A non-surgical, single-dose contraceptive has remained effective for at least three years in six female cats, and the effect is expected to last for the rest of their lives. If it does, the injected contraceptive could replace surgery as a means of preventing pregnancies in cats.
It would also be the first permanent, non-surgical contraceptive for any animal. “If this is truly lifetime contraception, then it would be the first lifelong contraception that I’m aware of,” says William Swanson at the Cincinnati Zoo & Botanical Garden in Ohio, who worked on developing it.
It should be possible to adapt the technique for other mammals, too, perhaps including people.
Creating a one-dose permanent contraceptive is a goal many researchers are pursuing to control populations of animals – especially stray, feral or invasive ones – without having to kill them. There are already a number of products that provide long-lasting contraception in some species.
For instance, some wild horses in the US are darted with a vaccine called PZP, which provokes an immune reaction in females that prevents pregnancies. However, repeated doses are required.
In cats, spaying is the only permanent way to prevent females having kittens. This is a costly procedure that involves removing the ovaries, or ovaries and uterus, under anaesthetic.
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Swanson and his team have instead developed an approach that involves a single injection into the thigh muscle of cats. The injection contains DNA that codes for a protein called anti-Müllerian hormone. The DNA is carried by a virus that delivers it to the nucleus of muscle cells, where it remains indefinitely.
Mammals naturally produce anti-Müllerian hormone, but the injection raises the levels in female cats to between 100 and 1000 times higher than normal, which prevents the development of egg follicles in the ovaries, says Swanson.
In a trial, six female cats received the contraceptive injection and three cats received a dummy injection. Each cat was housed with a male with proven fertility for two four-month periods. All of the untreated cats became pregnant, but none of those given the contraceptive did. “The cats are completely healthy. We are not seeing any adverse effects,” says Swanson.
Muscle cells persist for a lifetime, and those with the added gene should keep producing the hormone all this time, says Swanson. “That’s why we target muscles.”
Technically it isn’t sterilisation, he says, as lowering levels of the hormone would restore fertility.
Different species have different versions of anti-Müllerian hormone, so the injection would only function in them if it were altered to include their gene for the hormone. This has safety advantages, as it means that if anyone accidentally poked themselves with a needle while injecting cats, their fertility should be unaffected.
Swanson’s team was funded by a foundation set up by billionaire surgeon Gary Michelson, which also offers a $25 million prize for “a low-cost, permanent, nonsurgical sterilant for male and female cats and dogs”. The aim of the prize is to reduce the number of companion animals euthanised in shelters, which is done in some cases because it is too expensive to sterilise stray or feral animals.
“Surgery, especially in feral animals, has extensive stress and cost involved in trapping the animals, relocating them to a surgery facility, doing the surgery, holding them overnight and then releasing them,” says Aime Johnson at Auburn University in Alabama. “A simple injection would allow trapping, injection and release immediately.”
However, the approach of sterilising stray animals, known as trap-neuter-return, or TNR, is controversial.
“The issues around TNR are not prevented by having an injectable contraceptive – cats would still be left on the streets to encounter trauma, injuries, toxin ingestion, predation risk, human persecution, parasite loads, exposure and potential starvation,” says Patricia Fleming at Murdoch University in Perth, Australia. “Their wildlife impacts would also continue.”
But the contraceptive injection would have fewer animal welfare implications than spaying, says Fleming.
The Michelson prize will remain unclaimed for now, though. The method developed by Swanson and his team won’t win as it doesn’t work in males.
Nature Communications DOI: 10.1038/s41467-023-38721-0