Reviving ancient viruses could help fight modern viruses

But a new study led by Steven Fiedman of the University of Oxford and published in Science rejects this conventional wisdom. It shows how the resurrection of an ancient strain of virus could unlock the secrets of its evolution – and even improve the health of chickens today.

The virus in question is Marek’s disease virus (MDV), a killer of poultry. MDV causes inflammation and tumors in the lymphatic systems of birds, and kills more than 90% of unvaccinated chickens. Controlling it costs the poultry industry $1 billion per year. Yet when it was discovered in 1907, MDV rarely caused death.

Dr. Fidaman wondered whether its newly discovered toxicity was a result of the aggressive growth of industrial poultry farming. To find out, he and his colleagues turned to nearly 1,000 chicken bones from archaeological digs across Europe and Asia, some of which were as old as 2,000 years. DNA fragments from these remains were mapped onto the genome of today’s viruses. This approach led to the identification of MDV infection in chickens as early as the tenth century.

However, as the authors pieced together the ancient MDV genomes, they saw that the genes were organized very similarly to those of modern strains. This suggests that the increased toxicity was not caused by major structural changes, but by point mutations. These are genetic changes that affect only one or two bases in the DNA sequence of a gene. Taking a closer look at the genome, scientists found that a series of individual mutations had acted as a ratchet to increase virulence over the past century. In particular, there were changes in the sequence of a gene called mek, which has an essential role in tumor formation.

This finding suggests that ancient MDV may not have been capable of causing tumors. To test this hypothesis, Dr. Fidaman conducted a daring experiment. They synthesized the ancient form of the mek gene and inserted it into living chicken cells. It did not activate any genes associated with tumor formation. In comparison, a modern mek gene quickly showed the ability to cause tumors.

Over the past century humans have made chickens the most common birds on the planet (now numbering 35 billion), which has accelerated the evolution of viruses. It is unusual that MDV has evolved towards greater lethality; The virus will die out if the birds die before they can infect others. But a combination of overcrowding and the use of “leaky” vaccines, which ease symptoms but do not prevent transmission, have kept them alive long enough for deadly strains to flourish. By focusing on breeding more chickens, humans also managed to domesticate a killer virus, says Laurent Franz, a senior author of the paper. This sounds even scarier than reviving something old in a lab.

Combining archeology and modern genetic biology, the paper introduces pioneering methods into how and, more importantly, why any virus mutates. This could help scientists tackle other parasites that line the pockets of livestock farmers – for example by designing new vaccines – or even work on how to prevent another global pandemic.

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© 2023, The Economist Newspaper Limited. All rights reserved. From The Economist, published under license. Original content can be found at www.economist.com

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Published: Feb 13, 2024, 05:00 PM IST