By Ann A. Kiessling, PhD.
The take home message from internationally known tick expert, Dr. Sam Telford, during his Tuesday evening talk at Bedford Town Hall, was there are three ways to control the population of the ticks that carry Lyme disease: (1) decrease the deer population, (2) reduce the deer population, and (3) control the deer population. “Our grand children will thank us if we begin deer control now.”
Dr. Telford illustrated the direct relationship between the increase in deer population and the increase in incidence of Lyme disease with maps from the U.S. Department of Agriculture. He cited experiments that reducing or eradicating deer from some New England islands markedly reduced the tick population and Lyme disease.
For the 2014 tick season in Bedford, however, Dr. Telford stressed the importance of personal safety measures, and highlighted the complex life cycle and extreme humidity requirements of the deer tick (“black-legged tick”, Ixodes scapularis). An arthropod, not an insect, I. scapularis, is an obligate blood-feeder and has a two year life-cycle. It is the adult tick that feeds on deer (or human) blood, the engorged female of which lays her thousands of eggs in leaf litter, sometime between October and the end of May, and then dies.
The eggs mature during a spring warming period and hatch in mid-July into 6-legged larvae; the larvae themselves are not infected with Borrelia burgdorferi, the causative agent of Lyme disease. The larvae attach themselves to a small animal, probably near their hatching location, consume a blood meal that will carry them to the next stage of development, and drop back into forest litter to winter over and molt into the next stage, the 8-legged nymph.
So tiny it is rarely seen, even engorged with blood, it is the nymph that is infected with B burgdorferi, perhaps during the larval feeding stage, and is probably the principle source of Lyme disease transmission. Nymphs emerge in May, June and July and feed on any passing mammal. Dr. Telford noted that reported cases of Lyme disease peak in July. The nymphs must go through one more molt to emerge as adults by fall, find another blood meal, this time on a larger host, such as deer or humans, and possibly pass on B burgdorferi if infected. The adult females then lay their thousands of eggs that over-winter, to hatch the following May and begin the cycle again.
Dr. Telford noted the value of the Tick Management Handbook prepared by the Connecticut Agricultural Experiment Station as a tick reference: https://www.ct.gov/caes/lib/caes/documents/publications/bulletins/b1010.pdf
Re-forestation of farm land has not only allowed the marked increase in the deer population, but also provides ideal, moist habitat for I. scapularis which requires humidity of greater than 85% to survive. Hence, reducing ticks near human populations involves reducing areas of moisture accumulation, e.g. leaf litter, unmowed grass, tree shade, etc. Walking paths should be cleared of leaf litter and at least 10’ wide to avoid tick contact. He noted that I scapularis can only tolerate a few minutes of humidity less than 85% before it dies — hence the clothes in the dryer suggestion.
Dr. Telford touted the value and safety of common insect repellents, such as DEET, and the tick killing ability of the permethrin family of insecticides, the commercial form of the natural insecticide pyrethrins in chrysanthemums. The practice of tucking pant legs into white socks, and doing a thorough body search in the shower, have protected him from Lyme disease for his 30 years of tick research.
Tick nymph bites frequently go unnoticed, so the first signs of Lyme disease may be fever and the characteristic “erythema migrans” (migrating red rash), that can be in the shape of a bulls eye, and represents the migration of B burgdorferi through the skin. The rash generally appears a few days after the tick bite, but probably before antibodies have developed against B burgdorferi, which is why tests for Lyme disease on the day the rash appears are generally negative. Fortunately, B burgdorferi is susceptible to several antibiotics, and a course of 2 to 3 weeks of antibiotic therapy is generally prescribed for patients presenting with fever, erythema migrans and a history of possible tick exposure.
In contrast, Dr. Telford pointed to a double-blind stuffy of 100 subjects in Israel (NEJM, 2006, https://www.nejm.org/doi/full/10.1056/NEJMoa053884) that demonstrated 100% avoidance of Lyme disease in subjects receiving a few doses of doxycycline prophylactically, before the onset of fever and rash, and if the tick had been attached fewer than 48 hours. This has become standard of care for people in areas endemic for Lyme disease, such as New England.
According to Heidi Porter, Bedford’s Director of Public Health, the state gathers Lyme statistics from the blood samples submitted to the Massachusetts State Lab for antibody testing. But because treatment decisions should be made before the patient actually tests positive for Lyme disease, the follow-up test is rarely performed and the disease is, therefore, markedly underreported. The only way to know the true incidence of Lyme disease in Bedford would be to do blood tests on every Bedford resident.
Dr. Telford was asked if it helps to save the tick and have it analyzed for Lyme disease. Historically, surveillance of the incidence of B burgdorferi in tick populations is on the order of 5% to 15%. Because B burgdorferi is very temperature sensitive, it doesn’t start to multiply inside the nymph/tick until 24 to 48 hours after the tick has attached itself to a warm mammal. Therefore, whether or not it is infected, transmission of B burgdorferi does not occur at initial contact. And because treatment should be started immediately, it should not wait for the test results of the tick.
Dr. Telford was asked if squeezing the tick while attempting to remove it would force B burgdorferi into the bitten area. He responded “No. The tick has a small valve that allows blood to enter, but not exit, through the mouth parts.” Attached ticks should be grabbed as close to the skin as possible and jerked out. If mouth parts remain behind in the skin, they do not increase the possibility of Lyme disease, and they will be rejected from the skin like a splinter.
Bottom line: enjoy our great outdoors, but be “tick-smart” — protect legs and ankles from tick passengers, use the repellent DEET and the pyrethrin family pesticides, and search carefully for ticks with every shower.
(Next week: Part Two of Tick Talk: more ticks, more tick-borne diseases and results from communities that control the deer population)