Silicosis

What is silicosis?

Silicosis is a respiratory condition caused by inhaling silica dust (most commonly cytotoxic silica dioxide (SiO2) crystals). These small dust particles travel to the lungs where they lodge themselves in the airways and air sacs. The body responds with a strong immune reaction, but cannot break down and remove silicates, which leads to scarring of the lungs and chronic lung disease.

In humans, silicosis is a well-documented occupational hazard from mining, masonry, quarrying, stone cutting, sandblasting and other industries with exposure to easily inhaled dust from crushing rock. There is no cure, but occupational safety measures have greatly reduced disease incidence.

In horses, the form of silicate that primarily causes silicosis is cristobalite, but quartz and tridymite can also be responsible for the condition. It is abundant in the central and coastal regions of Northern California (Monterey/Carmel Valley and surrounding areas). Described as “chalk rock”, the dusty, light soil is typically found in foothill areas. It is easily disturbed and released into the air through activities such as construction. Drought conditions, which have persisted in the western United States, cause plants to dry out, which exposes more dust and further exacerbates the problem in these regions.

Horses in these environments can inhale the dust over days to years, in some cases developing severe pulmonary fibrosis, generalized granulomatous pneumonia, and hilar lymphadenitis. Affected horses may also develop a bone fragility syndrome known as silicate associated osteoporosis (SAO), characterized by extensive bone remodeling.

What are the clinical signs of silicosis?

Clinical signs of silicosis in horses can range from no observable signs (asymptomatic) to severely compromised breathing. Affected horses may exhibit coughing, high respiratory rates, shortness of breath, flared nostrils when resting, body soreness, exercise intolerance, and a prolonged history of weight loss. They may develop accentuated muscles in the chest and abdomen since their muscles require extra effort to breathe.

How is silicosis diagnosed?

X-rays (radiographs) of the lungs, along with a physical examination and the geographic history of the patient, are used to diagnose silicosis. Samples of fluid and cells from the lungs may be obtained through transtracheal or bronchoalveolar wash. Some advanced cases may require ultrasound evaluation and lung biopsies.

How is silicosis treated?

Treatment for silicosis in horses involves supportive care. Zoledronic acid is used as part of the therapy to reduce the wearing down of bone (osteolysis) and minimize the rate of progression of osteoporosis. Ideally, the affected horse should be removed from the dusty environment and relocated to a different geographic region. Non-steroidal anti-inflammatories (NSAIDs) and corticosteroids may be administered to alleviate pain. Bronchodilators may be used to make breathing easier, especially during flare-ups. Exercise should be restricted.

What is the prognosis for silicosis?

There is no cure for silicosis. Horses with long-term exposure to silicate dust can develop silicate associated osteoporosis, which causes bone deformities and predisposes horses to fractures.

How can silicosis be prevented?

To prevent silicosis, eliminate or reduce silicate dust by avoiding housing horses in or near areas of new construction that stir up dirt. Plant grass and irrigate dry lot areas if possible. Add materials such as wood chips or topsoil that minimize dust to stalls and runs.

Factors such as genetics, behavior, environment, and length of exposure likely affect development and severity of disease.