Black-Legged Ticks and Lyme Disease
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The ubiquity of black-legged or deer ticks (Ixodes scapularis) combined with their insidious, blood-sucking physiology evoke chimerical fears among hikers, campers and trail crews. They are virtually invisible, they can penetrate through several layers of clothing, and they can attach themselves to the skin with alacrity, potentially imparting a microbe that can lead to months if not years of illness. To offset their stealth advantage, it is necessary to understand their behavior, the Lyme Disease that they may carry, and prophylactic measures that may be taken to deter their predation.
The black-legged tick is a three-host parasite with a two year life cycle. Eggs that are laid in the first spring hatch to become six-legged larvae in summer, reaching their peak activity in August. The miniscule six-legged larvae attach primarily to small mammals such as the white-footed mouse and engorge with blood over several days. On satiation, they drop off and molt into the eight-legged nymph stage, to remain inactive through the first winter, emerging during the second spring. Nymphs become active in May and attach to medium size mammals such as raccoons, possums, and (alas) humans to feed for several days and then drop off as in the previous instar. Adult ticks seek a third and final host through the remainder of the second summer, peaking in late September. They typically ascend a grassy stalk to attach to the a large mammal host such as the white-tailed deer or the surrogate human. The female feeds for about a week while the male rarely feeds, seeking the deer host in search of a fecund female. After mating, the female drops to the ground and lays about 3,000 eggs under the leaf litter at the end of the second summer to complete the two year cycle. If the adult tick is unsuccessful in mating, it will remain dormant in the leaf litter until temperatures begin to rise above 40 degrees Fahrenheit the following spring to try again.
Lyme disease was known in Europe in the late 19th century by a number of different names and in the United States by its symptoms for about 100 years. However, it was not linked to the black-legged tick until the 1970's when residents of Lyme, Connecticut noticed an alarming outbreak of arthritis among young children. They enlisted the services of a rheumatologist from Yale named Allen Steere, who focused his epidemiological investigation not on the painful arthritic symptoms, but on the patients' recall of having gotten an unusual bull's eye shaped rash. This rash was known in Europe to be the symptom of a tick bite called erythema migrans. In 1977, one of his patients saved a tick that was subsequently identified as the black-legged tick. The true culprit was not isolated until five years later, when Willy Burgdorfer of the National Institutes of Health identified a spirochete (spiral-shaped) bacterium as the source, the eponymous species Borrelia burgdorferi establishing its provenance. Lyme Disease was declared by the CDC to be a nationally notifiable disease in 1991. The black-legged tick is not the source of Lyme disease, but a vector that transmits it from an infected animal to an uninfected animal. The infected animal is the white-footed mouse, the main food source for the larvae, which then molt to become infected nymphs. About 25 to 50 percent of adult Ixodes scapularis ticks are carriers of Lyme disease.
The prevention of tick bites is dependent on adequate prophylactic measures and most importantly careful scrutiny. Ticks become active with the warming weather in the spring and remain active until the onset of winter. They locate a host with sense organs on their front legs that detect carbon dioxide, odors and heat. They crawl along the ground and typically ascend several feet up the stalk of a plant to attach themselves to a host. They do not jump or fly, contrary to popular belief. Once on the human host, ticks crawl along the skin until they find a good spot to pierce the skin. Prophylactic measures such as wearing long pants tucked into boot tops help to impede the tick's access to the skin. Insect repellents such as “deet” (N,N, diethyl-meta-toluamide) can be applied to the skin or clothing to act as an additional deterrent . Field testing of deet in tick-infested areas has demonstrated it to be about 90% effective in preventing ticks from gaining access to the skin. The most aggressive anti-tick measure is to wear gaiters (upper shoe and sock covers) that have been treated liberally with permethrin, a chemical that kills ticks on contact. It is so effective that black-legged ticks are controlled by placing cardboard tubes filled with permethrin treated cotton in areas where the white-footed mice reside. The rodent vector mice use the cotton for their nests and concomitantly distribute the poison directly to their tick parasites. However, the only sure preventive measure is to carefully inspect the entire skin surface for ticks carefully after a hike or a day with a trail crew. Since it takes about 48 hours for an infected tick to transmit Lyme disease during the blood sucking , there is enough time for a thorough inspection everywhere on your body (yes, even there); use a partner or a mirror as appropriate. There are many folk remedies for removing ticks. Experimentation has demonstrated that the only effective method to remove ticks is to use tweezers applied as close as possible to the mouth parts and exerting a steady pull.
According to the literature, Lyme disease has many variable symptoms that are frequently placed in three categories: early localized, early disseminated and late disseminated. Early localized is indicated by flu-like symptoms such as fatigue, fever, and aching joints. The characteristic erythema migrans or bull's eye rash only occurs in about half of all cases so it is not a reliable indicator. Early disseminated occurs months later and is characterized by neurologic symptoms that may include severe headaches, cognitive problem and inflammation of the spinal chord. Late disseminated manifests over the course of years and may consist of severe arthritis, debilitating fatigue and chronic neurological problems. Having been infected twice, my personal experience is that the most telling symptom of early localized Lyme Disease is the overwhelming need for sleep even when thoroughly rested, especially during periods of prolonged physical activity like hiking. If you suspect you have Lyme disease you should consult a physician, who will likely order blood samples for the immunofluorescent antibody test (IFA) and the enzyme-linked immunosorbent assay (ELISA). These detect the presence of antibodies that your immune system has generated due to the B. burgdorferi spirochete. However, the immune system can take months to react and the first blood test will likely be negative (like mine were). Lyme disease tests are, in a word, unreliable. Accordingly a diagnosis will be made based on the symptoms and the activities of the victim and a 30 day treatment of either 100 mg of doxycycline twice a day or 500 mg of amoxicillin four times a day. It works, or I would surely be late disseminated by now and would not therefore be able to write this article.