Email from Ruth Watkins Sept 22
I would like to share with you a worry I have about bluetongue.
Now in the second year of bluetongue infection in Europe and with the midge numbers favoured by the weather, this year the exposure of animals will probably be different because there will be a greater number of infected midges and a greater number of bites - so that infected animals this year might be exposed to an inoculating dose of virus that is on average a thousand fold say greater than last year.
More animals in sheep flocks will be infected and greater number will have disease.
I believe last year they thought on fairly limited sampling of whole flocks that few sheep were infected in a flock in contrast to most cattle in a herd were infected.
I don't know what Ab Osterhus and other virology experts are saying. Nor the midge experts.
I do wonder whether bluetongue could have been present late in 2005 for instance in November and the one or two animals diseased unnoticed and undiagnosed.
This is because the outbreak begun this year at almost the same time as last year, in late July. I had expected that it would start in June perhaps. But it started explosively in the same areas as last year.
Were there carrier animals left amongst the cattle herds? Can wild deer carry it? (viz outbreaks of epidemic haemorrhagic disease annually in deer in USA, particularly bad this year)
It does occur to me that England should fall partly in the surveillance zone from France and Belgium.
It does of course change the focus of attitudes to vaccination even to FMD
I am glad to hear the German vets advocating FMD control by vaccination to live without penalties greater than stamping out by culling. When it comes to bluetongue diagnosis I hope we will be doing what our European colleagues do and that is RT-PCR as well as serology. None of this penside testing with a lateral flow device to the exclusion of RT-PCR as the first line test for FMD. As bluetongue animals are not being culled there is not pressure to use RT-PCR at the farm gate.
The main limitation of not doing farm gate real time PCR on screening farms within the protection zone for instance to detect spread is that they are waiting for the development of recognised clinical signs in cattle and the development of antibody in sheep before screening and will miss early infection on a farm or infection without any clinical signs viz farm no 5 where they had been three times and must have missed clinical disease if there was any.
It is characteristic of virus infections, with very few exceptions such as rabies, that infection may go unremarked or be without illness yet such infections are fully infectious to others and play a role in the chain of onward transmission.
I don't have any objection to their use of the penside lateral flow device but I do object to the exclusion of the use of the more sophisticated and sensitive technology of real time RT-PCR as a first line screening test.
Dr Ruth Watkins BSc Hons, MSc, MBBS, MRCP, MRCPath. e-mail firstname.lastname@example.org is an expert virologist and farmer
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