Britain's just-released Equine Quarterly Disease Surveillance Report reported seven infected properties - all geographically isolated from each other - in the last three months of last year.
Cases on two of the affected properties included vaccinated thoroughbreds.
Testing was done and found one strain was identified as being of the American variant-American lineage.
"This virus is thus similar to virus strains identified in North America, Australia and Japan in 2007 and to the best of our knowledge is the first isolate of this lineage identified in Europe," the report said.
"This highlights the importance of ongoing surveillance to allow the emergence and distribution of new equine influenza virus strains to be monitored.
"Other isolates identified in the 4th quarter 2007 were of the European variant American lineage, and similar to strains identified in the UK throughout 2007."