I'm in southeastern New York we have WNV issues here on occasion. All the South American sheldgeese and swans (and even some of the SA ducks) seem especially prone to WNV and the equally frustrating avian plasmodium, which shows very similar outward signs (loss of balance, general lethargy). Red-breasted and some of the other arctic geese are also pretty sensitive to WNV but don't seem to have as much a problem with plasmodium, though when folks don't submit samples for histopath it's hard to tell what's really going on.
Yes, I vaccinate all the sheldgeese now as a preventative. I know, I know, you'll say there's no real proof of it's effectiveness and you're right, I know that, but it makes me sleep better at night. I'm not a big believer in the immunity concept. Young birds have very weak immune systems compared to adults and can't fight off WNV or plasmodium like older birds. Even previously exposed adult birds can drop dead of WNV. Is it an especially virulent strain? Who knows?
Fortunately for magellans they're still by far the most common of the South American sheldgeese in captivity in North America, though the barred breasted type is not as well represented as they once were. There's a slew of extra females out there at the moment but that will even out in time. The shelgeese that concern me most are Ashy-headed, they are few and far between these days. Ruddy-headed aren't much better. Do you keep either of these species?
Danny