The parasite that causes toxoplasmosis has become widespread in Hawaii and now infects as many as 48 percent of some Hawaiian geese populations, a new study finds.

Hawaiian geese, also known as nēnē (Branta sandvicensis), nearly went extinct in the last century due to overhunting and introduced predators. By 1949, just 30 geese remained. Today that has turned around. Thanks to intense captive breeding efforts, an estimated 2,800 geese live on four of Hawaii’s main islands, where they remain protected by the U.S. Endangered Species Act.

Now the nēnē’s remarkable recovery faces a new challenge. According to a study published in the Journal of Wildlife Diseases, the geese have become increasingly infected by Toxoplasma gondii, the protozoan parasite that causes toxoplasmosis. Researchers found that the birds on the island of Molokai, the island which had “a conspicuously consistent presence of feral cats,” also had the highest rates of infection, an astonishing 48 percent. Infections rates were 23 percent on Maui and 21 percent on Kauai, where feral cat populations were lower.

Lead author Thierry Work, a wildlife disease specialist with the U.S. Geological Survey, says the geese are probably contracting T. gondii by either accidentally ingesting cat feces or eating insects that are carrying the parasite.

Toxoplasmosis doesn’t kill a huge number of geese by itself. Work says it’s only directly responsible for about 4 percent of deaths, but it may contribute to mortality in other ways. “There is increasing body of evidence to suggest that animals infected with T. gondii have altered behavioral patterns, one of which is that infected animals are more prone to trauma,” Work says. Trauma—such as predator attacks or being struck by vehicles—is leading cause of death in the birds. Work thinks the parasite could make the geese more prone to “traumatic events,” but that still needs additional confirmation.

Work says he hopes this new research—which also indicates a greater threat of toxoplasmosis for humans and other wildlife—could “prompt some additional soul searching on how to address feral cats in an ecosystem where they don’t belong.”

Conservationists echoed this sentiment. “While we appreciate cats as pets and acknowledge the important role pet cats play in many people's lives, it is clear that the continued presence of feral cats in our parks and neighborhoods is having detrimental impacts on people and wildlife,” Grant Sizemore, director of invasive species programs at American Bird Conservancy, said in a prepared release. “Before another species goes extinct or another person is affected by toxoplasmosis, we need to acknowledge the severity of the problem and take decisive actions to resolve it. What is required is responsible pet ownership and the effective removal of free-roaming feral cats from the landscape.”

Previously in Extinction Countdown:

Other Hawaiian Extinction Stories by John Platt: