Download 4/4/12 Epidemiology and Transmission

32.1 The Science of Epidemiology
• Epidemiology
– The study of the occurrence, distribution, and
determinants of health and disease in a
population
• In developed countries infectious diseases cause
fewer deaths than noninfectious diseases
– Worldwide, infectious disease accounts for 30%
of all deaths
• Even in developed countries, new diseases
are emerging
– Example: West Nile fever
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Figure 32.1
Mortality per 100,000
1000
Mortality per 100,000
800
600
80
60
40
20
0
1970
1980
1990
400
200
0
1900
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1920
1940
1960
Year
1980
32.2 The Vocabulary of Epidemiology
• A disease is an epidemic when it occurs in a large
number of people in a population at the same time
(Figure 32.2)
– A pandemic is widespread, usually worldwide
• An endemic disease is constantly present in a
population, usually at low incidences
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Figure 32.2
Endemic
disease
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Epidemic
disease
Pandemic
disease
32.2 The Vocabulary of Epidemiology
• A disease outbreak occurs when a number of
cases of a disease are reported in a short period
of time
• Mortality is the incidence of death in a population
• Morbidity of a disease refers to the incidence of
disease including fatal and nonfatal diseases
• Reservoirs are sites in which infectious agents
remain viable and from which infection of
individuals can occur
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32.3 Disease Reservoirs and Epidemics
• Zoonosis is any disease that primarily infects
animals, but is occasionally transmitted to
humans
– Control of a zoonotic disease in the human
population may not eliminate the disease as a
potential public health problem
– Certain infectious diseases have complex life
cycles involving an obligate transfer from a
nonhuman host to humans followed by transfer
back to the nonhuman host
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32.4 Infectious Disease Transmission
• Epidemiologists follow transmission of a
disease by correlating geographic, climatic,
social, and demographic data (Figure 32.3)
– Used to identify possible modes of
transmission
– For example, a disease limited to a tropical
area may suggest something about its vector
(e.g., malaria)
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Figure 32.3
50
45
40
Cases
35
30
25
20
15
10
5
0
2000
2001
2002
Month–year
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2003
32.4 Infectious Disease Transmission
• Pathogens can be classified by their mechanism
of transmission, but all mechanisms have the
following stages in common:
– Escape from host
– Travel
– Entry into new host
• Pathogen transmission can be direct or indirect
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32.4 Infectious Disease Transmission
• Direct host-to-host transmission
– Infected individual transmits a disease directly to
a susceptible host without the assistance of an
intermediary (e.g., flu, common cold, STDs,
ringworm)
• Indirect host-to-host transmission
– Occurs when transmission is facilitated by a
living or nonliving agent
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Vector borne disease
Pathogens are carried by someone or something
that does not contract the disease
Arthropods (mosquitos, mites, ticks, fleas)
Water
Medical equipment
Vector borne diseases tend to be more severe since
there is no need to keep previous host healthy
enough to transmit pathogen
Transmission and symptoms
Successful pathogens tend to manipulate the host immune
system to increase the likelihood of transmission to new hosts
•respiratory pathogens cause coughing or sneezing
•water-borne pathogens cause diarrhea
•arthropod-borne pathogens often cause lethargy which
allows bites by other insects
•STDs cause lesions on genitals which secrete pathogen
during sex with new potential host
32.5 The Host Community
• Coevolution of a host and its parasite is common
– Virulence of the parasite in host-to-host
transmission diminishes and resistance of the
host increases (e.g., myxoma virus introduced to
control rabbits in Australia; Figure 32.5)
• A host-to-host pathogen that kills its host before it
can infect another host may become extinct
– If a pathogen does not rely on host-to-host
transmission it may remain extremely virulent
(e.g., E. coli in hospitals)
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Figure 32.5
100
100
Rabbit mortality
60
90
40
Virus
virulence
20
80
0
0
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1
2
3
Years
4
5
6
Virus virulence (%)
Rabbit mortality (%)
80
32.5 The Host Community
• Herd immunity is the resistance of a group to
infection due to immunity of a high proportion of
the group (Figure 32.6)
– If a high proportion of individuals are immune to
an infection then the whole population will be
protected
– Immunized people protect nonimmunized people
because the pathogen cannot be passed on and
the cycle of infectivity is broken
• Diseases such as influenza tend to occur in cycles
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Figure 32.6
Susceptible
B
A
Infected
Immune
C
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32.6 The HIV/AIDS Pandemic
• Acquired immunodeficiency syndrome (AIDS) is
a viral disease that attacks the immune system
– First reported cases were in the U.S. in 1981
– At least 70,000,000 people have been infected
worldwide with HIV
– More than 25,000,000 people have died from AIDS
• Studies in the U.S. suggested the virus was
transmitted through sexual contact or blood
(Figure 32.8)
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Figure 32.7
Total AIDS cases, United States
80,000
70,000
60,000
50,000
40,000
30,000
20,000
10,000
1981
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1985
1990
1995
Year
2000
2005
Figure 32.8
0.89%
4% 0.3%
Male-to-male
sexual contact
9.9%
Heterosexual
contact
14.5%
71.3%
Males
Injection
drug use
Male-to-male
sexual contact
and injection
drug use
Other
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16.5%
82.7%
Females
32.7 Healthcare-Associated Infections
• Healthcare-associated infections (HAIs)
– Result from an infectious agent acquired at a
healthcare facility (Figure 32.9)
– Also referred to as nosocomial infections
– There are about 1,700,000 HAIs per year in
the U.S.
– Some are acquired from other patients, but
others are caused by pathogens that are selected
for and maintained by the hospital environment
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Figure 32.9
Other
17%
Surgical
site infections
22%
Respiratory
tract
infections
15%
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Bloodstream
infections
14%
Urinary tract
infections
32%
32.7 Healthcare-Associated Infections
• Infectious diseases are spread in hospitals for
several reasons:
– Patients have low resistance to infectious disease
– Healthcare facilities treat infectious disease
patients
– Multiple patients in the same room
– Healthcare personnel move from patient to patient
– Healthcare procedures may breach the skin and
introduce infection
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32.7 Healthcare-Associated Infections
• Infectious diseases are spread in hospitals for
several reasons (cont’d):
– Newborn infants are susceptible to infection
– Surgical procedures expose organs to
contamination
– Certain drugs increase a patient’s susceptibility
to infection
– Use of antibiotics has selected for antibioticresistant organisms
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32.9 Global Health Considerations
• Infectious diseases in Americas vs. Africa
(Figure 32.10)
– Death rate in Africa is much higher
– Most African deaths are due to infectious
diseases (10 as many as in the Americas)
• Concern for people traveling to other areas
– Travelers can be immunized
– Drink only decontaminated water
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Figure 32.10
Africa 2004: 11.2 million deaths
Intentional 3%
Injuries 5%
Other 8%
Respiratory
3%
Cardiovascular
diseases 11%
Infections
56%
Maternal and
perinatal 10%
Cancer
4%
The Americas 2004: 6.2 million deaths
Intentional
4%
Injuries
10%
Other
13%
Infections
10%
Cancer
20%
Diabetes
4%
Respiratory
6%
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Cardiovascular
diseases 32%
Maternal and
perinatal 3%