Q: Every three months, I give my dog a beef-flavored chew that kills any ticks that bite her. She has also been vaccinated against Lyme disease. Why don’t these options exist for people?
“It’s funny, in Lyme disease, animals have so many more options than humans do,” said Dr. Linden Hu, a professor of immunology at Tufts University School of Medicine. That includes several Lyme vaccines, as well as oral and topical tick-prevention medications.
Safety concerns and doubts about public acceptance have hindered the development of these types of drugs for people. But with rates of Lyme and other tick-borne illnesses increasing in recent years, researchers are exploring new (and old) options, and a few are now being tested in human clinical trials.
Vaccines
Between 1999 and 2002, there actually was a human vaccine for Lyme disease available in the United States. The drug, called Lymerix, was approved by the Food and Drug Administration in 1998 after clinical trials deemed it safe and effective for preventing infection with Lyme-causing bacteria. It was recommended for people between the ages of 15 and 70 who were living or working in areas where Lyme disease was common.
Shortly after people started receiving the shots, reports of side effects emerged, most notably symptoms of arthritis. Federal health officials “looked at it very carefully” and didn’t find evidence that the vaccine was unsafe, said Dr. Erol Fikrig, an infectious disease expert at the Yale School of Medicine, who was involved with developing the drug.
But the reputational damage had been done. Sales of the Lyme vaccine plummeted, and in 2002, GlaxoSmithKline, which manufactured the drug, pulled it from the market.
The Lyme vaccines that currently exist for dogs are similar to the one that was developed for humans. Both vaccines work primarily by prompting the immune system to create antibodies to a protein called OspA, which is produced by the Lyme-causing bacteria that are transmitted through tick saliva. When a tick ingests a dog’s blood, the antibodies kill the bacteria residing in the tick’s gut, preventing them from causing an infection.
In recent years, the pharmaceutical companies Pfizer, Valneva and Moderna have developed two new vaccines for humans that target the same OspA protein, though other aspects of the shots have been changed from the original version, Dr. Fikrig said. Both vaccines are currently being tested in clinical trials, and results are expected in a few years.
Topical and Oral Drugs
The other main prevention methods for pet owners are topical and oral treatments. These drugs, called acaricides, get distributed through the animal’s body after they’re swallowed or applied to the skin and kill ticks, along with fleas and mites, when they bite. A major advantage is that they protect against multiple tick-borne infections, not just Lyme disease.
For many of these illnesses, a tick must remain latched onto its host for a day or two to cause an infection, said Dr. Janet Foley, a professor of medicine and epidemiology at the University of California, Davis School of Veterinary Medicine. “So as long as you can kill that tick within hours, you’ll abort the possibility of transmitting anything like Lyme disease.”
While both the oral and topical medications are toxic to ticks, they’ve largely been proven safe for dogs at the doses used. However, Dr. Hu said that to his knowledge, few thought of investigating whether humans could also use them as prophylactics against tick-borne infections until recently.
An acaricide’s “toxicity for ticks is far, far, far greater than its toxicity for mammals,” Dr. Foley said. But, she added, it’s understandable that some people may be concerned about taking a drug that suffuses the body with a toxin just in case they get bitten by a tick. “I think that there wouldn’t be much market for that for people,” Dr. Foley said, though that’s “pure speculation,” she added.
One small drug company, Tarsus Pharmaceuticals, is betting that there might be such a market. In collaboration with Dr. Hu, it is testing whether an oral acaricide used in some veterinary tick medicines, called lotilaner, is safe and effective in people. (The drug was also recently approved as an eye drop for humans, to treat eyelid inflammation caused by mites.)
“We actually are one of the few examples of bringing a medicine from the veterinary side to the human side,” said Dr. Bobak Azamian, the co-founder, chairman and chief executive of Tarsus Pharmaceuticals.
According to the company, early clinical trials of lotilaner — provided to people as a pill, not a beef-flavored chew — showed that it was about 90 percent effective at killing ticks that bit the participants both on the day they took the medication and 30 days later. No major safety concerns have surfaced during the tests, Dr. Azamian said.
Still, it will be several years, and several more rounds of clinical trials, before any of these medications might be considered for F.D.A. review. And even then, said Dr. Hu, who has also worked on the Moderna vaccine, “it’s always a wild card how people are going to feel about it.”