Lyme Disease and Tick Control
By Kirby C. Stafford III
Department of Forestry and Horticulture; The Connecticut Agricultural Experiment Station
123 Huntington St.; P.O. Box 1106; New Haven, CT 06504
Tick-borne diseases, particularly Lyme disease, have increased dramatically in recent years. This is largely because of reforestation and increases in the abundance of tick host animals. This summer has seen a lot of tick activity and I suspect that Connecticut will continue to have the highest rate of Lyme disease in the United States. There were 1,548 human cases of Lyme disease reported to the Connecticut Department of Public Health in 1995, 3,104 cases in 1996, and 2,297 cases in 1997. One study suggested that only 16% of the diagnosed cases in the state are actually reported. In other words, there may have been over 14,000 diagnosed cases in 1997, alone. Lyme disease is multi-system illness of people and animals caused by the spirochete, Borrelia burgdorferi, and transmitted by the bite of the blacklegged tick, Ixodes scapularis. This tick is commonly known as the deer tick. It is also the vector for two other, less familiar, pathogens, one which causes human babesiosis, and the other human granulocytic ehrlichiosis, or HGE for short.
Babesiosis is a malaria-like illness caused mainly by Babesia microti, a rodent protozoan parasite of red blood cells. Symptoms include fever, chills, headache, fatigue, muscle pain, and anemia. All ages can be affected, but it is most likely to be severe, even fatal in the elderly, immunocompromised, and people without functioning spleens. Dr. John F. Anderson and other scientists here at the Experiment Station first isolated Babesia microti from a white-footed mouse captured in 1988 in southeastern Connecticut, the same year the first human case was recognized in the state. There were 172 cases documented in Connecticut as of 1997 with 26, 48, and 31 cases reported for 1995, 1996, and 1997, respectively. Most of these cases still occur in the southeastern part of the state. Because infections can be mild or asymptomatic, the true incidence of babesiosis is unknown.
Human granulocytic ehrlichiosis was first recognized from cases in Wisconsin and Minnesota in 1994, although Dr. Magnarelli here at the Experiment Station has found evidence of infection in wild animals going back many years. Nonspecific symptoms for both HGE and the other human ehrlichia disease, human monocytic ehrlichiosis (HME) include fever, headache, muscle pain, nausea, vomiting, and malaise. Illness may be mild, moderate, severe, or even fatal. Like babesiosis, the number of cases increases with age. Surveillance for ehrlichiosis in most states is sparse. Connecticut is part of a special surveillance program for ehrlichiosis (and other emerging infectious diseases) that involve epidemiologists and scientists at the Centers for Disease Control and Prevention (CDC), Yale University, the Connecticut Agricultural Experiment Station, and the Connecticut Department of Public Health. In Connecticut from 1995-1997, there were 173 confirmed or suspected cases of ehrlichia reported, 37 cases in 1995, 40 cases in 1996, and 96 cases in 1997. These cases were reported from all eight Connecticut counties. Additional information on these and other tick-associated diseases is available from an Experiment Station fact sheet or at our web site.
I have been monitoring the abundance of Ixodes scapularis nymphs in the region of Lyme, Connecticut, since 1989. This graph illustrates the close association I found between the abundance of nymphal ticks infected with Lyme disease spirochetes and the incidence of Lyme disease in Connecticut each year for the period from 1989 through 1997. The number of cases per 100,000 population of Lyme disease was increasingly higher in 1992, 1994, and 1996 with slight reductions in 1993, 1995, and 1997. Similarly, the abundance of infected nymphs, and presumably the risk of infection, was up or down during the same years. Tick numbers were up in 1998 and nymphal ticks were active a week or so earlier than usual this year. The number of nymphal ticks received at the Experiment Station has increased compared with last year as well. While the increase in Lyme disease, babesiosis, and ehrlichia cases may be partly because of increased public awareness and reporting, much of the increase in tick-borne disease is because of the increase in abundance and geographic spread of the tick.
The importance of reducing the risk of tick bite has increased with the number of human cases of Lyme disease and the presence of other tick associated diseases. A human Lyme disease vaccine may soon be available that will help reduce the incidence this disease, but Lyme disease precautions should still be followed. Personal protection measures against tick bite (long pants in socks and repellents) and the prompt removal of attached ticks can be highly effective in preventing infection, but this approach may not be consistently followed around the home or on hot summer days. There are a number of tick control tactics I have examined for their potential to suppress tick populations. These are the reduction or exclusion of animal hosts that carry ticks, host-targeted acaricides, biological control, landscape modifications to reduce the suitability of the area for ticks, and the application of acaricides to high risk areas around the home. I am going to focus on the results of two projects. My first project, funded by the CDC, has evaluated landscape and chemical control measures around the home with the objective of finding the least toxic tick control alternatives. Requests to commercial pesticide applicators for tick control services has increased in recent years. The second project, funded by the CDC and USDA, is evaluating the control of ticks on white-tailed deer to obtain community-wide reductions in the tick population.
There were five pesticides and two landscape modifications, sometimes in combination, evaluated from 1995 through 1997 at homes in the towns of Lyme and Old Lyme. One application of the pesticides was made in early June each year with either a low-volume back-pack mist blower or a higher volume hydraulic sprayer. One application of the first two ornamental-turf pesticides, cyfluthrin (Tempo) and fluvalinate (Mavrik), was highly effective regardless of how they were applied with an average of 90% or better reduction in tick abundance most of the time. Neither insecticidal soap and pyrethrin, with an average reduction of 36.8%, nor pyrethrin and the synergist piperonyl butoxide, with an average reduction of 54.4%, provided good control as a low volume spray. However, the application of a combination of insecticidal soap, pyrethrin, and piperonyl butoxide by hydraulic sprayer in the 1995 trials produced an average of 97% control indicating a lower toxic tick control may be possible. Clearing leaf litter from the perimeter of the yards also reduced tick abundance in the targeted areas by almost half, on average, to nearly 90% at one home. The installation of a 3-foot wide wood-chip barrier at the lawn edge reduced the number of ticks on the lawn by roughly one-half, on average. The combination of certain landscape practices and the selective use of pesticides can reduce tick numbers substantially. This year, I am evaluating two granular pesticides and one organically based product containing pyrethrin, piperonyl butoxide, and diatomaceous earth. The diatomaceous earth acts as an abrasive and desiccating agent to kill by dehydration.
White-tailed deer are the preferred host for adult Ixodes scapularis and numerous studies have linked the number of deer with the abundance of this tick. In an earlier study, I found that the exclusion of deer from a relatively large area of about 15 acres with an electric deer fence reduced the abundance of larval ticks by 100%, the nymphal ticks by 84% and that of adult ticks by 74%. The USDA has patented a device for the topical application of pesticides to deer for the control of ticks feeding on the animals. This device is called a 4-poster or sometimes a feeder because of the paint rollers on each corner to apply the pesticide and the troughs containing corn to get deer to brush up against the rollers. Originally developed to control ticks on deer in south Texas, studies funded by the USDA and CDC were begun last year in Connecticut and other states in the northeast to determine if this technology could be used to provide community-wide control of the blacklegged tick. These 4-posters were established at two sites in Connecticut. There are 23 of the devices placed in a community in the town of Old Lyme and 5 of the 4-posters located at a privately owned, forested tract in Bridgeport, Connecticut.
The application of pesticide to the rollers was begun in late September or early October of 1997, was stopped December 19th, and resumed for 3 months in March 1998. The pesticide we use is Point-Guard (2% amitraz), which is currently registered to control mites, lice and ticks on hogs. Corn consumption was monitored and usage by the deer was estimated by the use of a marking agent on the rollers once each month. We found that deer quickly adapted to the devices and the response exceeded our expectations. In Old Lyme, corn consumption was greatest in November, averaging 336 pounds per feeder, although this ranged widely from 50-700 pounds. In Bridgeport, corn consumption dropped in October and November with increased acorn production. Daily corn consumption data for Bridgeport in the spring has not yet been tabulated. Corn consumption corresponded roughly with deer usage rates. In Old Lyme, usage was moderate initially but peaked at 100% in December and 90% in April. Usage in Bridgeport was only moderate through the fall, in part because of the mast, but reached 95% in April 1998. Treatment of the deer was stopped in May and will resume this September.
The goal of treating at least 90% of the deer was reached in the first year of study. I expect this level of use to continue when treatment resumes in the fall. We found that the coverage of individual deer by the amitraz needs to be increased. We are not reaching the ticks on the entire body of the deer. A computer model suggests that killing 95% of the ticks on 90% of the deer population should reduce the local tick population by around 90% by the fourth year of treatment. The length of time required to affect tick numbers is partly because of the 2-year life cycle of the tick. With some adjustments, we are optimistic that this approach will work. Tick populations have been monitored at the two treatment sites for several years and the number of tick collection sites was expanded this year. However, any measurable impact on the tick population may not be evident until next year.
In conclusion, Ixodes scapularis is the vector for at least three human pathogens. As long as deer remain abundant and this tick continues to spread, the incidence of disease will increase and additional tick-borne diseases may emerge to affect the citizens of Connecticut. The reduction of Lyme disease and other tick-associated diseases will require a multifaceted approach utilizing a variety of tick control measures as well as the use of an effective vaccine.