While some medical textbooks still maintain that the Influenza B Virus is only infecting humans, current research has indicated that this view is redundant.
Here we have already demonstrated that the Influenza B Virus is very much capable of infecting seals. The researchers speculated that the probable cause of this jump from humans to seals was unvaccinated staff (!) from the Seal Rehabilitation and Research Centre in Pieterburen (The Netherlands).
 managed to detect antibodies to Influenza B Virus in horses. A survey conducted among a population of horses in an area near Toronto (Canada) showed a high frequency of antibodies to the Influenza B Virus. Although only a limited number of horses was tested, the evidence nevertheless suggests that natural infections of equines does occur. The horses did not exhibit any symptoms of illness.
The story in Influenza viruses remains a complicated one and now we know of three distict species in which the Influenza B Virus might mutate.
 Ditchfield et al: Upper Respiratory Disease in Thoroughbred Horses: studies of its viral etiology in the Toronto area, 1960 to 1963 in Canadian Journal of Comparative Medicine and Veterinary Science - 1965
 Kasel et al: Experimental Human B Influenza Virus Infection in Chincoteague Ponies in Nature - 1968