Lec 9: Zoonosis

Question 1

Describe Walkerton - May 2000

Answer 1

• E coli and campylobacter caused the death of 7 people and resulted in 2300 additional infections
• well water serving the community of Walkerton Ontario was contaminated by surface water carrying livestock waste from neighbouring farms during spring run off and heavy rains
• outcomes of this outbreak drove an overhaul to Ontario’s water safety policy and generated ripples that continue to be felt across the Canadian cattle industry
• estimated direct and indirect costs to Canadian economy $155 million

Question 2

Describe SARS - March 2003.

Answer 2

Feb 15 - China reports 305 cases of atypical pneumonia
March 12 - WHO issues global SARS alert
March 17 - 11 suspected cases in Canada
May 8 - 15% of people who get SARS will die
- estimated cost to Canadian economy 2 billion

Question 3

Describe BSE May 2003.

Answer 3

• May 8: first case confirmed in Canada
• May 21: Mexico, Japan and South Korea temporarily ban Canadian beef
• Canadian loss 5.5 billion
• 10 years later, decreased beef producers, beef exports and beef consumption

Question 4

What is an emerging infectious disease?

Answer 4

• a new agent appears in a new geographic area
• a known agent or its close relative affects a new host species
• a previously unknown agent is detected for the first time
• more loosely, term also relates to a pathogen that has been present historically but which has recently increased in incidence

Question 5

Define zoonoses.

Answer 5

any disease or infection that is naturally transmissible from animals to humans or vice versa

Question 6

Define zooanthroponosis.

Answer 6

A zoonosis normally maintained by humans but that can be transmitted to other vertebrates

Question 7

Define anthropozoonosis.

Answer 7

A zoonosis maintained in nature by animals and transmissible to humans

Question 8

3 methods of transmission

Answer 8

• direct contact
• environmental transmission
• vectors

Question 9

Define outbreak

Answer 9

occurrence of cases of disease in excess of what normally would be expected in a defined community, geographical location or season

Question 10

Define epidemic

Answer 10

a widespread outbreak of an infectious disease; many people infected at the same time

Question 11

Define pandemic.

Answer 11

A large epidemic with:

• wide geographic extension
• significant transmission that leads to disease movement and extension of geographic range
• highly contagious and short incubation times
• minimal population herd immunity contributes to disease spread
• generally associated with infectious disease
• often severe (fatal)

Question 12

What is herd immunity?

Answer 12

• protection of a proportion of the population or herd provides vaccination to other individuals
• affects rates/efficiency of disease transmission
• in diseases passed from person to person, it is more difficult to maintain a chain of infection when large numbers of a population are immune
• the more immune individuals present in a population, the lower the likelihood that a susceptible person will come into contact with an infected individual

Question 13

What is a species barrier?

Answer 13

• natural mechanisms that prevent a pathogen from spreading from one species to another

Question 14

Pathogenicity

Answer 14

• qualitative (not quantitative) trait, referring to the inherent, genetic capacity of a microorganism to cause disease
• mediated by specific virulence factors

Question 15

Virulence

Answer 15

• degree of pathogen’s infectivity or disease severity an organism has the potential to cause
• ie a highly virulent pathogen is likely to cause significant disease whereas an avirulent microorganism is likely to cause little or no disease

Question 16

Virulence factor

Answer 16

• pathogen product or strategy that contributes to its ability to cause infection
• some examples include microorganism production of toxins that kill the host cell, enzymes that act on the host cell walls, or substances that alter the normal cell growth

Question 17

Prevalance

Answer 17

• the proportion of a population that is infected, or diseased, at any one time
• the actual number of cases alive, with the disease either during a period of time (period prevalence) or at a particular date in time (point prevalence)

Question 18

Incidence

Answer 18

• the rate of new (or newly diagnosed) cases of the disease

- it is generally reported as the number of new cases occurring within a period of time (e.g. per month, per year)

Question 19

Infectivity

Answer 19

• refers to the ability of a microorganism to invade and replicate in a host cell or tissue

Question 20

Transmissibility

Answer 20

• refers to the efficiency with which a pathogen is transmitted to naive hosts
• there are valid arguments that at epidemiological level, transmissibility could be considered a component of virulence

Question 21

Describe global trends in emerging infectious diseases.

Answer 21

• have risen over time
• 335 infectious diseases have emerged between 1940 and 2004
• dominated by zoonoses (60% of EID)
• majority of these originate in wildlife

Question 22

What are 3 common misconceptions about emerging diseases?

Answer 22

1. Medicine and technology will progressively lead to a decrease in infectious disease emergence
2. The majority of emerging infectious disease events occur in geographic areas near the equator
3. Viral pathogens (particularly RNA viruses) represent a major threat and most likely to result in EIDs

Question 23

How have the leading causes of death in the US changed from 1900 to 1997?

Answer 23

• infectious diseases have been replaced by chronic diseases as most important causes of death

Question 24

What are some factors that influence the decrease in infectious diseases in the 20th century?

Answer 24

• better nutrition, antibiotics, improved hygiene and sanitation, immunizations, safer food and water, better housing
• have lead to decrease in host susceptibility and disease transmission
• which has lead to decrease in infectious diseases

Question 25

Describe the link between infectious diseases and chronic illnesses.

Answer 25

• infectious diseases are now being recognized as the causes of important chronic illnesses
• mycoplasma as cause of chronic asthma
• H pylori cause of peptic ulcers and cancer
• HPV cause of cervical cancer

Question 26

What are some factors leading to the re-emergence of infectious diseases?

Answer 26

• changes in demographics and behaviour, environmental change and land use, breakdown of public health measures, microbial adaptation and change, international travel and commerce, changes in technology and industry
• have lead to increases in host susceptibility, increases in disease transmission, and new diseases
• which has lead to emerging infectious diseases

Question 27

What did we previously think about EID distribution?

Answer 27

• latitudinal spatial gradient of human pathogens increases towards equator
• due to richness in taxonomic groups as well as higher temperatures and amounts of precipitation

Question 28

What do we currently think about EID distribution?

Answer 28

• actually concentrated in higher latitudes
• highest concentration found between 30-60 degrees north and 30-40 degrees south
• main spots in north eastern USA, western europe, Japan and southeastern Australia

Question 29

Why do people think that viral pathogens are the major threat?

Answer 29

• appearance of HIV, SARS and influenza
• high rates of nucleotide substitution and poor mutation error correction ability may lead to increased potential to adapt to new hosts

Question 30

What are the actual causative pathogens of emerging diseases?

Answer 30

• newly evolved pathogen strains due to resistance (multi drug resistant TB, vancomycin resistant S. aureus)
• pathogens that have recently entered human populations (HIV)
• pathogens that have been present in humans historically but have seen an increase in incidence (Lyme)
• actually 54.3% bacterial pathogens and only 25.4% viral or prion