The Northeast Area-wide Tick Control Project (NEATCP) included studies of tick control. The U.S. Department of Agriculture exposed white-tailed deer to acaricide, which are chemicals that kill ticks. They bait the deer using 4-Poster devices, which safely attract and apply the acaricide to the deer. Ixodes scapularis is the deer tick that mostly targets the white-tailed deer. Nymphs of the tick cause Lyme disease. So the more nymphs there are, the more cases of Lyme disease. After six years of treating the deer, there were 71% less tick nymphs. Thus, the risk of getting Lyme disease in the five states studied has decreased by 71%.
The study aims to show the effectiveness of the 4-Poster device in reducing tick nymphs.
The NEATCP treated deer in Rhode Island, Connecticut, New York, New Jersey, and Maryland from 1997 to 2004. The acaricide they used was 2% amitraz. The nymph stage in a tick life cycle occurs after the larva stage and right before the adult stage. During this stage, ticks are most likely to bite deer and humans. Lyme disease is caused by the bacterium Borrelia spread by ticks. The disease can lead to skin rashes, fever, joint pain, among other symptoms.
In the first year of treatment, there was a variety in results from the acaricide treatment. All states showed a decrease in nymphs, indicating tick control. But, some sites in New York and Connecticut did not show control (Table 2). Overall, nymphs decreased by 22% compared to the control site the first year.
With each year of treatment, the nymph population continued to decline. The amount of population decline increased every year (Figure 1). Nymphs did not decrease as much in Connecticut and New York in the first couple of years. By the last treatment year, or year 6, there was a 71% total reduction in tick nymphs.
When acaricide treatment was removed in year 7, the positive effect wore off. While the population of ticks still decreased by 18%, the results varied greatly between sites. In Connecticut and New York, the tick population was not controlled anymore. Thus, the researchers cannot conclude if the acaricide treatment has any benefits once it is stopped.
The results in this study are consistent with results from past studies. The variety of results indicate that perhaps the acaricide was not effective in controlling nymphs during the first two years. However, all sites showed nymph control from years 3 to 6, which support that the acaricide was effective in the later years. But, the time until effectiveness varied based on the location of the sites.
The researchers find it surprising that so many nymphs were controlled the first year. They think that the acaricide targeted the nymphs and maybe adults. Or, the interaction between the nymphs and the deer may be affected. Also, there may have been different numbers of ticks to begin with in the treatment and control sites.
It is not possible to tell how the tick population may have changed if treatment was continued past six years. The results in this study could have been an underestimate. Or, at year 6, the maximum possible ticks that could be reduced was reached. Other animals, such as birds and raccoons, may also bring ticks into the community, which will affect the nymph control.
The proportion of ticks that were infected remained the same throughout the study. Thus, the reduction in the number of ticks definitely decreased the risk of Lyme disease.
The researchers sampled for tick nymphs in the five states. Sampling methods varied between states. So, researchers accounted for this by making some statistical adjustments. The researchers also accounted for the season. They divided the total nymphs by the percent of nymphs during that season on that date. They compared treatment sites to control sites, which had no treatment from 4-Poster devices. They combine various past studies to compile this meta-analysis.