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In mountainous areas
of the West, some cattle at higher elevations suffer pulmonary
arterial hypertension, which leads to congestive heart failure.
This condition has been called brisket disease, mountain
sickness, big brisket, high mountain disease, or dropsy. Many of
the affected animals develop edema (swelling) in the neck and
brisket, due to high blood pressure forcing fluid out of the
vessels and into surrounding tissues. The swelling may spread up
the neck to the jaws, or along the underline of the belly. There
can be swelling under the tongue/jaw, but this can also be due
to other conditions (like wooden tongue); the most common
location for swelling is in the brisket area. Unless the
condition is reversed before permanent damage is done, the
animal dies. Sometimes, however, you don’t see outward signs;
you just find the animal dead, possibly from heart failure.
If cattle are taken
to higher elevations for summer pasture, you may see signs of
trouble at any time. Symptoms may not become obvious until years
later, or may appear as soon as 12 hours. Cattle living at
elevations above 5000 feet are at risk, and the incidence of
brisket disease increases at higher elevations. In thinner air
of high elevations, low oxygen availability triggers the problem
in susceptible cattle, and the heart tries too hard to supply
blood to the oxygen-starved body tissues.
Susceptibility to
this problem is inherited. Affected animals often start to have
problems early in life if they live at a high elevation, or
develop the problem within a short time if brought to high
elevation from lower altitudes, and some of them die. Cattle
with brisket disease are often lethargic. Other signs may
include weakness, diarrhea, bulging eyes, and difficult
breathing. If you are moving cattle and they travel very far,
the ones with brisket disease drop to the rear of the herd or
may lie down because they are short of breath and start to
exhibit heart failure due to exertion. The problem is often more
common in young cattle (calves through yearlings) on high
mountain pastures, than in older ones.
A number of factors
can contribute to onset of brisket disease, including
respiratory disease (such as pneumonia), migrating parasitic
larvae and lung worms, lung abscesses, cold weather stress,
breathing dust or smoke (anything that causes lung irritation
and damage), but the biggest risk is genetic susceptibility. A
susceptible animal may be more apt to develop brisket disease if
any of these other factors are present.
There are other
diseases that mimic high altitude disease (including parasites,
congenital heart disease, locoweed poisoning, etc.) and it can
often be difficult to differentiate them. In some ways they are
the same because the end result is congestive right heart
failure. Locoweed can have cardiotoxic effects. Brisket disease
is merely a disease of the lungs that leads to congestive heart
failure, and has a genetic predisposition. The other diseases,
in the long run, may still result in congestive right heart
failure, but have nothing to do with genetics.
Dr. Tim Holt
(Colorado State University) has been studying brisket disease
for more than 25 years and began testing cattle at various
elevations, from sea level to 15,000 feet (in Ethiopia). He
utilizes a test that was developed in humans for measuring
pulmonary artery pressure (PAP). The PAP test is a way to
determine which animals are most at risk for brisket disease,
detecting the early stages of the problem.
He explains that
the inner diameters of the small arteries in the lungs decrease
in size; the muscle layer in the wall thickens over time,
reducing blood flow into and through the lungs. This is what
increases blood pressure (the smaller the diameter of the
vessel, the greater the pressure). The muscle of the heart’s
right ventricle thickens due to extra effort needed to pump the
blood (causing enlargement of the right side of the heart) and
eventually the right ventricle loses tone and ability to
contract. As blood pressure increases and starts to back up into
the heart, it can blow out the valves of the right ventricle.
“You may start to see jugular pulsation because the heart valves
are failing due to the high blood pressure,” says Holt.
“Brisket disease is
a disease of the lungs, so the animal gets high blood pressure
on the right side of the heart. The PAP test measures that
pressure. It tells me if an animal is hypertensive or not. There
are multiple reasons why an animal may develop this problem, but
the big reason is that it is genetically susceptible to this,”
says Holt. The PAP test is a way to measure pressure within the
lung and blood flow resistance, thus making it possible to
predict that animal’s welfare at high altitudes.
Every animal
develops this type of hypertension in response to low available
oxygen levels at higher elevations, but cattle do it at a rapid
rate; they are high responders, he explains. They shunt blood
flow away from the poorly oxygenated lung tissue by constricting
those blood vessels—shunting more blood flow to the upper part
of the lungs where there is more oxygen. “In cattle this
situation develops more rapidly and more severely than in
humans,” says Holt. Due to the anatomy of the bovine lung and
the fact that lungs of cattle are small in size compared to the
rest of their body, this shunting process occurs to much greater
degree. This exaggerated degree of shunting, blood vessel
constriction and artery thickening (resulting in increased
pressure within the lungs and vessels) is highly heritable,
however.
There are some
cattle that seem less susceptible than others. A few breeds are
slightly less susceptible, but brisket disease has been seen in
all breeds. “Cattle that have been at high elevation for years
(many generations) have no problems. I just came back from
Ethiopia, and cattle grazing there at 10,000 to 14,000 feet are
not hypertensive at all. They have very low PAP scores,” says
Holt. Probably all the cattle in that region that were
susceptible to this condition died off long ago, and thus did
not pass this genetic tendency to offspring (survival of the
fittest).
In breeds used in North America we’ve selected for various
characteristics like meat (or milk) production, fertility, etc.
but since brisket disease is limited to a small geographic area
we have not paid attention to how animals function at high
altitudes, and have therefore not eliminated individuals that
might have problems. So this tendency is found in many breeds
and bloodlines used today. Most cattle live at lower elevations,
so there is no natural selection to eliminate the problem. There
are, however, some breeds and some family lines within breeds
that are more naturally resistant than others, and by using the
PAP test these animals can be identified, to aid ranchers at
high elevations in selecting the more naturally resistant
animals to use for breeding stock.
The problem
sometimes appears in feedlot cattle, even at low elevations, as
these animals get heavier and closer to finish weight, because
the heart must work harder. “It may be a different mechanism,
but it’s the same type of issue; they are dying of right heart
disease. There is research being done right now in feedlot
cattle at 3000 feet elevation, because these cattle can have the
same clinical appearance. We’re still looking at this,” says
Holt.
Brisket disease,
per se, generally only appears in cattle above 5000 feet, with
7500 feet (and higher) being more critical. Cattle in Colorado,
for instance, are often grazing at 9000 to 12,000 feet. Brisket
disease can become very costly unless any new animals brought
into these herds can be tested in order to predict which ones
might suffer this problem. PAP testing can help ranchers select
genetically resistant animals. A growing number of seedstock
producers who market bulls to customers at high elevations are
PAP testing their animals. The opportunity to purchase bulls
with low PAP test scores has helped reduce but not completely
eliminate the problem of brisket disease in many herds.
PAP testing is only
accurate when done at altitudes above 5000 feet and the accuracy
increases at higher elevations. The higher the elevation, the
more animals you’ll find with this problem. If you test at 5000
feet you may discover some of the animals that are highly
reactive, but you won’t get all of them, says Holt.
There are only a
few people who do the PAP testing. “It’s not difficult to do; it
just takes a lot of time, equipment and traveling, to go to
ranches to test the animals,” says Holt. If breeders want their
bulls tested, those bulls must live at a high elevation or be
taken to a high elevation for testing. Since most of the cattle
population and AI sires in the U.S. are at lower elevations,
this is a challenge for ranchers at high elevations—to find
breeding stock that do not have this genetic susceptibility.
There’s also some
research being done to develop other testing methods to look at
genetic markers, to determine which animals might carry this
susceptibility—to be able to predict whether or not the
offspring would inherit this tendency. Results of this research
are still several years away, however.
“Another
complicating factor is that for many years we’ve concentrated on
testing the bulls (the ones being produced at or for use at high
elevations), but have overlooked the female side,” says Holt. On
some ranches it appears as though the females may be more
indicative of the problem than the males; some of them may be
carriers (passing the genetic susceptibility to their calves)
without developing clinical signs themselves, he says.
You can’t totally
eliminate this problem in a herd without a lot of effort. You
might have to use PAP tested bulls (with low scores) for many
generations in order to produce cattle that can be raised at
high elevation without problems. The genetic factor is not
simple; it’s a very complicated gene.
PAP Testing
The PAP test can be done at any age, but accuracy is better if
the animal is at least a year old. A calf can be tested, but a
low score at that age does not mean he won’t develop brisket
disease later. His body is still growing, and the lung capacity
has not yet reached its limit. “I still find a lot of young
animals that have brisket disease (chronic oxygen shortage), but
they tend to develop it more as they get older. By 11 to 12
months of age the accuracy of the test becomes much better,”
says Holt, and by 16 months or older the scores are even more
reliable. If a young animal is tested and has a low score, that
animal should be retested later, to determine whether or not it
may be at risk for brisket disease.
To test an animal,
a catheter is inserted into the jugular vein in the neck,
threading it through the vein and into the right ventricle of
the heart and out into the main pulmonary artery between the
heart and the lungs. The pressure in this artery is measured to
determine the blood pressure in the right side of the heart. The
higher the blood pressure is, the less capable the animal is of
living at that particular elevation; the animal may already be
in the process of dying, due to right heart failure. Thus a PAP
test gives the animal’s risk for brisket disease. To be most
accurate, however, the tests must be done at elevations above
6500 feet, and cattle must have been at the high elevation for
at least 3 to 6 weeks in order to get a proper measure,
especially if you are wanting to evaluate the animals for
genetic susceptibility (to use them for breeding purposes).
Holt has
demonstrated that an animal’s PAP score will go up as the animal
is taken to higher altitude. Thus the elevation of the test site
must be considered, along with the elevation where the animal is
(or will be) living or grazing for summer pasture. It is
important to realize that cattle raised at low elevations will
have a higher probability of experiencing brisket disease when
taken to higher elevation than cattle raised at high altitudes,
says Holt.
The resistance to
brisket disease (of cattle already living at high elevation) is
probably due to natural selection (the fact that the most highly
susceptible animals died off) and to culling efforts of
ranchers. Most ranchers get rid of any animals that show signs
of brisket disease or any cows that give birth to calves that
develop brisket disease. Thus most herds that live at higher
elevations have more natural resistance than do herds at low
elevations that have had no selection against this problem.
There are some
things that can affect the accuracy of a PAP score including
illness, stress, or vaccination. The score is simply a
measurement of lung blood flow resistance, so anything causing a
temporary or permanent decrease in lung space can create an
increase in the PAP measurement, according to Holt. Any type of
respiratory or lung impairment can lead to a high PAP score. If
a high score is thought to be due to a temporary disease, the
animal should be retested later. But PAP scores of more than 50
to 55 have rarely been shown to drop back to an acceptable
level—probably because once the measurement gets this high,
extensive and permanent damage has taken place in the arteries
of the lungs.
Score Evaluation
Following scores are based on tests of cattle 12 months old or
older, tested at or above 6500 feet. A PAP score of 30 to 35 is
considered excellent; 36 to 39 is excellent for animals over 12
months. Any score less than 41 is acceptable, but Holt
recommends that yearling cattle measure less than that
(depending on the altitude of the test). Any animal with a score
of 41 and higher should always be retested prior to use. A PAP
score of 41 to 45 is acceptable for animals more than 16 months
of age. Animals less than 16 months in this range should be
retested later to more accurately predict their future. A PAP
score of 45-48 is acceptable only for older animals that have
been at high elevation for an extended period of time; animals
in this range are more susceptible to environmental stresses
leading to brisket disease and should be considered at risk.
Animals that score greater than 49 must always be considered
high risk candidates for brisket disease, and their offspring
will be at risk also.
Treatment
The best treatment is to immediately take the affected animal to
a lower elevation, or treat it in a hyperbaric chamber (where
oxygen concentration in the air it breathes can be greatly
increased). “This is what I did for years, putting cattle into
these chambers to simulate lower elevation,” says Holt.
Other treatments,
which can alleviate symptoms (and keep the animal from dying
until it can be taken to lower elevation) involve use of
diuretics to lower blood pressure and fluid volume in the body.
“You treat the animal just like you would a human patient with
right side congestive heart failure. You put them on Lasix, with
is a diuretic, eliminate fluid and salt intake, and put them on
a high broad-spectrum antibiotic, and vitamin B complex. One of
the things that’s been shown to be very beneficial is draining
the chest cavity of fluid,” he says. This can be done by
inserting a large diameter needle through the chest wall,
between the ribs.
Many ranchers are
doing this themselves, after being shown how to do the procedure
by their veterinarian. “You poke it between the ribs, into the
chest cavity, to drain out all the fluid that’s accumulated in
there,” he says. This must be done carefully and in the proper
location, to avoid draining the heart cavity or poking a hole in
the heart. The fluid you see on the outside of the animal
(brisket, neck, etc.) is just a small part of the total. There
is even more fluid inside the animal, and fluid inside the
thoracic cavity (chest) is putting pressure on the heart and
lungs. Draining the fluid can relieve the animal until you can
move it. All these treatments are helpful, but you still have to
get the animal to lower elevation. You’re just buying it time
until you can move it. |
