From Dr Ruth Watkins, livestock farmer, formerly head of the Virology Laboratory at St Mary's Hospital, London
There are two ways of dating the probable date of infection of the herd.
The one I heard the CVO refer to was the histology of the lesions. Acute virus ulcers develop then heal and the appearance of the cellular infiltrate can tell one where the lesions are along this path. One then adds on either the shortest or longest incubation period to get a range for exposure.
There is another route and that is virology. If they bled all the animals in the herd so they could test for antibody, as well as take samples to determine if virus is present in the blood, mouth, nasopharynx etc they could see if there were any animals who had developed IgG class antibody but were virus detection negative; any such animal must have been infected before the current group of lesion positive ill animals.
In fact both could be used in conjunction. There is great biological variability with an incubation period to illness (either the prodrome of fever or the definitive illness of ulceration etc.) of between 3 to 21 days I believe. The formation of detectable antibody would also vary but for IgG I would think it is about 7 - 10 days after the onset of illness as above. Shedding of the virus is also variable but where it is cleared it is probably not longer than 3 weeks from just before the onset of illness.
One has also to bear in mind that a number of acutely infected animals would be asymptomatic. I hope they sampled all the animals in the first herd they slaughtered for the presence of antibody and virus as well as looking at the hisotology of their lesions.
What has struck me from the start is the number of animals ill at the same time. It seems a considerable proportion were infected and a significant proportion of these were ill. A large dose of virus is both more likely to result in an infection with illness and to shorten the incubation time. The outbreak of FMD in these two cattle herds, the only two proven infected, could not have been missed by any farmer.
Either there was an index case earlier with a heavy exposure of the rest of the herd, 1 animal infecting 30 for instance, or there is the rather remarkable finding of simultaneous exposure of the 10 or so animals.
In virus infection within a family group, an epidemiological unit, the index case is often milder than the subsequent cases because there is a longer and more intense exposure to virus in the subsequent cases- for example chicken pox or measles - whilst in the home living with the first case who was exposed more fleetingly in the community.
I don't know if the results on the two herds point to a possible exposure of one index animal in the first herd to be infected (if indeed both of the herds were not simutaneously exposed) with subsequent cases or whether there is likely to have been a mass exposure event in the first herd. The CVO did say they had some interesting epidemiological information. I haven't heard a squeak as to what this is.
As you have pointed out I wonder if there is a missing piece in the jigsaw, a Pirbright deliberately or accidentally infected animal(s). With all the finger pointing at the labs on the site and the HSE investigation I can only suppose this is not the case. I think that question should be put to the HSE investigators.
Ruth Watkins August 17 2007
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