email received May 7 2008 from Dr Ruth Watkins See also emails of May 4th and May 8th
Vaccination of sheep now in May would I think cover them for the rest of the season, ie 6 months. As there will be a 3 week period before immunity is maximal, the peak response to the vaccine, vaccination now is certainly the most prudent (immunity by July when clinical cases started to emerge in large numbers in 2007).
Vaccination should be done again in the midge down time, even if that is only 6 months after the first dose / course in sheep to ensure immunity next seaon 2009.
Obviously if there is any illness suggestive of bluetongue in a vaccinated sheep a test for virus presence by PCR should be done. In that way confidence in the vaccine will be built. Rumour of virus infection or reinfection etc is just not good enough: there must be diagnostic virology investigation. Field testing for infection could be done on sheep (and cattle) more than 6 months after vaccination now, ie next Dec 2008 or Jan 2009 using a BTV-8 PCR test to determine if there have been any infections, whether or not there have been any symptoms.
In the case of possible congenital infection in cattle or sheep the length of virus postivity in the blood is unknown, so the protection of adult animals by vaccine should be assessed in the field, as if infection should occur the blood especially the red blood cells will retain virus for up to 6 months.
It would be interesting to see the intervet BTV-8 bovilis testing results but if a good antibody response is made with a high level of protective antibody then protection could be expected to persist for at least 6 months. I understand tests can be done in-vitro to determine the level of protective antibody, by neutralising virus infectivity in tissue culture, and this will correlate directly with in-vivo protective levels of antibody. I am sure these tests are time consuming and expensive to do but I would have thought they would have been done in the evaluation of the vaccine effectiveness given the very short time frame for vaccine development. The second part of of the evaluation of the vaccine will be in the field by testing flocks and herds as I have suggested.
I think the private vet's remarks are counterproductive for infection control.
PS in the case of my sisters longhorn herd she had all the adults and older heifers and young bulls tested this winter 2008 as they thought some had ulceration on an udder or in the mouth or were lame without explanation and were concerned that there were other animals in the herd infected with BTV-8. The only infection in the herd was the original cow Duchess (who has had a normal calf but infection status of the calf is not yet known, Duchess ate the placenta before it could be gathered for testing. They do hope to test the calf when they vaccinate the whole of the rest of the herd except for Duchess herself. I have suggested they vaccinate Duchess's calf; it will do no harm and if it is uninfected then it will need protection when the colostral antibodies wane and disappear). It just shows, as in FMD of 2001, when you look hard you can worry about minor symptoms and wrongly attribute an infectious cause on clinical grounds but laboratory testing will exclude infections of concern such as bluetongue or in 2001 FMD.
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