The insect-borne bluetongue virus has now been found on over 503 sheep farms in the Netherlands amid doubts over the efficacy of the two vaccines currently used to combat the disease.

In April then agriculture minister Piet Adema fast-tracked the use of the new vaccines. But despite the vaccination campaign that followed, the number of infections rose from 94 last week to over 500 at the beginning of this week, figures from food safety watchdog NVWA showed.

Most of the affected farms are in the provinces of Overijssel, Gelderland, Noord Brabant and Limburg.

The bluetongue virus was first identified on four Dutch sheep farms in early September last year and spread to hundreds of farms nationwide. It was the first time since 2009 that the disease, which does not pose a threat to humans, had been found in the Netherlands.

Following a winter with fewer cases, the virus flared up again in June. It is not clear if the vaccines are not working or if their efficacy diminishes over time. Medicine assessment agency CBG calls on all sheep farmers to report cases of bluetongue in sheep that have been vaccinated.

Agriculture minister Femke Wiersma has asked for an investigation into the development of the disease in vaccinated animals. “Much is unclear about how the disease progresses in a situation in which more than one vaccine is being used,” a spokesman for the GD laboratory, which is carrying out the research, told the AD.

Farmers’ organisation LTO said it had received several reports of vaccinated sheep that had died but said that it is too early for conclusions.

Bluetongue, which is spread by infected midges and can be lethal, is characterised by high fever, swelling of the lips and red mucous membranes in the mouth and tongue. It can also affect goats, cows and deer.