Cities turn to birth control for rodents

After California and other states restricted the use of highly toxic rat poisons, cities have turned to a new method to try to control their rat populations: birth control.

Rather than a pill, the contraceptive, known as Contrapest, is deployed in bait boxes as a liquid that is high in fat and water and targets their reproductive capabilities, preventing them from contributing to the population.

“The problem is that rats reproduce at an incredible rate,” said Ken Siegel, CEO of Senestech, a biotechnology company that makes Contrapest. “You just can’t kill them fast enough to offset that reproduction rate. And so Contrapest is really the only solution that’s out there that deals with the second half of the equation, which is birthrate.”

Just two sexually mature rats could account for about 15,000 rats within a year, according to Siegel. Once consumed, the liquid contraceptive leaves rats infertile for about 100 days, preventing them from producing offspring.

San Francisco and Los Angeles – both among the top five “rattiest” cities in 2021, according to Orkin’s annual report – started using Contrapest after California banned anticoagulant rodenticides. Those include brand names such as d-con, Hot Shot and Talon.

Anticoagulant rodenticides thin blood to prevent it from clotting, causing a rat to die from internal bleeding. The same fate befalls other animals that encounter poison, whether they eat the rat or are exposed through other means, the British Columbia Society for the Prevention of Cruelty to Animals says.

Contrapest was found to help decrease rat populations by more than 90%, based on studies at various locations, including in Washington and a poultry facility in California, according to Brandy Pyzyna, vice president of research and regulatory affairs at Senestech.

The San Francisco Recreation and Parks Department has used Contrapest since 2019, ensuring there isn’t a sanitation issue, reducing vegetation that might harbor rats and monitoring and trapping rats when necessary, according to Tamara Aparton, the department’s spokesperson. “It can be really effective if other food sources have been eliminated,” Aparton said.