It is not a good time to be a feral cat.
Evidence is mounting that the small predators are responsible for an outsized number of wildlife deaths around the globe. Australia plans to kill 2 million feral cats by 2020, to prevent the felines from hunting native species to extinction. In the United States, a 2013 estimate put the annual toll from free-ranging domestic cats between 1.3 billion to 4.0 billion dead birds and 6.3 billion to 22.3 billion dead mammals.
And in a strange but no less lethal twist, on the Hawaiian Islands, cats are responsible for the deaths of rare monk seals. The 10-pound carnivores do not kill animals 50 times their size outright, not with little claws and needle teeth, but via a deadly parasite that resides within feline excrement.
In the last 15 years, the germ has claimed eight monk seals, according to National Oceanic and Atmospheric Administration data reported by the Associated Press on Monday.
Even without parasites, it is not a particularly good time to be a Hawaiian monk seal, either. Although they lack feral cats' reputation as harbingers of extinction – with their blunted silhouettes and scruffy, smiling muzzles, the monk seals are the Pacific's answer to a pre-blockbuster Chris Pratt – the animals still have a bad rap. The rare mammals have been found beaten to death on beaches, possibly due to pernicious rumors the seals are to blame for depleted fishermen's stocks.
Where cats and Hawaiian monk seals wholly diverge is in population. The Hawaiian Humane Society calculated in 2013 that 300,000 feral cats roamed the island of Oahu. All told, the seal population is just over 1,000 individuals. This low number of Hawaiian monk seals is why Hawaiian officials are so concerned that eight monk seals are dead from the cat-borne parasite, Toxoplasma gondii.
"While eight seals may not sound like a lot of animals, it actually has pretty large ramifications for an endangered population where there's only about 1,300 seals in existence at this point in time," Michelle Barbieri, a veterinarian with NOAA's Hawaiian monk seal research program, told the AP.
The Toxoplasma gondii parasites reproduce only in the bodies of domestic cats. But once cats pass the parasite's eggs, called oocysts, through feline feces, other animals may be infected.
Toxoplasmosis, the disease caused by the single-celled parasite, can alter the behavior of mice and rats, and is why pregnant women should steer clear of stray kittens. (The infection, somewhat dubiously, is also blamed for road rage in humans.)
On islands like those in the Hawaiian archipelago, where cats are numerous and their feces may spread through runoff, toxoplasmosis can be devastating. Biologists point to the infection as a reason the Hawaiian crow is extinct in the wild.
In January 2004, a male seal named RK07 was found dead in the surf on island of Kauai, researchers with the Armed Forces Institute of Pathology reported in the Journal of Parasitology in 2005. In their examination of RK07, they made the first observation of a fatal toxoplasmosis infection in monk seals.
The scientists found lesions in the animal's adrenal glands, brain, diaphragm, lymph nodes and spleen. "The most severe lesions were within the lymph nodes," they wrote. "The character of lesions suggests that the seal acquired T. gondii infection recently, most likely through ingestion of oocysts."
Five years later, scientists were "only just beginning to understand the prevalence of the disease in the population and determine ways to mitigate the impact," as NOAA monk seal researcher Charles Littnan told Scientific American in 2010. At the time, there were four confirmed deaths from toxoplasmosis and an additional two speculated. By 2016, as the Associated Press reported Monday, the confirmed deaths had doubled, to three male and five female seals.
To stop the seal deaths, and also conserve other native wildlife, NOAA supported initiatives that included euthanizing feral cats. A bill in the Hawaiian State legislature to allow that was ultimately deferred in early 2016, withering under criticism from pro-cat advocates.
"It struck a nerve with lots of people, and the vast majority of testimony was from animal advocates who were strongly opposed," Hawaiian state Sen. Mike Gabbard told North Hawaii News in February, when the bill was deferred.
In Hawaii and elsewhere, some animal advocates argue for trapping, neutering and returning feral cats to the wild as a nonlethal option. In general, however, mathematical models and historical evidence indicate euthanasia is able to reduce cat populations far more quickly and cheaply than trap-neuter programs.