The Musculoskeletal Health of Danish Occupational Fishermen: Occupational Epidemiology

Introduction
Occupational fishing has been universally recognized to be a physically strenuous and hazardoustrade owing to a combination of several work-related risk factors within this challenging occupational setting. Fishery workers carry out manual handling (lift, pull, and push) of heavy gear despite constant movements and vibrations onboard the vessel in wet, slippery, and moving surfaces. Additional factors when on board, such as small crew sizes, long working hours, and limitedopportunities for rest and sleep, contribute to the high risk seen within this occupational trade.The consequences of working within these occupational hazards may, in turn, lead to the development of musculoskeletal disorders (MSDs). Musculoskeletal disorders are a common designationfor multiple disorders and discomforts, which vary in etiology, pathogenesis, and prognosis.Among these, back disorders (BDs) are most dominant. Over the last two decades, several newpreventive health and safety initiatives have been introduced to the fishery trade to improve thephysical work environment. Despite these efforts, the physical strain from the occupational workload of frequent manual handling of catch (lift, push, and pull) will never be completely eliminated. The literature suggests that the high prevalence of MSDs in occupational fishermen existsworldwide. However, the knowledge about occupational fishery risk factors for MSDs is sparce andinconsistent.

Aim
This thesis examines the impact of working within occupational fishery on the incidence of anyMSDs and reoccurrence of musculoskeletal back disorders. Thereto, several work-related factorsare investigated to clarify which act as risk factor of these musculoskeletal conditions. The overallaim of the thesis was to address the following hypotheses, working within occupational fishery:- are related to a high prevalence of MSD complaints and disorders and are influenced byseveral work-related risk factors of MSDs (paper 1).- involves work-related risk factors that cause a higher risk of hospital contact due to anyMSD (paper 2) and recurrent back disorders (paper 3) in Danish fishery workers.

Materials and methods
The data used in the papers for this thesis were derived from various data sources. A systematicliterature review was carried out to create a narrative synthesis with a quality assessment of available literature concerning the prevalence and risk factors of MS complaints and disorders withinoccupational fishery worldwide (paper 1). Next, two studies addressed the incidence of first hospital contact due to any musculoskeletal disorder (paper 2) and the risk of recurrent musculoskeletalback disorders (paper 3). Papers 2 and 3 were conducted with register-based data from the DanishOccupational Cohort with eXposure (DOC*X), which included several national registers within Statistics Denmark. The statistical methods included descriptive multistate model and stratified Coxregressions.

Results
In the systematic literature review (paper 1), substantial evidence supported the contention thatworking within occupational fishery may increase the risk of MSDs. High prevalence was seen forMSDs in fishery worldwide, ranging from 10-93% depending on the case definition. The variouswork-related characteristics that were investigated, however, showed no consistency regardingthe risk of causing MSDs. In the register-based analysis of fishery work-related risk of a hospitalization due to any MSDs (paper 2), an incidence of 40% was shown, with back disorders as the mostdominant complaint. Moreover, the study revealed a significant non-linear risk from occupational7seniority, calculated by the accumulation of years in trade. The occupational seniority risk washighest with fewest years in trade (less than five) for both genders, while men again had similarlyelevated risk when working more than 15 years compared to working more than 20 years. Thehealthy worker effect was documented in this occupational group. Additional occupational riskfactors for first MSD in fishers were more years in the workforce, a captain education, and primarily working part time, all of which significantly reduced the risk of experiencing a first MSD formen. Period effects did not affect the association but significantly confounded and reduced therisk from occupational seniority. In the register-based analysis on the lifetime occupational fisheryof recurrent back disorders (paper 3), 16% experienced a first back disorder (n=2165). Of thosefishermen, more than half experienced a second back disorder (n=1172). A significant risk of having the second back disorder was illustrated (HR 2.36, 95%CI 2.19,2.54) compared to having thefirst back disorder when adjusting for several confounders. Stratified analysis of fishery statesshowed that only age had a significantly influence on the risk of recurrence of second and thirdback disorder.

Conclusion
The findings presented in this thesis suggest that working within occupational fishery causes a significant risk for the development of first hospital contact with any MSD and the recurrence of musculoskeletal back disorders. Strong associations have been demonstrated between the risk of fistMSDs and multiple characteristics from occupational fishery. Especially noteworthy is that occupational seniority demonstrated a strong but a non-linear relationship to MSDs. The risk of recurrentback disorders was only affected by age, where increasing age were less and less protective for recurrence. Conclusively, the risk from occupational fishery varies widely across a lifespan. The effect of several additional covariates varies widely from the risk of first MSD to the risk of recurrence of back disorders, including comorbidity and working part time. In all the analyses, age hadan important and a significant influence on the effect estimates seen, and the influence of thehealthy worker effect was also illustrated.