Around five years ago, bobcats at one of the Midpeninsula Regional Open Space District’s preserves were in rough shape. Twelve bobcats at Rancho San Antonio Open Space Preserve died after being struck with notoedric mange — a disease that occurs when mites burrow inside the skin of animals with compromised immune systems — between 2009 and 2013. During that window, dozens more were reported sick.
The bobcats’ suffering was difficult to ignore. Visitors, volunteers and staff members were shocked to see emaciated, lethargic bobcats, and even carcasses, dotting the landscape at the Cupertino preserve.
“The issue was really visible in Rancho San Antonio because it’s our most used preserve,” said Leigh Ann Gessner, a Midpen public affairs specialist. “It’s in an area where open space is adjacent to a highly populated urban area.
“But that’s not the only place this is going on,” she continued. “This is really happening statewide across California.”
It was initially unclear what was causing the illnesses. But Midpen specialists soon discovered a common culprit behind the parasitic disease: rat poison.
“We worked with folks at the laboratories at (University of California) Davis, and also with the California Department of Fish and Wildlife,” said Karine Tokatlian, a resource management specialist at Midpen. “What we were able to find was that (the rat poisons) were causing internal bleeding and a lot of other complications.”
In recent years, research has demonstrated a clear link between the anticoagulant chemicals found in certain second-generation rodenticides, or rat poisons, and mange-related deaths among bobcats. A 2017 study published by the Royal Society attributed the rapid population decline of bobcats in urban Southern California to mange.
Tokaltian explained that rodenticides afflict animals, like bobcats, that prey on rats that have ingested anticoagulant poison. The predators then become sick as well.
“They (experience) depression or an inhibition of their immune system because of the effect that the rodenticide is having within the body,” she said.
The poisons also impact other wildlife, including hawks and owls.
“The issue of secondary poisoning means that the bobcats, owls and other predatory animals who eat these rodents are affected by this,” said Tokatlian. “Animals like great-horned owls, burrowing owls along the coast, and barn owls in particular.”
In response, Midpen worked with the Santa Clara County Vector Control to educate the district’s neighbors, particularly near the preserve, about the impact of rodenticides on wildlife. The district also joined ongoing efforts to lobby the Legislature to prevent anticoagulant poison from being widely used.
In 2014, the Legislature passed AB 2657 prohibiting the use of rodenticides containing anticoagulants in wildlife habitat areas. Last month, the California Department of Pesticide Regulation announced it would be re-evaluating the use of anticoagulant rat poisons.
In recent years, Midpen has received consistent reports that the bobcat populations in the district’s preserves are much healthier, without any noticeable signs of mange. Tokatlian said she is comfortable saying that the district has effectively addressed the issue.
But the district also warns that some anticoagulant rodenticides are still available in California — and continue to impact wildlife across the state. Tokatlian advised Coastside residents to keep homes clean and clear, keep food properly stored away and use non-chemical methods, like barn owl nesting boxes, to control rodent populations.
To learn more about Midpen’s efforts, visit www.openspace.org/rodenticides.