Flock worker's lung

INTRODUCTION — An outbreak of work-related interstitial lung disease among employees at a Rhode Island textile plant specializing in the manufacture and application of nylon flock was described in 1998 [1,2]. Based on a case definition of persistent respiratory symptoms, previous work in the flocking industry, and histologic evidence of otherwise unexplained interstitial lung disease, a cluster of eight cases of flock worker's lung was recognized among fewer than 200 at-risk workers at a single production facility [1]. More than two dozen cases have now been reported among nylon flock workers in Rhode Island, Massachusetts, North Carolina, and Ontario [1,3-5].

The epidemiology, clinical features, and natural history of flock worker's lung are described here. General issues related to the diagnosis of interstitial lung disease are discussed separately. (See "Approach to the adult with interstitial lung disease: Clinical evaluation" and "Approach to the adult with interstitial lung disease: Diagnostic testing".)

NYLON FLOCK EXPOSURE — In the flocking industry, short fibers (flock) are cut from cables of synthetic monofilaments and applied to adhesive coated fabric surfaces to produce velvet-like materials often used to produce upholstery, blankets, and clothing [1,3,4]. Exposures common to the nylon flocking industry include nylon, tannic acid, acrylic adhesive, nonfibrous zeolite, heat transfer oil, thermal degradation products, and ammonium ether of potato starch [1].

Although most nylon flock manufacturers worldwide use guillotines to cut flock, two large North American companies use rotary cutters. The rotary cutters apparently generate substantial quantities of respirable-sized nylon particles [4,6]. Inhalation of these fibers resulted in pulmonary toxicity in one animal model; however, these findings were not replicated in a separate industry-sponsored experiment [7,8]. All affected nylon flock workers described thus far have had occupational exposure to rotary-cut flock [4].

The polycationic nature of nylon flock is reminiscent of the polycationic compounds in Acramin-based spray paint, which caused occupational outbreaks of organizing pneumonia in Spain and Algeria, and the humidifier disinfectant polyhexamethylene guanidine, which caused an outbreak of interstitial lung disease in Korea [9]. Experimental studies in animals support the idea that the polycationic nature of these agents contributes to lung toxicity when the agents are inhaled despite apparent absence of toxicity in dermal and oral exposure studies. In one notable study, nylon microfibers showed much greater inhibition of airway development and repair compared with similarly sized polyester fibers; moreover, it appears that rather than the nylon fiber per se, one or more unknown compounds leaching from the nylon is responsible for impairing normal airway epithelial cell differentiation [10].

EPIDEMIOLOGY — Fibrotic interstitial lung disease is an uncommon, if not rare, disease. At a Rhode Island textile plant, seven of the eight identified cases of flock worker's interstitial lung disease (together with three additional cases of other forms of interstitial lung disease) occurred among a cohort of 165 workers. This yields a sex-adjusted standardized incidence ratio for all interstitial lung diseases of 48 (95% CI 23-88). The age- and sex-adjusted standardized incidence ratio for idiopathic pulmonary fibrosis (used as a surrogate for flock worker's lung) was 258 (95% CI 104-530) [1].

It remains unknown whether this greatly increased risk of interstitial lung disease applies only to workers exposed to rotary-cut nylon flock or is shared among other flock workers. Production processes involving other man-made organic fibers may also pose risk of flock worker’s lung, as evidenced by a case report from Spain of a symptomatic polyethylene flock worker with comparable histopathologic abnormalities, the finding of abnormal pulmonary function and mild high-resolution computed tomography (HRCT) scan abnormalities in a cross-sectional study of Turkish flock workers exposed to guillotine-cut polypropylene, and a report of mildly abnormal pulmonary function in a cross-sectional study of rayon flock workers [11-13].

CLINICAL FEATURES — Individuals with flock worker's lung have presented in a variety of ways. In the Rhode Island outbreak, those initially referred were previously healthy young male textile workers who developed respiratory symptoms. The first patient described the rapid onset of pleuritic chest pain and dyspnea, which progressed over one year to chronic interstitial lung disease initially believed to be hypersensitivity pneumonitis. The second patient described a chronic cough, intermittent pleuritic chest pain, and dyspnea over an eighteen-month period. Most of those initially diagnosed presented with subacute or chronic symptoms. However, in the Canadian outbreak, 5 of 88 exposed workers developed severe disease reportedly over a five to seven day time frame following several months of mildly progressive symptoms [14].

In the Rhode Island outbreak, patients included seven men and one woman, with ages ranging from 28 to 58. All manifested cough and dyspnea, and in six, symptoms were chronic and progressive. Three patients reported intermittent atypical chest pain. On average, affected employees had a six-year latency period from hire to symptom onset. Only two reported work-related symptoms, and no high-risk job categories were identified. Additional case finding on the basis of symptoms alone was confounded by the high prevalence of respiratory symptoms temporally related to work [15].

On physical examination, crackles were auscultated in all but one of the first eight cases identified in Rhode Island, but in only one of the five subsequently diagnosed cases [1,4].

Radiography — Routine chest radiography appears to have low sensitivity as a diagnostic test. When abnormal, chest radiographs most often show diffuse reticulonodular or patchy opacities. High-resolution chest computed tomography (HRCT) has been abnormal in every case, although the changes are often subtle (image 1A-E). Diffuse micronodular opacities, patchy ground glass opacities, patchy consolidation, and peripheral honeycombing have been described most frequently [1,4,16]. (See "Evaluation of diffuse lung disease by conventional chest radiography" and "High resolution computed tomography of the lungs".)

HRCT may be abnormal even in patients without pulmonary symptoms. This was illustrated in a study in which HRCT scans were performed in 87 individuals, including 11 with flock worker's lung and 32 flock-exposed workers not meeting criteria for the disease; the studies were blindly reviewed by three pulmonary radiologists [16]. Diffuse abnormalities were noted in all 11 previously diagnosed individuals and in 19 of the 32 asymptomatic flock-exposed individuals.

Pulmonary function — Pulmonary function tests are generally abnormal. In the Rhode Island outbreak, only one of eight patients had normal pulmonary function. A restrictive pulmonary defect with reduced diffusing capacity for carbon monoxide was most frequently observed, although an obstructive defect also was seen in some individuals [1,5]. (See "Overview of pulmonary function testing in adults".)

Laboratory studies — No consistent abnormality on routine blood testing, including antinuclear antibodies, serum precipitins, and cellular differential, has been observed.

PATHOLOGY — Bronchoalveolar lavage is almost always abnormal, with eosinophilia (>25 percent), lymphocytosis (>30 percent), or neutrophilia commonly observed. In most patients, lung histopathology is that of nonspecific interstitial pneumonia/fibrosis, with nodular peribronchovascular interstitial lymphoid infiltrates with or without germinal centers, and a lymphocytic bronchiolitis [1,3,4]. (See "Role of bronchoalveolar lavage in diagnosis of interstitial lung disease" and "Role of lung biopsy in the diagnosis of interstitial lung disease".)

Co-existing diffuse alveolar damage has been observed twice. Organizing pneumonia (previously called bronchiolitis obliterans organizing pneumonia) and desquamative interstitial pneumonia have each been observed once, but even in these cases sentinel nodular peribronchovascular interstitial lymphoid infiltrates were present. Notably, granulomas (suggestive of hypersensitivity pneumonitis) have never been observed. A pulmonary pathologist consensus group concluded that, while not always present, a characteristic and distinctive pattern of lymphocytic bronchiolitis and peribronchiolitis with lymphoid hyperplasia was common [17]. (See "Idiopathic interstitial pneumonias: Classification and pathology".)

One patient who experienced slowly progressive interstitial lung disease with eventual respiratory failure and death underwent post-mortem examination, which revealed end-stage fibrosis and superimposed acute interstitial pneumonitis [5].

TREATMENT AND PREVENTION — Removal from exposure is the only effective therapy and is usually associated with stabilization or improvement in respiratory symptoms [1,3]. All individuals in the initial Rhode Island cohort displayed clinical, radiographic, and functional improvement within weeks to months of leaving work, and similar regression has occurred in subsequently reported patients. In a series from Ontario, Canada, five of nine had complete resolution of ILD after leaving the workplace [5]. However, one patient in Rhode Island and two in Canada required long-term oxygen therapy; the latter two patients died two decades later with slowly progressive interstitial lung disease despite cessation of workplace exposures [1,5].

Treatment with immunosuppressive therapy has been instituted in several cases, but there is no evidence that it provides benefit beyond removal from the workplace. Indeed, the two Canadian patients who suffered slowly progressive respiratory failure were initially treated with systemic glucocorticoids without clear benefit [5].

It is not known if patients can ever safely return to work within this industry once they have developed flock worker's lung. There is, however, evidence to the contrary [3]. It is unclear whether respirator use offers adequate protection to those with established disease.

It is sensible to recommend engineering and work practice controls to reduce respirable-sized fragments of flock dust and the use of personal respiratory protection for primary prevention. However, in the absence of a clearly defined inciting agent, the efficacy of this approach is uncertain.

Patients with suspected flock worker's lung should be reported to the National Institute for Occupational Safety and Health (NIOSH) (1-800-CDC-INFO).

COMPLICATIONS — Long-term occupational exposure to nylon flock may be associated with an increased risk of lung cancer. In a retrospective cohort study of lung cancer incidence in nylon flock workers, the Rhode Island Cancer Registry detected five cases versus 1.61 cases expected on the basis of age-sex-era specific rates of lung cancer incidence in Rhode Island for a standardized incidence ratio of 3.1 (95% CI 1.01-7.23). The observed threefold increase in lung cancer incidence could not be readily ascribed to chance, study bias, or uncontrolled confounding [18]. Two of the case-patients had clinical evidence of flock worker's lung, but three did not [18].

SUMMARY AND RECOMMENDATIONS

●Etiology – Flock worker's lung occurs in textile workers in the nylon flocking industry and most likely results from inhalation of respirable-sized fragments of rotary-cut nylon monofilament. (See 'Introduction' above.)

●Other risk factors – Workers exposed to flock made of polyethylene, polypropylene, and rayon fibers may also be at risk. (See 'Epidemiology' above.)

●Presentation – Affected patients usually present with chronic respiratory symptoms, restrictive pulmonary function abnormalities, and abnormal high-resolution CT (HRCT) scans. (See 'Clinical features' above.)

●Invasive testing – Bronchoalveolar lavage is almost always abnormal with eosinophilia (>25 percent), lymphocytosis (>30 percent), or neutrophilia commonly observed. Lung biopsy usually reveals the histologic findings of nonspecific interstitial pneumonia/fibrosis, together with bronchiolocentric lymphoid nodules and a lymphocytic bronchiolitis. (See 'Pathology' above.)

●Treatment and prevention – Removal from exposure is the only effective therapy and is usually associated with stabilization or improvement in respiratory symptoms. There is no clear therapeutic role for immunosuppressive agents. It is not known if respirator use is protective or if previously affected individuals can be safely rechallenged. (See 'Treatment and prevention' above.)

●Complications – Long-term occupational exposure to nylon flock may be associated with an increased risk of lung cancer. (See 'Complications' above.)

●Reporting – Patients with suspected flock worker's lung should be reported to the National Institute for Occupational Safety and Health (NIOSH) (1-800-CDC-INFO). (See 'Treatment and prevention' above.)