The patients lingered behind the wooden fence, eyes downcast, chests heaving. If they could, they'd be wearing oxygen masks. "Their hearts are the size of volleyballs," said Mark...

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LARAMIE, Wyo. — The patients lingered behind the wooden fence, eyes downcast, chests heaving. If they could, they’d be wearing oxygen masks.

“Their hearts are the size of volleyballs,” said Mark Stayton, a professor of microbiology at the University of Wyoming. “They should be the size of softballs.”

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Stayton and colleague Rich McCormick are on the cutting edge of research into elevation sickness — in cows.

At more than 5,000 feet above sea level, some cattle get woozy. Fluid collects in their hearts and lungs. Listlessness sets in. Death follows. Symptoms are similar to those seen in people suffering from emphysema, where every breath is a struggle.

The scientists are taking blood samples and compiling a database, looking for a genetic marker that might indicate which cows are prone to altitude sickness. That way, animals could be identified before being brought to ranches in the Rocky Mountain West, where the problem is most acute.

“The ability to predict what animals are predisposed to this … would be invaluable,” McCormick said.

The University of Wyoming, which maintains a working cattle ranch and sits at an elevation of about 7,100 feet, is perfectly suited for such experiments.

Young cows suffering from the ailment stand in muddy pens beside healthy animals. The sick ones have swollen necks; they weigh about one-third less than they should.

“What’s happening is that it’s hard for these cows to force blood through their lungs,” McCormick said. “They have a lot of stress, their hearts enlarge and they begin to do poorly.”

As physically distressing as altitude sickness is for the cows, it’s financially painful for ranchers. On average, according to researchers, between 3 percent and 10 percent of calves in areas above 5,000 feet die each year because of elevation sickness — also called brisket by ranchers. In extreme cases, 20 percent of a herd can be lost.

Cattle ranches in Colorado and Wyoming are hit hardest because so many pastures are at high elevations. Animals sometimes are brought up from lower locales and get sick; other times they are born susceptible to the condition.

Many ranchers depend on Tim Holt, one of a handful of veterinarians who travel the West testing cows for brisket.

Holt has done 137,400 tests over the past 20 years and is collaborating with Stayton and McCormick on their research.

His technique is not for the squeamish. Cows are placed in a chute, their heads are turned sideways and a large needle is inserted into the jugular vein. A catheter is threaded through the pulmonary artery, and the pressure is taken. If the animal is sick, it will have trouble pumping blood to its lungs, and pressure will be high.

Animals that pass the test are given certificates. A prize bull can sell for $5,000, but if that same animal fails its physical, a seller would be lucky to get $1,000.