Isegoria

Dozens of rodents carry plague, Ed West notes, but it would only become deadly to humans when Yersinia pestis infected the flea of the black rat (Rattus rattus):

Black rats are sedentary homebodies and don’t like to move more than 200 metres from their nests; they especially like living near to humans, which is what makes them so much more dangerous than more adventurous rodents. Black rats have been our not-entirely-welcome companion for thousands of years, and were living near human settlements in the Near East from as far back as 3000 BC; the Romans and their roads helped them spread across the empire and brought them to Britain, the oldest rat remains here being found from the fourth century, underneath Fenchurch Street in London.

Black rats were especially comfortable in the typical medieval house, and while stone buildings became a feature of life in the 12th century, most were still made of wood and straw. In the words of historian Philip Ziegler, ‘The medieval house might have been built to specifications approved by a rodent council as eminently suitable for the rat’s enjoyment of a healthy and care-free life.’ This type of rat is also a very good climber, so could easily live in the thatched roofs which were common then.

Because of its preferred home, the black rat is also called the house rat or ship rat, while the brown rat prefers sewers. On top of this, the animals are fecund to a horrifying degree; one black rat couple can theoretically produce 329 million descendants in three years. So the typical medieval city had lots of rats, and with them came lots of fleas.

Fleas are nature’s great survivors. They can endure in all sorts of conditions, and some have developed the ability to live off bits of bread and only require blood for laying eggs. The black rat flea, called Xenopsylla cheopis, is also exceptionally hardy, able to survive between 6-12 months without a host, living in an abandoned nest or dung, although it is only active when the temperature is between 15-20 centigrade. As John Kelly wrote in The Great Mortality, the Oriental rat flea is ‘an extremely aggressive insect. It has been known to stick its mouth parts into the skin of a living caterpillar and suck out the caterpillar’s bodily fluids and innards’. What a world.

There are two types of flea: fur fleas and nest fleas, and only the former travels with its host rather than remaining in the nest. The rat flea is a fur flea, and while it prefers to stay on its animal of choice, they will jump on to other creatures if they’re nearby – unfortunately, in the 14th century that happened to be us. (In fact, they will attach themselves to most farmyard animals, and only the horse was left alone, because its odour repulses them, for some reason.)

As part of the great and disgusting chain of being, the rats inadvertently brought the plague to humans, but it wasn’t fun for the rats either, or the fleas for that matter. When the hungry flea bites the rat, the pestis triggers a mutation in the flea guts causing it to regurgitate the bacteria into the wound, so infecting the rat. (Yes, it is all a bit disgusting). Y. pestis can be transmitted by 31 different flea species, but only in a rodent does the quantity of bacillus become large enough to block the fleas’s stomach.

The flea therefore feeds more aggressively as it dies of starvation, and its frantic feeding makes the host mammal more overrun with the bacterium. The fleas also multiply as the plague-carrying rat gets sick, so that while a black rat will carry about seven fleas on average, a dying rat will have between 100 and 150. Rats were infected with the disease far more intensely than humans, so that ‘the blood of plague-infected rats contains 500-1,000 times more bacteria per unit of measurement than the blood of plague-infected humans.’

When the disease is endemic to rodents it’s called ‘sylvatic plague’, and when it jumps to humans it’s called ‘bubonic’ plague. For Y pestis to spread, there will ideally be two populations of rodents living side by side: one must be resistant to the disease so that it can play host, and the other non-resistant so the bacteria can feed on it. There needs to be a rat epidemic to cause a human epidemic because it provides a ‘reservoir’ for the disease to survive. Robert Gottfried wrote: ‘Y pestis is able to live in the dark, moist environment of rodent burrows even after the rodents have been killed by the epizootic, or epidemic. Thus as a new rodent community replaces the old one, the plague chain can be revived’. The rat colony will all be dead within two weeks of infection and then the fleas start attacking humans.

The first human cases would typically appear 16-23 days after the plague had arrived in a rat colony, with the first deaths taking place after about 20-28 days. It takes 3-5 days after infection for signs of the disease to appear in humans, and a similar time frame before the victim died. Somewhere between 20-40 per cent of infected people survived, and would thereafter mostly be immune.

What happened next would have been terrifying. ‘From the bite site, the contagion drains to a lymph node that consequently swells to form a painful bubo,’ or swelling lump, ‘most often in the groin, on the thigh, in an armpit or on the neck. Hence the name bubonic plague.’

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