Study: Killing Rodents Might Spread Plague
Oct. 20, 2000 -- To control a disease outbreak, it doesn’t always help to kill its agent, a new study has found.
In a newly-created population dynamics model, Matthew Keeling, a Cambridge University mathematician, has found that although rats can spread the bubonic plague to people, killing infected rats can actually make the outbreak worse. That’s because there is another creature involved in the infection chain.
“As the rats die, the fleas don’t have anything to feed on,” says Matthew Keeling, a Cambridge University mathematician who studies disease models and author of the study published this week in Nature.
Known from medieval times as the Black Death, the plague infects humans mainly through flea bites — not nips from the rodents they live on. Fleas live in the fur of infected rodents, usually rats, but also mice, squirrels, chipmunks, prairie dogs and others. When an outbreak occurs, people are often tempted to kill the rodents because they harbor infected fleas in their fur. But Keeling found that decreasing populations of rodents may actually trigger outbreaks.
Medieval and Modern
“If you do see human cases, you shouldn’t do the instinctive thing and go out and [kill] rats,” he says.
Studies show that plague outbreaks among people occur when rodent populations are low and hungry fleas seek warm blood from other sources. When that happens and human cases start to appear, killing more rodents only forces the fleas to seek other sources of sustenance.
Many people regard the plague as a medieval disease, known for killing about 25 million people in Europe, or a quarter of the population, in the 13th and 14th century. Historical accounts indicate the disease was probably first introduced when trading ships arrived in Mediterranean ports with dead or dying sailors, infected by the rats on board their vessels. The last pandemic happened in 1894-1904, originating in Asia and affecting ports on most continents. Millions of people died.
Modern cases of the plague are usually of the bubonic form of the disease, causing the lymph nodes in the groin, armpit and neck to swell. The swellings are called buboes, stemming from the Greek word for groin. Untreated, as the disease was in the Middle Ages, the plague eventually infects the internal organs and causes bleeding. Tiny blood vessels under the skin burst and turn black, giving the infection its medieval name, Black Death.
Pneumonic plague, another form of the disease, attacks the lungs of the infected person and can be spread from person to person by cough droplets and contaminated clothing. And septicemic plague infects the blood.
Thousands of plague infections are reported every year around the world, mostly in Asia and Africa. Infection can be cured with antibiotics if diagnosed early, according to the Centers for Disease Control. About 10-20 cases are diagnosed in the United States each year (see sidebar), but health officials say the plague doesn’t pose much of a threat to Americans.
The Spread of Infection
The bacteria that cause the plague, Yersinia pestis, occur naturally in rodent populations all over the world, and the rodents often die as a result. But up to five percent of rats can survive an infection of the plague, and their offspring are often resistant to infection and become carriers, Keeling says. His population models show the disease can circulate in relatively small populations of rodents — about 60,000 animals — without killing them off completely.
Keeling and his co-author Chris Gilligan say the population models also help explain a question that has long puzzled epidemiologists: how has the plague persisted throughout history, popping up after decades free from the disease?
In this case, it seems swelling populations of rats may have been our best buffer against infection.