Study shows parasite killing Hawaiian monk seals also fatally infecting spinner dolphins

A parasitic disease attacking the Hawaiian monk seal population for the past 20 years is now showing up in the carcasses of spinner dolphins, according a recent study by the University of Hawaiʻi at Mānoa Health and Stranding Lab.

The research stems from the discovery of two dolphins — one on Hawai‘i Island in 2015 and the other on O‘ahu in 2019 — that tested positive for toxoplasmosis, a parasite commonly found in cat feces.

Kristi West, associate researcher at UH Mānoa’s Hawaiʻi Institute of Marine Biology who directs the UH Health and Stranding Lab, said biologists were aware that the disease could kill a spinner dolphin but thought it was a rare occurrence. In the study, researchers looked at the impacts toxoplasmosis has on the animals in greater detail.

“We suspect that many more spinner dolphins may succumb to toxoplasmosis and die than the animals that are recovered dead and examined for cause of death,” West said. “A better understanding of toxoplasmosis infections and infectious cycles is important to developing effective conservation strategies for protected and endangered Hawaiian wildlife.”

The first documented Hawaiian monk seal death due to toxoplasmosis occurred in 2001. The disease has killed 14 animals — the last being a year ago. The parasite has also been identified in the Hawaiian crow, ʻalalā. 

Other animals that harbor the parasite include pigs, mongoose, chickens and rats. West said it’s unclear how the marine life is ingesting this parasite that attacks an animal’s organs, causing them to be inflamed and ultimately shut down.

After performing necropsies on both dolphins, UH researchers found the animals were infected with Toxoplasma gondii genotype 24. This same genotype was identified in feral pigs on Oʻahu in 2020 and described in previous studies in bobcats in Missouri, and chickens in Costa Rica and Brazil.

While she’s fairly confident the parasite is in water runoff to the ocean, West said it’s possible that it’s getting into their food chain.

“It’s the nearshore dolphins that are more susceptible,” she said. “Animals like the monk seals and the spinner dolphins are especially vulnerable.”

The first spinner dolphin death from infection by this parasite was documented in an adult spinner dolphin that was stranded in Haleʻiwa in 1990. Since then, this parasite has claimed the lives of two more dolphins, but it is likely that many additional dolphins have died from this infection.

The UH Health and Stranding Lab only recovers and examines approximately 5% of the spinner dolphins that die in Hawaiian waters, which could suggest that at least 60 spinner dolphins may have died of toxoplasmosis, researchers hypothesize in the study.

West said this is a problem people can do something about, adding researchers and the National Oceanic Atmospheric Administration rely entirely on public reporting of a dead or distressed dolphin. Sightings can be reported to the toll-free statewide NOAA Marine Wildlife Hotline at 888-256-9840.