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BDD in context: a literature review |
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This literature review covers general aspects of BDD,
including economic impacts, welfare issues and our current understanding of its
epidemiology.
In the 20 years
since BDD was first observed in the UK, the majority of UK dairy farms have
become infected, and there are indications that the infection is still
spreading further (Laven, 2003). BDD is currently considered to be the leading
infectious cause of lameness in the dairy sector in the UK. Given the intense
painfulness of the disease, it is a leading welfare concern, which seems bound
to further increase in significance.
Studies over the past decade
have showed a trend of rising prevalence (Somers et al., 2005; Holzhauer et
al., 2006), both in terms of the proportion of farms infected as the morbidity
on the farm level; Holzhauer et al. (2006) suggest that this may be due to
prolonged housing periods. Laven and Lawrence (2006) found that the seasonality
of BDD incidence has decreased since 1997, ascribing this to the disease having
become endemic on most farms, and persisting into the grazing
period.
BDD is a complex condition. The current understanding of the
aetiology, pathogenesis and epidemiology of BDD is far from complete. The rapid
and pervasive spread of BDD may have altered the infection dynamics of the
disease, posing new challenges for investigation and the design of practical
and effective control measures.
On the basis of published
information and existing knowledge, causal mechanisms of BDD are discussed in
detail; the hypotheses presented here were used as a basis for subsequent
investigation. The evidence that BDD-associated Treponema spp. bacteria
represent the ‘necessary cause’ is becoming increasingly strong
(Demirkan et al., 2006; Edwards et al., 2003; Stamm et al., 2002; Trott et al.,
2003; Walker et al., 1995); we have assumed this to be the case. However, it is
clear that this is a highly multifactorial disease, and hence a complex of
other component causes has been identified. These consist of demonstrated as
well as hypothetical (‘biologically plausible’) factors. It is
currently unknown which of these causes combine with the ‘necessary’
cause to result in a ‘sufficient’ cause which will bring about
clinical BDD. A set of component causes was incorporated into a causal web,
which exhibits a hierarchical structure, from proximate (physiological) causes
to more distal effects (management factors). This causal web was useful for
directing our thinking about the aetiology, pathogenesis and transmission; the
definition of variables of interest; and identifying possible confounders and
sources of interaction. As the causal web had BDD as the outcome and explicitly
defined a hierarchical structure, it also lent itself to the fitting of
multivariable multilevel models. |
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Fig.
W1. A causal diagram showing the hierarchical relationship between the
development of BDD lesions and physiological
factors (animal level), proximal effects (management
group level) and distal effects (farm and sectorial levels) |
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©
Willem Daniel Vink 2006 |
