The parasite Toxoplasma gondii is smaller than a single dust mite burrowing in your mattress. In fact, it’s about the same size as the excrement of a dust mite. But don’t be fooled by size—these microscopic, single-cell organisms are surprisingly hardy and exceedingly destructive. Their eggs—known as oocysts—can survive in soil, smeared onto foliage, and floating in seawater for months up to years at a time. Just one is enough to kill an animal as big as a dolphin, sea otter or Beluga whale.

Now, they’re taking down the country’s most endangered marine mammal: the Hawaiian monk seal.

Last month, Michelle Barbieri, lead veterinarian with the National Oceanic and Atmospheric Administration’s Hawaiian Monk Seal Research Program (HMSRP) announced the deaths of three of Hawai`i’s official state mammal due to toxoplasmosis, a disease triggered by the tiny parasite. The total of known deaths to the species due to toxo is now 11—a significant tally, given that just 300 animals total reside in the main Hawaiian Islands.

Prior to this century, nearly all Hawaiian monk seals lived throughout the mostly uninhabited Northwestern Hawaiian Islands, which arc like stepping stones northwest of the islands more popular with vacationers. Then, starting in the late 90s, seals started to re-populate their historic range at the southeast end of the archipelago, helping stem the decades-long population decline that got them listed under the Endangered Species Act in 1976. HMSRP has posted population upticks of 3 percent over the past several years, totaling an estimated 1,400 individuals.

Shortly after the monk seals started hauling out again on Hawai`i’s famous beaches, in 2001, one turned up dead of toxoplasmosis. Two more would die by the close of the century’s first decade. But since 2010, eight more have died, making clear the threat of toxo was no fluke.

Last month’s three deaths gave Barbieri something else to consider: all three were female. That put the victims at eight females and three males. “In large part, females are responsible for carrying on the species,” Barbieri said. “When we lose a female, we don’t just lose one individual seal, we lose all her future pups and all the future pups of any of those female pups and so on.”

The deaths also coincided with a series of extreme rainstorms in Hawai`i that many fear may be the new normal as climate change is expected to create extreme storms on a more regular basis.

Barbieri acknowledged the sample size of 11 is small, and they haven’t pinpointed any factor that would make females would be disproportionately vulnerable to toxo, but she’s watchful. Monk seals can live to be 25 to 30 years of age. Females start pupping as early as five. Two of the recently deceased females had already contributed four offspring to the population—five if you count the full-term pup born dead as a result of toxo (which can pass from mother to pup).

But how is a land-based parasite is killing animals in the ocean?

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“In Hawai`i, everything moves from land to sea,” Barbieri explains. Much of what is inferred on how toxoplasmosis is killing Hawai`i’s state mammal comes from scientific discoveries made with Southern sea otters along coastal California—primarily that freshwater runoff is carrying the parasite to the sea. Similarly, toxoplasma has infected other marine mammals around the world—Beluga whales off Canada, Risso’s dolphins in the Mediterranean, and Hector’s dolphins off New Zealand.

“Monk seals have multiple potential roots of exposure,” Barbieri said. First, their diet. Monk seals dine on nearly the entire buffet of foods offered in the mid-Pacific Ocean, from octopus to eel to lobster to benthic fish, any of which could be infected. They may also be ingesting the parasite directly in water.

When the parasite enters the stomach, it jump-starts into action, exiting the wall of the gut and kicking asexual reproduction—basically, cloning—into high gear. Toxoplasma courses through the body, targeting any combination of organs, including, in the case of females, the uterus, and for fetuses, the placenta. Basically, the clones go anywhere they want.

Once they choose a place to reside, they form cysts. The infected monk seal’s inflammatory response is quickly overpowered by these invaders, leading to cell death, organ death, and, finally, mortality of the host itself.

Not all infected seals die. “There is still a lot to be learned in what turns an infection into a disease and mortality,” Barbieri said. She suspects different strains of toxo are present, some more deadly than others. There could be other contributing environmental factors. It could be something in the seals themselves. To try to tease out answers, HMSRP is going back to its tissue archives and doing additional testing.

Toxo is now considered the number one disease-related threat to the recovery of the species. It’s also the most frustrating for researchers, because there’s little they can do to help—compared to a seal with a fishhook embedded in its jaw. Even for sick seals, like RB24, whose behavior suggests disease, it’s usually too late to intervene.

Plus, there’s currently no vaccine to inoculate monk seals from toxoplasmosis the way that HMSRP has been doing as a preventative against a morbillivirus outbreak. Compound that with the fact that Hawaiian monk seals spend two-thirds of their life at sea, and an untold number of toxo deaths are going undetected.

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Generally, a good parasite doesn’t kill its host—which is why Barbieri is pondering is whether this parasite is a relatively new organism to Hawai‘i. Maybe toxoplasma and its host are still getting to know each other, which could be bad news for monk seals.

Known as the most isolated landmass in the world, the Hawaiian Islands possess a history of species arriving here without their predators and eventually losing their immunities. Hawaiian monk seals, it’s believed, arrived from the Caribbean as far back as 12 million years ago when the watery passage between North and South America still existed. According to Barbieri, the situation with toxoplasmosis might be similar to Australia, “where wallabies are highly susceptible to toxoplasmosis, because cats are not native.”

Cats are not native to Hawai`i, either. They were introduced by European explorers on their arrival to the islands, sometime in the late 18th or early 19th centuries.

Cats. Any discussion of toxoplasmosis always lands blame at the paws—or feces—of cats. (Really, the entire carnivorous mammal family of felids, because felids are the definitive host of T. gondii .) The only felid in Hawai`i are house cats, tame and feral, found in homes and yards and boat harbors and beach parks and deep in the backs of valleys and high on the summit of mountaintops and basically everywhere. Estimates of feral cats on O`ahu alone range from 50,000 to 300,000.

It seems T. gondii and cats have formed the perfect parasite-host relationship. When a cat ingests toxo-infected prey, it rarely gets sick. But its digestive tract allows toxoplasma to sexually reproduce, and for the next several weeks, the cat will excrete oocysts into the environment through its feces by the hundreds of millions. Then, along comes a terrestrial animal that accidentally ingests the parasite while grazing or eating an infected insect. Or heavy rains wash the millions of parasites out to sea, and one gets swallowed by a monk seal.

The animals and seal become intermediate hosts—intermediate, because they may get infected with the parasite, but they don’t shed oocysts in their scat. “No other species is putting oocysts into the environment,” said Barbieri. “And ultimately, it doesn’t matter where the oocysts are deposited on the islands, they’re posing a risk to animals. Not just monk seals but terrestrial wildlife, too.” That parasite has been implicated in the deaths of Hawai‘i’s native birds, as well.

The toxoplasma parasite was first discovered in 1908, but the complete understanding of its lifecycle was not figured out until 1970 when Dr. Jitender P. Dubey described its sexual phase in the small intestines of felids. But not even Dubey knows the answer to why only felids seem to be primary hosts.

The irony is that in the very spot where hope for the future of the Hawaiian monk seal species was springing, the main Hawaiian Islands, a new threat is emerging—and it’s incubating in the gut of pets. Now the challenge for Barbieri and her colleagues will be partnering with cat advocates on how to reduce the hundreds of millions of T. gondii oocysts being shed into the environment by free-roaming cats in Hawai‘i.