FRIDAY, April 4 (HealthDay News) -- One injection of a long-acting version of the antibiotic doxycycline appears to protect mice from developing the tick-borne illnesses Lyme disease or anaplasmosis, new animal research reveals.

The finding -- not yet replicated in people -- raises hope for developing a safer and more effective way to combat transmission of both diseases among humans.

"We're the first to show that you can use a sustained-release formulation of this antibiotic to completely inhibit both infections when transmitted simultaneously by ticks," said study author Dr. Nordin Zeidner, chief of the U.S. Centers for Disease Control and Prevention's Vector-Host Laboratory, in Fort Collins, Colo.

Zeidner stressed, however, that the treatment shouldn't be thought of as a vaccine for either disease but rather as a potentially novel method to inhibit infection following exposure.

"But this is, nevertheless, an important proof of concept," he added, "because we know that a lot of ticks infected with Lyme disease also carry this co-infection."

The study was published in the April issue of the Journal of Medical Microbiology.

Lyme disease is the most common tick-borne illness in the United States, with approximately 20,000 new cases diagnosed in 2006, according to the CDC. It's transmitted by blacklegged ticks infected with the bacterium Borrelia burgdorferi. The ticks are common in the upper Midwest and Northeastern regions of the United States and can also carry other diseases such as human granulocytic anaplasmosis, posing a risk for combined infections.

People infected with Lyme disease can experience flu-like fever, weakness, headache, fatigue, and skin rashes. If left untreated, the disease can spread to the joints, as well as to the heart and the nervous system. Symptoms of anaplasmosis can include fever, headache, lethargy, rashes and gastrointestinal problems, according to the CDC.

Efforts to develop a vaccine for either Lyme disease or anaplasmosis haven't met with much success. So, doctors stress prevention measures when outdoors, such as the use of insect repellants with DEET and covering up with clothing.

As for treatment, the CDC suggests that two to four weeks of repeated oral antibiotics -- such as doxycyline, amoxicillin, and cefuroxime axetil -- can effectively treat most patients, particularly when the infection is caught early.

But, some doctors and patient-advocacy groups have argued that chronic infections may require a much longer antibiotic regimen -- despite treatment guidelines issued in 2006 by the Infectious Disease Society of America that warned that long-term antibiotic use raises the risk for drug resistance and medical complications.

Searching for a way to address such concerns, Zeidner and his colleagues focused on the potential benefits of a single dose sustained-release version of the antibiotic doxycycline. They noted that a standard single oral dose of the drug is quickly cleared from the body (about eight hours) -- requiring continuous and repeated use. By contrast, the sustained-release version is injected and continues to circulate for approximately 19 days after delivery.

In their mice study, the researchers exposed 6-week-old female mice to ticks infected with both Lyme disease and anaplasmosis. Three days after infection, some mice were randomly assigned to receive a single dose of either oral or sustained-release injected doxycycline, while others were given just water or a non-antibiotic compound.

The results: 100 percent of the mice injected with the long-acting antibiotic were fully protected from developing either disease. Only 20 percent to 30 percent of the mice given the oral antibiotic were similarly protected, the researchers said.

"I want to emphasize, however, that this is an animal model of disease we're looking at, and this formulation is not ready to put in people tomorrow," Zeidner cautioned. "It will take some time before it's ready for the clinic."

Dr. Raphael B. Stricker, recent past president of the International Lyme and Associated Diseases Society, described the new research as "outstanding."

"This work is very interesting," he said, "because they've found a way to target both Lyme disease and another very significant disease that up until just four years ago wasn't even recognized. And since the current treatment could involve taking antibiotics for months at a time, a single shot like this -- giving long-term protection and maybe even treatment -- could certainly end up being preferable."

More information

For more on treating Lyme disease, visit the CDC.

SOURCES: Nordin Zeidner, DVM, Ph.D., chief, Vector-Host Laboratory, U.S. Centers for Disease Control and Prevention, Fort Collins, Colo.; Raphael B. Stricker, M.D., California Pacific Medical Center, San Francisco, and past president, International Lyme and Associated Diseases Society; April 2008 Journal of Medical Microbiology

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