Wednesday, Aug. 15, 2007 | Turns out the bacteria that caused the Black Death is still around, but it isn’t so fatal these days.

The pestilence that wreaked havoc throughout Eurasia in the Middle Ages, leaving putrid piles of festering corpses in its wake, has maintained a small but sustained presence in the United States, particularly the Southwest because of its climate and mountains. About five to 15 human cases of the plague are reported in the country each year, 14 percent of which are fatal, according to the Center for Disease Control and Prevention. County officials said there have been no cases of locally acquired plague reported in San Diego County.

The Bubonic Plague

  • The Issue: Six ground squirrels on Palomar Mountain tested positive for the bubonic plague last week, bringing the total number of plague-infected squirrels at the Doane Valley Campground to seven for the summer.
  • What It Means: Campgoers are advised to steer clear of squirrel burrows to avoid getting bitten and infected by plague-carrying fleas.
  • The Bigger Picture: Five to 15 cases of plague are reported yearly in the United States in humans, mainly in the Southwest, where the plague has maintained a presence for more than a century.

Rodents in Colorado, Arizona, Utah and Los Angeles County have been wiped out by the plague this summer. The plague made local headlines last week when six ground squirrels at the Doane Valley Campground on Palomar Mountain tested positive for it. Another squirrel in the same location tested positive in July. But the chance of a human contracting the disease is slight, officials said. The plague is transmitted by infected fleas, and the squirrels tested were flea-free.

“We’re not saying, though, that plague in the United States will cause another Black Death,” said David Engelthaler, director of programs and operations for TGen North, a pathogen genomics laboratory in Flagstaff, Ariz. “Those were different times; people lived in different conditions, different sanitation. They didn’t understand germ theory. They didn’t know how to treat disease.”

Now that people don’t live in such squalid conditions, where human fleas run rampant, the plague is a much less fearsome disease.

The bubonic plague, the most common of three forms the plague can assume and the type found in the squirrels, is now highly treatable through the use of antibiotics. It has a much longer gestation period and infects the lymph nodes in the neck, armpit or groin, inflating them into large, painful cysts called buboes. The other two forms, septicaemic, which causes bleeding in the skin and other organs, and pneumatic, which infects the lungs and can be spread through water droplets projected by coughing, are more fatal.

When a flea is infected with bubonic plague, the bacteria, called Yersinia pestis, forms a plug in its stomach, forcing it to starve. The flea will continue to bite into new hosts in attempts to satiate its hunger, but instead vomits the plague-tainted blood into the wound, infecting the host. The pests favor rodents like the prairie dogs, field mice and ground squirrels like those affected this summer.

Fleas are specific to their hosts, so it is unusual that a rodent flea would bite a dog or a human. “If you’re dealing with dead ground squirrels that have some fleas on them and they’re looking for another blood meal and those fleas are infectious, they could potentially bite a human,” Engelthaler said.

He said there have been a few cases where cats have gotten infected with pneumatic plague and transmitted it to their owners. “The cat can develop the plague pneumonia and cough in their owner’s face and transmit it that way.”

When the infected fleas do jump to humans and bite them, people will experience fever, chills and lymph node swelling, said Francesca Torriani, a doctor specializing in infectious diseases at the University California, San Diego, Medical Center. Though these symptoms can also accompany the flu or other ailments, when the person has recently been in a place where the plague thrives, the person should seek immediate medical attention, she said.

But unlike centuries ago, one case isn’t likely to spread disease on a massive scale. Though the plague has a dark connotation, most cases are bubonic, which isn’t transmitted through person-to-person contact, Torriani said.

Despite the unlikelihood of transmission, the plague has been used since the Middle Ages and during World War II and the Cold War as a potential biological weapon, Engelthaler said. Japan reportedly infected fleas and let them loose into domestic rodent populations in China during World War II. The former Soviet Union developed an enormous stockpile of biological weapons using the plague.

“Plague does not easily survive in environment like an anthrax might; it’s not easily spread,” he said. “It probably would be a very targeted weapon, probably would be aerosolized and breathed in.”

When identified early, the bubonic plague is highly treatable with antibiotics that target the bacteria. As with anything, though, prevention is always the best medicine. That’s why it’s so important to take precautions in areas known to be home to the plague-infested squirrels, said Chris Conlan, supervising vector ecologist for the county’s Department of Environmental Health.

Signs are posted around Doane Valley Campground where the plague has been confirmed, advising campers to resist the urge to handle or feed wild or dead animals, watch for rodent burrows before setting up camp and keeping household pets at home if possible. If campers do bring pets, they are advised to use flea control.

Conlan said his team tests squirrels around the county one to two times per week from April to October or November, the season squirrels are most likely to be above ground and active. On average, he said five or six squirrels will test positive for plague. “It’s a little out of the ordinary to find that many squirrels in one spot, so that definitely raised an eyebrow,” he said.

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