Richard Collins: Shared habitats may result in disease spillover

The year 1347 saw the beginning of one of history’s most notorious ‘spillovers’.

Twelve ships arrived in Sicily from ports in the Black Sea. Most of the sailors aboard were dead and those still alive had ugly black boils.

The disease responsible for this horror would go on to kill one third of the European population. Nor was it the first such catastrophe; eight centuries earlier, the Plague of Justinian had killed millions.

The pandemic was caused by a bacterium, Yersinia pestis, which originated in Asia. Rats are usually blamed for introducing it. Some of them had become immune to the disease but those in Europe, it’s claimed, were not.

When they succumbed to the disease, their fleas jumped onto people. But, if rats were responsible, how did it spread so rapidly? Other commentators claim that the Black Death was carried from person to person by human fleas and body lice.

Nor are all spillovers destructive, as the subsequent history of plague in Europe shows; brown rats, arriving from the Asia in the 18th Century, ousted the black rat in a rodent spillover, reducing the frequency of outbreaks.

Rats are notorious vectors of disease. So are bats, implicated in the Covid-19 pandemic. Both belong to highly successful families; there are over 60 rat species worldwide and a quarter of all mammals are bats.

Exploiting man-made environments, urban and rural, rats have vastly increased their numbers and range. With the destruction of their traditional habitats, bats now roost and breed in buildings.

Creatures living in large concentrations close to people present spillover opportunities to viruses. Poultry in close proximity to soldiers may have caused the 1918 pandemic, killing more people than had died in the war preceding it. The culprit, ‘an invisible antagonist’, was a flu virus.

Christine Johnson is lead author of a paper just published on spillovers of infectious disease in mammals.

Her team examined 142 human viruses, known to have come from animals.

Pets and livestock, she found, carry eight times as many such viruses as wild mammals, a result of close contact with domesticated creatures.

Species in severe decline, from hunting habitat destruction or capture for the wildlife trade, may carry twice as many animal-derived viruses than those declining for other reasons.

Wild creatures, such as rats and bats, living in human-dominated environments, share more viruses with us than those keeping their distance. The risk of spillover increases when habitats are damaged and wild creatures are forced to live closer to humans.

"Infectious diseased that originate from animals", the authors write, "are one of the greatest challenges facing public health".

Disease spillover is probably vastly under-reported, particularly in remote regions where people have limited access to healthcare.

The study, the researchers hope, "provides new evidence for assessing spillover risk from mammalian species".

It highlights "processes whereby the causes of wildlife population declines have facilitated the transmission of rogue viruses to humans".

We have behaved abominably towards other creatures. Now the chickens are coming home to roost. What goes around comes around!

• Christine Johnson et al. Global shifts in mammalian population trends reveal key predictors of virus spillover risk. Proceedings of the Royal Society B: Biological Sciences. 2020.