Saturday, November 02, 2013

BCG in Spain - a modelled scenario.

In GB, we are not alone in torturing the old BCG (Bacillus calmette-guerin)  vaccine into some sort of alternative control for  zTuberculosis in wild life reservoirs of the disease.

We understand that if it works at all, BCG is better at preventing 'extra pulmonary' lesions. Which means that lung (or pulmonary) lesions are still a problem and a source of spread of the disease - including or even especially to BCG vaccinated human contacts - [link]

Reporting earlier in August this year is a paper - (link) which discusses the modeled effectiveness of BCG, in controlling zoonotic Tuberculosis in wild boar, one of the wildlife reservoirs of the disease in southern Spain.

 The abstract is as follows:

"Bovine tuberculosis is a persistent disease of livestock in many parts of the world, especially where wildlife hosts co-exist with livestock. In south-western Spain, despite the widespread implementation of test-and-cull strategies for cattle, the herd prevalence in areas with high wild boar densities remains stable. The control of M. bovis infection in wild boar is likely to be essential for effective disease control in livestock."
The control of a zoonotic disease in any wildlife reservoir is essential, but we all know that don't 'we'? The authors of this paper, including British scientists from York University, developed an individual-based model to evaluate whether vaccinating wild boar piglets with oral BCG bait would be an effective strategy to reduce the prevalence of M. bovis infection in wild boar populations and thus the effect on farmed cattle.
The abstract explains their modelled method:
"Specifically, we quantified the proportion of piglets requiring vaccination and the number of years the vaccination programme would need to continue to eradicate bTB from wild boar within 25 years, comparing ‘managed’ populations on hunting estates where supplementary food is provided, [and populations controlled - ed] with ‘unmanaged’, free-living populations. Successful vaccination was defined as the proportion of piglets that were delivered the vaccine and were effectively protected from infection."
The key results of this exercise were as follows:
"Longer-term (25-year) vaccination strategies were more successful than short-term (5-year) strategies at either eradicating M. bovis or reducing it to below 90% of its original prevalence.

M. bovis infection could be eradicated under a 25-year vaccination strategy if 80% of piglets were vaccinated in a managed population or 70% of piglets were vaccinated in an unmanaged population. In contrast, 5-year strategies in which 80% of piglets were vaccinated reduced only by 27% or 8% in the managed and unmanaged populations, respectively."
Just so there is no misunderstanding of this result, the model showed that when oral bait was thrown at 'unmanaged'  free living, wild populations of wild boar in Spain, the take up necessary by the young piglets to give any protection again z Tuberculosis required 25 years of baiting and a coverage of 70 per cent of the population. A lot of 'ifs' and 'maybes' but the gist is, coverage has to be very comprehensive and the time scale very long. And that the programme is aimed at a population whose size, health and welfare is controlled or 'managed'.

 Conversely if vaccination was offered over 5 years to an 'unmanaged' population the effect was just 8 per cent drop in disease spill over.

 Compare this more realistic (if mathematically modeled) scenario to the outrageous claims (- link) being made for a single dose, ad hoc vaccination programme on an unmanaged, wild population of grossly infected British badgers - and weep.

The paper concluded that:
"The results of our simulation model, coupled with the promising results of initial vaccine and oral bait- uptake trials in wild boar indicated that vaccination could be an effective strategy to reduce the prevalence of M. bovis infection in wild boar if used in conjunction with other disease-control measures."
Before anyone gets over excited and does an abbreviated cut/paste on that snippet, please note the end of that particular sentence:
 Vaccination of unmanaged wild boar in Spain, may have a part to play over a 25 year zTB eradication strategy, "if used in conjunction with other disease-control measures".  

Disease control measures as in 'managing' the population? And removing the grossly infected pockets?

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