Parasites also need protection

More worms in the sushi

In addition to meticulously analyzing the museum specimens, Wood’s team is now turning to other resources. Although there are hardly any long-term data sets for the occurrence of a single parasite species, there are isolated studies that document parasitism at a specific place and time. In a 2020 paper in Global Change Biology, Wood’s group summarized such results for two species of parasites found in raw fish, which is commonly used in sushi and ceviche. To do this, the researchers looked at a total of 123 studies from 1967 to 2017. They found that one of the worms is as common today as it was in the past, while the other worm has seen an incredible 283-fold increase since the 1970s had recorded.

Sushi containing worms can cause vomiting and diarrhea in humans, but Wood is more worried about marine mammals — the worm’s actual hosts. Normally, a single worm does not drain much energy from its host. However, if numbers of these parasites skyrocket, they could pose a problem for marine mammals, particularly populations that are already under stress. The critically endangered orcas in Puget Sound, for example, suffer from pollution, ship noise and a lack of salmon for food. In 2018, an emaciated killer whale calf was found in the sea bay. Authorities tried unsuccessfully to save the calf. Even before it died, scientists found that its feces were full of parasite eggs from the same sushi worm family that Wood’s study had identified.

This alone does not prove that parasites played a role in the calf’s death. But it does suggest that the parasitic organisms could be making life even harder on an already ailing population, says Wood. Natalie Mastick, a graduate student in Wood’s lab, is using a variety of methods to learn more about whether whales face a greater threat from intestinal parasites today than they did in the past. For example, she collects whale feces found by sniffer dogs on boats and analyzes them for hormones, nutrition and parasite levels. “If parasites turn out to be a huge stressor, then at least that would be a treatable disease,” says Mastick. For example, wildlife managers can hide anti-wormers in the salmon they feed to infested marine mammals, or they can use small darts to deliver medication remotely.

When parasites hurt the economy

Aside from the potential impact on human and wildlife health, increases in parasite populations can also harm certain sectors of the economy. The Puget Sound bay, for example, is famous for producing Pacific oysters with pearly, flawless shells. But in 2017, a colleague brought a specimen to Wood’s office whose shell was pitted with gouges and dark, ugly spots — signs of an oyster pest called Polydora burrowing through the shell. Although the parasites themselves are not dangerous if eaten, they form blisters filled with mud and worm droppings on oyster shells, scarring them with their voracious tunnel digging. Of course, restaurant visitors do not want something like that on their plate.

Since the 1860s, Polydora outbreaks have devastated the oyster industries in Australia, Hawaii and the US east coast, but Washington state – the largest producer of farmed mussels in the US – was long spared. In March 2020, Julieta Martinelli, one of Wood’s postdocs, and her colleagues wrote in the journal Scientific Reports that an infamous species Polydora websteri, had actually entered Puget Sound. Martinelli is now studying the parasite’s ecology in hopes of finding ways to help oyster farmers treat and contain the parasite. Additionally, Woods and Martinelli are attempting to unravel the spread of Polydora and other bivalve-boring polychaetes. Accidental introduction may seem like the obvious answer at first, but the reality may be more complex.

The remains of early gourmets help researchers

To unravel the history of Polydora in the Pacific Northwest, Martinelli searches prehistoric rubbish heaps for the remains of oyster remains from past feasts. In such shell waste from the “Jamestown S’Klallam Tribe”, a tribe of Native Americans, she found, for example, 1000-year-old Olympia oyster shells that show signs of worm infestation. Although Martinelli suspects that it is a different species than Polydora, it is also possible that Polydora was present in very small numbers at the time and has only now spread due to an as yet unknown environmental trigger.

Martinelli is now planning to dig up and analyze newer oyster clusters as well. In this way, it may be possible to find out when the parasite first infested the local mussel populations. “The difficult thing about paleo work is that we’ll never get a definitive answer,” she says. But at least there are traces of the past that can be compared with the present.

Save the parasites

The main focus is on the increase in parasites, so Wood focuses on population declines and their impact on humans and wildlife. Sometimes a decline is cause for celebration, as in the case of the eradication of guinea worm, a spaghetti-like parasite that grows up to eight feet in the digestive tract of an infected human before migrating into and eventually piercing the skin. But the vast majority of parasite species do not affect humans, and here the decline is partly worrying: A 2017 article in Science Advances estimated that up to 30 percent of parasitic worms could die in the coming decades due to the virus climate change and other influences could die out.

Nobody knows what that means. We are only just beginning to understand how such an extreme loss of biodiversity could play out. Take, for example, the manipulation of the host that some parasites are capable of. “They shift energy from lower to higher trophic levels by making prey ruthless,” explains Wood. Euhaplorchis californiensis is a flatworm that looks a bit like a sperm in the larval stage, with a large head and long tail. The flatworm begins life in a snail, then migrates into a California killifish, and finally into the gut of a predatory water bird, such as a heron or little egret. Killifish are actually cautious animals that tend to hide well, which defeats the flatworm’s goal. As a result, the parasite forms cysts in its host’s brain, causing the fish to splash around on the water’s surface and display its shiny belly to attract the birds.

Researchers have found that infected killifish are 10 to 30 times more likely to be eaten by a bird than uninfected ones. Overall, the trematodes contribute to making a significant part of the killifish populations more readily available as a meal for birds. In a way, the parasite subsidizes the diet of these birds of prey. If certain parasite species decline or even disappear, some predators could find it much more difficult to survive, Wood explains.

Another example is a 15 centimeter long string worm from Japan. It causes infected crickets to jump into streams, where the adult worms abandon their hosts to start a parasitic reproductive orgy. Meanwhile, the doomed crickets become food for the endangered Japanese char, providing up to 60 percent of the fish’s caloric needs. So not only does the worm help feed an endangered species, but it also relieves pressure on other invertebrate species that are also eaten by the fish. In this way, the parasite significantly affects the overall ecology of the river.

How to protect against parasites?

As it becomes more apparent that parasites play an important role in ecosystems, a small but growing group of scientists is beginning to seriously consider the need for targeted parasite protection. In August 2020, parasite ecologist Colin Carlson of Georgetown University in the US, along with Wood, Hopkins and nine other scientists, published a 12-point plan for protecting parasites over the next decade. First of all, they write in the journal Biological Conservation, we cannot care for or conserve something that we do not know exists. They therefore call on the scientific community to shed light on the diversity of parasites and to describe more than 50 percent of parasite species by 2030. “We’ve barely scratched the surface, really,” Hopkins says.

Once data on the ecology and life cycle of each species is available, the authors say, vulnerable parasites could be identified and then integrated into existing conservation programs with relative ease. Conservation of parasites can be piggybacked, so to speak, on existing efforts to save other endangered species. Threatened parasites can also be included in various registers for recording and protecting endangered plants and animals. Only one animal parasite, the blood-sucking pig louse, is currently on the International Union for Conservation of Nature’s Red List of Threatened Species.

Hopkins, Wood and their colleagues know that parasites have a serious image problem. Nevertheless, they are confident that this can change. They compare the status of parasite protection with the situation in which predator protection found itself just a few decades ago. At the time, many researchers and the public considered bears, wolves and other predators to be dangerous to humans and livestock, and even harmful to nature. The latter in particular turned out to be fundamentally wrong. Scientists now know that predators are key species — species on which entire ecosystems depend for their existence. Removing them can trigger a cascade of negative impacts, from disease outbreaks to disrupting nutrient cycling to moving species to entirely different habitat types. But as soon as science recognized the importance of predators, so did the general public. So Wood hopes people will be willing to take a peek inside the black box in which the parasitic animals have been kept. Because: “Parasites are not a massive threat,” she says.