Download Hypothesis of Disease Causation - Wk 1-2

Hypothesis of Disease Causation
1. Proving Hypotheses & Disease Causation (History & Philosophy of
Science and Medicine)
When proving hypothesis and disease causation, the principle model that is used is the
Bradford Hill Criteria. Sir Bradford Hill established the following nine criteria for
causation, outlining the minimal conditions needed to establish a causal relationship
between two items. Although developed for use in the field of occupational medicine
(ie does factor A cause disorder B), these criteria can be used in most situations. The
nine factors to consider include;
1. Strength of the association. How large is the effect? This is defined by the
size of the association as measured by appropriate statistical tests. The
stronger the association, the more likely it is that the relation of "A" to "B" is
causal.
2. The consistency of the association. Has the same association been observed
by others, in different populations, using a different method? The association
is consistent when results are replicated in studies in different settings using
different methods. That is, if a relationship is causal, we would expect to find
it consistently in different studies and in different populations. This is why
numerous experiments have to be done before meaningful statements can be
made about the causal relationship between two or more factors
3. Specificity. Does altering only the cause alter the effect? This is established
when a single putative cause produces a specific effect. Can be determined if
altering only the cause alters the effect.
4. Temporal relationship. Does the cause precede the effect? Exposure always
precedes the outcome. If factor "A" is believed to cause a disease, then it is
clear that factor "A" must necessarily always precede the occurrence of the
disease.
5. Biological gradient. Is there a dose response? An increasing amount of
exposure increases the risk. If a dose-response relationship is present, it is
strong evidence for a causal relationship
6. Biological plausibility. Does it make sense? The association agrees with
currently accepted understanding of pathological processes. In other words,
there needs to be some theoretical basis for positing an association between a
vector and disease, or one social phenomenon and another.
7. Coherence. Does the evidence fit with what is known regarding the natural
history and biology of the outcome? The association should be compatible
with existing theory and knowledge. In other words, it is necessary to
evaluate claims of causality within the context of the current state of
knowledge within a given field and in related fields.
8. Experimental evidence. Are there any clinical studies supporting the
association and can the condition be altered (prevented or ameliorated) by an
appropriate experimental regimen?
9. Reasoning by analogy. Is the observed association supported by similar
associations? In judging whether a reported association is causal, it is
necessary to determine the extent to which researchers have taken other
possible explanations into account and have effectively ruled out such
alternate explanations. In other words, it is always necessary to consider
multiple hypotheses before making conclusions about the causal relationship
between any two items under investigation.
Historically speaking, discussions of causal criteria arose from the recognised
limitations of the Henle-Koch postulates, formalised in the late 19th century to
establish causation for infectious agents. These postulates required the causal factor to
be necessary and present, applied only to a subset of infectious organisms, and
conflict with multifactorial causal explanations. The non-specificity of causal agents
was of particular concern Two lists of criteria published before 1960 did not include
specificity, and instead included consistency and replication, strength, dose-response,
experimentation, temporality and biological reasonableness. The term ‘criteria for
causation’ applied to Hill’s list of factors to consider before inferring causation from
an observed association. Hill himself however never called these considerations
criteria, but rather referred to them as viewpoints and never saw them as the ‘hardand-fast’ rules of evidence. Nevertheless, the publication of his landmark address to
the Royal Society of Medicine is frequently cited as the authorative source for causal
criteria in epidemiological practice.
(NB- Guys, I know this is kind of the same as the LO Ash did in wk 15 but this is what I understood
the LO to be about and from what I asked other ppl about this is what they said to do. If you think it is
wrong or want me to do something else let me know!)
2. Identify and Discuss Ethical Issues in Immunization
While there are many views on what constitutes acceptable child rearing, in Australia
parental discretion is limited primarily by legislation against abuse or neglect. In
decisions involving medical treatment, the legal starting point is that the united view
of both parents is correct in identifying the child's welfare. Parental decisions are
usually overridden when the court feels that the child's life is endangered. While
vaccine administration is not considered in the category of immediately life saving,
vaccinations satisfy criteria for preventive interventions in children. So in essence,
should vaccinations be compulsory?
Ethical Considerations
A need to balance the rights of individuals in deciding whether to be vaccinated with
rights of society to be protected from infectious disease
 If people believe they have the right to be free from infectious disease, then
they should just vaccinate themselves. Transmission from an un-vaccinated
person to a vaccinated person is very low.
 The community at large may have a right to be able to receive immunisation if
a vaccine is available, the risk of contracting disease is high, and the cost is
not prohibitive
If parents do not wish for their child to be immunised, can health care workers be
justified in vaccinating anyway as they believe it to be in the Childs best interests?
 Dealing with issues of capacity and competence
 Paternalism
 It can be argued that parents have the right to raise their children without
interference from others. Also, the family has a right to privacy
 Putting child at risk as child is not yet autonomous
 A child has the right to vaccination if the danger of severe infection is great
and a safe vaccine is available. Parents must maintain a minimum level of care
for their children
Risk Perception
 Informed consent
 Risks may seem more real than advantages
 Risks are more visible than disease when vaccination rates are high
 There is a perceived low risk of illness, even if not immunised (but this is
actually dependant on maintaining herd immunity through high immunisation
levels).
Individuals and the Community
 Herd immunity requires high vaccination rates.
 Where this exists, unvaccinated people get benefits without risk.
- Herd immunity is in essence a type of common good. There is an
obligation to contribute to common good. Failure to immunise can lead
to harm to others.
- As herd immunity fails the benefits of vaccination are once again seen.