Gabriel Miller, Reuters – 3/3/2014
NEW YORK (Reuters Health) – Vaccinating mice can lower the proportion of ticks carrying the bacteria responsible for Lyme disease, according to a new study.
Deer ticks become infected with the Lyme disease spirochete Borrelia burgdorferi after feeding on infected mice. The new strategy targets mice as the pathogen’s “reservoir” and avoids the controversy surrounding human vaccination.
“The main novelty of this approach is the delivery method-an oral vaccine that can be distributed in a bait that is eaten by wildlife,” said Dr. Jean Tsao, associate professor in the Department of Fisheries and Wildlife at Michigan State University, in an email. Dr. Tsao was not involved with the study, but has done similar work targeting Lyme disease reservoir hosts.
“The entire purpose of (this) strategy…is precisely to circumvent vaccinating humans, given that human vaccination has fallen out of favor with the public at large in the last decade or so,” said Dr. Maria Gomes-Solecki, the study’s lead author, in an email to Reuters Health. “Using this method we can prevent (or) block transmission of the bacterium which causes disease at the source, in animals, before it reaches humans.”
A human Lyme vaccine was approved by the FDA in 1998 but was controversially withdrawn from the market in 2002 after claims were made by vaccine recipients that the compound caused arthritis.
In the new study, published online February 11 in the Journal of Infectious Diseases, four plots of land in New York were baited with oatmeal containing 200 mg of E. coli expressing a vaccine that prevents transmission of B. burgdorferi. The vaccine was deployed for three to five years, depending on the plot. Three control plots were also followed for the same time period.
After two years of feeding mice the vaccine-laced oatmeal, 23% fewer tick nymphs were infected with B. burgdorferi. After five years of treatment, 76% fewer tick nymphs were infected.
Because plots were treated for different lengths of time, the investigators were also able to see the cumulative effect the mice vaccine had on tick nymph infection rates.
“We were able to plot differences in the infection rate of the ticks as regards vaccination over time,” said Dr. Gomes-Solecki. “That is, the longer you treated the field sites with the reservoir-targeted vaccine, the sharper the decline in tick infection with B. burgdorferi.”
Dr. Gomes-Solecki said “there is very little research remaining to be done on” the vaccine. She said it is not likely that the reservoir-targeted vaccine’s effect on human Lyme disease incidence will be studied. Though not impossible, it would difficult because a control group would have to be knowingly exposed to ticks carrying B. burgdorferi.
One limitation of the study in terms of the vaccine’s effect on human Lyme disease rates, Dr. Tsao said, is that it “does not take into account the density of nymphs, which is the other important component of the environmental risk of a tick-borne disease.”
Thus, even if vaccination lowers the prevalence of infected nymphs, if the overall population of tick nymphs increases during the same time period, the actual risk of Lyme disease to humans may not change in lock step.