Livestock farmers have been warned to be on their guard against bluetongue after the virus was detected in two cattle imported from France.

The Animal and Plant Health Agency (APHA) identified the disease through routine post-import testing after the infected animals were brought to North Yorkshire from an assembly centre in central France, where bluetongue continues to spread slowly.

The virus can cause reduced milk yield in cows, infertility in sheep and, in the most severe cases, is fatal for infected animals – but it does not affect people or food safety.

It affects cows, goats, sheep and other camelids such as llamas, and is transmitted by bites from midges, which are most active between May and October. Not all susceptible animals show immediate signs of contracting the virus.

Animal health officials said the two infected cattle had been humanely culled and, following the successful interception, the UK remains officially bluetongue-free, the risk of the disease remains low and exports are not affected.

But deputy chief veterinary officer Graeme Cooke said it was a stark reminder about the potential threat to farmers.

'This detection is an example of our robust disease surveillance procedures in action but must highlight to farmers the risks which come with bringing animals from disease-affected areas into their herds,' he said. 'Regulations and systems are in place for the benefit of our UK livestock industry.

'It is also a clear reminder for farmers that the disease remains a threat, despite coming towards the end of the season when midges are active.

'Farmers must remain vigilant and report any suspicions to APHA. Farmers should work with their importer to make sure effective vaccination needs are complied with, source animals responsibly and consider the health status of their own herd if they are not protected.'

The APHA said farmers have the option to send animals without fully compliant paperwork back to France or to cull them as a measure to reduce the risk of disease spreading to susceptible UK livestock.

The UK's most recent case of bluetongue came in 2007, but the nation has been officially free from the disease since July 2011.

• For more information about bluetongue see the government website.