Identifying causes of diseases

Question 1

What is the difference between necessary cause and sufficient cause?

Answer 1

necessary cause = the disease cannot occur without the cause

sufficient cause= the disease will occur if the factor is present

Question 2

What type of cause is this

“Ingesting V Cholerae bacteria as a cause of acute cholera”?

Answer 2

you cannot have cholera wihtout ingestion of the bacteria so this is necessary cause

but not all those who ingest the bacteria become ill with cholera so it is not ‘sufficient’

Question 3

what type of cause is the following

“A Large overdose of paracetamol as a cause of liver failure”?

Answer 3

a large ovedose is sufficient in itself to cause liver failure, there are other ways of getting liver failure - so this is a ‘sufficient’ cause

Question 4

what type of cause is this?

“smoking as a cause of lung cancer?”

Answer 4

you can smoke without developing lung cancer so it isn’t sufficient cause

and you can develop lung cancer without smoking so it is not necessary cause

therefore it is neither necessary nor sufficient

Question 5

what type of cause is the following?

“Tay-Sachs disease is caused by a gene mutation”

Answer 5

Tay- Sachs disease - requires the presence of the gene mutation = necessary cause

and all those with the gene develop the disease so it is also sufficient cause

Therefore it is both!

Question 6

What are the factors you need to consider when determining causality?

Answer 6

1. temporal relation- cause must come BEFORE the disease
2. Plausability - is the association between the factor and the disease consistent with what we know already?
3. consistency - are the results of the studies consistent with each other?
4. Strength - what is the size of the relative risk associted with the factor? (at least 1.5 in factors associated with common serious diseases)
5. Dose - Response Relationship -the more exposure - more chance of developing disease
6. Reversitibility - if you remove the cause does this remove the risk?
7. Sources of Evidence - Does the evidence come from a well designed study?

Question 7

Once you have found probable cause, what factors do you need to take into account before acting?

Answer 7

1. cost/benefits of action - is intervention effective, specific, affordable?
2. what are the costs if we’re totally wrong?
3. how much effort to avert one event?
   4.