According to ISIS there are only 9 zoos with musk oxen in their collection in North America, and 6 of those are situated in Canada. Only the Point Defiance Zoo (2), Denver Zoo (2) and Minnesota Zoo (a large herd of possibly 10) keep muskox throughout the entire United States. Is it a coincidence that all three of those American zoos are within close proximity to snowy winters? Does this mean that muskox might suffer greatly in warm climate collections? Perhaps the need for cold weather, combined with the sheer size of the species, contributes to the fact that they are extremely rare in North American zoos.
San Diego tried keeping them in the 80's. I understand they didn't fare well. Although San Francisco has kept (and bred, I believe) them in the past, not sure how they did there. Portland is a reasonably temperate climate and they until recently kept the species as well.
Ungulate (a member here who is a zookeeper at the Toronto Zoo) said that they require very expensive fencing, hence why the zoo has decided to phase out the species. They were originally going to be placed in the new Tundra area that will be opening in the fall, but there were budget cuts. This could be another factor.
I saw the San Francisco musk oxen years ago. It seemed to be an exhibit that they were very proud of. It surprised me quite a bit to hear they were gone. It was a good sized field exhibit for such an urban zoo. They were kept near where the gorilla exhibit is today on the edge of the zoo near the ocean. I was under the impression that they'd had them awhile.
Brookfield zoo held musk ox in the early eighties. I also remembered seeing musk ox at the san francisco zoo quite a few years ago. Imagine my amazement to see a fine breeding herd at the Berlin tierpark, which has every animal possible except apes. Have to go to zootierliste to check, but i suspect nothern european collections have some musk ox herds.
Tacoma does not have snowy winters like Minneapolis and Denver, but the muskoxen seemed to be doing well when I saw them there. Minneapolis has very hot summers although they didn't seem to be suffering when I saw them there years ago. I had read somewhere that muskoxen were being phased out of North American zoos in favor of Sichuan takins.
I know Cheyenne Mountain Zoo had Musk ox, but there exhibit is gone and that space is now used as an extension of the Reticulated giraffe exhibit. They also had them at the Los Angeles Zoo on the Hillside