Aging Hosts Seen to Ratchet up Viral Virulence

Older adults are more susceptible to viral diseases because their immune systems are not as robust as they were during their prime. Now, Agricultural Research Service (ARS)-funded scientists have found that a relatively mild, or benign, strain of a common virus became more virulent after passing through older animal hosts. The study was published in the September 12 print edition of the Proceedings of the National Academy of Sciences.

The study was led by Simin Nikbin Meydani, associate director of the Jean Mayer USDA Human Nutrition Research Center on Aging (HNCRA) at Tufts University in Boston, Mass. She is also director of the HNRCA's Nutritional Immunology Laboratory. The findings suggest a new way of thinking about why older hosts are more susceptible to viruses, according to the authors.

In the animal study, none of a group of younger adult mice infected with a mild strain of a common virus died, while 14 percent of older infected mice did.

The scientists then isolated and studied the mild virus from the infected older mice. Although the mild viral strain didn't affect young adult mice in previous tests, the scientists found that after it had cycled through an older mouse host, it killed 43 percent of other younger mice later infected and 71 percent of other older mice later infected.

Because the results indicated that the older host environment had allowed the virus to change to a more virulent strain, the scientists performed a DNA sequence analysis on the mild virus after isolating it from the older mice. They found that a DNA segment related to virulence had mutated along 13 nucleotides, enabling it to match that of a more virulent, disease-causing strain.

The scientists don’t know the mechanisms through which the viral strain mutated to mimic its virulent cousin in the older mouse hosts. They concluded that because of the world’s increasingly older population, the potential impact of age-associated viral evolution on public health warrants further investigation.

ARS is the U.S. Department of Agriculture's chief scientific research agency.