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Strategies for Control
Reading: pg: 441 – 451
Control of infectious diseases
•
Infection control can be effected by three approaches
(fig. 31.1)
•
drugs (chemotherapy)
•
vaccines (immunization)
•
better public health conditions
• Epidemiologic considerations
•
classes of infected hosts (fig. 31.2)
–
susceptible
–
infected but latent (non-infectious)
–
infected and infectious
–
recovered and immune
•
understand the
infectious agent
–
microparasites
(viruses, bacteria, fungi, protozoans)
–
macroparasites
(helminths, arthropods)
–
spread is
relative to reproductive rate of pathogen
–
fig. 31.3, fig.
31.4
–
case
reproductive rate or transmission potential (R0)
– average number of secondary cases of infection produced by
one primary infection in a completely susceptible population
–
effective
reproductive rate (R)
•
importance of
reproductive rates
•
influence of
behavior on spread of infection
•
transmission
between groups
•
Mixing of
different groups may affect the transmission of an infection
•
determined by
the timing of school terms and vacations
•
changes in
incidence of infection
•
Common directly
transmitted viral and bacterial infections show regular
peaks in incidences
•
Inter-epidemic
period
•
Seasonal
•
Longer term
•
transmission
success (fig. 31.7, fig. 31.9)
•
Varies due to
differences in demographics and behavior
•
Mass vaccination
can alter the incidence of infection, age of infection, and
pattern of fluctuation
•
Detection and Diagnosis
–
Important in understanding the epidemiology of a
disease so that control measures can be implemented
–
Descriptive epidemiology: used to identify the
pathogen and determine the source of infection
•
Case definition is important
Chemotherapy vs. vaccination
•
Vaccination
–
Edward Jenner
– first to vaccinate
–
Louis Pasteur
– discovered killed or weakened (attenuated)
microbes
•
Chemotherapy
–
Ehrlich –
discovered that certain chemicals can cause specific
damage to microbes
•
The central
concept of both chemotherapy and vaccination is selectivity
or specificity (fig. 31.10)
–
Specificity
of antimicrobial drug resides in its ability to damage the
microbe and not the host
–
Binds
specifically to certain microbial structures and damage
them – does not affect host due to lack of components or
differences in components.
–
Bacterial
cells: (Fig 31.12)
•
Antimicrobial agents
•
Specificity of vaccines depends
on antigenic determinants
•
Antigens introduced into the
body will induce host immune responses to protect against
reinfection by same pathogen (long-term protection)
•
Drugs and
vaccines have drawbacks:
–
Both are not
risk free
•
incorrect use
can give rise to toxicity
•
Hypersensitivity can result in severe consequences
•
Vaccines can
have unexpected side effects
•
Viruses are
more difficult targets for chemotherapy
–
Viral life
cycle uses host components
–
Viral targets
for chemotherapy:
•
Enzymes for
viral replication
•
Acyclovir
– for herpesviruses; targets DNA polymerase
•
Zidovudine
– for HIV; AZT, target reverse transcriptase
–
Vaccine –
many virus infections are prevented by vaccines
•
Drugs and
vaccines may lead to resistance by:
–
Penicillin
resistance – b-lactamase breaks down penicillin
b-lactam
ring
–
Chloroquine
resistance in malaria – pumps drug out of the parasite at
a very fast rate
–
Resistance to
vaccine – influenza virus due to antigenic variations
•
Drugs – treat
diseases, usually given regularly
•
Vaccine –
prevent diseases, often only given a few times
Control vs. eradication
•
Infections can be controlled by drugs or prevented by
vaccines
•
Eradication is more difficult
–
Persistence of reservoirs of
infections
–
Movement of people from
endemic areas
•
Smallpox – only disease to be
eradicated
•
fig. 31. 13 Factors important
in eradication programs
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