause massive no. of deaths:
- Pneumonia
- Influenza
- HIV
• acute infectious diseases still leading cause of deaths among
children and young adults in low income countries accounting
for 50% of all deaths in such countries.
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POVERTY AND INFECTIOUS DISEASES
• Malnutrition and micronutrients deficits are associated with
increased risk of morbidity and mortality from a wide range of
infectious diseases
• Lack of education [assoc. with poverty] impairs basic
measures to prevent infectious diseases [e.g.: hand washing
after defecation]
• Lack of access to basic sanitation measures [e.g.: lack of
access to drinkable water and lack of access to basic
devices to dispose human waste]
• Household crowding
• Lack of access to health care
WORLDWIDE MORTALITY DUE
TO INFECTIOUS DISEASES
Rank Cause of death
Deaths 2002 Percentage of
(in millions)
all deaths
Deaths 1993
1993 Rank
(in millions)
N/A
All infectious diseases
14.7
25.9%
16.4
1
Lower respiratory infections
3.9
6.9%
4.1
1
2
HIV/AIDS
2.8
4.9%
0.7
7
3
Diarrheal diseases
1.8
3.2%
3.0
2
4
5
6
7
8
9
10
11
Tuberculosis (TB)
Malaria
Measles
Pertussis
Tetanus
Meningitis
Syphilis
Hepatitis B
1.6
1.3
0.6
0.29
0.21
0.17
0.16
0.10
2.7%
2.2%
1.1%
0.5%
0.4%
0.3%
0.3%
0.2%
2.7
2.0
1.1
0.36
0.15
0.25
0.19
0.93
3
4
5
7
12
8
11
6
12-17 Tropical diseases
0.13
0.2%
0.53
9, 10, 16-18
32.2%
Note:
Other causes of death include maternal and perinatal conditions (5.2%), nutritional
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deficiencies (0.9%),noncommunicable conditions (58.8%), and injuries (9.1%).
FACTORS LEADING TO EMERGENCE OF
INFECTIOUS DISEASES
•
•
•
•
•
•
•
•
•
•
•
•
•
AIDS
Population growth
Speed and ease of travel
Dam building
Global climate change
Increased antibiotic use for humans and animals
Encroachment of human populations on forest habitats
Industrial commercial agriculture
War and social disruption
Relocation of animals
Growth of daycare
Aging of the population
Human-animal contact
MAIN APPROACHES TO
INFECTIOUS DISEASES
• Preventing exposure
• Immunization
• Treatment:
– to restore health
– to prevent further spread
• Improving diagnosis and early detection
systems
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TYPES OF CONTROL
INTERVENTIONS FOR INFECTIOUS
DISEASES
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1. Vector control [malaria, dengue, yellow fever, river
blindness]
2. Vaccination [smallpox, measles, polio, neonatal tetanus,
diphtheria, pertusis, tetanus, hep B, meningococcal
meningitis, yellow fever]
3. Mass chemotherapy [hookworm, Guinea worm, STIs]
4. Improved sanitation [diarrheal diseases]
5. Animal and food control interventions
6. Improved care seeking and provision [diarrheal diseases,
acute respiratory infections, neonatal tetanus]
7. Behavioural change [HIV and other STIs, diarrheal
diseases]
8. Occupational measures
CONTROL OF COMMUNICABLE
DISEASES: SURVEILLANCE
Def: “continuous scrutiny of all aspects of occurrence and spread of disease
pertinent to effective control of that disease” (Tulchinsky, 2009)
Surveillance of infectious diseases. Aspects to survey:
1. Morbidity reports
2. Mortality reports
3. Reports from selected sentinel sites
4. Special field investigations of epidemics or individual cases
5. Laboratory monitoring of infections and therapeutic response in population
samples
6. Data on supply, use and side effects of vaccines, toxoids, immunoglobulin
7. Data on vector control activities [e.g.: mosquito nets use]
8. Immunity levels in samples of the populations at risk
9. Review of literature
10. Epidemiologic and clinical reports for other neighbouring jurisdictions
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PREVENTION AND CONTROL AS GLOBAL
HEALTH ISSUE. A CASE STUDY:
POLIOMYLITIS
• Caused by poliovirus serotypes
• Faecal-oral route
• 0.1 – 1% of infections will develop
symptomatic polio
• Of those developed paralysis, 10% die
acutely, 10-15% permanently unable to
walk, 10-15% unable to walk normally
• Eradication intervention: Polio Vaccines
(oral & injectable)
• 1980s 350,000 cases reported, by 2006 it
had been reduced to under 2000
CASE STUDY: POLIOMYLITIS
• 1st > POLITICAL WILL: 1988 WHA*’s goal
- To interrupt polio transmission worldwide by 2000
• 2nd: DEFINITION AND IMPLEMENTATION OF STRATEGIES
AND INTERVENTIONS (goal focused):
- Routine immunization of infants
- Annual national immunization days
- House-to-house mop-up campaigns for missed cases
- Sensitive surveillance for and lab testing of cases with acute
flaccid paralysis to identify cases
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World Health Assembly. The governing body of WHO
A CASE STUDY: POLIOMYLITIS
[cont.]
• RESULTS
- Last case in Western Hemisphere: Peru 1991.
- Three WHO regions (Americas, Western Pacific
and Europe) polio free since 2002
- Decrease of no. of polio endemic countries: 125
in 1988 to 6 [Afghanistan, Egypt, India, Niger,
Nigeria and Pakistan] nowadays.
- Mayor resurgences of polio observed in some
Indian states, northern Nigeria] prompting
reimplementation of interventions.
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RECOMMENDED READING
• Tulchinsky T and Varavikova E. The New Public Health [2nd edition]. Chapter
4.
• Nelson (2007) Epidemiology of Infectious Diseases. Johns Hopkins
University.
• Barreto et al (2006) Infectious diseases epidemiology. Jurnal of epidemiol
community health 60: 192-95:
• Liu L, Johnson HL, Cousens S, et al. Global, regional, and national causes of
child mortality: an updated systematic analysis for 2010 with time trends
since 2000. The Lancet. 2012; 379(9832): 2151-2161.
• WHO (2008) Disease and injury regional estimates: cause-specific mortality:
regional estimates for 2008. At:
www.who.int/healthinfo/global_burden_disease/estimates_regional/en/index.
html.
• And even something for fun!
http://www.mclph.umn.edu/watersedge/play.html