Parasites are not all bad, and in a rapidly changing world, they need our protection, but they don't seem to be getting it.
In fact, in the second-largest estuary in the United States, scientists have cataloged a mass die-off among marine organisms that rely on free-living hosts to survive.
Over the past 140 years, from 1880 to 2019, parasite numbers in Puget Sound dropped by 38 percent for every degree Celsius of warming in sea surface temperature, researchers at the University of Washington (UW) have found.
The study is the largest and longest dataset on parasite abundance collected anywhere in the world, and the results are even worse than some conservationists had feared.
Parasites are the invisible threads that help tie food webs together. How ecosystems will cope without their influence is unclear.
"The findings are a real bummer if you care about biodiversity or you know anything about parasites," parasitologist Chelsea Wood from UW told ScienceAlert.
"The declines that we observed shocked even me."
If the same degree of loss were observed among mammals or birds, Wood says it would trigger conservation action immediately.
Birds in North America, for example, have declined by just over 6 percent a decade from 1970 to 2017, and already they feature heavily in conservation plans.
In comparison, no one really cares about parasites. A decreasing number of creatures that leech off the life of others is usually seen as a good thing. But that's an outdated view that neglects the bigger picture.
Today, many scientists agree that climate change has Earth hurtling towards a mass extinction event, but the scenario looks even worse when you consider that we haven't really factored in just how heavily life-forms on Earth depend on parasites (the vast majority of which are undescribed).
At the moment, very few ecological surveys consider parasites, and conservation efforts almost always overlook their connective role in a habitat, despite their widespread and essential role in maintaining ecological balance.
Only when parasites proliferate and become a problem do we tend to pay them any notice.
In 2020, for instance, Wood's lab at UW made headlines when it found a specific parasitic worm in raw seafood had increased 280-fold since the 1970s.
But not all parasites are faring so well. In fact, many of them are probably suffering in the current climate crisis. Like bubbles in a boiling pot, they are disappearing faster than we can count them.
In the recent findings from Puget Sound, parasites with three or more hosts (just over half of all the parasites sampled) seemed to be particularly vulnerable to warming waters.
As for why, it's possible higher temperatures might place parasites at direct physiological risk, or, alternatively, warming waters could be impacting the availability and viability of their host or hosts.
Either way, the more hosts a parasite must bounce between, the more imperiled it probably becomes by changes in climate.
Of the 10 parasites Wood identified that had gone extinct by 1980 in Puget Sound, nine of them had life cycles that relied on three or more hosts.
"What we expect when we look at a changing environment is winners and losers," says Wood.
"But what we found here were a whole bunch more losers than we were anticipating."
If Puget Sound is anything like other ecosystems in the world, then Wood thinks parasite losses could match or even exceed the mass rate of extinction taking place among free-living species.
But no one can say for sure if that's the case without other researchers following in Wood's footsteps.
Wood thinks the current view of parasites is similar to how people once considered apex predators, like wolves or bears, in the 1960s and 1970s. For centuries, large carnivores were hunted by humans to the point of near extinction out of fear and anger.
Only in the mid-20th century did it become clear to scientists what had been done. The world had systematically removed some of the most profoundly important movers and shakers in ecosystems to the detriment of habitats worldwide.
Apex predators, as it turns out, weren't always disruptive pests; they were essential habitat stabilizers. Reintroducing them to habitats helped ecosystems flourish once again.
"That's where we are for parasites," Wood says, "We're at this moment when research is starting to accumulate to suggest how awesomely powerful parasites are in an ecosystem. But that information hasn't yet leaked out to the public."
In 2017, a study on 457 parasite species predicted that up to 10 percent could go extinct by 2070, including 30 percent of parasitic worms. Spurred by the results, the authors created the first endangered 'red list' for parasites.
In 2020, Wood joined forces with like-minded researchers from around the world to detail a 12-goal parasite conservation plan for the future.
Colin Carlson, a co-author on the paper, told The Atlantic in 2015 that the starting point is to stop destroying parasites the moment we find them.
"The most fundamental idea, and it's a bit silly that we've missed this, is you don't destroy something if it's doing okay," Carlson told reporter Ed Yong.
The next step is data collection and synthesis, and in this subfield, Wood is leading the way. Her lab at UW is the very first to use museum samples of fish to create a historical timeline of marine parasite abundance.
"No one has noticed anything like this," says Wood. "And part of it is that no one's looking."
Unlike apex predators, parasites are harder to see if you aren't actively searching them out. And finding them is not exactly glamorous work.
"Your fieldwork is sitting in the basement of a museum, dissecting fish that are suffused with disgusting chemicals," says Wood.
"It doesn't have sex appeal. But it gives us the opportunity to time travel. And if I get the chance to time travel, I'll sniff some formalin fumes."
The parasites of the present and the past are there for us to count. Now we just need to plug our noses and dive.
The study was published in PNAS.