A silent killer has been stalking
the horses of Carmel Valley and South County for years.
The culprit is the disease silicosis, which is caused
by inhalation of fine silica dust particles that lodge
in the smallest airways and air sacs of the lung. The
disease is incurable.
In recent years, veterinarian Matt
Durham of Steinbeck Country Equine Clinic in Toro Park
has been seeing more and more horses suffering symptoms,
which affect not only the lungs, but apparently contributes
to bone degeneration in horses. The source of the dust,
Durham said, is chalk rock, the sedimentary Carmel stone
that breaks down into a pumicelike dust. The breakdown
occurs when the rock is exposed by plowing or earth moving,
of which there has been a lot of the past few years in
Carmel Valley. Horses inhale the dust while feeding off
the ground. Silicosis is a well-known occupational
disease in humans, Durham said, typically caused by inhaling
rock dust created in mining, masonry work, sandblasting
and other industries where rock is crushed into an easily
inhaled dust.
In a report co-written by University
of California-Davis veterinarian John Madigan, Durham
noted that the disease was first diagnosed in horses
in the late 1970s in the Monterey-Carmel area. Before
then, area veterinarians noted signs of respiratory
disease, but were unsure of its cause. For years, silicosis
in horses was believed to be isolated to this region.
But as awareness of the disease increased, cases started
to be diagnosed in other parts of the state, notably
Napa and Sonoma counties and southern Monterey County.
"Unfortunately," Durham
said, "we're
in the hot seat of it. All throughout Carmel Valley
are pockets of chalk rock, Carmel stone." The
first medical reports mentioning silicosis in horses
appeared in 1982, and it was distinctly noticed in
areas where there was new development. "We felt
it was directly correlated to bulldozing and exposing
the soils with no organic matter," Durham
said.
Carmel Valley resident, horse
owner and author Jane Smiley lost horse Percy to silicosis
two years ago. She recalled the mare seemed to have first
been affected by breathing smoke from a forest fire in
1999. Percy became worse when he was moved to a boarding
stable in Hidden Hills, where land was being bulldozed
nearby.
"All
the bad things happened," she said. "It
was clear by 2002 that she was in distress." When
Smiley learned that there was no cure, she decided
to move the mare to live at Holman Ranch until her
death in 2006.
When dust particles become lodged
in the lung, Durham said, the body mounts a strong
immune reaction. While bacteria or pollen can be
broken down and removed from the lung, silicates cannot,
and this causes an ongoing process that can lead to
scarring of the lung. In people, it can lead to autoimmune
disorders.
Amanda Murray, veterinarian at
UC-Davis, has done recent research into an apparent link
between equine silicosis and equine osteoporosis, because
horses suffering from silicosis are showing bone disease.
She is researching development of a blood test to aid
diagnosis.
"No one knows exactly how they're correlated," Durham
said, but horses he has seen from areas where chalk rock
is found had metabolic bone diseases. "It seems
there's a direct correlation."
A likely explanation, he said, is that irritation of
the lungs causes a chronic stimulation of the immune
system. "The
body can't shut off the immune system, so we know
with both diseases like that, there can be effects on
the bones."
Humans with lupus or rheumatoid arthritis often have
osteoporosis, and humans suffering from silicosis develop
arthritis or lupus at greater rates than the general
population.
Indicators
of silicosis are shortness of breath, more rapid
than normal breathing — a horse normally
takes eight to 12 breaths per minute and affected horses
will take 30 or more — flared nostrils and intolerance
to prolonged exercise, Durham said. Those are symptoms
related to other heart and lung diseases.
There
is no surgical procedure to correct silicosis, Durham
said, and limiting a horse's exposure to the dust
is critical. But "it's easier
said than done." Some steps that can be taken
are adding wood chips or topsoils to turnout paddocks
or stalls, and planting and irrigating grass where
horses live. But that is problematic in an area with
chronic water shortages. And it would seem impractical,
he said, to fit horses with respiratory masks. "They'd
have to have the masks on 24/7," he
said.
In some experimental studies,
veterinarians have tried to completely flush the lungs
with saline solution to wash them out. "That may
work to a degree," he said, "before
the white blood cells have a chance to go to work,
but in a chronic case, it won't do the job."
There is no cure or good treatment
for humans or horses, Durham said. With
mild cases, there may be no need to treat at all,
and moderate cases can be managed with some medication — anti-inflammatories
or dilators. "Bone disease is much tougher to
manage," he
said. "There is a significant amount of pain and
it doesn't respond to typical pain therapies that are
normally effective in other diseases." Common
horse painkillers such as phenylbutazone don't work,
he said. A drug called Tildren administered to osteoporosis-suffering
horses shows some short-term improvement, but not during
the long term, Durham said.
And "it's
an expensive drug."
Kevin Howe can be reached at 831-646-4416 or khowe@montereyherald.com.
