Source: USA Today (Extract)
Posted: June 10, 2023

For our feline companions, the vast outdoors offer tantalizing temptations beyond the occasional avian snack.

Their lives outdoors are dangerous ones where the threat of infectious diseases and larger predators constantly loom. But when given the chance, cats — whether strays or domesticated pets whose owners allow them to roam free — will, well, breed.

While 71% of the estimated 80 million pet cats in the U.S. are kept indoors, the Humane Society of the United States estimates there are still anywhere from 30 to 40 million stray or feral cats in the country.

They often live in poor conditions and, due to their predatorial instincts, inflict harm on the local wildlife. Those who find their way to an animal shelter face the additional threat of euthanasia, a sad method for controlling the population.

Surgical sterilization has long been another method to stop cats from reproducing, albeit an expensive one. But another method may be on the horizon after a study published Tuesday in the journal Nature Communications found that long-lasting injections to prevent ovulation in female cats showed promise.

What did the study find?

Conducted by researchers at Massachusetts General Hospital, the study found that just one shot of a gene therapy prevented female cats from becoming pregnant for at least two years.

Though the study was small — six female cats who received the gene therapy shot were compared to three who did not — only those in the “control” group were able to become pregnant in the two years that researchers studied the animals.

The contraceptive shot was of the anti-Müllerian hormone, or AMH, which interferes with the development of egg follicles in the ovaries. Female cats and other mammals naturally produce AMH in their ovaries, while males produce it inside their testes

Scientists had previously researched AMH as a way to protect ovarian reserves in women undergoing chemotherapy. However, that research led to the study’s authors finding that raising the level of AMH beyond a certain threshold suppressed the growth of ovarian follicles, effectively preventing ovulation and, thus, conception.

To raise AMH levels in female domestic cats, the researchers created an adeno-associated viral (AAV) gene therapy vector with a slightly altered version of the feline AMH gene. The Food and Drug Administration has approved human therapies that use similar AAV vectors.

Calling the study a “breakthrough,”  Gary K. Michelson, the founder and co-chair of the Michelson Found Animals Foundation, which funded the work, said the non-surgical sterilization method “is long overdue and will transform animal welfare.”

“This breakthrough discovery is a major milestone in our quest to provide pet owners with an alternative to surgical spay and neuter,” Michelson said in the prepared statement.

How did the study unfold?

In the study, six female cats were treated with the gene therapy at two different doses, which produced AMH in their muscles rather than their ovaries. Another three cats who were part of the “control” group were not given a shot.

Researchers then brought male cats into the female colony for two four-month long mating trials, during which time they studied the female cats for more than two years to assess the effect of the treatment on reproductive hormones, ovarian cycles, and fertility.

All three control cats gave birth to kittens, while none of the cats that received the gene therapy became pregnant. Hormones like estrogen did not appear to be affected, researchers found, and no other adverse effects were observed.

However, researchers cautioned that additional studies are needed to confirm the validity of the preliminary findings before the surgical expertise of veterinarians is no longer required to sterilize cats.

“This technology may be a little ahead of its time,” said David Pépin, a reproductive biologist at Massachusetts General Hospital in Boston, in a written statement. Pépin added that the infrastructure needed to produce enough doses to sterilize millions of cats via gene therapy does not yet exist.

“Our goal is to show that safe and effective permanent contraception in companion animals can be achieved using gene therapy.”