Comment for warmwell.com from Dr Ruth Watkins November 10 2007....who is pleased that the farmers who visited France and who, says Ruth Watkins ..
"..see for themselves the results of intense infection in an area with BTV-8 and do not believe DEFRA and other ill-informed people who say the a good cold winter will kill off the midges and the disease won't come back next year.
However I would like to make two important points about their observations.
So this year, each susceptible ruminant is likely to be bitten by many infected midges when it is initially infected which in virology can lead to more severe infection.
- Once infected a ruminant does make a strong immune response (if it survives) that protects it from any significant infection again with that same serotype for the rest of its life.
The reason why it is worse this year in any country where it appeared before is that this year instead of 10 - 15% of the cattle herd being infected as in 2006 in N France, in 2007 90%of the herd was infected.
For sheep, in 2006 the infection rate was perhaps1% and now it is 80 to 90%.
All farms in the areas of intense infection will have infected ruminants whether they are documented so or not. That is why so many more animals were sick this year
- Why did this happen?
BTV-8 is an arbovirus and all arboviruses can be hugely amplified in the right conditions for infection of the arthropod and lots of susceptible hosts to be infected (the midge and ruminants in the case of BTV-8). Cattle can remain infectious for up to 100days and sheep up to 54 days.
Midges prefer biting cattle to sheep- perhaps for various reasons, easier to bite? more CO2 on the breath so more attractive?-one infectious cow can infect thousands of female midges, perhaps 10,000s of female midges.
Even so, many infectious ruminants are not noticeably ill, though they may become sterile or have cracked feet later, and these of course also amplify the infection.
This is why a good vaccine (we have every reason to believe that the inactivated vaccine will be a good vaccine) offers hope and help to ruminant livestock farmers.
Next year all the young and new born ruminants in N Europe- once they lose the maternal protective antibody - will themselves be susceptible so the infection will continue again in 2008 - unabated - unless vaccination is applied.
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