Bluetongue is spread to livestock by biting midges
A farming union has said it is "disappointed" that many farmers in Wales have not yet vaccinated their animals against the bluetongue virus.
Stocks of the drugs, worth around £3m, may have to be thrown away if farmers do not inoculate their animals by June.
Dai Davies, the president of the National Farmers' Union (NFU) Cymru, said the low uptake could be disastrous for the farming industry.
It is thought the vaccine is the only protection against the disease.
Bluetongue is spread by a species of midge and can be fatal to animals such as sheep and cows.
Under European livestock rules, once an area has been designated a protected zone vaccinations can be issued.
A bluetongue protection zone was extended to cover the whole of Wales in September and the Welsh Assembly Government ordered 7.5m doses of the vaccine.
Mr Davies said the uptake was not as widespread in Wales as it could have been because vets had advised farmers it was better to vaccinate in the spring rather than in the autumn of 2008 when it first became available.
"There were some unfounded stories going about that if animals were vaccinated they would abort or become infertile," he said.
But with stocks of doses due to go out of date, Mr Davies said it was important farmers who had not inoculated did so quickly.
"We are emphasising this in order for farmers to wake up and get on with the job," he said.
"As long as they do it this spring. The sooner they do it the better because as the temperature rises the midges become more active and therefore the danger of this disease moving to this country is so much greater.
"The jab is quite reasonable, something like 60p per ewe and £1.20 per animal but of course there are the other costs of handling and the dangers involved in handling a lot of animals."
Mr Davies said the vaccine uptake in Wales was around 23% whereas in Scotland it was 40%.
Defra said enough vaccine had been sold to date to vaccinate 60% of the susceptible livestock population in England.
"However, take up was very high in eastern and southern counties in England (up to 90%) and lower in Northern counties," said a Defra spokeswoman.
Bob Stevenson, a vet in an agricultural practice in Usk, Monmouthshire, said the benefits of vaccination far outweighed any costs or inconvenience.
"But it has to be fitted in with the normal animal health programme," he said.
Judy Lewis, a farmer of sheep and pedigree cattle in Newcastle Emlyn, said she had not vaccinated her animals yet because their annual TB tests had been due at the end of September.
"We got in touch with the State Veterinary Service, which is in Carmarthen and they emphasised that it would not to wise to vaccinate for the bluetongue before doing our TB tests," she said.
"We are definitely going to vaccinate but it will be in the near future once the cows have finished calving and the lambing has finished."