MILWAUKEE -- Scientists have discovered that a very common type of dog tick can spread Rocky Mountain spotted fever, an often-fatal illness that reached historic highs in the United States last year.
Two types of ticks already were known to transmit the disease, but they are not as common and are carried mostly by rodents and dogs that live near wild or rural areas.
This is the first time that a tick that routinely plagues house pets has been implicated.
The discovery was made through an investigation of Arizona's first outbreak, involving 16 cases and two deaths in the past few years.
Health officials said they do not want people to panic or think this will become a nationwide epidemic, because they have found these infected ticks only in Arizona. But the newly implicated tick lives everywhere in the world, and specialists have been stumped by many unexplained cases of the disease around the United States.
''We may have been missing this in the past," said Linda Demma, who led the study for the federal Centers for Disease Control and Prevention.
Dr. J. Stephen Dumler, a specialist on the disease at Johns Hopkins University, said, ''It's almost certainly occurring in other places and not diagnosed." Dumler wrote an editorial accompanying a report of the CDC study for today's issue of The New England Journal of Medicine.
Rocky Mountain spotted fever was first recognized a century ago in Idaho but has spread through much of the United States. More than half of cases are reported from the south-Atlantic areas -- Delaware, Maryland, Washington, D.C., Virginia, West Virginia, North Carolina, South Carolina, Georgia, and Florida. Oklahoma and North Carolina have reported the most.
The disease is caused by bacteria that infect ticks, which then bite and infect animals and people.
Symptoms occur five to 10 days later and can include fever, nausea, vomiting, muscle pain, lack of appetite, and severe headache -- signs often mistakenly attributed to common viral ailments.
Late symptoms include a spotted rash, abdominal pain, joint pain, and diarrhea.
Antibiotics, particularly doxycycline, are effective when given early. But fatality rates as high as 20 percent have been reported when cases are not recognized, and the disease is especially severe in children.
From a low point of 365 cases in 1998, cases have risen to 1,514 last year, but officials suggest that far more have gone unreported.
The CDC and Indian Health Service officials from Arizona and New Mexico investigated a cluster of cases in rural eastern Arizona from 2002 through 2004.
Blood and tissue samples confirmed that 11 people had the disease; five more were called probable cases. Most were under age 12, and two died.
Researchers found infected common brown dog ticks in all of the victims' yards.
Ticks turned up in the cracks of stucco walls inside homes, in crawl spaces underneath them, and on furniture that children played on outside.
The investigators have since found three other people they suspect had the disease in 2001 from the same area of Arizona.
Until now, the only ticks known to spread Rocky Mountain spotted fever were the less common American dog tick and the Rocky Mountain wood tick.
''No longer can we consider Rocky Mountain spotted fever a disease of only rural and Southern venues; it has emerged and reemerged again," Dumler wrote.
''The disease is in the midst of its third emergence since 1920, after peaks from 1939 to 1949 and again from 1974 to 1984," according to Dumler, who has compiled numbers from published accounts and cases reported to the CDC.