Translation - Spanish-English - Metritis contagiosa equina

In English, "MEC" (Metritis equino contagiosa) is "CEM" (Contagious equine metritis).

Also, to convey the meaning of the original, I think "that year" should be placed at the beginning of the sentence as in the original.

Also, "veneral" should read "venereal."

Also, "baterium" should read "bacterium."

Thus, my suggested version is:
"CEM is an extremely contagious venereal disease in equines caused by a bacterium named Taylorella equigenitalis. The first case of CEM was diagnosed in England in 1977. That year, there was an outbreak of venereal infections in breeding animals of the United Kingdom and Ireland."

Oops, thanks Spasty, "bacterium" was a typo, indeed. I'll correct that, but "venereal" is correct.
I think you are right about the acronym.

Oops!

CC: Spasty

I figured that those were typos. Thought I'd mention them anyway, just to list all the edits I had in my version. ^^