Dear Colleagues, 
 
If I correctly understand the situation we are in it is that the international authorities see PPR as an eradicable disease but wish to see that accomplished by progressing progressive control.
 
I'm sure PPR is a relatively easily eradicable disease. The point at issue is whether to follow a "big bang" pathway with lots of mass vaccination campaigns or follow a more intelligent pathway based on trying to understand and impinge on the mechanisms by which the virus is entrenched as an endemic infection across very important small ruminant populations.
 
At this point there are several issues we need to think about. At this time the virus is still expanding its range which it does by 
 
gaining access to new (international) naive small ruminant populations and causing epidemics. Thereafter (broadly speaking) the virus assumes an endemic profile with an overall reduced level of livestock loss but with the possibility of local (national) epidemics. It is from this point that we want to progress control to whichever of several ends later appear most feasible. These might amount to a) the virus dying out of a livestock population under the influence of progressive control or b) remaining endemic at either lower or unreduced levels or c) becoming so avirulent as to be a trivial and un-worrisome condition.
 
At the outset you (meaning me -the moderator) suggest that we know enough about the epidemiology of the virus to embark on control efforts akin to those which worked for rinderpest but you need to bear in mind that with attempts to control rinderpest with vaccine lasted 60 odd years, punctuated by periodic epidemics as campaign fatigue occasionally created vulnerable susceptible cattle populations. Agreed,  we showed that mass vaccination campaigns such as the African JP15 could eradicate the virus from a number of countries but the lack of synchronicity built into its planning ensured that the virus remained endemic across the continent and subsequently re-established a temporary pan-continental distribution (east-west that is). But we never really analysed the relevance of supply chains, the ability of the virus to seemingly remain endemic for years within a population and then later reappear in epidemic form (the Egyptian experience) and the
 inability of local authorities to control cross-border movement of infected stock.
 
From this stand point my argument is that I doubt that we know as much as we need to about the micro-epidemiology of ppr with respect to the different livestock husbandry systems within which it exists. How exactly does it move from village to village in Pakistan and India – shared grazing? Purchase of incubating stock at market? Gifts between neighbours? And there is much we need to know about traditional husbandry methods such as nomadic movements. We probably know not very much about major small ruminant supply chains along which the virus may well move to be introduced into new populations and we do not understand the impact of local commercial stock rearing and marketing at festival time. Nor do we know whether local owners appreciate the dangers of infected animals and are mindful of the dangers of introducing infected stock or the value of sequestering such animals until clearly healthy – although most interestingly one of your correspondents
 discussed traditional public health measures with fines for non-compliance. This is an area where there should be much wider harvesting of community understandings (or absence of same).
 
The late Bob Sellers, my one time Director  at Pirbright once gave a lecture in which he determined that vaccine might be a virus’ best friend. With PPR, the idea  would be that vaccine, unless used to immunosterilise a population, will always end up creating a partially immune population within which the virus will remain endemic - to which I would add that given that the post - -epidemic outcome will be similar why waste public resources vaccinating?
 
So, I would suggest that the international authorities use national public services to develop a much broader understanding of the ways by which the virus is able to persist at the endemic level concentrating on international and national and local community supply chains with a view to finding points of vulnerability whereby this endemicity could be constrained (= progressive control), with or without vaccine through a broad level of support to local vet. services by way of tools (advice, training, tactics). At the same time there needs to be a detailed look at the way in which communities may or may not have taken it upon themselves to undertake veterinary public health measures and if seen as helpful, ensure that these too become additional tools (more widely employed etc. etc.) along the way to progressively reducing the impact of endemicity.
 
If you are a country on the edge of the PPR map you can of course immunosterilise the virus out of the host population without understanding the epidemiology but you ought to know how it entered in the first place (presumably from a neighbour) and how to stop it regaining entry.

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