Pneumoconiosis (Silicosis)
Peggy S. Marsh
Pneumoconiosis refers to lower respiratory tract disease associated with deposition of small particles (dust) within the lungs. In human medicine the majority of cases are related to occupation and include coal worker's lung, silicosis, and asbestosis.1,2 There are two related reports of silicosis in horses.3,4 The majority of the horses originated from the Monterey-Carmel Peninsula in California.
Silicosis is a disease caused by inhalation of crystalline silica. There are several clinical forms of silicosis in humans, including simple and complicated chronic forms as well as acute silicosis (silicoproteinosis). Silicosis has been associated with mycobacterial diseases, airway obstruction, and pulmonary neoplasia.5 Freshly fractured silica is more toxic to alveolar macrophages than aged silica. Crushing silica leads to the formation of reactive radials.6 The small (0.5 to 5 mm) particles are deposited in small airways and alveoli, then subsequently phagocytosed, initiating a cycle of granulomatous inflammation, oxidative stress, and apoptosis leading to chronic fibrotic interstitial disease.1,5,6The clinical signs reported in horses are similar to those of severe equine asthma (heaves) and include weight loss, cough, exercise intolerance, and sometimes exercise-induced respiratory distress (sweating, nostril flaring, abducted elbows, reluctance to move, and shaking). Resting respiratory rates are often increased. Physical examination revealed a restrictive pattern of breathing with harsh breath sounds and exercise-induced wheezing on auscultation.
Diagnosis was based on the presence of birefringent crystalline inclusions within pulmonary macrophages from samples obtained via tracheal aspiration, BAL, lung biopsy, or at necropsy.4 In some cases repeated tracheal sampling was required.
Radiographs of the thorax revealed a variety of findings, including most commonly an interstitial pattern (miliary, reticulonodular, and linear) most severe in the cau- dodorsal lung fields. Other radiographic abnormalities noted included lymphadenopathy, pleural effusion or thickening, hyperinflation, and pulmonary consolidation.4Gross pathologic analysis revealed interstitial granulomatous pneumonia and fibrosis. Histologic examination with both light and electron microscopy revealed peribronchitis, perivasculitis, and cellular infiltrates of refractile inorganic dust particles within macrophages and giant cells.3,4
In general the horses reported to have silicosis had a poor prognosis. The original report presented postmortem findings of nine horses diagnosed with silicosis.3 In a later, related paper, a total of 19 horses were identified.4 Short-term (2 to 12 months) follow-up evaluation was available in eight of these individuals, with a progressive loss of respiratory function noted in four of the cases, leading to euthanasia. Treatment consisted of environmental change and glucocorticoid therapy. Novel forms of therapy, including inhibition of inflammatory mediators as well as inhibition of apoptosis, may be seen in the future6; however, silicosis remains with no cure.
Recent work has revealed a systemic osteoporosis in a group of horses associated with fibrosing pulmonary silicosis.7 Also, there are reports of pneumoconiosis in other domestic species that was caused by environmental contamination.8 It may be prudent to keep environmental contamination in mind when investigating chronic noninfectious forms of lower respiratory tract disease in the horse.
Mycoplasma infections. Thus routine cultures for aerobic and anaerobic bacteria should be performed in any case of pleuritis. Culture for the presence of Mycoplasma bacteria is difficult to obtain, and Mycoplasma-specific media should be used. Seroconversion can also be an aid in diagnosis, with seroconversion occurring in a large number of the reported cases of M. felis infection.1,2,7 Experimental infection revealed that once seroconversion has occurred, the organism can no longer be isolated from culture samples.3 Seroprevalence in the United States is not known. Diagnosis may be aided by the use of RT-PCR methods for the detection of M. felis.8
Experimentally infected ponies recovered without treatment in one study, so the requirements of treatment specific for mycoplasma are unclear.3 Where available, sensitivity testing should be performed on any Mycoplasma spp. isolate to help with antimicrobial selection. Macrolides and tetracyclines are generally effective for infections in other species. The role these organisms play in equine respiratory disease is still unclear, and investigation continues.