Madrid - Two Spaniards have become the first mother and son to die of the variant of the Creutzfeldt-Jakob Disease (CJD) linked to the bovine spongiform encephalopathy (BSE) or mad cow disease, experts said Thursday.

Investigations had confirmed that the death of a 40-year-old man in April and of his 64-year-old mother in August had been caused by the CJD variant, the regional authorities of Castile and Leon said.

The case of a mother and son both succumbing to the disease was unique worldwide, said Juan Jose Badiola, president of the Spanish College of Veterinarians.

He said the circumstances of the transmission would be investigated.

Veterinary experts believe the son caught the disease by eating infected beef before 2001, when preventative measures were adopted. Belonging to the family of an infected person has not been considered a risk factor so far, according to Badiola.

The two deaths raised the Spanish death toll for the CJD variant to four. The first victim died in 2005. (dpa)