**Acknowledgement**

Work at CBMSO was supported by grants from Ministerio de Ciencia e Innovación (MICINN, Spain) BIO2008-0447-C03-01, and BIO2011-24351, and by an institutional grant from the "Fundación Ramón Areces".

Work at INIA was supported by grants RTA2011-00036 from the Spanish Ministerio de Ciencia e Innovación (MICINN) and the Network of Animal Disease Infectiology and Research-European Union NADIR-EU-228394. MAMA is the recipient of a JAE-Doc fellowship from Spanish Research Council (CSIC).
