Veterinarians recommend sterilizing cats, both to live with them (heat can be very annoying), and to prevent them from suffering some pathologies that arise when they do not reproduce. It is also the plan that is contemplated in the animal welfare law, which speaks of the CER method (capture, sterilization and return), to control feline colonies and prevent them from becoming overcrowded. This Tuesday, the journal Nature has published an article with the first results of an injection developed in the United States to sterilize cats. Researchers have used a virus to introduce a hormone, called anti-Müllerian (AMH), into the body of these animals to inhibit ovulation and prevent pregnancy.
The study was conducted with nine cats, six treated and three as a control group. The first data show the effectiveness of this method and open an alternative route to surgery. None of the treated cats became pregnant and no side effects have been observed four years after the test, something that has not surprised the researchers, as explained by the lead author of the study, William Swansson: "It is a natural protein, which is already produced by females." The concentration of the hormone at higher than normal levels is what slows ovulation.
A possible protective effect has even been observed against pathologies such as cystic endometrial hyperplasia (an alteration of the endometrium) and pyometra (an infection of the womb). Swansson, of the Cincinnati Zoo and Botanical Garden (USA), attributes it to the suppression of ovulation and the successive lower production of progesterone, which is closely related to the development of these diseases. Four years later, the cats continue to express a higher than normal level of AMH, something the researcher considers a good sign.
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The currently available methods of neutering cats are medical and surgical. In the first group are hormonal treatments, which inhibit reproduction for a certain period of time, but can have long-term side effects, as explained by Raquel González, co-author of the work. Some, such as progesterone drugs, can only be used on occasion. There are also implants of deslorelin, a substance that in the long term regulates the hypothalamus to decrease the production of hormones that influence the maturation and production of eggs, adds the veterinarian.
González, who specializes in reproduction, explains that the only permanent option is surgery and consists of an ovariohysterectomy (removal of uterus and ovaries) or an oophorectomy (removal only of ovaries). But, like any surgical process, it involves risks and recovery. You can even modify your behavior. If it is done at very early ages, it can make them more shy and elusive, adds Joaquín Cerdeira, a specialist in reproduction of small animals and professor at the Complutense University of Madrid (UCM), who has not participated in the project.
There's no concrete number [of cats] that you can sterilize and say, it's done. There are many cats that do not let themselves be seen
Miguel Clavero, researcher at the Doñana Biological Station (EBD-CSIC)
The goal of the researchers is that the injection becomes an alternative to surgical intervention, in order to be able to castrate cats permanently with a single dose. However, they acknowledge that there is still much work ahead. Laura Abril, a postdoctoral researcher at the Faculty of Veterinary Medicine at the University of Murcia, who has not collaborated in the Nature study, argues that a larger sample is necessary. "It would be interesting to know if I use this method with my cat, in 10 years it can cause ovarian cancer, for example," she adds.
The UCM professor also believes that the sample is insufficient, although he acknowledges that it is a "very novel and promising" project. Swansson said the goal of this trial was to use a few animals to make sure it could work before starting to work with larger numbers.
Although none of the cats became pregnant, two of them did have reproductive behavior in the two reproduction trials they did at eight and 20 months after the injection. The author is not concerned about this and attributes it to individual variability. In each of the females, the amount of hormone that enters the body and then they produce can vary, and even "it is possible that there are some immune responses in cats that could affect [the result]," says the scientist.
Cerdeira believes that now the important thing is to discover whether or not it is reversible and from what age it would be advisable to apply it, something in which Swansson agrees. For it to work, the expression of AMH has to remain elevated and, for now, three years later, it remains so. The author calls it a "very good" finding, but they need to make sure this holds for at least five or ten years. "If protein levels drop a lot, they may become fertile again," he concludes. Cats are still under observation and are routinely checked to make sure of this and check for side effects.
Colony control
There are 600 million cats in the world and 80% of them have no owner, says the Nature article. Feral cats, or domestic cats with access to the outdoors, pose a real problem for the biodiversity of the environment in which they live, says Miguel Clavero, a researcher at the Doñana Biological Station (EBD-CSIC), who is also independent of the research. In territories such as the Canary Islands or the Balearic Islands, where they have no natural predators, they pose a danger to birds and reptiles and have even caused the extinction of some native species, Clavero said. In addition, they can also be a risk to human health, as they can transmit diseases such as toxoplasmosis, which is only dispersed through their feces.
For these reasons it is so necessary to control cats that live in colonies and the rest that are in freedom, says the ecologist. One of the objectives of the US project is to be able to contribute to the control of animals, but Clavero believes that it will have little effect on the impact that these felines cause in their environment. He argues that it is very difficult to capture all the cats that can reproduce: "There is no concrete number that you can sterilize and say, it is already done. There are a lot of cats that don't let themselves be seen."
However, the EBD-CSIC researcher does recognize that it can be an advance for the welfare of domestic cats and for those in colonies that allow themselves to be captured. "An injection is always better than an operation with an open wound and then releasing them in the middle," he summarizes.
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