A study on occupational asthma among workers of silk filatures in South India.

BACKGROUND: The production of silk is a multidimensional and multistep process involving exposure of workers to allergens at work place. The silk allergen has been implicated in the development of bronchial asthma.
OBJECTIVES: To identify the prevalence of occupational asthma (OA) and to identify sensitization to silk allergen and among workers in silk filature units.
MATERIALS AND METHODS: A community-based cross-sectional study was conducted in silk filature units of Ramanagara (Silk City) in Karnataka, South India, for a period of 6 months. One hundred and twenty workers of silk filature units who met the inclusion and exclusion criteria were recruited into the study group. For comparison, a control group comprising of 120 individuals not working in silk filature units was constituted. All the subjects were interviewed using the standardized International Union against Tuberculosis and Lung Diseases (IUATLD) Questionnaire and subjected to the skin prick test, which used the extracts of silk allergen.
RESULTS: The study group comprised of 35 males and 85 females, whereas the control group comprised of 58 males and 62 females. The prevalence of occupational asthma among workers in silk filatures was 20.83%. It was observed that 35.83% of those in the study group and 20.83% of those in the control group were found to be sensitive to silk allergen. This difference was statistically significant (χ(2)= 6.64; P < 0.05).
CONCLUSION: There is a high burden of sensitization to silk allergen and occupational asthma among silk filature workers in South India.

INTRODUCTION

Occupational asthma is a form of work-related asthma characterized by variable airflow obstruction, airway hyperresponsiveness, and airway inflammation attributable to a particular exposure in the workplace and not due to stimuli encountered outside the workplace.[1] It has become the most common work-related lung disorder in the developed countries and is on the rise in developing countries. The prevalence of occupational asthma varies from 5% to 30% in different parts of the world.[23]

Sericulture, the technique of silk production is a type of agro industry playing an important role in the development of the rural economy of India. India is the second largest producer of silk in the world with an annual silk production of approximately 20,000 million tons. The contribution from the state of Karnataka has been noteworthy with 85% of the country's silk being produced here.[4]

Cocoons raised by the farmers are delivered to the factory, called a filature, where the silk is unwound from the cocoons and the strands are collected into skeins. Operations within a filature consist of sorting of cocoons, boiling/cooking, and reeling the filament. Sericin and fibroin, which are high-molecular weight proteins, mainly produced during the production of silk, have been implicated in the causation of occupation asthma.[56]

There are no further studies on occupational asthma in sericulture in India after a study by Harindranath et al., in 1985.[2] Thus this study was undertaken to identify the prevalence of occupational asthma and silk allergen sensitization among workers of silk filature units in Ramanagara also known as Silk City of Karnataka, India.

MATERIALS AND METHODS

After obtaining approval from the Institutional Ethics Committee, a community-based cross-sectional study was conducted in silk filature units of Ramanagara in South India for a duration of 6 months. The sample size was calculated based on the prevalence of 16.9% in the previous study[2] and precision of 10%, which amounts to 56. With doubling effect, it was rounded off to 120. Silk filature units in Ramanagara town were visited randomly and their owners were briefed about the purpose of the study. All the workers of filature units who were willing to participate were included after taking their informed consent.

One hundred and twenty subjects aged between 18 and 60 years, working in silk filatures for a minimum period of 6 months were included in the study group. Individuals who were unable to comprehend instructions and successfully perform spirometry as per ATS guidelines, and those in whom spirometry and skin prick test were contraindicated, were excluded. The control group consisted of 120 subjects who were selected from the same geographical area and fulfilling the inclusion and exclusion criteria but not working in silk filatures.

All the subjects were interviewed using standardized International Union against Tuberculosis and Lung Diseases (IUATLD) Questionnaire.[7] All the respondents who answered affirmatively to both (1) whistling sound from chest, or chest tightness, or breathlessness in the morning, and (2) having suffered from asthma, or having an attack of asthma in the past 12 months, or using bronchodilators, were labeled as “positive” in accordance with the questionnaire definition for the diagnosis of bronchial asthma.[89] These respondents were asked the following questions: (1) Do your symptoms increase when you are at work; and (2) Do your symptoms decrease or are absent when you are away from work. Those with an affirmative response were labeled as “positive” as per the questionnaire definition of asthma of occupational origin.[10]

All subjects underwent spirometry as per ATS guidelines.[11] Postbronchodilator spirometry was done for subjects who were “positive” as per questionnaire definition of bronchial asthma. Skin prick test using silk allergens was performed on all the individuals, to identify the silk allergen sensitivity. Three types of silk allergens, namely, Pupa, Cocoon, and Sericin were used in this study. Commercial extracts of silk allergens were not available in India. Raw materials of Pupae, Cocoon, and Sericin extract were obtained from the Karnataka State Sericulture Research Development Institute (KSSR and DI) and sent to Creative Diagnostic Medicare Pvt Ltd, Navi Mumbai, India, for preparation of allergen extracts for the skin prick test. Skin prick test was performed on the flexor aspect of the arm and interpreted according to the Indian College of Allergy Asthma and Immunology guidelines. Grade 2+ (>3 mm with good positive and negative control) and above was considered as positive.[12]

Data were tabulated using Microsoft Excel software and analyzed using SPSS version 19.0. Statistical analysis was done using descriptive statistics such as proportions and inferential statistics such as Chi-square test.

RESULTS

The study group consisted of 35 males and 85 females of whom 45 (37.5%) were in the age group of 21-30 years. 58 males and 62 females made up the control group, of whom 33 (27.5%) were aged between of 21 and 30 years [Figure 1]. The mean age was 34.14 ± 2.84 and 40.59 ± 14.40 in the study and control groups, respectively. 83 (69.17%) individuals and 50 (41.67%) individuals in the study and control groups, respectively, were illiterate [Table 1]. In the study group, 90 (75%) were working in reeling units, 7 (5.83%) in the boiling section, 3 (2.5%) in sorting, and 2 (1.67%) in weaving. Forty-eight (40%) of them had been working for a period ranging between 11 and 20 years.

Figure 1

Distribution of age groups between Study group and control group

Table 1

Baseline information of study subjects

The prevalence of bronchial asthma based on the questionnaire was 35 (29.16%) in the study group and 8 (6.6%) in the control group. This difference was statistically significant (χ2= 20.65; P < 0.05 [Table 2]. In the study group, 25 (20.83%) of these 35 individuals had increased symptoms at work and a decrease/absence of symptoms away from work along with sensitivity to silk allergen, thus suggesting occupational asthma [Figure 2]. Of these 25 subjects, 18 (15%) were confirmed to be having obstruction by spirometry as per ATS guidelines. A patient with a positive history and with evidence of sensitization to a known immunologic agent has an 80% chance of reacting to a specific agent on challenge, which is diagnostic of occupational asthma.[13] Hence the prevalence of occupational asthma was calculated as 20.83% in our study.

Table 2

Prevalence of bronchial asthma based on questionnaire

Figure 2

Flow chart

Out of the 35 subjects who are “positive” as per the questionnaire definition of asthma, 23 had an FEV1/FVC ratio <0.7, 28 had a FEV1 <80%, and 33 subjects had PEF <65%, which is suggestive of obstruction as per ATS guidelines. Eighteen subjects showed an FEV1 reversibility of >12% and 200 mL in response to bronchodilator administration. Among the eight subjects in the control group who are “positive” as per the questionnaire, five subjects had an FEV1/FVC ratio <0.7 and FEV1 <80%, and all eight subjects had a PEF <65%, which is suggestive of obstruction.

Sensitization to silk allergen was identified by the skin prick test. Results from the study group showed that 22 (18.33%) individuals were sensitive to pupae, 42 (35%) to cocoon, and 35 (29.17%) to sericin. In the control group, the skin prick test revealed that 4 (3.33%) individuals were sensitive to pupae, 21 (17.5%) to cocoon, and 6 (5%) to sericin. Overall silk allergen sensitivity (sensitivity to one of the three allergens) was 35.83% and 20.83% in the study and the control groups, respectively [Table 3]. The difference in allergen sensitivity between the study and the control groups was statistically significant (χ2= 6.64; P < 0.05) [Table 4].

Table 3

Distribution of study subjects according to allergen sensitivity

Table 4

Comparison of allergen sensitivity between study and control groups

A positive family history of asthma was found in 16 (13.33%) study subjects and 9 (7.5%) control subjects. Among the study subjects, 42% had allergic rhinitis, 31.67% had allergic conjunctivitis, and 19.17% had urticaria.

DISCUSSION

This study revealed that the prevalence of occupational asthma among workers in silk filature units was 20.83%, and 29.16% of study subjects were suffering from bronchial asthma. Extensive review of literature showed that there was only one study conducted among silk workers in India by Harindranath et al., in 1985, which showed that 16.9% of subjects had asthma of occupational origin and 36.2% of the subjects who were engaged in processing of natural silk suffered from bronchial asthma.[2] In their study, the skin prick test was performed using crude silkworm cocoon and pupal allergen extract. Results showed that 28.8% of the subjects were sensitive to silk allergen. However, in our study we observed a higher prevalence of occupational asthma. Sericulture is also an important industry in China. In a community-based cross-sectional study conducted in China in 2001, by Juan et al., it was seen that 8.5% of asthmatics were sensitive to silk allergen.[14] However, our study showed that sensitization to silk allergen was higher at 35.83%, which is more than that seen in both of the above studies.

In our study, it was observed that 18 (15%) asymptomatic workers in the filature units showed a positive skin prick test for silk allergens. Twenty-five (20.83%) individuals in the control group (who are not working in silk filature units but are living in that area) were also sensitive to silk allergens. This indicates that these subjects are at risk of developing occupational asthma and other allergic disorders due to silk allergen in later years, due to its continual release into the environment from filature units.

We thus conclude that there is high burden of silk allergen sensitivity and bronchial asthma of occupational origin among the workers of silk filature units in Ramanagara. It is recommended that health education to be given regarding the signs and symptoms of bronchial asthma, to enable its early diagnosis and treatment and to prevent complications. Appropriate preventive measures to reduce/prevent exposure to silk allergen among silk industry workers is advised. Adoption of newer technology such as Automated Reeling Machine, which reduces the release of silk allergen into the environment and contact with human beings will greatly reduce the burden of silk-induced asthma in this geographical area.

LIMITATIONS

Serial peak flowmeter reading and specific allergen challenge tests, which increases the authenticity of diagnosis of occupational asthma, were not done due to feasibility and resource constraints.