May 9, 2003

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COLLEGE STATION – An unusually mild fall and winter with adequate rainfall in many areas of Texas are adding up to an abundance of ticks and possible tick-borne diseases for people and their pets, said Dr. Pete Teel, Texas Agricultural Experiment Station entomologist.

Rocky Mountain Spotted Fever is the most severe and most frequently reported rickettsial illness for humans in the United States, said the Centers for Disease Control and Prevention.

The disease is caused by Rickettsia rickettsii, a species of bacteria that is spread to humans by ixodid (hard) ticks, the CDC said.

Initial symptoms of the disease are sudden onset of fever, headache and muscle pain, followed by development of a rash. The disease can be difficult to diagnose in the early stages, and without prompt and appropriate treatment it can be fatal, the CDC said.

The second most-reported tick-borne disease in humans is Lyme disease, named in 1977 when arthritis was observed in a cluster of children in and around Lyme, Conn., the CDC said. Other clinical symptoms and environmental conditions suggested this was an infectious disease probably transmitted by an arthropod. Further investigation revealed that Lyme disease is caused by the bacterium, Borrelia burgdorferi.

These bacteria are transmitted to humans by the bite of infected deer ticks and cause more than 16,000 infections in the United States each year.

Symptoms of Lyme disease are a characteristic "bull's-eye" rash, also called erythema migrans, accompanied by nonspecific symptoms such as fever, malaise, fatigue, headache, muscle aches (myalgia), and joint aches (arthralgia), the CDC said. The incubation period from infection to onset of erythema migrans is typically seven to 14 days but may be as short as three days and as long as 30 days.

Most people have heard of Rocky Mountain Spotted Fever and Lyme disease, but they probably haven't heard about the third – a disease called ehrlichiosis, Teel said.

According to Teel, there are two forms of human ehrlichiosis in the U.S. One is called human monocytic ehrlichiosis or HME, and the other is human granulocytic ehrlichiosis or HGE. They are transmitted by two different ticks.

While the number of cases of Lyme disease in Texas declined from 125 in 2001 to 55 in 2002 and the cases of Rocky Mountain Spotted Fever remained the same at three, the number of cases of human ehrlichiosis increased from none in 2001 to three in 2002, according to Texas Department of Health statistics.

Texans will be most exposed to the one called HME, he said. Originally discovered in an Arkansas patient in the late 1980s, state health departments, university researchers, and the CDC are now unraveling its mysteries.

Teel said the actual name of the causal agent is Ehrlichia chaffeensis, named for its discovery at a military base in Fort Chaffee, Ark.

Ehrlichia chaffeensis is believed to normally cycle between the Lone Star tick and white-tailed deer.

"However, when human beings are bitten by an infected tick, the tick will pass the organism along to humans," he said.

The disease can be life-threatening, but those cases are generally rare.

Since the symptoms are very similar to other diseases, it can be difficult to diagnose. Most tick-borne diseases are described in terms of the flu-like symptoms at their onset.

"You may run a fever, have muscle aches and pains, weakness, and generally there's a headache involved," Teel said.

"This disease produces no rash, but a person may experience some confusion, nausea and vomiting, and in some cases it causes joint pain."

The lack of a rash is important; while it helps to differentiate this disease from Rocky Mountain Spotted Fever, it does not necessarily discriminate from Lyme Disease as Lyme patients do not always present a rash.

The treatment for ehrlichiosis, like several other tick-borne diseases, is tetracycline antibiotics.

Early diagnosis – awareness on the individual's part about both ticks and tick-borne diseases and communication with the physician – is important to ensure early diagnosis and treatment, Teel said.