Epidemiology and the Spread of Disease

Question 1

What is epidemiology?

Answer 1

Epidemiology is the study of when and where diseases occur and how they are spread.

In most developed countries, infectious disease is no longer perceived to be a significant threat to public health because of sanitation, vaccination and antibiotics.

A few infectious diseases still remain a major public health concern (e.g., tuberculosis).

Furthermore, new infectious diseases are continuously emerging.

Question 2

How many deaths worldwide are due to infectious diseases?

Answer 2

Worldwide, 30% of all deaths are due to infectious diseases, many of which could be prevented with scientific, medical, economical and educational solutions.

Question 3

What is the incidence of disease?

Answer 3

The fraction of a population that contracts (i.e., newly acquires) the disease during a specific time period.

Question 4

What is the prevalence of disease?

Answer 4

The prevalence of disease is the fraction of a population having symptoms of the disease during a specific time period (includes both newly acquired and established cases).

Question 5

What is a sporadic disease?

Answer 5

Occurs occasionally.

They are often recorded as individual cases in geographically separated areas, implying that they are cases that are not related.

Question 6

What is an endemic disease?

Answer 6

Occurs continually at a relatively low level.

The pathogen that causes the disease may not be very virulent, or the majority of the individuals in a given area may be immune to the pathogen.

Question 7

What is an epidemic disease?

Answer 7

Occurs sporadically at an elevated level and occurs continuously at a low level between these sporadic outbreaks.

Question 8

What is a pandemic disease?

Answer 8

A widely distributed epidemic.

Question 9

What is a disease outbreak?

Answer 9

When the number of disease cases increases in an area which previously had experienced only sporadic cases of the disease.

Question 10

What are resevoirs of infection?

Answer 10

Sites in which viable infectious agents remain and from which infection of individuals may occur.

Question 11

Humans that carry and shed the infectious agent are human resevoirs.

What three categories do they make up?

Answer 11

These individuals may be:

symptomatic, i.e., currently have the disease

convalescent, i.e., recovering from the disease and still harbour the infectious agent

carriers, i.e., have no disease symptoms but carry the pathogen. Carriers may be in the incubation period of the disease which precedes the development of symptoms. These individuals would be described as acute carriers because their carrier state is transient.

Question 12

Some diseases occur in both humans and animals, so there are animal resevoirs of disease as well.

What are diseases that occur primarily in animals that can be transmitted to humans called?

Answer 12

zoonoses

Question 13

What are chronic carriers?

Answer 13

Individuals who have recovered from the clinical disease but still harbour the infectious agent, or they may be individuals that carry the pathogen as part of their microbiota without the occurrence of disease.

Question 14

Discuss insect reservoirs.

Answer 14

Many diseases are transmitted by insects such as fleas, lice and flies.

Insects that fly also have significant epidemiological implications as they have broader home ranges than non-flying insects, thus making control more difficult.

Question 15

Discuss inanimate reservoirs.

Answer 15

Inanimate or non-living reservoirs – some diseases are present and are transmitted from water or soil.

Question 16

What is contact transmission?

Answer 16

Disease is transmitted by direct human contact (e.g., skin pathogens).

Question 17

What is common vehicle transmission?

Answer 17

Disease is transmitted by inanimate objects (fomites), food, water, blood, drugs (e.g., gastrointestinal pathogens).

Question 18

What is airborne transmission?

Answer 18

Disease is transmitted by droplets or dust (e.g., respiratory pathogens).

Question 19

What is vector transmission?

Answer 19

Disease is transmitted by living agents such as insects, ticks, mites, fleas, rodents and other animals (e.g., skin and blood-borne pathogens).

Question 20

What is virulence?

Answer 20

The relative ability of a pathogen to cause disease and can be quantified by experimental studies to determine the infectious and lethal doses.

Question 21

What is the infectious dose?

Answer 21

The infectious dose (ID) is the number of bacteria required to produce disease.

The infectious dose varies considerably between different organisms.

• Shigella dysenteriae* – the causative agent of dysentery has an ID of 1–10 bacteria.
• Salmonella typhimurium* – a causative agent of food poisoning has an ID of 1,000,000.

Question 22

What is the lethal dose?

Answer 22

The lethal dose (LD) is the number of bacteria required to kill.

This is usually determined in laboratory animals and is expressed as the number of bacteria required to kill 50% of the population i.e., the LD50.

The route of entry of the pathogen into the body affects the dose.

• S. typhimurium* injected intravenously into mice has an LD50 of 10
• S. typhimurium* given orally to mice has an LD50 of 5,000,000.

Question 23

What is a nosocomial infection?

Answer 23

Nosocomial infections are those infections that are acquired by patients as a consequence of hospitalization.

This occurs in about 5% of all patients admitted to hospital. In the U.S., there are approximately 2 million nosocomial infections leading to 100,000 deaths each year.

Question 24

What factors may result in nosocomial infection?

Answer 24

• hospitals treat patients with infectious disease, and these patients may be reservoirs of pathogenic organisms
• patients are exposed to numerous pathogens in a hospital setting, many of which may be resistant to antibiotics
• patients often have weakened immune systems (i.e., they are a compromised host) which make them more susceptible to infection
• pathogens can be transferred from patient to patient by hospital staff or visitors. Often these drug resistant pathogens can be found as the microbiota of the hospital staff.

Question 25

What is the difference between an endogenous nosocomial infection and an exogenous one?

Answer 25

Nosocomial infections can be endogenous (i.e., caused by the microbiota) or exogenous (i.e., transmitted from another individual).

Question 26

What is an iatrogenic infection?

Answer 26

Iatrogenic infections (literally meaning doctor induced) are a result from surgery, invasive diagnostic procedures, or medical implant devices such as catheters.

Question 27

How may the spread of nosocomial infections be minimized?

Answer 27

By isolating highly infectious patients and by good housekeeping measures (e.g., disinfection of all surfaces such as floors, door handles and beds), barrier protection (e.g., the wearing of gloves, caps, gowns), and good hand-washing practices.

Question 28

What is the Walkerton crisis?

Answer 28

An example of a tragic acute outbreak involving concepts covered in the last three sections: ‘Therapy and Prevention’, ‘Isolation and Identification’ and ‘Epidemiology and the Spread of Disease’.

Question 29

How was the Walkerton crisis controlled?

Answer 29

A boil water advisory

(the tap water was contaminated)

Question 30

What bacteria were found in the stool samples of those infected during the Walkerton crisis?

Answer 30

Two major enteric pathogens were found in these samples: approximately 2/3 were positive for E. coli and approximately 1/3 were positive for Campylobacter jejuni.

Question 31

What was the primary source of contamination of the Walkerton crisis?

Answer 31

Ultimately, the primary source of the contamination was found to be manure that had been spread on a farm near one of the municipal wells.

The manure was introduced into the well following heavy rainfall a few days later.

This well had been insufficiently chlorinated.

Question 32

What is generally added to water supply to ensure it is safe to drink?

Answer 32

Chlorine

Question 33

In terms of epidemiology, how was the infection of the Walkerton crisis acquired?

Answer 33

Through contaminated water from a zoonotic (animal-borne) origin.

Question 34

How were the bacteria involved in the Walkerton crisis identified?

Answer 34

Identification involved testing both human and water samples, most likely using culture-based methods that take some time to display results.