For all the cats who share our homes as companion animals,
there is a vast shadow world of strays — a sprawling and fast-breeding crowd.
Their lives are plagued by the threat of infectious
diseases, predators, and fast-moving cars. And they are major predators
themselves, hunting down millions of birds and small mammals annually.
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In the US, volunteers are especially active in trapping the
cats, bringing them to clinics to get surgically sterilized, and then returning
them to their colonies. But controlling stray cat populations is costly and
logistically cumbersome. Many communities, especially in countries outside the
US and Europe, lack the veterinary and economic resources to coordinate such
efforts.
“Coming up with an alternative to surgery has been a goal
for a lot of people for decades, and there just hasn’t been anything else
that’s proven to be effective,” said William Swanson, director of animal
research at the Cincinnati Zoo & Botanical Garden.
Such a method might finally be on the horizon. In a study
published last week in the journal Nature Communications, a single shot of a
gene therapy prevented pregnancy in cats for at least two years. The study was
extremely small: Six female cats that received the gene therapy shot were
compared to three who did not.
By limiting the study size to just a few cats, the
researchers were able to track each one extensively, analyzing 15,220
freeze-dried poop samples for estrogen and progesterone levels and examining
1,200 hours of video of mating behavior, Swanson said.
The contraceptive shot delivers a gene that enters muscle
cells, enabling them to pump out a substance called anti-Müllerian hormone, or
AMH, which interferes with the development of egg follicles in the ovaries.
Researchers cautioned that much more research would be
needed to test the preliminary findings. And if larger studies confirm that the
treatment — the first gene therapy developed specifically for animals — is safe
and effective over a cat’s lifetime, controlling cat populations will not
require the surgical expertise of veterinarians, Swanson said.
David Pépin, a reproductive biologist at Massachusetts General
Hospital in Boston, was originally studying AMH as a potential therapy for
ovarian cancer, but decided to look at its effect on ovaries. When he injected
the hormone into mice, their ovaries shrunk to newborn size, suggesting AMH
might have contraceptive properties.
Pépin is investigating the potential use of AMH in people,
not as a gene therapy but as a pill or injection that must be taken
continuously. Most contraceptives today prevent ovulation, but AMH would act
earlier, blocking follicles from maturing.
He thinks that it might be useful for women who could not
take birth control pills with progesterone or estrogen for medical reasons or
that it could help women undergoing cancer treatments preserve their fertility.
“It’s a hormone that we didn’t get to play with before that potentially has
many different applications in women’s health,” he said.
As a gene therapy that could be permanent, the use of AMH in
people is unlikely. “But it’s actually the perfect tool to control cat
overpopulation,” he said. Four of the cats in the study did not show behaviors
indicating they were ready to mate, and two allowed male cats to mate with
them, but did not ovulate.
Pépin and Swanson, an expert in feline reproduction (and a
scientific advisory board member of the Michelson Found Animals Foundation,
which funded the work), are planning a larger study that could support an
application to the Food and Drug Administration to consider approving the
therapy to be marketed for use in cats.
They are also testing the therapy in kittens, which can be
treated starting at eight weeks of age, as well as in dogs, which also have
enormous stray populations, particularly in other countries.
“This is really exciting, and I hope it will pan out,” said
Julie Levy, a veterinarian at the University of Florida College of Veterinary
Medicine in Gainesville, Florida, who was not involved with the study.
“Wouldn’t it be great if we could send out a technician into the field to
inject cats and then let them go?”
The study is an example of the Michelson foundation’s
practice of “throwing a lot of big money at the problem” to find nonsurgical
contraception for stray cats and dogs, said Levy, who works with cats in
outdoor colonies and shelters, both in the United States and abroad.
But she cautioned that there was still much to learn from a
larger study, such as how long the shot lasts, whether it is as safe as it
seems, and what proportion of cats it will actually protect from pregnancy,
“because it probably won’t be 100%.”
Others note that it might not be quite so easy. If the shot
is effective, long-lasting and cheaper than spay and neuter surgery, it could
be very valuable, said Autumn Davidson, a veterinarian at University of
California, Davis, School of Veterinary Medicine. But to receive the injection,
animals have to be captured, and queens who are adept at evading people’s traps
might still make population control a struggle.
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