Chang Evaluates Antibiotics Used to Treat Lyme Disease

Lyme disease persists as the most prevalent tick-borne disease afflicting horses, humans, dogs, cats, and cattle. The illness is caused by the corkscrew-shaped bacteria, Borrelia burgdorferi, and is characterized by nonspecific signs such as fever, stiffness, muscle pain, and swollen joints. Veterinarians often need to diagnose the disease by using a combination of medical history, clinical signs, risk of probable exposure, nondefinitive blood tests, and whether the signs respond to antibiotics.

When veterinarians suspect Lyme disease in a horse, they typically do prescribe an antibiotic.

"However, treatment is often prolonged, expensive, and carries some chance of toxicity," says veterinary microbiologist Yung-Fu Chang DVM, PhD.

"Although using these antibiotics is justified based on what we know about Lyme disease in other animals-such as the successful use of antibiotics for acute Lyme disease in humans-we know less about the efficacy of these medications in horses, especially those with chronic Lyme disease," he says. "Most importantly, we don't know which antibiotic would be most effective in treating the infection and its associated joint pain and swelling."

With a new grant from the Zweig fund, Chang will compare the effectiveness of two antibiotics: doxycycline and ceftiofur. To mimic infection in the wild, he will employ an equine Lyme disease model that he developed with a prior Zweig grant, which successfully infects horses with B. burgdorferi and causes arthritis and the other signs of a Lyme infection. First, Chang and his colleagues Thomas J. Divers DVM, Dipl ACVIM and Sean McDonough DVM, PhD will expose 10 six-month-old ponies to 40 B. burgdorferi-infected ticks for seven days on two separate occasions. Then for 12 weeks, the animals will be observed for signs of illness, such as depression, fever, stiffness, and lameness. In addition, the researchers are using blood tests and skin biopsies to monitor the infection.

At the end of the 12-week observation period, during which time he will have confirmed infection in the horses, Chang will treat half of them with either doxycycline or ceftiofur. The horses will then be observed for clinical signs and monitored with blood tests, bacterial culture, and other tests. After 10 months, Chang will euthanize two horses a week-one that was treated with an antibiotic and one horse that was infected but received no antibiotic-and examine and compare their tissues in detail to determine how the medications impacted the horses' condition. Chang and his colleagues will attempt to isolate the spirochetes from various tissues, such as the muscles, joints and connective tissues, nerves, brain, lymph nodes, and skin.

"This kind of information is very critical to both veterinarians and horse owners who want the most effective treatment for equine Lyme disease," says Chang, who has also worked on vaccines for Lyme disease in both horses and dogs. He is currently evaluating the least toxic pest control products on the market for controlling ticks in home gardens. In a previous study, Chang also looked at antibiotic treatment in dogs infected with Lyme disease.

Lyme disease is the most prevalent tick-borne disease not only in the country but in the world. Most common in the Northeast, the upper Midwest, Texas, and northern California, equine Lyme disease is transmitted via infected adult deer ticks, also known as "black-legged ticks," which emerge in the fall and early spring. Ticks pick up the bacteria as larvae and nymphs when they feed on infected mice.

Humans, horses, and other animals get the disease when infected tick nymphs or adult ticks feed and transmit the infection through the skin, particularly during the summer and fall.

"Deer are not a reservoir for Lyme disease, as previously thought," Chang says. "Rather, deer serve as the main definitive hosts for vector ticks and provide the blood meal that adult ticks require for reproduction during the winter season."

Until vaccines under development are approved for use in horses, owners can minimize infection by controlling tick populations through daily grooming and tick removal. Ticks may also be controlled with the use of tick repellents and by keeping pastures mowed, removing brush and woodpiles, and controlling rodent populations.

"Even if we find no differences between the two antibiotic therapies or if the treatments do not eliminate the organisms, the findings still will be useful toward designing intervention protocols," says Chang."