Ailments and Age Groups: What Makes Illness Age Dependent?

By Chris Healey

An uncommon and underreported virus has affected children in states across America. State health departments around the country have reported an unusual number of enterovirus D68 infections this season. Many hospitalizations – but no deaths – have been reported.

Enterovirus D68 was first isolated in California in 1962 from four children with pneumonia. Enteroviruses generally inflict a wide range of symptoms, but species D68 almost exclusively affects the respiratory system. D68 also shares genetic similarity with rhinoviruses—the viral species responsible for the common cold.

Past outbreaks of enterovirus D68 have occurred mostly in children. Although health experts aren’t sure why children are vulnerable to the illness relative to other age groups, the answer probably lies with the immune system. Age effects immune function. In prepubescence, the immune system is immature and naïve toward host threats. In old age, deterioration of essential immune system tissues – such as bone marrow – contribute to immune system decline.

Due to dampened immunity in childhood and late adulthood, illness is more common – and more often fatal. However, one historical exception stands out: the Spanish Flu of 1918.

Flu subtypes undergo antigenic drift, a process resulting in subtle genetic changes prompting the need for new flu vaccines each year. However, the Spanish Flu of 1918 was a result of dramatic genetic change called antigenic shift. The result was a new subtype to which the population had no immunity.

Many health experts consider the Spanish Flu of 1918 the worst pandemic in history – with at least 40 million deaths worldwide. By comparison, the Black Death was responsible for 25 million deaths. The Spanish Flu pandemic was caused by a direct transmission of influenza subtype H1N1 from bird to human.

The Spanish Flu of 1918 was unique because of its W-shaped mortality curve. When Spanish Flu mortality among age groups are plotted on an x-y axis – with x as age groups and y as specific death rate – the graph shows there were more deaths among the 18-to-40-age group than any other. That trend is unusual – 18-to-40 age groups typically have the highest immune function of all age groups, providing the greatest defense against pathogens.
For the Spanish Flu, the immune system actually worked against the host. The immune system reacted so violently to the novel Spanish Flu that it damaged the host more than the flu infection itself, leaving those with the strongest immune systems – ages 18-to-40 – most grievously affected.

 

Image Credit: Fox6Now

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