Why the U.S. Allows Pig Kidneys in People but Still Bans Growing Human Organs in Pigs

In October 2025, surgeons in New York made history by transplanting a gene-edited pig kidney into a living patient enrolled in a clinical trial. The kidney was engineered to resemble human tissue and grown in a pig as an alternative to waiting for a human donor.

What Happened

The recipient was one of six participants in the first pig-to-human kidney transplant clinical trial. The effort is part of a growing push to address the severe shortage of human organs: more than 100,000 Americans are currently on transplant waiting lists, and many die while waiting.

Two Scientific Paths

Researchers have pursued two different approaches to solving the organ shortage. One approach modifies animal organs to be more acceptable to the human immune system by inserting a few human genes and deleting some pig genes. These gene-edited pig organs remain the closest-term option, but recipients still require strong immunosuppression and face the risk of rejection. A January 2025 case in New Hampshire illustrated these challenges when a gene-edited pig kidney implanted in a patient was removed nine months later after declining function.

The other approach aims to grow organs made entirely of a patient's own cells inside animals. This would involve disabling the pig embryo's genes for a specific organ and seeding the embryo with human stem cells so the animal develops a genetically matched organ. In theory, this chimera-based strategy could eliminate immune rejection, but it is technically complex because human and pig cells develop at different rates. A proof of concept—growing a mouse pancreas in a rat—had been demonstrated before the federal funding pause.

Why Funding Was Paused

In 2015 the National Institutes of Health paused funding for experiments that introduce human stem cells into animal embryos. Regulators cited ethical concerns: notably the possibility that human cells might migrate to an animal's brain and alter its cognitive capacities. The NIH warned of potential "alterations of the animal's cognitive state," and organizations like the Animal Legal Defense Fund warned that if chimeras developed humanlike awareness they would require far greater protections.

The Ethical Debate

The core worry is about moral status—whether and how an animal's interests and protections should change if it acquires humanlike cognitive capacities. If an animal became self-conscious in ways akin to humans, harming it could be morally more serious than harming an animal with only basic sentience.

Critics of the NIH rationale, including some bioethicists, argue that the reasoning is inconsistent. They note that if cognitive capacities alone determined moral status, regulators should be equally worried about inserting cells from other cognitively complex animals (for example, primates or dolphins) into pigs. In practice, research protections are generally organized around species membership: humans are protected because they are human, not because every human meets a specific cognitive threshold.

Importantly, the mere presence of human cells in an animal does not make it human. Pigs engineered to support human-compatible organs already carry human genes but are not treated as "half-human" beings under current regulations. Legitimate concerns remain, however, including animal welfare, the risk of unintended biological consequences, and public attitudes about treating animals as organ producers.

Where Things Stand

Gene-edited pig organs are advancing in clinical trials and offer a nearer-term path to expanding the organ supply, but they still rely on immunosuppression and face rejection risks. Growing patient-matched organs in animals could, in principle, reduce rejection but is paused in the United States because of ethical uncertainty and scientific hurdles. The debate centers on how moral status should be assigned, how to weigh animal welfare, and how to regulate research responsibly without blocking potentially lifesaving innovation.

Updated to correct the location and date of the first pig kidney transplant clinical trial.

Author: Monika Piotrowska, University at Albany, State University of New York. This article is republished from The Conversation, a nonprofit news organization.