Slime, rhyme and snot in the time of COVID-19

Written by Nobel Laureate Professor Peter Doherty

Kids’ nursery rhymes can reflect all sorts of things, from the depths of history, to the obsession of small children with various body excretions and secretions. In the ‘acceptable in polite society category’, an easy one to relate to infection and immunity is:

Ring a ring o’ roses,
A pocket full of posies,
A’ tishoo, A’tishoo,
We all fall down.

The grimmest (and not necessarily most favoured) interpretation of this rhyme is that it refers to the sneezing and coughing of pneumonic plague (caused by the bacterium Yersinia pestis) accompanied by the skin reddening, or rose-colour of fever. Unlike the plague, which we treat now with antibiotics, children are at low risk of dying of COVID-19: a recent French study showing mortality rates of 0.001% for the under-20s compared with 0.7% across the population.

Other rhymes are clearly meant only for the ears of kids. They’re easy to find online, but don’t look close to a mealtime. The one that starts with ‘Yellow belly custard, Green snot pie’ goes on to be especially revolting. Like the Beatles, that rhyme is claimed by Liverpool. ‘Green snot’ could reflect infection with the bacterium Pseudomonas aeruginosa: ‘yellow belly custard’ pus from a boil carrying a lot of dead inflammatory cells. There are booger rhymes in English and Spanish. ‘Vulture vomit, camel snot, Deep dish boogers…’ comes from The Greasy Grimy Gopher Guts US boy scout song. Camel snot! The MERS-CoV is thought to have jumped into humans from camels that were infected by bats.

Apart from having fun and grossing-out adults, the kids are spot-on when they celebrate the glories of snot, boogers and the rest. They all start with slime, the mucus that is among the most important of our frontline defence molecules against infection.

From the nose, via the pharynx – we can also be mouth breathers – and larynx (voice box) to the big tube of the trachea to the smaller bronchi, the epithelial cells lining the respiratory tract are bathed in proteins called mucins, which are loaded with the attached carbohydrates (sugars) that give mucus its gel-like, slimy characteristic.

Mucus is secreted by specialised cells, like the goblet cells that are right at the surface of the respiratory tract epithelium. Others, the seromucus glands, are in the underlying connective tissue. Deep down the respiratory tree at the terminal bronchioles and alveoli, the mucus gives way to a very different mix of lipids (fats) and proteins called surfactant, which works to lower surface tensions and facilitate the O2/CO2 exchange that is basic to lung function.

We make 1.0 to 1.5 litres of mucus a day, so it can’t stay in the lungs. The mucus is swept towards the nose, larynx and pharynx by hair-like structures, the cilia, that beat on the surface of the epithelial cells. Optimally, incoming dust, pollens, bacteria and virus particles get stuck in this slime layer and are moved straight back out again. That retrograde flow also removes dead cells and other solid waste products that have been discarded into the lung lumen. If we are suffering a respiratory infection, the trash collection will include ‘used’ inflammatory cells, like neutrophils and monocytes that came into the lung from the blood, bits of damaged host cells and pathogenic invaders like bacteria or fungi.

Much of the mucus we make is swallowed but, for the remainder that is coughed or sneezed out, the more cell debris, the thicker and creamier the snot or the phlegm we view in a tissue or handkerchief. A red or brown colour means there’s some bleeding high up or lower down. Back in 1821, after coughing, the poet John Keats observed: ‘that drop of blood is my death-warrant; – I must die’, at age 25, from tuberculosis (TB). Now, children are to some extent protected against TB by the BCG vaccine that we’ll hear more about later and, while most TB cases can be cured with antibiotics, there is great concern about the emergence of multi-drug resistant strains of the causative Mycobacterium tuberculosis. And, while smokers may have black snot, discoloured mucus is not a feature of COVID-19.

Sneezing and coughing relatively clear mucus is what we usually experience when infected with viruses that stay localised to the upper-respiratory tract. Of the 100 or so different viruses that can cause symptoms of the common cold, several are β coronaviruses related to SARS-CoV-2. And, while a dry cough and sneezing can be experienced in the early stages of COVID-19 those symptoms are not, as we have found recently from the extensive, negative testing of nasal and throat swabs, in any sense characteristic.

Especially under low humidity conditions, dried out mucus and snot stick as ‘boogers’ in our nose. They aren’t of particular interest in COVID-19, except from the aspect of prevention. Vigorous hand washing with soap and water while reciting ‘Ring a ring O’ roses’ twice before touching the face, and especially the nose, remains one of the best mechanisms for avoiding this disease. Next week, we’ll look more at both mucus and the possible role of the nose in COVID-19.