All about the Black Death flea that killed 100 million people

Oriental Rat Flea

(Xenopsylla cheopis)

SIZE: Up to 1/6 in (4 mm)

FAMILY: Pulicidae

HABITAT: Near rats, their primary food source

DISTRIBUTION: Worldwide, particularly tropical and subtropical climates, but some temperate zones as well

MEET THE RELATIVES: The cat flea, Ctenocephalides felis, is a relative, as is the dog flea C. can is—but in the United States, it is primarily the cat flea that preys on both cats and dogs. They are known to transmit tapeworms.

On an autumn day in 1907, two brothers in San Francisco found a dead rat in the cellar. Inspired by their father, an undertaker, they decided to find a coffin for the rat to give it a proper funeral.

When they ran home for dinner that night, the boys brought along a souvenir of their adventures—bloodthirsty fleas, starved for a meal after their rat host had died. Along with the fleas came a deadly disease—the plague.

The rat flea would prefer to leave humans, cats, dogs, and chickens alone, but when rat populations experience a massive die off—as they do during epidemics of the plague—the fleas turn to other warm-blooded creatures for their food. This is exactly what happened to those two unfortunate boys. Within a month, the plague had killed their parents but spared the boys, leaving them orphans.

This particular rat had died during an outbreak of the Black Death that began just after the turn of the twentieth century. A steamer called the Australia left Honolulu, Hawaii, and passed through San Francisco's Golden Gate with its load of passengers, mail, and plague-ridden rats. The rats made their way through the city, which, at that time, was not a clean place. Garbage piled up,

and makeshift sewers allowed germs and rodents to multiply. The rats felt right at home. Soon, a few people exhibited the dreaded symptoms of the plague—severe fever and chills, headaches and body pain, and telltale red lumps the size of boiled eggs in their armpits and groins. Before long, bleeding would give way to enormous black bruises, and death would not be far behind.

The flea's role in this dreaded disease had been discovered in the late 1800s, but the exact mechanism was still a mystery. It was not until 1914 that scientists realized that the gut of the flea held clues as to how it managed to spread the plague so swiftly and efficiently. They discovered a phenomenon called blocking, in which the plague bacteria build up in the gut of a flea to such an extent that the flea can barely swallow. Instead, it is only able to draw the host's blood into its esophagus, the part of the body that runs from its throat to its stomach. There the blood mingles with live plague bacteria. Unable to swallow because it is so full of plague itself, the flea regurgitates the blood and the bacteria back into the host's bloodstream. Flea vomit is the true culprit in a plague epidemic.

But that's not all. The fleas are so hungry because of their inability to digest a blood meal that they feed voraciously, moving from host to host in a desperate attempt to fill their bellies. Ultimately, the fleas die of starvation and exhaustion—if the plague itself doesn't kill them first.

The Oriental rat flea is just one of over eighty species of fleas that transmit the plague. The disease would have killed many more San Francisco residents during the so-called Barbary Plague except for one lucky fact: Oriental rat fleas were in the minority during this outbreak. The species most often found during the San Francisco plague were less likely to engage "blocking" and less likely to regurgitate plague bacteria.

The plague appears to have evolved from a more benign gastrointestinal bug about twenty thousand years ago. It has run its destructive course through human civilization several times, reputed to have killed more people than all wars combined in the course of human history. An African and European pandemic in the sixth century known as Justinian's plague killed about forty million people, which represented about a fifth of the world's population at that time. When it reappeared in Europe in the Middle Ages, the plague was called the Black Death. For two centuries, it ravaged Europe, killing another one-third to one-half of the continent's population.

Doctors at the time believed that the plague circulated in the air. They ordered patients to keep their windows closed and refrain from bathing, which they believed would expose the skin to the sickening air. Keeping the windows closed wouldn't stop the plague, but it might have stopped the stench of the dead and the dying. In large cities like London, there was no choice but to pile bodies in thinly covered mass graves. The rat population thrived in such a horrific mess. And because cats were believed to be witch companions in the Middle Ages, they were killed, nearly eliminating one of the rat's natural predators just when Europeans could have used the cats' hunting skills the most.

The plague then moved from China to India to the United States in the early twentieth century. Today, cases of the plague still occur from time to time in the American Southwest, but modern antibiotics can usually treat a case that is caught early.