Mother Nature didn't endow cattle with overly large lungs. In fact, they tend to have less lung capacity for their body weight when compared to most mammals.
At high elevations, chronic low-oxygen tension causes cattle's small pulmonary arteries to thicken, resulting in high pulmonary blood pressure. This results in congestive right heart failure.
It's a very common condition in cattle that range the high altitudes of Colorado, Utah, Wyoming and New Mexico. The condition is also known as dropsy, hypoxia, brisket disease or high mountain disease (HMD).
The development of pulmonary artery pressure (PAP) testing made it possible to predict which animals are susceptible to pulmonary hypertension, the early signs of congestive right heart failure. The signs and symptoms are lethargy, weakness, collapse, diarrhea, bulging eyes and edema — with the brisket region being most pronounced. The altitudes at which cattle are affected are 5,000 ft. or greater — with 7,500 ft. and greater becoming the most critical elevations.
The altitude at testing
Research has shown that the condition leading to HMD is highly heritable. Therefore, PAP testing provides the ability to genetically select resistant breeding cattle for use in high altitudes.
Most affected ranchers are aware of the need to PAP test the cows and bulls they bring into their herds. Many of the region's seedstock producers routinely test their cattle and make PAP scores (see sidebar on page 73) available to prospective buyers. But, the elevation at which PAP testing is done has recently become a hot topic.
Tim Holt of Fort Collins, CO, is the consensus world expert on HMD. He's an assistant professor at Colorado State University's veterinary teaching hospital, and also has a private individual veterinary practice. Holt is also known as the guru of PAP testing.
Holt says the lower the elevation at which cattle are tested, the lower the accuracy of the test. By “lower” elevation he means most definitely below 5,000 ft. and probably below 6,000 ft.
“This disease is stimulated by hypoxic stress secondary to high altitude,” Holt explains. “Therefore, it's impossible at this time to conduct accurate PAP tests in lower elevations.”
That's not to say it's not done at lower elevations, but this should be kept in mind at all times when testing cattle in lower elevations. The test at low elevations should be used only to evaluate the cattle in that herd, Holt explains.
“It is not a ‘sellable’ test,” he says. In other words, cattle he's tested at lower elevations, regardless of how low their PAP score, have still developed HMD — or scored a high PAP test — when taken to high altitude.
“At this time, a low elevation test should be used strictly to eliminate animals that test high at that low elevation, so as not to put any more money into them,” he explains.
Holt emphasizes that “lowland” ranchers need not be concerned with the PAP test, nor worry about HMD at this time. It's only a concern if there's any chance those cattle might go to high elevations.
Breeds and other variations
Holt's testing of more than 130,000 cattle indicates no single breed is totally resistant to the effects of high altitude. There have been high-measuring animals found in all breeds tested.
“There does, however, seem to be some breeds, and pedigrees within breeds, that are more naturally resistant to the effects of high altitude,” he points out.
When evaluating a PAP score, the age of the animal at the time of testing should be considered. The accuracy level of the PAP test is less predictable for animals tested at 12 months of age or younger.
Cattle tested at 16 months of age and older appear to be the most consistent and accurate.
“An exception to this inconsistency is in animals with a high PAP measurement,” Holt reports. “The accuracy level for a PAP measurement, regardless of age and/or elevation of the test, is extremely high in these animals.”
There also appears to be some breed variation in this inconsistency as well. Holt says some breeds show a greater degree of inconsistency and variability than others.
Is there more to PAP testing?
While only 3% of the nation's cattle are susceptible to HMD, the science behind PAP testing is getting noticed outside the mountain states. Some cattle feeders, for instance, suspect calves with low PAP scores might out-perform those with high PAP scores — even when fed at low elevations. These low-altitude cattle feeders, particularly some in Nebraska, are starting to pay more attention to the genetic practices related to avoiding HMD.
“There's no scientific evidence of this; it's purely anecdotal at this time,” Holt clarifies. “But, the feedlot performance of low-PAPing calves is something one might want to look into down the road.”
Another concern Holt is hearing is that incidence of right heart failure in calves fed at low-altitude feedlots appears to be rising. Holt's not ready to speculate why.
“We don't have solid data to support these observations. It's just what we're being told,” he adds. “There's a huge science behind pulmonary hypertension, and a lot we simply don't know about how it affects general cattle populations.”
Holt says some scientists are working to find a genetic marker associated with HMD so high-risk cattle can be identified by a blood test.
“This is probably the most promising new development linked to this very costly disease,” he says.
Holt notes, though, that an animal's innate resistance to high altitude effects is secondary to the effects of both natural selection and the culling processes by the rancher.
“Ridding the herd of animals with HMD themselves, and/or giving birth to calves that develop HMD, can help to build a more naturally resistant herd in high altitude,” he explains. “This is compared to lowland herds in which no natural selection takes place. That's an important consideration in testing and/or using animals originating from lowland herds.”
Interpreting PAP scores
PAP 35mmHg — this is considered an excellent and highly reliable score.
PAP 36-39mmHg — considered an excellent score for any animal more than 12 months.
PAP less than 41mmHg — reliable measurements in all animals greater than 12 months of age.
PAP 41-45mmHg — an acceptable range for older animals, i.e. greater than 16 months.
PAP 45-48mmHg — an acceptable range for only older animals that have been in high elevations for an extended period of time.
PAP greater than 49mmHg — animals scoring in this range must be considered high-risk, not only for themselves but offspring.