In the letter, she said that the Welsh Assembly government made 7.5 million doses of bluetongue serotype 8 vaccine available in 2008, but that the uptake of the vaccine to date has been "disappointing".
Only around 25% of Welsh farmers have vaccinated against the disease, apart from the South East of the country, where the figure is more like 70%.
Glossop has written to all vets in Wales about bluetongue vaccination in 2009, saying, "As spring approaches, I would urge the industry to vaccinate as many of their livestock as possible to ensure the highest level of protection."
Adding that some farmers and vets have raised concerns about the possibility of adverse reactions resulting from the use of bluetongue vaccine last autumn, she said: "The three bluetongue vaccines authorised for use in the UK are killed vaccines. All adverse reactions reported have been investigated and, to date, there has been no real evidence of problems relating directly to the use of vaccine."
She said that in order to prevent BTV-8 from becoming endemic in Great Britain, it is necessary to aim for the highest possible levels of immunity in cattle and sheep.
"If there are cases in 2009, unvaccinated animals will continue to be at risk. Immunity takes up to 21 days after completion of the primary vaccination course, so waiting until cases are disclosed may be too late to protect stock," she said.
"For these reasons I am urging farmers to vaccinate now. This message is being communicated to all farmers: those who vaccinated last year, those who planned to delay vaccination until the spring, and those who have not yet decided to vaccinate."