one health lecture 4- epidemiology

Question 1

what does epidemiology aim to do?

Answer 1

understand why diseases occur where/when they do, and how infections spread through populations

Question 2

what is epidemiology?

Answer 2

“the study of the distribution and determinants of disease frequency…”
“ …and the application of this to control health problems”

Considers infection at the population-level rather than the individual

Question 2

what is one health?

Answer 2

recognises the interconnectedness between animal and human health within a shared environment

Question 3

what is zoonoses?

Answer 3

diseases transmitted to humans from other vertebrates

Question 4

what is the impact of infectious diseases to humans?

Answer 4

Responsible for 16% of human deaths each year
Main cause of mortality in low income countries

Main killers:
-Diarrheal diseases
-Malaria
-Tuberculosis
-HIV/AIDS

Question 5

what is the impact of infectious diseases to livestock?

Answer 5

Massive economic impacts-
-Direct loss
-Costs of control
-Trade restrictions

-Important link to people’s livelihoods
-Relevance to human health (zoonotic diseases)

Question 6

what is the impact of infectious diseases to wildife?

Answer 6

-Threat to rare and endangered species

-Disease-induced mortality can regulate host populations

Wildlife as potential reservoirs of infection
-Zoonotic diseases
-Livestock diseases

Question 7

what is morbidity?

Answer 7

frequency of disease within a population – measured by incidence/prevalence

Question 8

what is incidence?

Answer 8

the number of new cases per unit time

eg 10 new cases of Zombievirus were reported in Glasgow (population 1 million) today. That’s a daily incidence rate of 1 case per 100,000.

Question 9

what is prevalence

Answer 9

the proportion of a population that has the disease at a given moment

Seroprevalence – proportion of serum samples reacting positively (containing antibody

eg There are 1000 people who test positive for Zombievirus in Glasgow today. The current prevalence in this city is 100 per 100,000, or 0.1%.

Question 10

what is the case fatality rate?

Answer 10

proportion of those with clinical disease that die

eg It’s estimated that around 50% of people diagnosed with Zombievirus will die from this infection. This means the case fatality rate is 50%.

Question 11

what is the mortality rate?

Answer 11

relative number of deaths within the population per unit of time

eg If the case fatality rate stays constant, and there are 50,000 cases of Zombievirus in Glasgow this year, what will be the annual mortality rate in this population?

Question 12

what are the causes of infectious diseases?

Answer 12

-viruses
-bacteria
-parasites both micro and macro
-fungi
-prions and transmissible cancers

Question 13

what is vertical transmission?

Answer 13

Vertical: parent to offspring eg from mother during pregnancy, labor or breastfeeding

Question 14

what is horizontal transmission?

Answer 14

to any other individual in the population

eg
Close contact/ body fluids
-Ebola, smallpox

Sexually transmitted infections (STIs)
-HIV/Aids, syphilis

Aerosol/respiratory
-Influenza, measles, tuberculosis

Question 15

what are types of indirect transmission?

Answer 15

Vector-borne- Malaria, Lyme disease, dengue fever

Food-borne- E. coli, Staph aureus endotoxins

Environmental contamination- anthrax, cholera

Question 16

what are multi host pathogens? Example: Influenza viruses

Answer 16

-Majority of emerging infectious diseases are caused by multi-host pathogens: able to infect more than one species

-Transmission might involve wildlife, domestic animal and human populations

Question 17

what is a reservoir?

Answer 17

a reservoir is a critical ecological niche where pathogens can survive and persist, potentially leading to disease outbreaks in other populations.

maintains the pathogen independently of the target community

Question 18

what is an example of a reservoir?

Answer 18

Example : elk and bison as reservoir of Brucella for cattle in Yellowstone ecosystem

Question 19

what are carriers and examples?

Answer 19

Carrier: infected with limited/no clinical signs

Examples:
-Staph aureus (~1/3 of people)
-Taenia multiceps – canine tapeworm that causes ’ormilo’ (neurological disease) in sheep, but no signs in dogs

Question 20

what are the factors affecting infection outcome?

Answer 20

• the virulence factors and resistance of the pathogen
  -the Determines whether host/pathogen interact
  and the Impacts on both other factors from the environment
• the host and the Varying levels of susceptibility (breed, immunity)

Question 21

what is the probability of anthrax?

Answer 21

anthrax occurs mostly in areas characterised by low organic matter and in proximity to water bodies. This suggests that the transmission of B. anthracis to animals likely drives the risk of disease, more than factors that favour the survival of spores in the environment.

Question 22

what is the basic reproductive number?

Answer 22

The average number of secondary infections caused by a single infected host in a completely susceptible population

Question 23

what happens when R0 = 1

Answer 23

If R0 equals 1 each infection replaces itself

Question 24

what happens when R0 = 2

Answer 24

-this is the number of infections will occur from one infected individual
-Susceptibles needed to maintain chain of infection

Question 25

what happens when R0 < 1

Answer 25

no replacement so the disease goes extinct

Question 26

what is an epidemic?

Answer 26

-Outbreak: number of infections is growing exponentially
-R0 >1
-If affecting populations on large geographic scale (e.g. globally): pandemic

Question 28

what is an endemic?

Answer 28

-Numbers of infections remaining more or less stable through time (R0 ∼1)

-Transition from epidemic to endemic might take some time (see COVID-19)

Question 29

what is an emerging disease?

Answer 29

Defined as disease with
growing number of cases following its introduction into a host population
OR
growing number of cases in existing population due to changes in epidemiology

Question 30

what are ways to prevent transmission?

Answer 30

-Hygiene
-Isolation (quarantine)
-Social distancing
-Personal protective equipment
-Drugs, antibiotics
-Vaccination

Protecting population vs. protecting the individual

Question 31

what is the role of pathogen evolution?

Answer 31

Emergence of Venezuelan encephalitis virus in horses/humans required a single point mutation in the virus’ envelope gene

Question 32

what does the effort required for control depend on?

Answer 32

R0

Question 33

what is transmission?

Answer 33

Critical process: required for any infectious agent to persist
_ no infection without transmission
Wide range of transmission mechanisms
Linked to pathogen fitness: traits determining transmission are under selection and thus might evolve