Plague Hunters on Guard Against an Ancient Enemy
A man and a woman leaned over a picnic table in a sunny glade at Griffith Park, wearing gas masks and surgical gloves, and wielding syringes, spray cans of automotive starter fluid and toothbrushes. Five small animals shuffled back and forth in cage-like traps under a nearby tree.
The duo’s appearance and equipment were puzzling, almost comical, but their task was grim. They were preparing to test the captured animals for plague, then kill them to avoid the possibility of spreading the disease.
They are part of a six-person unit run by the Los Angeles County Department of Health Services that monitors “vector-borne” diseases, those carried from one organism to another by some intermediary--fleas, mosquitoes or the like. A large portion of their time is devoted to tracking plague.
This ancient scourge, the same disease that ravaged Europe in the Middle Ages as the “Black Death,” is alive and well and living in ground squirrels in the hills that ring Los Angeles.
Once a week, inspectors disperse into the hills, checking ground squirrel populations and capturing rodents to test their blood and the fleas in their fur for evidence of plague.
The first positive tests for plague in Los Angeles County this year came back from the laboratory last month, said Frank Hall, the senior biologist in charge of the surveillance program. The samples were taken from three squirrels captured in Vasa Park, a private picnic ground in Agoura.
The park and surrounding areas have been posted with warning signs and dusted with insecticide to kill the fleas that spread the disease. County agricultural officials plan to set out poison to reduce the squirrel population.
The bacteria that cause plague, Yersinia pestis , often exist quietly in the blood of burrowing rodents, which remain healthy, Hall said. But occasionally, for reasons that scientists cannot explain, the bacteria become virulent, causing a “die-off” or “epizootic,” the animal equivalent of a human epidemic.
The ground squirrel is what scientists call the reservoir, or host, for the plague bacteria. Some rat species are good plague hosts, too, but plague has not been seen in Los Angeles’ urban rat population for decades, Hall said. Fleas constitute the vector, the path by which the bacteria spread from animal to animal, or to man.
A colony of ground squirrels in a maze of burrows can be quickly decimated as the bacteria are carried from one squirrel to the next by fleas, he said.
Such a die-off is the time when the risk to humans can heighten, said Dr. Shirley Fannin, associate deputy director of disease control programs for the county.
“While ground squirrels stay alive, the fleas will stay on their bodies,” she said. “But if you get a die-off, then an area can be hopping with infected fleas. When some warm body walks by, they jump on and take a blood meal wherever they can.”
The warm body can belong to a person or a pet, she said. House pets that become infested with plague-carrying fleas can become a bridge, carrying the fleas into contact with humans.
Six Cases in U.S. This Year
Human cases are rare, according to Allan Barnes, director of the plague branch of the federal Centers for Disease Control. Only six cases have been reported this year in the United States; one of those was in Northern California. The last two cases of plague in Los Angeles County occurred in 1984.
Even so, Barnes said, as long as rodents, fleas, the plague bacteria and people exist in the same area, the risk of an epidemic is real. “There is tremendous potential if we let our guard down,” he said. “If we just let this squirrel population go, problems could arise.”
Plague has been monitored by the federal government since the turn of the century, when it was included on a list of potentially catastrophic diseases by an international body that later became the World Health Organization, Barnes said.
When the bacteria infect a human, the disease can take two forms--bubonic or pneumonic. Bubonic plague is characterized by swollen lymph nodes and a high fever. It develops slowly over a few days and is not very contagious.
Both forms of plague can be treated with two common antibiotics, but pneumonic plague, which attacks the lungs, progresses so swiftly that treatment frequently comes too late. “The mean time from start to death is 1.8 days,” Barnes said. Moreover, pneumonic plague can be spread by a cough or sneeze as easily as the common cold.
The full destructive potential of plague was last felt in the United States in 1924, he said, when an epidemic swept a small section of downtown Los Angeles and killed 33 people in three weeks.
“The situation was really ripe,” said Fannin, adding that there were huge populations of ground squirrels in the Chavez Ravine area, where Dodger Stadium now stands, and of rats in Chinatown.
“The disease went rapid-fire from one human to another,” she said, and was stopped when 10 square blocks were cordoned off by armed guards. Within a decade, Los Angeles had put together a major rat-control program, she said, and plague was not seen in the city again until the 1970s.
When it reappeared in Los Angeles County, plague came back not in the urban slums, but in communities next to the mountains, health officials said, places like Diamond Bar and Bradbury. Instead of slum-dwellers, its new victims were canyon residents and campers.
The county began its own plague surveillance, Fannin said. “We decided we’d rather not find out about plague after the fact.”
The goals of the program are to understand how the disease is distributed in ground squirrels and to control it. Members of the surveillance unit conduct surveys of squirrel populations, periodically visiting about 400 sites around the county.
The highest priority is given to areas that are shared by squirrels and people, particularly campgrounds and public parks, said county biologist Hall. The population is determined by counting burrows, counting squirrels that are above ground and estimating the number that might be out of sight, he said. A ranking is assigned: low, medium, or high. On subsequent visits, a significant drop in the squirrel population can thus be quickly noticed.
What the scientists do not want to find is evidence of a die-off in progress, Hall said. One indicator is abandoned burrows, which are sometimes covered with cobwebs. Flies buzzing around a burrow entrance also signal a die-off, he said.
At some sites, traps are set--wire-mesh containers the size of shoe boxes in which animals are caught alive, so that samples of blood and fleas can be taken from them.
Portable Lab at Park
On a typical trapping run recently, two county inspectors set up their portable laboratory on a picnic table in a quiet section of Griffith Park. They had captured five animals in traps set out the night before. Three were caught on the grounds of the Los Angeles Zoo, and two on the slopes above Victory Boulevard. There were three ground squirrels, a pack rat and an opossum.
The inspectors picked the isolated glade because the health department is concerned that the trapping, testing and killing of squirrels and other wild animals might incur the wrath of militant animal-rights groups, Hall said.
“It seems cruel,” he said, but if a blood sample from a tested animal later turned out to be positive for plague, and that animal had been released, the department might be held responsible for any consequences.
As they started the testing procedure, the reason behind each item in the inspectors’ odd armamentarium became clear. The spray cans of starter fluid, normally used to drive water from a damp carburetor, contained ether and thus made an economical anesthetic. All the animals remained unconscious throughout the procedure and were killed with an extra dose of ether.
The ether, along with the possibility of disease in the animals, made the gas masks a necessity.
The toothbrushes were used to brush fleas from the animals’ fur. The opossum produced the largest number; Hall said he once brushed 240 fleas from one opossum.
The fleas are counted and placed in tubes filled with a salt solution. Along with blood samples, the fleas are sent to the Plague Branch of the Centers for Disease Control, in Fort Collins, Colo.
There is often a six-week wait before results are returned, Hall said. The blood samples taken in Agoura that produced this year’s first positives were drawn in June, he said.
“The slow turnaround time has been a thorn in our side,” Fannin said. “We know about things four to six weeks after they happen, but we keep worrying that something is going to happen in that gap.”
She said the county is planning to run the plague tests in its Veterinary Services laboratory, rather than send all the samples to Colorado.
In areas where there are signs of infection--such as Agoura--inspectors have also gone door-to-door, asking pet owners to have dogs or cats tested. Pets and other animals brought to animal shelters around the area are also frequently tested, Hall said. In 1980, county tests pinpointed a resurgence of plague in the Topanga Canyon area, Hall said.