New Tick-Borne Pathogen Identified

Ticks, already infamous as disease carriers, get another black mark in tomorrow's New England Journal of Medicine. A paper there reports that ticks can infect people with a new, potentially fatal, disease. The culprit is a bacterium known as Ehrlichia ewingii, which so far was thought to infect only dogs.

The microbe is a relative of E. chaffeensis, which humans can also contract from ticks. Attacking its victim's white blood cells, E. chaffeensis causes a disease called ehrlichiosis, characterized by fever, malaise, body aches, and, if left untreated, organ failure. Ehrlichiosis was first reported in 1986; since then, hundreds of cases have popped up in the U.S., with the highest incidence in Missouri.

Because its flulike symptoms closely resemble those of many other diseases, ehrlichiosis is difficult to diagnose. Physicians can get an early clue from patients' white blood cell counts, says Gregory Storch, a pediatrics professor at Washington University in St. Louis, but to confirm their diagnoses, clinicians use the polymerase chain reaction to search the patient's blood for a ribosomal gene typical of several Ehrlichia bacteria.

Between 1994 and 1998, Storch's team ran this test, and a more specific one that detects only E. chaffeensis, on 413 people suspected of having ehrlichiosis. Sixty patients tested positive on the general test, but of those, only 56 were confirmed to have E. chaffeensis. Suspecting that the other four carried a new Ehrlichia species, the researchers sequenced the bacterial gene in their blood and compared it to a database containing the sequences of known infectious bacteria. To their surprise, the "new" species had already been identified--by veterinarians. Called E. ewingii, it commonly infects dogs that have been bitten by ticks. "This is the first time anyone saw this in humans," says Storch.

The symptoms of an E. ewingii infection seem indistinguishable from E. chaffeensis, but much else about the disease is unknown. "It could be rare or common," says Storch, "and it could be carried by deer ticks or Lone Star ticks." Three of the four patients already had a weakened immune system--which may have helped E. ewingii gain a foothold--but the fourth one was previously healthy.

"They have opened another chapter in the history of tick-borne pathogens," says physician Jesse Goodman of the University of Minnesota. The next step is to try to grow E. ewingii in the lab and perhaps learn how to prevent ehrlichiosis, he says. "We will continue to work on the medical aspects," says Storch, "maybe in collaboration with veterinarians."

Posted in Health, Biology