Concepts of Causation

Question 1

What are Koch’s postulates?

Answer 1

to be considered a cause for a disease:

• the microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms
• the microorganism must be isolated from a diseased organism and grown in pure culture
• the cultured microoganism should cause disease when introduced into a healthy organism
• the microorganism must be reisolated from the inoculated, diseased experimental host and identified to the original specific causative agent

Question 2

What are some problems with Koch’s postulates?

Answer 2

• pathogen isolation can be difficult
• disease not reproducible
• mutlifactorial (many pathogens causing one disease)
• disease production may require co-factors
• viruses cannot be cultured like bacteria because viruses need living cells in which to grow
• pathogenic viruses can be present without clinical signs
• not applicable to non-infectious diseases

Question 3

What is the epidemiological triad?

Answer 3

• disease occurs when, within a defined environment, a susceptible host encounters an agent of disease

Question 4

What are the environmental factors?

Answer 4

• geographic - inc altitude, climate, soil type, vegetation, season, interaction with wildlife or vectors
• physical - inc managed environment: fences , shelter, floors, ventilation, light, equipment and maintenance, transport
• husbandry- diet, stocking density, routine prophylaxis, biosecurity, handling, stockmanship, productivity
• economic - value of a dairy bull calf, feed costs, access to capital for farm improvements, access to vet care
• contact network
• proximity to other holdings

Question 5

Give examples of the Parasites and infectious agents

Answer 5

• virulence - the severity of disease caused, shown by the case fatility rate
• Pathogenicity- the ability of the agent to cause disease in a particular host
• infectious dose- number of organisms required to predictably initiate disease
• transmission rate
• life cycle, inc vectors
• route of infection- direct, indirect, droplet, airborne, vector-borne, fomites

Question 6

What are the chemical, physical and radiological agent factors?

Answer 6

• dose - deficiency/ excess
• toxicity
• duration of exposure
• rate of accumulation/ degradation
• absoprtion
• route of exposure

Question 7

What is Ken Rothmans Sufficient Cause Model?

Answer 7

• pie model - each slice is a contributory cause, the whole pie is a sufficient cause
• diseases have multiple causes
• each sufficient cause has multiple component causes
• ofetn 1 component is non-redundant
• removal of any 1 component cause from a sufficient cause will prevent disease

Question 8

What is the sufficient, component and necessary causes of TB?

Answer 8

• sufficient cause = whole pie
• component cause = parts of pie
• necessary cause = exposure to TB

Question 9

What are Bradford-Hill’s Criteria for Causation?

Answer 9

1. strength of association
2. consistency
3. specificity
4. temporality
5. biological gradient
6. plausibility
7. coherence
8. experiment
9. analogy

Question 10

What is the difference between association and causation?

Answer 10

• association is about relationship between an exposure (a potential cause) and a disease
  
  • when we measure association, look at the magnitude of the relationship: put in x effort
• causation - about consequences

Question 11

How do you measure strength of association? What else should you consider?

Answer 11

Relative Risk

• 1.1-1.3 = weak
• 1.4-1.7 = modest
• 1.8-3.0 = moderate
• 3-8 = strong
• 16 = very strong
• 40 = dramatic
• 40+ = overwhelming

Also consider the confidence interval around the RR as well as point estimate

Question 12

How is consistency measured?

Answer 12

• replication of the findings:
  
  • by different investigators
  • at different times
  • in different places
  • different methods
  • ability to convincingly explain different results

Question 13

Specificity?

Answer 13

• an association between an exposure and a disease is more likely to be causal if it is only linked with one disease
• however - now recognise that one exposure may be linked to more than 1 disease outcome and also that one disease has a multifacotial aetiology

Question 14

What is the significance of Temporality?

Answer 14

• the putative cause needs to precede in time the presumed effect
• exposure must precede effect

Question 15

What is meant by Plausibility?

Answer 15

• the proposed causal mechanism should be biologically plausible
• causal mechanism must not contradict what is known about the natural history and biology of the disease
• cause/ effect interpretation of an associations should fit with the known facts and observations

Question 16

Explain Coherence

Answer 16

• related to plausibility and consistency
• the putatuve cause should be in line with accepted knowledge
  
  • publications
  • knowledge of underlying biological mechanisms
  • epidemiological findings should be in line with lab findings

Question 17

What is ‘Experiment’?

Answer 17

• the demonstration that under controlled conditions changed the exposure causes a change in the outcome is of great value, some would say indispendable, for inferring causality

Question 18

What is Analogy?

Answer 18

• we accept arguments easier that resemble others we accept
• have there been similar situations in the past??