Rocky Mountain Spotted Fever Spread by Dog Ticks

NEW YORK (Reuters Health) - An unusual outbreak of the sometimes-fatal tick-borne infection Rocky Mountain spotted fever in eastern Arizona has been traced to a previously unrecognized carrier, the brown dog tick.

Of concern is that "the brown dog tick is actually widely distributed around the world," Dr. Linda J. Demma told Reuters Health. "It's usually found where there are dogs."

Writing in this week's New England Journal of Medicine, Demma from the Centers for Disease Control and Prevention in Atlanta and her colleagues also say the broad distribution of this common tick could lead to "the introduction of Rocky Mountain spotted fever into areas where it has not previously been recognized."

Rocky Mountain spotted fever is spread from ticks to humans. Its initial symptoms include fever, muscle pain and headache followed by a characteristic spotted rash. If recognized, it is treatable with the antibiotic doxycycline.

In their article, Demma's team notes that between 1981 and 2001, three cases of Rocky Mountain spotted fever were reported in the entire state of Arizona. From 2002 through 2004, sixteen cases were diagnosed in patients from rural eastern Arizona and three more cases were suspected.

These figures translate to an incidence rate of 1800 cases per million persons age 19 or younger, compared with the average national annual incidence of 5.6 cases per million.

The affected area is characterized by sparse, low vegetation, an arid environment and large populations of feral dogs.

Ecological surveillance revealed high densities of brown dog ticks in the area, but none of the kinds of ticks that are the usual carriers of the disease. DNA from the causative organism, Rickettsia rickettsii, was detected in three brown dog ticks.

"Physicians need to be aware of this outbreak because this type of ecology can occur in other places in the US where known tick vectors are not prevalent," Demma pointed out. "No matter where they are in the country, physicians should be aware that Rocky Mountain spotted fever can show up in their area even though it's not expected."

In a related Perspective article, two experts comment that the number of cases of Rocky Mountain spotted fever is probably under-reported. Also, in addition to a mortality rate of 5 to 10 percent, the illness can lead to amputation, deafness or permanent learning impairment.

"Far too few physicians consider a diagnosis of Rocky Mountain spotted fever or take the time to inquire about tick bites or exposures," they write, "critical information that can lead to the diagnosis and to lifesaving empirical therapy with doxycycline."

SOURCE: New England Journal of Medicine, August 11, 2005.