Getting a grip on the yuck factor

Experiments combining human and pig cells provoke strong responses from some people, but is the reaction justified. Paul Biegler explores the issue.

A not altogether accurate artist's impression of a pig-human chimera.
A not altogether accurate artist's impression of a pig-human chimera.
Tim Archibald/Getty Images

For the millions of diminutive fans of Peppa Pig there is almost nothing cuter than a talking piglet. Outside cartoon land the boundary between pig and human is also blurring; and the response, in some quarters, is less enthusiastic.

In January 2017 a team led by Juan Carlos Izpisua Belmonte, at the Salk Institute in California, announced it had introduced human stem cells into pig embryos. The result was a hybrid embryo, part pig, part human. Such creations are called chimeras, after the mythological creature that blended lion, goat and snake.

No Peppa Pig resulted. Just one in 100,000 of the embryo cells were human, and the embryos were destroyed after 28 days. The researchers’ stated goal was to test which types of human stem cells would best engraft into a pig embryo, with the ultimate aim of directing those cells to grow into human organs that might, one day, be used for transplants.

Nonetheless, such experiments often elicit a repugnance that can morph rapidly into moral condemnation.

In August 2016 the US National Institutes of Health (NIH) invited public comment on the idea of lifting its moratorium on funding chimera research, in place since 2015. Before that time federal government funding for chimera research had been available so long as primate (including human) embryos weren’t used. Privately funded research, such as Belmonte’s, was not subject to government restrictions.

The NIH received more than 21,000 submissions. There were common themes: scientists would be creating monsters and generally blurring the line between human and non-human animals.

In philosophical parlance such arguments appeal to what bioethicist Leon Kass has called “the wisdom of repugnance” or, less formally, the “yuck factor”.

The question is how much weight we should give the yuck factor. Kass, arguing against human reproductive cloning in 1997, thought we should give it plenty: repugnance, he wrote, “is the emotional expression of deep wisdom, beyond reason’s power fully to articulate it”.

As a guide to what is good, though, emotions have a very chequered history. A feeling of disgust on smelling spoiled food, for example, is obviously conducive to good health. But many people also find the thought of a faecal transplant repulsive, despite recent evidence it is effective for some types of bowel inflammation. So it is worthwhile interrogating emotions.

Consider the lofty goal of researchers. Human organs, most likely from pig-human hybrids, could one day reverse the massive donor shortfall that sees people awaiting transplants die each day. That goal received a big boost in February 2017 when a team led by Hiromitsu Nakauchi at Stanford University grew mice pancreases in rats. The pancreas cells were then implanted back into diabetic mice, curing their disease for a year.

The good tidings were tempered, however, by residual yuckiness from a 2013 experiment, led by Steven Goldman at the University of Rochester Medical Centre, New York, that found mice embryos injected with human brain cells turned out significantly smarter.

That experiment stoked speculation pig-human chimeras might inadvertently acquire human intelligence and, with that, a commensurate set of moral rights, even a right to continued existence. Julian Savulescu, a bioethics professor at the University of Oxford, has argued that, if a pig achieved sufficiently advanced cognition, “the default position should be that we assign them high moral status”.

The awkward corollary might be a pig with human organs that could not, in good conscience, be killed to retrieve them.

It is likely, though, that personable porcines will remain in fantasy land. For one thing, pig gestation takes a mere three months, hardly long enough for a human brain to form. Furthermore, genetic tricks are possible, such as inserting instructions into the DNA of human stem cells so brain cells will self-destruct.

In Australia the Prohibition of Human Cloning for Reproduction Act outlaws the creation of chimeric embryos. However, Megan Munsie, of the Centre for Stem Cell Systems at the University of Melbourne, points out the law only bans putting animal cells into human embryos. “That act is silent on the reverse scenario,” Munsie says. That topic will be squarely on the agenda at the annual meeting of the International Society for Stem Cell Research in Melbourne in mid-2018.

The yuck factor will no doubt continue its strident contribution to the debate. We’d do well to ensure, however, the voices of people on transplant waiting lists are not drowned out in the hubbub.

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Paul Biegler is a philosopher, physician and Adjunct Research Fellow in Bioethics at Monash University. He received the 2012 Australasian Association of Philosophy Media Prize and his book The Ethical Treatment of Depression (MIT Press 2011) won the Australian Museum Eureka Prize for Research in Ethics.
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