They're largely invisible, these disease detectives, working without the recognition of their scientific peers who win prizes and publish articles in prestigious journals. And yet they often are the first line of defense to prevent the spread of disease lurking in food, water, air or blood.
While their searches often start with hunches, their training teaches them not to rely too much on this. Suspicions are to be tested through statistics, fieldwork and investigation.
Their task is to solve mysteries in which the answers often are uncertain. These sleuths often are left with a best guess about what befell a group of adults or children, at home at work or out on the town.
Here are three of their cases: *
Dr. Katherine Feldman was called in to solve a mysterious cluster of "rabbit fever" cases on Martha's Vineyard.
It was July 2000, and Feldman, who specializes in the study of bugs we catch from animals, called zoonoses, was soon on a plane. She was headed to the 13-mile-wide island off Massachusetts that swells with vacationers each summer.
The patients had contracted tularemia, a rare bacterial infection caused by a bacterium called Francisella tularensis . The illness gets its name from an infection found in ground squirrels in Tulare County, Calif., in 1911, and from the scientific work of Dr. Edward Francis. It usually infects hunters and others who handle carcasses of infected animals.
Just 100 to 200 cases are reported each year, most transmitted by the bites of infected ticks, which leave behind a characteristic ulcer and cause high fever and lymph node swelling. The infection usually passes, and most people recover.
But these patients had a more severe form of the illness, known as primary pulmonary tularemia.
Untreated, it kills as many as 60% of its victims. In such cases, the infection usually comes from exposure to animal urine or feces, airborne particles of which enter the lungs, resulting in pneumonia and high fever.
Feldman, an Epidemic Intelligence Service officer, is posted in Fort Collins, Colo., at the CDC's Division of Vector-Borne Diseases.
Her job is a perfect fit for her background: She loves computers and problem-solving, was trained as a veterinarian and has a fascination with public health.
By the time she landed on the island, all five patients had recovered, but it was important to determine why so many people turned up with this rare form of the disease.
"We worried about other people getting it," she said.
Feldman said she and her colleagues looked first to rabbits but knew that in other parts of the world, tularemia had been linked to muskrats, moles and beavers. So they didn't rule out other animals as the source of infection.
They tried to determine if there was some increase in the island's rabbit population and if there had been extra deaths among rabbits or other animals. Had there been unusual fevers in domestic cats?
"Nothing really came up."
Next, she interviewed the five patients, compiling detailed histories of everything they had done in the two weeks before getting sick. One patient, a 40-year-old landscaper, recalled running over a rabbit with a brush-cutter. But none of the other patients described similar encounters with animals.
Feldman visited their homes and the places they worked. She scoured hospital records for other cases of unexplained pneumonia that might be tularemia.
By month's end, having found no new cases and thinking the outbreak was over, she returned home.
As she was leaving, one fact stuck in her mind: There had been only one previously reported outbreak of primary pneumonic tularemia in the U.S. It was on Martha's Vineyard, in 1978.
"It was intriguing yet it was also frustrating," she said. "We weren't any closer to figuring out why it was occurring on Martha's Vineyard with a greater frequency than anywhere else."
A break in the case came in August: six new cases in men, including a previously healthy 43-year-old man who died. Four of the six had used lawn mowers; three of them were landscapers. The dead man was a part-time landscaper.
Only once before had rabbit fever been linked to lawn mower use. Two boys in Tennessee had become sick after running over a rabbit with a mower.
Feldman had a theory. Could the men have breathed in tiny infectious particles spewed into the air when lawn mowers or brush cutters came in contact with infected animals or their waste?
By late August her hunch was confirmed with more interviews among the first patients. As it turned out, three of the original five had used lawn mowers too.
How did she overlook that initially? Feldman explained that one of the men had run over a rabbit, so "we didn't pin that on using the lawn mower so much as having contact with a dead rabbit. Another man had been bitten by a tick, and we knew the third guy used a mower and had played golf and neither of those activities was especially suspicious early on."
Feldman returned to the island and compared the risk factors of the 11 people who developed pneumonic tularemia with 99 healthy residents of the island who were selected at random.
At the same time, the investigators donned white protective gear and respirators as they mowed their way across fields, collecting grass cuttings and air samples. They set animal traps, which yielded a skunk and a rat with evidence of infection.
By now, Feldman felt the lawn mower theory was looking stronger.
But in the end, as is often the case, the data settled the case. All of the infected adults had used a mower or brush cutter during the summer; slightly fewer than half of the healthy group had done so.