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Review of the work on contagious disease by Koch.
Epidemiology and Public Health.
Microbial interactions with higher animals.
Examination of the normal flora of animals.
Entry of the pathogen into the host. Colonisation
and growth.
Transmission of pathogens. Bacterial respiratory
infections and sexually transmitted bacterial
diseases.
Insect transmitted diseases.
Food-borne and water-borne bacterial diseases.
Epidemiology
• Study of occurrence, distribution, and control of
diseases within populations.
• Infectious disease control has been extremely successful
in developed countries (see previous lecture)
• However, worldwide, infectious diseases are still
responsible for 30% of 56 million annual deaths – a big
problem especially for developing countries.
• The developed world is also at risk (international travel,
climate change, etc).
• New infections like West Nile fever are emerging and
diseases like TB are reemerging.
• Scientific, medical, economic, educational solutions are
required to keep infectious diseases under control.
Microorganisms and Disease in
the USA
2008
1900
Influenza and
pneumonia
Tuberculosis
Heart disease
280
Cancer
205
Stroke
Pulmonary
disease
Accidents
Influenza and
pneumonia
Diabetes
Gastroenteritis
Heart disease
Stroke
Kidney disease
Accidents
AIDS
Cancer
Suicide
Infant diseases
Cirrhosis of
the liver
Homicide
Diphtheria
0
100
Deaths per 100,000 population
200
0
100
200
Deaths per 100,000 population
• Red: microbial disease
Brock 12th ed. Fig. 1.8
• Green: non-microbial disease
Deaths caused by infectious
disease in the USA
• A general decrease
throughout 20th century.
• Why a spike at 1918?
• A significant increase in
infectious disease
mortality from 1980
onwards
Pathogens
• Pathogens often kill the host – if so why doesn’t the
pathogen become extinct?
– A pathogen must grow and reproduce in the host in
order to cause disease.
– Pathogens can be host-dependent or hostindependent for survival.
– A well adapted pathogen lives in balance with host
leading to a chronic (long-term) infection. When
there is equilibrium between host and pathogen,
they both survive, however if the host’s defence is
low due to poor diet, age and other factors, then
the host might get damaged.
Pathogens
•A newly arisen pathogen may cause more
serious infection – such a situation may
arise when a species barrier is crossed
(e.g. bird flu, swine flu). Such pathogens
usually cause acute infections. In these
cases, both the pathogen and the host
play an important role in each other’s
evolution.
•In some cases, the pathogen might not depend on the host for
survival. Organisms of the Clostridium genus live in soil but may
accidentally infect humans (tetanus or botulism). In this case,
the death of the temporary host will not affect general survival
of the pathogen.
The epidemiologist
• Traces the spread of disease.
• Determines the origin of disease
and mode of transmission.
• Gathers data from:
– Clinical studies
– Disease reporting surveys
– Insurance questionnaires
– Patient interviews
• Aim is to define common
factors that constitute a
disease in order to improve and
maintain public health measures
and effectively control disease.
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Epidemiological terms
Prevalence of disease: Proportion of diseased individuals in a population in a
given period of time
Incidence of disease: Number of new cases of disease in a given period of time
Epidemic: Unusually high number of diseased individuals occurring in a
population at the same time
Pandemic: Widespread, usually worldwide epidemic
Endemic: Low incidence but constant presence of disease in a population
Epidemiological terms (cont.)
Sporadic cases of disease: Individual cases of disease
that occur in a scattered manner.
Outbreak: A sudden increase in the number of disease
cases in a relatively short period of time.
Subclinical infections: Infections where the diseased
individuals show mild symptoms or no symptoms at all.
Carriers: Individuals subclinically infected – showing few
or no symptoms. They maybe actively carrying and
spreading the pathogen.
http://www.microbiologyplace.com – Epidemiology: Occurrence of Diseases
Mortality and morbidity
• Mortality
– The incidence of
death in a
population
• Morbidity
– The incidence of
disease (fatal and
non-fatal cases).
– Many diseases cause
illness but not
death. Thus
morbidity statistics
reflect the status
of the general
health of a
population better
http://ps4h.org/communicable_diseases.html
Morbidity Rates for Poliomyelitis in Cuba
http://www.medicc.org/publications/medicc_review/I/varied/html/inmuno.html
Worldwide Deaths Due to Infectious Diseases
Typical progression of disease
for microbial infections
• Infection – organism invades, colonises and grows in host
• Incubation period- time period between infection and
symptoms (depends on organism virulence, inoculum size,
host defence status, distance between entrance site and
target site. Towards end of incubation symptoms appear .
• Acute period - height of disease (fever, aches etc)
• Decline period - symptoms subside
• Convalescent period - return to normal health
Reservoirs of disease and epidemics
• A reservoir is a site where infectious agents are located
– Reservoirs might be animate or inanimate (see next two slides
or table 33.2 in Brock)
– A number of pathogens require a living organism (humans and/or
animals) as a reservoir for their existence. These (especially
pathogens that live only in humans) can usually be controlled and
even be eradicated.
– On the other hand, some pathogens are saprophytic and only
incidentally infect humans and cause disease. These pathogens,
therefore do not need to infect a host for survival.
• E.g. Clostridium tetani – causative agent of tetanus, normally
inhabits soil.
- Zoonosis: A disease that primarily infects animals, but
occasionally transmitted to humans. E.g. SARS, bird-flu, swineflu.
Epidemic Diseases: Agents, Sources,
Reservoirs, & Control
Epidemic Diseases: Agents, Sources,
Reservoirs, & Control
Epidemic Diseases: Agents, Sources,
Reservoirs, & Control
Epidemic Diseases: Agents, Sources,
Reservoirs, & Control
Disease carriers
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•
A carrier is a pathogen infected
individual that has no obvious signs of
disease.
They might be at incubation stage of
disease – acute carriers.
Chronic carriers may be infected for
long periods with no outward symptoms
and therefore may spread disease for
extended periods of time.
Carriers can be identified by testing
(culture or antibody).
Typhoid Mary (Mallon) was a cook in
New York (early 20th century). She was
probably a carrier of Salmonella typhi
(typhoid).
Infected 47 people – 3 died! She
rejected all accusations that she might
be responsible.
She was eventually imprisoned because
she refused to have her Gall bladder
removed.
Infectious disease transmission
• Epidemiologists
correlate infection with
season, geography, age
group etc.
• California encephalitis
rises in summer and falls
in winter (mosquito
borne viral disease).
• Disease follows mosquito
prevalence
Infectious disease transmission
• Host to host transmission is the key to pathogen
survival.
• Pathogens can be classified by their transmission
mechanisms (direct or indirect), which have 3
stages in common:
(1) Escape from host
(2) Travel
(3) Entry into a new host
Infectious disease transmission
Direct host-to-host transmission:
•Direct transmission of infection to a susceptible host by an
infected host without the assistance of an intermediate agent.
E.g. Flu (influenza virus), common cold (rhinoviruses), STIs (syphilis
– Treponema pallidum; gonorrhea – Neisseria gonorrhoeae), boils and
pimples (staphylococci), ringworm (fungi) .
•Droplets resulting from sneezing or coughing most often cause
direct transmission of upper respiratory tract infections such as flu
and the common cold.
Infectious disease transmission
Indirect host-to-host transmission:
•Transmission of infection to a susceptible host by an
infected host with the assistance of an intermediate
agent (living or nonliving).
•Living agents are called vectors. E.g. arthropod insects
or vertebrates.
•Nonliving/inanimate agents are called fomites and
these include bedding, toys, books, cutlery, surgical
instruments.
•Food and water are potential disease vehicles.
Common source epidemics
•
Arises due to infection of a large
number of people from a
contaminated common source
(food, water supply).
•
Foodborne and waterborne
common source epidemics;
intestinal diseases - pathogen
leaves the body in faecal material,
contaminates food or water
supplies, enters the intestinal
tract of the recipient during
ingestion.
•
Classic example is cholera – faecal
contamination of drinking water.
Vibrio cholerae is the infectious
agent.
•
Usually a common source epidemic
has a rapid rise to peak
(simultaneous infection)
Cholera ward in Bangladesh
http://www.microbiologyplace.com –
Epidemiology: Transmission of Disease
Host-to-host epidemics
• Typically has a slow progressive
rise and a gradual fall.
• Might be initiated by a single
infected person in a population.
• Pathogen spreads to
susceptible individuals where it
replicates and is spread to
other susceptible individuals.
• Influenza and chickenpox are
examples of diseases that are
typically spread in host-to-host
epidemics.
Flu epidemic
Chicken pox blisters
Origins of epidemics (Brock Figure 33.4)
Herd Immunity
• Resistance of a group to infection due to immunity of a high
proportion of its members.
• Must assess immune state of population.
• If a high enough proportion are immune then whole population is
effectively protected.
• If immunity is absent, even poorly infective agents can be
transmitted person-to-person. E.g. H5N1 avian influenza cases in
humans.
• Need data from immunization programmes to estimate the
proportion of population that is immune in order to prevent
infection in the rest of the population.
• Polio: if 70% immunized then all population is protected but depends
on population density – this % depends on transmissibility (e.g.
chickenpox needs 90% immunization).
• Sometimes, immunized people can be carriers e.g. diphtheria. This is
due to immunisation being protective against the effects of
diphtheria toxin, but not necessarily against its causative agent
Corynebacterium diphtheriae.
Herd immunity
Infected person (red) transmits disease to susceptible people (blue).
Immunized people (yellow) prevent spread and thus protect person B
and C.
Epidemiology and public health
• Identification, containment and eradication of disease.
• Public health refers to the health of the general
population and the measures taken by public health
authorities in the control of disease.
• Improvement of basic living conditions (sewage
treatment, clean drinking water, better nutrition, less
cramped living- all help).
• Quarantine and vaccination have played a major role in
controlling several diseases, including small pox, typhoid
fever, diphtheria, poliomyelitis
Control of disease
• Control the reservoir
– Domestic animals – treat, immunise
– Wild animals – eradicate (badgers, bovine TB) – more difficult
– Rabies would require elimination of all wild animals!
– Insect (mosquito) – insecticides – but environmental risks.
– What if reservoir is human (e.g. HIV or TB)? – quarantine,
control spread, immunise if possible. Strategy used for
eradication of smallpox successfully and is currently being used
for eradication of polio.
Control of disease
• Transmission control
– Pathogens in food or
water eradicated by
treatment (example is
milk pasteurization and
control of bovine TB in
humans).
– Respiratory pathogens
more problematic – air
filtration not feasible.
Face masks (Japan)
Control of disease
• Immunization
– Many diseases have been controlled by immunization (e.g.
Smallpox, diphtheria, tetanus, pertussis, measles, mumps,
rubella, polimyelitis)
– 100% immunization not necessary (herd-immunity)
• Vigilance must be ensured to ensure appropriate level of
immunization. Complacency a problem. Due to decline of
immunity over time, a high percentage of adults lack
effective immunity to important infectious diseases.
Therefore, update of immunity in adults (boosters – 10
years) - a necessary measure in controlling disease.
• Autism scare in UK from MMR – probably unfounded but now
measles on rise due to low uptake of vaccination.
Control of disease
• Quarantine
– Restricting the movement of infected individuals until
they are no longer infectious.
– Time-limit will be the longest period of communicability
of that disease. May simply mean the individual stays at
home when infected.
– Isolation may be used for severe infectious diseases.
– International agreement for quarantine include:
Smallpox, cholera, plague, yellow fever, typhoid fever,
relapsing fever.
Control of disease
• Surveillance
– Observation, recognition, reporting of disease.
– The list might change and will vary from season to season
and from country to country.
– CDC – centre for disease control and prevention through
the National Centre for Infectious Diseases (NCID).
– National databases are formed through reported
diseases, which are then used to formulate and
implement plans for isolation, diagnosis, treatment and to
stop their spread.
Control of disease
• Pathogen eradication
– Aim is to remove all of a pathogen from any reservoir.
– Several pathogens have effectively been eradicated by
immunization programmes (WHO eradicated smallpox
1980)
– Polio eradicated from Western hemisphere using same
strategy.
– Leprosy is targeted and can be treated by multidrug
therapy
– Chagas’ disease (Trypanosoma cruzi) elimination underway
by vector control and treatment of active cases.
Emerging and Reemerging Infectious
Diseases
• Worldwide disease distribution changes rapidly.
• Emerging diseases are those that suddenly become
prevalent. E.g. syphilis, plague, legionellosis, AIDS and
Lyme disease.
• Reemerging diseases are those that have become
prevalent after having been previously under control.
E.g. TB
• See Table 33.8 in Brock.
Emerging and Reemerging Infectious
Diseases
• Emergence Factors
1.
2.
3.
4.
5.
6.
7.
Human demographics and behavior
Technology and industry
Economic development and land use
International travel and commerce
Microbial adaptation and change
Breakdown of public health measures
Abnormal natural occurrences
Emerging and Reemerging Infectious
Diseases
•Addressing emerging diseases:
1. Recognition of disease
-Surveillance
2. Intervention to prevent pathogen transmission
- In order to isolate outbreaks and prevent
spread of specific diseases; quarantine, immunisation
and drug treatment must be employed as means of
public health response.
•Vectorborne and zoonotic diseases, the animal host or
vector must be identified, life cycle of the pathogen
must be interrupted and transmission to humans must
be prevented. E.g. Controlling the emergence of SARS
Directed Reading:
•Hughes J. M. 2001. Emerging Infectious Diseases:
A CDC Perspective. Emerging Infectious Diseases,
7, 494-496.
•Madigan et al., 2009. Brock (12th ed). Chap.33
Biological Warfare and Biological
Weapons
• Biological warfare is the use of biological agents
to kill a military or civilian population
• Biological weapons are organisms or toxins that
are
– Easy to produce and deliver
– Safe for use by the offensive soldiers
– Able to incapacitate or kill individuals under attack in
a consistent and reproducible manner
Anthrax as a Biological Weapon
• Bacillus Anthracis is a preferred agent for
biological warfare and biological terrorism
– Endospores enhance ability to disseminate
B. anthracis in aerosols
• Three forms of the disease
– Cutaneous anthrax
– Gastrointestinal anthrax
– Pulmonary anthrax
Anthrax as a Biological Weapon
• Weaponized Anthrax
– Refers to preparations of B. anthracis that exhibit
properties to enhance dissemination
• Vaccination is only done on people at risk
• Treatment of anthrax is done with Ciprofloxacin
– Small particle size interspersed with a very fine
particulate agent (usually talc)
Bacillus anthracis
Cutaneous Anthrax
Inhalation Anthrax