Infections in Squirrels Remind That Victory Over Plague Isn’t Complete : County health workers issue warning after animals in two areas of Angeles National Forest are found to carry the bacteria.

The plague--the Black Death that ravaged the population of Europe in the Middle Ages and killed more than 100 people in early 20th-Century California--has lurked within the Angeles National Forest for as far back as anyone knows.

Every so often, it gets a little too close to us, invading our favorite campgrounds and contaminating the animals we see on our weekend sojourns.

The county health department again is warning that this is the time to be on alert: sylvatic plague, which in humans takes the form of bubonic or pneumonic plague, was discovered in two separate areas of the forest and is probably hiding in other spots.

The re-emergence of the infectious disease is a reminder of the potent killers that struck down people in another era. But it is also a testimony to the strength of modern medicine and the public health system.

“It’s a constant battle to keep this thing under control,” said Frank Hall, the chief of the county health department’s vector management program. “But if you think about the Dark Ages, we’ve come a long way.”

In the Middle Ages, people thought their best weapon against the so-called Black Death was prayer. Today’s antibiotics effectively kill the bacterium that causes the plague.

California has long been vulnerable to the disease, both because of its wildlife and its crowded urban centers. In 1924 in Los Angeles, near what is now Chinatown, 36 people contracted the plague within three weeks. Thirty-two died.

In turn-of-the-century San Francisco, health officials were afraid that acknowledging the presence of plague would close the city’s port, its main economic lifeline. So the disease smoldered for four years, killing more than 100 people.

Today, the Los Angeles County Department of Health Services regularly monitors the county for sylvatic--or animal--plague.

Southern California’s ubiquitous ground squirrels are a major host for the plague and the most common source of the disease in humans. Fleas that bite ground squirrels are the carriers.

When the squirrels sicken and die, the fleas need a new host; people or their pets are a fine substitute.

Once in a human host, the disease takes on the form of bubonic plague, characterized by flu-like symptoms and swelling of the lymph nodes. If untreated, the disease spreads to the lungs--pneumonic plague--and then can be transmitted through coughs and sneezes.

To get the right treatment, people who come down with flu-like symptoms after a camping trip or other contact with the wild need to tell their doctors.

That’s how the most recent human cases in Los Angeles County had happy endings. In 1984, two visitors to the Angeles National Forest came down with the disease. A third contracted it from a house pet that had been in the forest. All three were successfully treated after they told their doctors about their contact with the wild.

The forest has more than 200 recreational sites, from picnic areas to campgrounds, where humans commonly come into contact with animals. Because not all can be tested for presence of plague, health workers have devised a system for measuring likelihood of the disease by monitoring squirrel and flea populations.

Too many squirrels can increase the likelihood of plague because overpopulation wears down the squirrels’ resistance to disease. Too few--especially if there is a sudden die-off--can also spell trouble.

So the health department concentrates its efforts on keeping the squirrel population at a sustainable level.

When health workers find the disease--as they did this summer in squirrels in Switzer’s Picnic Area, north of La Canada Flintridge and at Camp Follows, north of Azusa--they dust the squirrels’ burrows with flea powder, which gives the animals a dose of pesticide every time they go in or out.

The next step is to use poison to rid the area of its sick squirrels and bring down the numbers of squirrels in general.