Toxoplasmosis, the disease that originates from the feces of cats, has been known in recent years to have killed at least 15 Hawaiian monk seals, as well as endangered birds and other wildlife in the islands.
Now new research has discovered the deadly disease in two spinner dolphins, one that stranded off Oahu in 2019 and the other off Hawaii island in 2015, and has estimated that as many as 60 dolphins have died from the disease over the past three decades.
“This is a significant threat to spinner dolphins,” said Kristi West, a researcher with the University of Hawaii’s Hawaii Institute of Marine Biology.
West, who directs the UH Health and Stranding Lab, was the lead researcher in a paper describing the discovery and published Thursday in the journal Diseases of Aquatic Organisms.
Toxoplasmosis is caused by a single-celled organism, Toxoplasma gondii, of which cats are the host. Toxoplasma eggs or oocysts are shed in cat feces and can survive in the environment for up to two years and carried to the ocean by rainwater.
When animals ingest the oocysts, the disease attacks the body’s tissues and can lead to a swift but brutal death.
To estimate the prevalence of Toxoplasmosis gondii in dolphins and whales in Hawaii, West’s research team screened archived tissues from 37 spinner dolphins and 18 other cetacean species that stranded from 1997 to 2021.
The good news is that no other deep-water species tested positive for the disease — a real possibility because it previously has been found in beluga whales from Canada, Hector’s dolphins from New Zealand, a Guiana dolphin and Bryde’s whale from Brazil and striped dolphins off Italy, according to the paper.
The bad news is that two spinner dolphins tested positive. And when you add that result to a previously discovered case of toxoplasmosis in a stranded Hawaii spinner dolphin from the late ’80s — previously thought to be an outlier case — it’s clear that the marine species is vulnerable in the islands.
The paper suggests that the vulnerability lies in the fact that spinner dolphins spend a lot of time in nearshore waters, and freshwater streams can carry cat feces or infected carcasses into estuaries and nearshore areas where spinner dolphins feed. The dolphins also spend part of their day resting in nearshore waters.
Any warmblooded animal, including humans, can contract toxoplasmosis by ingesting a single T. gondii egg.
Toxoplasmosis can cause severe disease in immunocompromised people and cause birth defects if a pregnant woman becomes infected with the disease by, for example, accidentally ingesting cat litter or fecal particles, consuming undercooked meat or unwashed produce, or coming into contact with the eggs in the soil.
There are currently no statistics on how many people have been infected with toxoplasmosis in Hawaii, according to the state Department of Health. The Centers for Disease Control and Prevention has estimated that 11% of the U.S. population age 6 years and older has been infected with the Toxoplasma parasite.
People with normal immune systems rarely develop signs of illness.
In animals, toxoplasmosis has been identified as contributing to the decline of the Hawaiian crow (alala) and Hawaiian goose (nene) and is considered one of the “Big Three” threats to monk seals in the main Hawaiian Islands (the others being trauma and encounters with certain types of fishing gear).
As for spinner dolphins, the UH Health and Stranding Lab recovers and examines only about 5% of the species that die in Hawaii waters.
West said the public can help scientists learn more about this issue by reporting dolphin and whale strandings in order to allow the recovery and examination of every carcass. Sightings of dead or distressed marine mammals can be reported to the toll-free statewide NOAA (National Oceanic and Atmospheric Administration) Marine Wildlife Hotline at 888-256-9840.
The public can also help reduce the spread of toxoplasmosis through responsible cat ownership by spaying and neutering cats, keeping cats exclusively indoors and reporting any illegal dumping of cats or kittens outdoors into feral colonies.
Funding for this work came from NOAA’s John H. Prescott Marine Mammal Assistance Grant Program; the U.S. commander, Pacific Fleet; and NOAA’s Pacific Islands Regional Office.
