Dr Christianne Glossop, Wales’ chief veterinary officer, said reaching the key temperature of 15C had made the need to protect livestock against the disease critical.
“The rise in temperature triggers midge activity and the ability of the bluetongue virus to replicate,” Dr Glossop claimed.
“We have heard upland farmers assuming their animals are not at risk, but they should not be complacent.
“It is entirely possible that there are areas within every locality where the midge that carries the virus can flourish.”
She also warned that it took three weeks after completing a course of vaccine for animals to have optimal immunity, so it was not sensible to wait until the disease was found close to a farm.
Vaccine was still available from the Welsh Assembly as only 23% of the 7.5m doses ordered had been taken up.
The cost of new stocks would not be underwritten, which could affect the price of vaccines once existing doses had passed their expiry date.
Rees Roberts, chairman of Hybu Cig Cymru (Meat Promotion Wales), said bluetongue would have a devastating impact on Welsh livestock production if susceptible animals were not protected in time.