A Johns Hopkins University scientist says he has identified the Lyme disease bacterium in two white-footed mice captured in Druid Hill Park.
Dr. Brian S. Schwartz, a medical epidemiologist, told a gathering of entomologists in Baltimore yesterday that, while the findings are still preliminary, "we think that's the first time Lyme disease has been found in an inner-city park."
The search for infected mammals in the park was prompted by a diagnosis of Lyme disease last year in an elephant keeper at the Baltimore Zoo, which is in the park.
Although the study doesn't prove the zoo worker caught the disease in the park, Schwartz said, the man reported he had not left the city in six months prior to his illness, and had to remove ticks from his body "daily" while on the job near the elephant display.
In any case, Schwartz said, the Lyme disease risk to zoo patrons and park-users is "very small," since most stick to paved walkways and open lawns.
He said 91 other zoo workers were tested for exposure to Lyme disease, and no one else was found to have been exposed.
Schwartz reported his still-unpublished findings to the 62nd annual meeting of the Eastern Branch of the Entomological Society of America, meeting at the Lord Baltimore Hotel.
Lyme disease is a bacterial infection transmitted by deer ticks. It is characterized by a skin rash near the site of the bite, fever, headache, soreness, fatigue, a stiff neck, joint pain and swelling.
Caught early, it can be treated with antibiotics before serious symptoms appear. Untreated, it can lead to arthritis, heart damage, paralysis or blindness.
First identified among children in Lyme, Conn., in 1975, Lyme disease has since been identified in a growing number of states, and the number of reported cases is growing.
Maryland last year reported 138 cases. It has now been identified in all Maryland counties, with the largest number of cases in counties surrounding Chesapeake Bay.
The Lyme disease bacterium is carried by the deer tick, which feeds primarily on white-footed mice and white-tail deer during the various stages of its life.
Because of where its hosts live, the tick and Lyme disease have been a problem mostly in rural areas, where hunters, hikers, outdoor workers and children are most likely to be exposed. Although urban residents have been diagnosed with the disease, most are thought to have contracted it during visits to the country.
Asked how the disease might have found its way to the park, which is deep inside the city and surrounded by heavily developed urban neighborhoods, Schwartz said deer have been known to wander down the Jones Falls valley to the park, and could have carried the tick into the city.
He said there are "two or three" deer living in Druid Hill Park, one of them within the zoo fence.
But birds, too, have been found carrying the deer tick. "Birds are hypothesized to be the reason for the geographical spread of the disease," Schwartz said, "and I would guess that birds is how it got there [to Druid Hill Park]."
Once present in the park, the ticks would need small rodents to feed on during early stages of their lives.
In his study, Schwartz captured 78 small mammals in the park, including mice, voles and shrews. Only six were found to have deer ticks, and only two of those tested positive for Lyme disease bacteria by the definitive polymerase chain reaction test.
"The fact that only two animals tested positive by the PCR test is what is underwhelming people," said Schwartz. He explained that the scientific community may want more convincing proof before it fully accepts his conclusions.
There is also the problem of the tiny number of deer in the park. The incidence of Lyme disease in the Northeastern U.S. elsewhere has been found to be linked to the deer population. Where the deer population is high, so is the incidence of Lyme disease.
With so few deer in Druid Hill Park, some scientists will ask, how could the deer tick survive?
It's possible that the disease is continuously reintroduced by birds carrying the ticks, Schwartz said.
But there is also a "raging debate" among scientists as to whether the deer tick really needs deer to feed on during its larger, adult stage, he said.
"The majority of people in the field think not," he said. The highly adaptable tick has been found on 50 different mammal species.