Winter roundups often result in the upper respiratory equine illness called strangles (equine distemper) because the helicopters chase the terrified horses in freezing cold temperatures and the horses catch a chill in the traps. You can see the steam coming from the horses in the trap site photo here.
Strangles is highly contagious–spread from horse to horse contact and spread from horse to human to horse contact.
Strangles is inevitable during winter roundups–putting all the wild horses at risk who are forced to live in close quarters for months in short term holding.
Strangles got its name from having difficulty breathing as if being strangled. Horses can die from strangles.
There is a vaccine for strangles but it is less than 50% effective and dangerous to give to horses already exposed to strangles. The illness shows up some time after exposure. It spreads quickly in holding facilities during winter. So the vaccine is not the answer. The answer is to refrain from sweating wild horses in the chill of winter which translates to no helicopter roundups in the winter.
Wild horses get strangles from living in confinement after being chilled not from living freely on the range. In freedom they are very healthy.
For more information on strangles visit wiki: http://en.wikipedia.org/wiki/Strangles