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Does disturbance increase or decrease parasite transmission among wildlife hosts? Ecologists cannot answer this controversial question, in part because few historical datasets rigorously document parasite abundance. Without such a baseline, it is difficult to determine whether contemporary ecosystems are experiencing elevated parasite burdens. Here, we investigate change over time in the abundance of a parasite that affects the economic value of a fish species. Clavinema mariae is a nematode parasite of benthic fishes that is common in English sole (Parophrys vetulus) of Puget Sound, WA. We obtained historical records of its abundance from the literature and from unpublished government agency data, and resampled the same locations using the same methods in 2017. We also used a new approach by estimating the C. mariae burden for museum specimens of English sole collected between 1930 and 2016. Both the historical data and museum specimen data suggested increases over time in C. mariae abundance, with robust agreement between the two approaches. In addition to documenting a previously unrecognised similar to eightfold increase in the burden of an economically important parasite, our work demonstrates-for the first time-that parasitological examination of liquid-preserved museum specimens can produce reliable data on long-term trends in parasite abundance, at a much greater temporal resolution than is possible to obtain from historical records. Synthesis and applications. Defining a baseline state of infection is vital for natural resource management and policy, which must respond to the threat of disease; without such a baseline, managers attempting to maintain or recover the health of ecosystems under their stewardship are shooting in the dark. The method we present here-using museum specimens to reconstruct detailed chronologies of parasite abundance change-offers a solution. Our approach would allow managers to accurately characterise past disease states, informing the development of appropriate disease management targets. Given the broad representation and availability of liquid-preserved specimens across ecosystems, geographic regions and host taxa, this solution may be feasible for the management of biological resources not only in marine ecosystems, but in freshwater and terrestrial ecosystems as well.