The biggest parasite study ever conducted revealed that they are dying

"Changing world where the fate of hosts is being reshuffled."
Nergis Firtina
These monogenean worms (Microcotyle sebastis) were dissected from the gills of a preserved copper rockfish specimen from the UW Fish Collection at the Burke Museum.
These monogenean worms (Microcotyle sebastis) were dissected from the gills of a preserved copper rockfish specimen from the UW Fish Collection at the Burke Museum.

Katie Leslie/University of Washington 

During the 140 years between 1880 and 2019, Puget Sound, their habitat and the second-largest estuary on the mainland U.S., warmed significantly, according to new research from the University of Washington.

Published on January 9 in PLOS, the largest and longest dataset available on the prevalence of animal parasites. According to this, parasites might be particularly vulnerable to climate change.

“People generally think that climate change will cause parasites to thrive, that we will see an increase in parasite outbreaks as the world warms,” said lead author Chelsea Wood, University of Washington associate professor of aquatic and fishery sciences.

“For some parasite species, that may be true, but parasites depend on hosts, and that makes them particularly vulnerable in a changing world where the fate of hosts is being reshuffled.”

As said in the release, analysis of historical fish specimens revealed an 11 percent average reduction in abundance per decade for parasites that depend on three or more host species for the duration of their lifetime, which includes more than half the parasite species found in the study's Puget Sound fish. Nine of the 10 parasite species that had entirely vanished by 1980 required three hosts or more.

The biggest parasite study ever conducted revealed that they are dying
A jar of fluid-preserved fish specimens from the UW Fish Collection at the Burke Museum. These fish were collected in Hood Canal in 1991.

“Our results show that parasites with one or two host species stayed pretty steady, but parasites with three or more hosts crashed,” Wood said. “The degree of decline was severe. It would trigger conservation action if it occurred in the types of species that people care about, like mammals or birds.”

A new method for resurrecting information

Wood's is one of the first studies to employ a new technique for recovering details on historical parasite populations. Taxidermy, which only preserves parasites on skin, feathers, or fur, is used to preserve mammals and birds. However, specimens of fish, reptiles, and amphibians are stored in fluid, which also retains any parasites present when the animal dies.

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The study concentrated on eight fish species that are frequently found in the hidden collections of natural history museums. Most of them were from the Burke Museum of Natural History and Culture's UW Fish Collection. Before returning the preserved fish specimens to the museums, the authors meticulously cut into each one, identified, and counted the parasites they found within.

“It took a long time. It’s certainly not for the faint of heart,” Wood said. “I’d love to stick these fish in a blender and use a genomic technique to detect their parasites’ DNA, but the fish were first preserved with a fluid that shreds DNA. So what we did was just regular old shoe-leather parasitology."

Study abstract:

Long-term data allow ecologists to assess trajectories of population abundance. Without this context, it is impossible to know whether a taxon is thriving or declining to extinction. For parasites of wildlife, there are few long-term data—a gap that creates an impediment to managing parasite biodiversity and infectious threats in a changing world. We produced a century-scale time series of metazoan parasite abundance and used it to test whether parasitism is changing in Puget Sound, United States, and, if so, why. We performed parasitological dissection of fluid-preserved specimens held in natural history collections for eight fish species collected between 1880 and 2019. We found that parasite taxa using three or more obligately required host species—a group that comprised 52% of the parasite taxa we detected—declined in abundance at a rate of 10.9% per decade, whereas no change in abundance was detected for parasites using one or two obligately required host species. We tested several potential mechanisms for the decline in 3+-host parasites and found that parasite abundance was negatively correlated with sea surface temperature, diminishing at a rate of 38% for every 1 °C increase. Although the temperature effect was strong, it did not explain all variability in parasite burden, suggesting that other factors may also have contributed to the long-term declines we observed. These data document one century of climate-associated parasite decline in Puget Sound—a massive loss of biodiversity, undetected until now.