MEDICINE II
Fractures and Bone Deformities in 18 Horses With Silicosis Matthew G. Durham, DVM; and Coral M. Armstrong, DVM
Horses with silicosis commonly develop progressive bone-related issues that often become the dominant problem. Although prevalent in Monterey County, California, the Monterey formation is present throughout the California coastal range. Practitioners should be suspicious of silicosisrelated bone disease in horses from this area exhibiting insidious lameness, neck stiffness, scapular deformities, ataxia, or lordosis. Authors’ addresses: Steinbeck Country Equine Clinic, 15881 Toro Hills Avenue, Salinas, CA 93908 (Durham); and Los Coches Animal Hospital, PO Box 125, Soledad, CA 93960 (Armstrong); e-mail:
[email protected] (Durham). © 2006 AAEP.
1.
Introduction
Silicate pneumoconiosis (silicosis), long known as an occupational lung disease of humans, was first recognized in horses in 1981. Schwartz et al.1 documented the Monterey Formation or spontaneously occurring equine silicosis in horses from Monterey County, California that had been exposed to the soil type known as Miocene Monterey shale. Typical lesions consisted of pulmonary fibrosis, which is associated with granulomatous lung disease, and resulted from silicate accumulation within the pulmonary macrophages. Although silicates are very common soil components, there is an unusually high concentration of the fibrogenic and cytotoxic cristobalite form in certain parts of Monterey County, and the disease is relatively common in that area. Many horses living in affected areas do not show the more severe pulmonary manifestations, such as tachypnea, exercise intolerance, and granulomatous pneumonia, despite long-term exposure to the typically dry, dusty environment that is associated with the condition.2
This report describes fractures and deformities of the axial skeleton in horses living in areas with the soil type known to be associated with silicosis. These changes, although observed by area practitioners over many years, have not been previously presented in the veterinary literature. Further clinical investigation of horses with these bone lesions has shown silicate accumulation in the pulmonary or lymphatic tissue. Early bone lesions can be subtle, but they progress over months to years. Although common locally, the Monterey Formaa tion is widespread. Therefore, practitioners examining horses with pain or deformity of the axial skeleton should consider the possibility of concurrent clinical or subclinical respiratory disease. 2.
Materials and Methods
Case records of patients with presumed silicosis were reviewed from the files of Steinbeck Country Equine Clinic and Los Coches Animal Hospital. From this group, horses with lameness, bony deformities, stiffness, ataxia, or fractures were identified. There were at least 75 horses that showed these signs.
NOTES
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Fig. 1.
Horse 12 showing scapular bowing and lordosis. Fig. 2. Photograph of Monterey Shale exposed by a road cut in Carmel Valley, California.
To be included in the study, it was decided that a horse must: 1.
Have a known location history from a region with documented Miocene Monterey shale. 2. Have a diagnosis of silicosis substantiated by bronchoalveolar lavage (BAL), thoracic radiographs, and/or necropsy findings.
3. Have bone disease substantiated by radiography, ultrasonography, pathology, or photography and showing obvious bone distortion. Eighteen of the seventy-five cases that were identified fit all of the above criteria. Signalment, presenting
Fig. 3. Sonograms of horse 6 showing bony irregularity and widening of scapular spine. image on the right is longitudinal. 312
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The image on the left is transverse, and the
MEDICINE II Table 1.
Signalment and History
Case No.
Breed
Sex
Age
Location History
Duration of Signs
Presenting Complaint
Respiratory Signs
Follow-Up
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Arab Arab App QH QH App App QH QH QH NSH Arab QH Arab QH QH QH QH
F G G G G F F G F F G G F F F G F G
8 15 27 12 20 15 16 28 25 15 6 19 14 24 25 13 14 11
Lockwood CV, Arroyo Seco Arroyo Seco CV, Jolon Pine Canyon Arroyo Seco Arroyo Seco Pine Canyon Pine Canyon Carmel Valley Carmel Valley Corral de Tierra Bradley Carmel Valley Carmel Valley Carmel Valley Carmel Valley Carmel Valley
2 yrs. 6 mo ⬎5 yrs. 1.5 yrs 1.5 yrs 2 yrs. ⬎4 yrs 12 yrs 4 yrs 1.5 yrs 1 year 1.5 yrs 6 mo 2 yrs. 5 yrs 6 mo 6 yrs 1 year
Stiff neck, lame in front Fx scapula Cough, lordosis, ataxia Lame, ataxia, lordosis Wt. loss, dyspnea, stiffness Neck pain, stiffness Neck pain, stiffness Lordosis Lordosis Lameness, cough Ataxia, neck stiffness ADR, lordosis, dyspnea Unresponsive pneumonia Lordosis, dyspnea Dyspnea, lordosis Dyspnea, ataxia, stiffness Dyspnea, fracture Dyspnea
No No Yes Yes Yes No No Yes Yes Yes No Yes Yes Yes Yes Yes Yes Yes
Euth Euth Euth Euth Alive Died Alive Alive Alive Euth Alive Alive Alive Euth Euth Euth Euth Died
App ⫽ Appaloosa, QH ⫽ Quarter Horse, NSH ⫽ National Show Horse.
complaint, presence or absence of respiratory signs, and duration of signs were also recorded. Additionally, lungs were evaluated sonographically in five horses, and scintigraphy was available in one horse from the study. 3.
Results
Results are summarized in Table 1 and Table 2. Mean age of the horses was 17.1 yr (range ⫽ 6 –28 yr). There were 10 Quarter Horses, 4 Arabians, 3 Appaloosas, and 2 National Show Horse. Nine were castrated males, and nine were females. Bone
Table 2.
deformities consisted of lordosis (8 horses) and scapular bowing (17 horses). Fractures were seen in vertebrae (two horses), scapulae (two horses), ribs (six horses), pelvis (four horses), and maxilla (one horse). Proliferative lesions were seen in the cervical vertebrae of four horses and the scapular spines of seven horses. All cases of scapular deformity or lordosis were confirmed by owner report or serial clinical examinations to be of adult onset with no known trauma. All horses had multiple bone lesions, but not all lesion types were seen in each case. Although all horses
Diagnostic Procedures
Method of Silicosis Diagnosis
Signs of Bone Involvement
Bone Disease Demonstrated by
Case No.
Path
BAL
Thoracic Rads
U/S Lungs
Deformity
Fracture
Pain/ Stiffness
X-ray
U/S Bone
Photo
Path
Scint
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
— Yes Yes Yes — Yes — — — Yes — — — Yes Yes — Yes
— — — No Yes Yes Yes Yes Yes Yes Yes Yes Yes — — — —
Yes — — — Yes Yes Yes — — Yes Yes — Yes — — Yes —
No — — — No No No — — — No — Yes — — — —
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No
Yes Yes Yes Yes No Yes Yes No No Yes No No No Yes No Yes Yes
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Yes — — — — Yes Yes — — — Yes — — Yes — Yes —
— — — — Yes Yes Yes Yes Yes Yes Yes — — — — Yes Yes
— — — — Yes — — Yes Yes Yes Yes Yes Yes — Yes — —
No Yes Yes Yes — Yes — — — Yes — — — Yes — — Yes
Yes — — — — — — — — — — — — — — — —
—, Not performed; BAL, bronchoalveolar lavage; path, necropsy findings; U/S, ultrasound; scint, scintigraphy. AAEP PROCEEDINGS Ⲑ Vol. 52 Ⲑ 2006
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MEDICINE II had both bone and respiratory lesions, 5 of 18 showed no signs of respiratory disease. Respiratory lesions included a diffuse, structured interstitial pulmonary pattern as described by Berry et al.2 in eight horses, a phagocytized refractile crystalline material within alveolar macrophages in nine horses, and a histologic diagnosis of respiratory silicate accumulation in eight horses. Respiratory signs, when present, consisted of resting tachypnea with nostril flare and exercise intolerance. Three horses exhibited a dry cough. 4.
Fig. 4.
Photograph of horse 8 showing scapular bowing.
Discussion
Chronic silicosis is often asymptomatic in humans, and two asymptomatic horses were previously reported.2 It is our clinical impression that horses with chronic forms of silicosis almost always eventually develop some type of bone pathology. Horses with acute silicosis tend to be less likely to develop bone pathology. The presenting complaint in this group of horses was commonly not a respiratory condition. Nonetheless, all had evidence of pulmonary silicosis. Not all horses in this study were living in suspicious areas at the time of diagnosis, but all had strong his-
Fig. 5. Map of California and inset of Monterey County region showing locations of horses. courtesy of NASA Visible Earth). 314
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(Regional map modified from image
MEDICINE II
Fig. 6.
Lung radiograph of horse 7 showing a diffuse miliary interstitial pattern.
tories of current or previous exposure to the cristobalite-laden soil specific to the Monterey Formation. Berry et al.2 documented several horses with silicosis from areas outside Monterey County and suggested that the disease was more widespread than previously described. The Monterey Formation is prevalent in the coastal ranges from Point Reyes to Los Angeles. The same conditions that created the Monterey Formation in California are present in southwest Africa and Peru.3 This may indicate a wider geographic distribution of cristobalite. In terms of pulmonary pathology, radiography was the most reliable antemortem diagnostic tool, revealing pathology in all eight horses that had available thoracic radiographs. Three horses (cases 6, 10, and 11) required a second BAL to obtain a positive diagnosis. These results are similar to those obtained by trans-tracheal wash in the study by Berry et al.2 Thoracic ultrasonography was very informative in one horse (case 13). In this horse, large areas of consolidated lung were visualized in the dorsal fields and were compatible with granulomas; additionally, a moderate increase in anechoic free pleural fluid was observed. However, ultrasonography was the least-reliable screening tool in this study, because only one of six horses showed pathology using this method. Because most pulmonary pathology occurs in the hilar and interstitial regions, the lung surface is often unaffected.
In terms of bone pathology, radiography showed a maxillary fracture (case 14), rib fractures (cases 6, 7, 11, and 16), scapular bowing (cases 1 and 7), and mixed productive and destructive lesions in the cervical and cranial thoracic vertebrae with arthritis at the articular facets and between vertebral bodies (cases 6, 7, 11, and 16). Ultrasonography showed pelvic fractures (cases 6, 7, and 17), rib fractures (cases 6, 7, 10, 11, and 16), thickening and bony irregularity of the scapular spine (cases 5, 6, 7, 8, 9, 10, and 11) with overtly fractured scapulas (cases 6, 7, and 10), and bony irregularity with arthritis at the cervical vertebral facets (cases 6, 7, and 11). Scapular fractures occurred in the proximal fourth of the bone and were difficult to image radiographically. Ultrasound was felt to be a useful screening tool for bone pathology, because it allowed for quick diagnosis of fractures that might otherwise require general anesthesia. The frequency of fractures and lack of trauma history suggest that they are pathologic in nature. One scapular fracture (horse 2) was observed to occur when the horse was walking along a level trail carrying a six-yr-old child. Some of the horses were found to have unsuspected old fractures at necropsy. Because many of them were geriatric, the owner simply thought their deformities and infirmities were caused by advanced age. Progression of signs is variable. A horse not included in this study did not develop overt bone paAAEP PROCEEDINGS Ⲑ Vol. 52 Ⲑ 2006
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Fig. 7.
Radiograph of horse 7 showing osteolytic and osteoproliferative lesions with marked arthritis and disk degeneration at C5-C6.
thology but developed a severe respiratory form of silicosis after an exposure of ⬍4 mo. Case 11 had been exposed since birth and started developing clinical signs at 5 yr of age. For the older horses in the study, the clinical development seems to have been slower in onset. We can speculate that these horses received lower doses of cristobalite over a long time, whereas those more acutely affected may have received a larger dose over a shorter time. However, differences in immune response from horse to horse may also play a role as well as countless unknown factors. This may partially explain why horses housed together in an affected area will display different clinical signs. The incidence of this disease is difficult to address at this time. We are only now beginning to understand the clinical manifestations, and it is likely that many cases have gone undiagnosed. We believe that subclinical pulmonary silicosis is common in affected areas and that the development of bone pathology is a common sequela. Bone pathology in humans and other species has not typically been associated with silicosis. However, there is a report of spontaneous rib fractures in two human silicosis cases in the literature.4 Rheumatoid 316
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arthritis and immune-mediated collagen vascular disorders are more commonly seen in people with the disease. We cannot draw any conclusions regarding the nature of the connection between respiratory silicate accumulations and bone lesions, but we have found both to be present in every horse presented for the skeletal syndrome described here. Further work is necessary to determine the exact nature of the bone disease. Possible mechanisms could include parathyroid-related osteoporosis, silicate-induced trace mineral imbalances, preferential uptake of cristobalite in the bone, and immune-mediated osteoporosis. Subjective improvement in lameness in some horses treated with corticosteroids supports an immune-mediated component, but many do not respond to this therapy. Because skeletal signs often appear before respiratory signs, practitioners should consider silicosis, especially in geriatric horses exhibiting a vague, insidious onset of pain or fractures of the axial skeleton. The authors thank the owners of the horses in this study, whose cooperation and generosity made this
MEDICINE II work possible, and Steinbeck Country Equine Clinic for tremendous support. References and Footnote 1. Schwartz LW, Knight HD, Whittig LD, et al. Silicate pneumoconiosis and pulmonary fibrosis in horses from the Monterey-Carmel Peninsula. Chest 1981;805:825– 855. 2. Berry CR, O’Brien TR, Madigan JF, et al. Thoracic radiographic features of silicosis in 19 horses. J Vet Int Med 1991;5:248 –256.
3. Durham DL. Uranium in the Monterey Formation of California. In: U.S. geological survey bulletin 1581-A. Washington, DC: United States Government Printing Office, 1987;A3–A4. 4. Balgairies E, Amoudru C. Spontaneous rib fractures in silicosis. J Fr Med Chir Thorac 1957;11:113–114.
a
Durham DL.
Personal communication. 2005.
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