Relative risks of the uncontrollable (airborne)spread of FMD by different species
The modellers did not define the mechanism of 'local' spread , but assumed that it would happen as a statistical probability.
Furthermore, the infectivity and transmission parameters used by the modellers were based on an average hypothetical species.
Given the very wide variation between different species in terms of the quantities of virus excreted ,their susceptibility to infection ,and the routes by which they are likely to be infected, the modelling of the spread of FMD using an average species is an over-simplification, and in certain circumstances would generate inaccurate forecasts.
The results presented in this short communication indicate that, when disease is diagnosed and movement control is fully implemented around an infected premises, the animals on contiguous premises should not be at risk from uncontrollable spread, that is, from airborne spread, unless
(a)there are pigs or very large numbers of cattle or sheep on the affected premises with early clinical signs
(b)the concentration of virus in the plume was at the same or higher
concentration than the threshold concentration required to infect them
Under those 'ideal'circumstances for airborne spread, the species at risk downwind would be sheep and cattle. Pigs under a plume would be unlikely to be at risk since very high doses of airborne virus are required to infect them (Donaldson and Alexandersen 2001).
The action taken on contiguous premises should, therefore, be determined by the species at risk on those premises. In the case of sheep which may have been exposed to an infectious plume of virus, culling would be justified since FMD in that species is often mild or inapparent (Donaldson and Sellers 2000)and so clinical surveillance would be of limited value in determining whether a flock was infected or not. For cattle, intensified clinical surveillance would be an appropriate alternative to immediate culling,since FMD in that species is easily recognised and any cases should be quickly identified and eliminated before there was a risk of infectious plumes of virus being generated.
For pigs, provided that the possibility of any dangerous contacts had been eliminated (see below),on-going clinical surveillance would be appropriate but no other special actions would be justified.
The action taken should be guided by the species at risk, the local circumstances and whether clinical and/or serological surveillance is appropriate.
The implementation of the 48-hour contiguous herd culling policy has resulted in the slaughter of hundreds of thousands of animals and created severe disposal problems. The potential benefits of culling all animals on all contiguous premises within 48 hours should be weighed against the likelihood that many of the contiguous premises did not contain infected animals, the impact of having to dispose of the resultant animal carcases and the diversion of very limited veterinary resources and support staff from surveillance activities.
Veterinary Record,May 12,2001