Statistics of wolf-on-dog attacks are not available, but we certainly seem to be experiencing an uptick of hunting dogs killed by wolves. And this would make sense as wolf populations continue to rise.
In a dramatic January attack, a Montana lion hunter lost all three of his hounds. And three separate attacks have occurred in Wisconsin just this month. On August 4, a Wisconsinite training his bear dog—a 9-year-old female black and tan—lost her to wolves in Rusk County. Subsequent attacks occurred on August 10, when wolves killed a pair of dogs in Lincoln County and another hound in Sawyer County. A fourth attack occurred on August 12, when a hound was killed in Florence County.
It's not known for certain whether these attacks were predatory or territorial in nature. Apparently wolves defend their dens most aggressively from May to September, when pups abound. The Wisconsin DNR set up a four-mile "caution" area for each attack, and already had plans in place to reduce its overall wolf population as part of its management plan.
I'm no biologist, but four dead hunting dogs in just over a week's time seems to vindicate Wisconsin's decision to increase its wolf quota by 75 animals for the 2013-2014 season. A drawing took place on August 15, in which 2,500 lucky hunters were selected from a whopping pool of 17,000 applicants. Total wolf quota this year is 275.