Got Blood? Deer Ticks (Ixodes scapularis) and Lyme Disease (Borrelia burgdorferi)Table of Contents:
Keywords: deer ticks, black legged ticks, Ixodes scapularis, Lyme disease, spirochete, Borrelia burgdorferi, babesiosis, ehrlichiosis, American dog tick, Lone star tick, Ixodes, Dermacentor variablis
Got blood? Then you’ve probably encountered, and undoubtedly detest, those pesky creatures we know as ticks. Of course, everyone hates ticks; even Aristotle referred to them as "disgusting parasitic animals." We abhor their unsavory appetites as obligate hemophagous ectoparasites, for female ticks must obtain a blood meal from an unwilling host to develop and nourish their eggs. Ticks are loathsome and unwelcome; they are the uninvited guests that stay for dinner and breakfast and then leave without a trace of gratitude, but only after a hearty lunch. They sip on us, our pets, and, in fact, on all manner of creatures furry, fuzzy, feathery and scaly. They are found on all continents and in a range of climates. Researchers have observed several hundred hard ticks feeding on the necks of hapless Adelie penguins near Palmer Station, Antarctica. Another describes a ticky situation with her dog, penned for two weeks in a Florida kennel, liberating thousands of brown dog ticks into her well-scrubbed bathroom upon his return.
It seems as though these invertebrate phlebotomists are everywhere we want to hike, play, work, and live. We can choose to ignore them, but they won’t away. However, if we choose to learn a little about their biological habits and life history… well, no, they’re still pretty repulsive. Yet, there are lessons to be learned from the ignoble tick. For example, their habits and their ability to dine on a host in a most tenacious manner, defeating host immune systems while imbibing precious blood and fluids, may provide us ultimately with biochemical tools to aid open heart surgery and to foil blood clotting. Their saliva is a pharmacopoeia of new and wondrous drugs and therefore, ticks, at least in the practical sense, are to be acknowledged.
In the taxonomic scheme of things, ticks are animals and, as classified according to the current, accepted hierarchy, are in a phylum noted for its invertebrate members with jointed appendages, the Arthropoda. Along with spiders, mites and scorpions, ticks make up an ostracized class known as the Arachnida. Here, two tick families (and a tiny third) create the order Acarina - the hard ticks of the family Ixodidae, of which there are about 650 species, and the soft ticks of the Argasidae, comprising about 170 species. The soft ticks have a life cycle somewhat distinct from their more chitinized ixodid cousins, are generally tropical, and are involved in this article only from the status of a distant relative.
Of the many ticks worldwide and the dozens of species in the United States, those found in Massachusetts include numerous ticks affecting primarily wild and domestic animals, and at least four medically important hard tick species. The deer tick, (Ixodes scapularis), the American dog tick, (Dermacentor variabilis), the brown dog tick, (Rhipicephalus sanguineus) and very recently, the lone star tick, (Amblyomma americanum) are within our borders, and certainly elsewhere in the United States. Each of these species may vector, or transmit, one or several human pathogens, and each species has generated considerable research attention. Additional and appropriate information about the latter three is located at several web sites, including the noteworthy web home from the University of Rhode Island, www.uri.edu/artsci/zool/ticklab/ticks.
In this article, a single tick species, I. scapularis, is featured. It is known both as the deer, or black legged tick (aka wood, aka bear tick) due to taxonomic confusion regarding its specific status. Note that the various names of I. scapularis are regionalized; the accepted common name, black-legged tick, may be replaced by the more popular ‘deer tick’, used throughout this text. The genus Ixodes is cosmopolitan, with about 250 species worldwide and about 35 in North America. Its species are important ecologically, economically and medically, but it could be argued that I. scapularis is preeminent, as it is a host of several human/animal pathogens. The deer tick carries a microbe, a type of bacterium known as a spirochete, that has generated a public wave of misinformation, confusion and contentious discourse.
Ticks can be a nuisance, and their tiny bite may produce a localized and swollen rash, but a more serious consequence is the potential transmittance of any of several disease-causing pathogens. A tick may serve as a vector for these causal agents into their unwitting host. In the US, there are at least nine tick-transmitted diseases - bacterial, viral, or protozoan in origin. These pathogens are responsible for human illnesses such as Rocky Mountain spotted fever, babesia, forms of ehrlichia, relapsing fever, tick paralysis and tularemia, but Lyme disease is the most prevalent arthropod borne disease in the United States. In the last decade, hundreds of thousands of cases have been diagnosed and nearly as many treated, with over three thousand confirmed cases in Massachusetts alone. This affliction is being acknowledged by clinicians and health care professionals, but is still misunderstood by the public and primary care practitioners. In fact, numerous web sites address this seeming rancorous polarity between researchers, clinicians and legislators; for example see www.lymenet.com for a regular update of advances and setbacks re the diagnosis of this disease.
Lyme disease, as a rash described as erythema migrans, was first noted as a clinical entity in 1909. The first American case was reported in Wisconsin in 1969 and the first recognized epidemic appeared in coastal Connecticut in 1975, surrounding the affluent town of Old Lyme, where 51 residents were found suffering from an arthritic-like ailment. The scientific scrutiny generated by this epidemic unraveled a knot of medical intrigue, and so Lyme disease, obscured for decades as a host of non-infective but debilitating symptoms, was named, and emerged as a microbial scourge. Today it is the primary arthropod-borne disease in the US, and the number of cases each year continues to escalate.
Lyme disease is triggered by the host invasion of a spiral bacterium known as Borrelia burgdorferi, isolated originally in 1982. The twenty or so identified ‘species’ of Borrelia are carried primarily by soft ticks, but B. burgdorferi is transmitted by several ticks in the Ixodes complex, including the western deer tick, I. pacificus in western US, and the deer tick to the east. Other hard tick species, such as the American dog or lone star tick, may harbor this bacterium, but have not been implicated, to date, as human Lyme disease vectors, due to physiological and immunological factors that obstruct propagation and migration of the spirochete from arthropod to host. Note, however, that other pathogens may be spread by these species, including tularemia and Rocky Mountain spotted fever.
Since the early 1970’s, the phenomenon of Lyme disease has generated an enormous amount of scientific and anecdotal information. For instance, the key words ‘deer tick’ into a Google search generates over 25,000 URL sites and ‘Lyme disease’ lists over 170,000. Most of these contain important, relevant information, but nestled in this infoweb is a reliable starting point, and the valuable reference, www.aldf.com, cyber home of the American Lyme Disease Foundation. As are appropriate, additional Lyme disease related links are sprinkled throughout the text.
In recent years, the deer tick seems to have increased its range throughout Massachusetts, as it is now found from the Berkshires to Cape Cod, but little baseline data actually exists to show geographical spread. Studies during the last decade provide evidence of the presence of ticks and Lyme disease in many counties, with endemic or ‘hot spots’ in the Berkshire range, on Cape Cod, Martha’s Vineyard, Nantucket, in the area surrounding the Quabbin reservoir, around Ipswich and in the Leominster/Haverhill region.
Likely, increased tick awareness, coupled with directed field surveillance has led to the rise in deer tick sightings. Other factors that may contribute to a crowded environment include the population explosion of the reproductive host, the white-tailed deer (Odocoileius virginianus), development and habitation of formerly wooded areas, moderate yearly temperatures and convergent lifestyles between hikers and ticks. An update of the status of the deer tick population may be found at the MA Department of Public Health’s site at www.state.ma.us/dph/cdc/epiimm2.htm. Quantification of tick populations has been conducted in this state, and is interpreted to provide an index of risk associated with the tick and pathogen each season. One such field surveillance project on Cape Cod and the islands of Martha’s Vineyard and Nantucket is described later.
The deer tick has a two-year life cycle that is actually an inverted life history, at least in areas with freezing winters, like Massachusetts (Ostfeld, 1997). Generally, up to three thousand eggs are deposited by each female tick, and six-legged larvae hatch in a bimodal distribution during spring, and later in summer. Note that two generations of deer ticks could hypothetically generate over 4.5 million new ticks from each female! This geometric population grows in response to severe mortality agents, primarily in the egg and larval stages. Larvae move little from their hatching point as they attempt to encounter a host within their very limited domain with a behavioral process known as questing. Camped near the ground, in the humid leaf litter, the miniscule larva sways its forelegs in a patient arc; each leg is studded with minute sensillae, or tiny sensory organs, capable of detecting movement, heat and CO2. A foraging rodent, bird or other mammal may pass by the outstretched legs, and the tick finds its prey, like a velcro patch.
Usually, the initial host is a small ground dwelling mammal, such as a chipmunk (Tamais striatus), a meadow vole (Microtus pennsylvanicus) or more typically, a white-footed mouse (Peromyscus leucopis). The deer tick has least 80 known hosts, including many mammals and avian species; birds are regarded as vagile hosts, capable of dispensing fed ticks many miles from an original site. After a single blood meal that may last five days, the larva drops off, molts to an eight-legged nymph and over winters in the leaf layer, usually within a wooded area. Nymphs become active in late April and early May and seek another host at ground level.
The deer tick’s host is usually a small mammal, but may be a bird or in more southern states, a lizard, the catholic tastes of the deer tick tuned to a vertebrate. After feeding for up to five days, the now replete nymph drops off, and molts for about 45 days to the adult stage. The adult tick seeks a larger mammal for its blood meal, and moves the quest higher into the underbrush. It appears that the white-tailed deer is the principal ‘reproductive’ host, and may carry and feed literally thousands of deer ticks. Ticks may mate on the deer; the females drop to the ground to renew the cycle, depositing eggs in autumn leaves, after which, they die. Males continue to locate females and mate. Hungry adults can remain active throughout the winter, resuming their quest on warmer winter days (above 45F) and may rendezvous with a white-tailed deer, a raccoon (Procyon lotor), a black bear (Ursus americanus), a red fox (Vulpes vulpes), or other host through the following spring. Again, eggs are deposited soon after the bloody banquet, giving rise to the summer, and larger, brood of larvae.
Within this arachnid life history is the enzootic cycle of disease maintenance (see Ginsberg, 1994 and references therein). The deer mouse serves as a competent reservoir for the spirochete; that is, the host mouse retains infection for the duration of its life. Subsequent uninfected tick larvae or nymphs become infected as they feed upon this host. Because the deer tick has a two-year life cycle, infected nymphs from the previous generation will appear in May and June, infecting young and old mice alike. The current larval tick population then becomes infected around July and August. Thus, the disease is maintained in a tick population even when mouse numbers are low during a season. Other small mammals — eastern cottontail rabbits (Sylvilagus floridanus), short-tailed shrews (Blarina brevicauda), woodchucks (Marmota monax) -- may provide a blood meal, but they are considered as less competent, or incompetent, hosts and do not brace the infectious cycle. In fact, an incompetent host feeding the adult tick may be regarded as ‘zooprophylactic’, a dead end for the disease cycle.
Once upon a host, a deer tick will settle, sometimes in minutes, and initiate the feeding sequence; larvae seem to find soft tissue around a mouse ear or underbelly as the preferred location, but may attach anywhere. Anyone with an outdoor cat has probably found tick nymphs around the feline eyelids; dogs will harbor ticks beneath their legs, on/in their ears, and wherever they cannot groom. Adult ticks move over the stiff hair of a deer and congregate on the neck, ears and belly; deer in endemic areas may carry thousands of ticks. On humans, ticks will usually move upward to an area of constricted clothing, like the ankle or waist, but any tissue will serve as their dining room. Usually the tick will crawl to a more hidden horizon — the back of a neck, knees, hairline, armpit, etc., and begin to feed. 
The sentry-like twin palps of the tick's mouthparts explore and affirm the epidermal barrier. The hungry tick will then anaesthetize the new wound and insert the mouthparts, the saw-like chelicerae rasping through tissue, the hypostome providing the conduit between tick and host, serving as anchor and straw. An epoxy fastens the tick to its meal port. Plasma is slowly pumped from host to parasite in the effort to consume vital red blood cells. At the onset of this blood meal, interesting microbiology begins to develop in an infected tick. Spirochetes in the midgut now multiply rapidly and move into the hemolymph, or blood of the tick. They migrate to the salivary glands and invade salivary cells, then moving in saliva from the tick into the host blood. Studies have indicated that transmission from tick to a human takes at least 24 hours (Piesman et al, 1987); it is generally accepted that infection may require between 24-48 hours. If the infected tick continues to feed, the probability of host infection increases.
Because ticks require several days to satiate, they must challenge the host immune system with saliva containing anti-hemostatic, anti-inflammatory, and immunosuppressive components in order to block activity of defensive cells such as neutrophiles, lymphocytes and T-cells. Ticks must unclot platelet coagulation and minimize inflammation, or the blood flow will cease and the meal will end prematurely. In a belletristic article, Cynthia Mills (1998) describes ticks as "miniature laboratories…with anticoagulants, and platelet-aggregation inhibitors, prostaglandins and latex-like cement." She speculates that many of these compounds may ultimately be identified and synthesized as biomedical breakthroughs. Indeed, researchers are eager to master the alchemy of unique ingredients manufactured by the ignominious tick. Imagine, for instance, a novel and transcendent aspirin, originating from tick spit, selected to thin blood, and thus saving the lives of many cardiac patients.
Getting back to practical biology, the tick is usually discovered before repletion (unless the person is hygiene-challenged) and is removed and discarded. Local swelling and subsequent inflammation may reveal the hidden diner, but its proper removal is necessary to prevent infection from mouthparts that remain within the wound site.
PreventionDuring the high-risk months of May through September, nymphs are present and seeking hosts, but adults may be active from October through April. The first step in prevention is to recognize that any trip into a wooded area, particularly in the endemic areas of Massachusetts, can have dire tick consequences, thus hiking during the risk periods should include repeated tick checks and a thorough examination at home. It is advisable to wear light-colored clothing, as ticks are more visible on a pale background.
Some careful hikers even tuck their cuffs into socks, preventing ticks from traveling on the skin, and first apply a tick barrier, or repellent. Many repellents contain n,n-diethyl-m-toluamide, known commonly as DEET. Never use a product with more than 30% of this compound as active ingredient. Apparently, DEET creates a vapor repellent to ticks for several hours. It is applied to the skin and many formulations contain sun block and soothing aloe, as well. Note that this material is not formulated for coating infants, nor should it be salved around the mouth or eyes. Application is most efficacious from the waist down, the zone of tick/human contact.
Another more lethal product, an acaricide or tick-killing compound, is a pyrethroid known as permethrin, and this material may be applied to clothing, not skin, for protection. Permethrin actually kills ticks that contact the treated material. It should not be used indiscriminately, but applied to work clothes to be washed apart from the usual laundry. In any case, be sure the label states that the material is to be used against ticks. Once home, clothes should be placed in the dryer at high heat for at least 10 minutes to kill any ticks before the soiled laundry is tossed into a hamper or corner. A hot shower following the outing is a good idea and may wash off ticks that have not attached.
Note that if a tick is found attached, remove it, circle the calendar and note the location and place where it was found, as this may aid a diagnostic decision regarding whether or not to treat. Removal of a tick may be accomplished with fine forceps or a tick removal tool. Grasp near to the point of attachment and pull straight back (do not twist) with a firm and focused motion. Do not apply vaseline, gasoline, kerosene, Listerine, nail polish remover or other substances to the attached tick. Do not touch it with a lit match! Do not crush it or scratch it out, as these techniques may force the bacteria from the tick’s gut or saliva into the wound site. Finally, apply antiseptic to the site; if the tick still seems to have imbedded mouthparts, see your doctor or clinician.
The syndrome of Lyme disease is complicated and controversial, and is given cursory attention here. Treatment and diagnosis are not defined clearly, but the mist surrounding this enigmatic disorder is dissipating, slowly. Essentially, there are three episodes of Lyme disease infection — early, disseminated, and chronic, complicated by the multiple expression of symptoms associated with Lyme disease, ‘the great masquerader’. 
In the early stage, a rash known as an erythema migrans can appear days to weeks after the initial infection, but appears in only about 60% of cases. This annular rash is described as dynamic and may spread considerably over several days. The hallmark rash appears as a ‘bullseye’ with a blanched central area and concentric reddish rings, but in fact, the rash can have many reddish shapes. Early stage symptoms are quite variable and may include swelling and aches in large joints (knees and elbows), headaches, and flu-like discomforts.
Dissemination or migration of the spirochete in the body can generate a range of symptoms, including secondary rashes (either singly or multiple), accompanied by joint pain, facial palsy and other nervous disorders, cardiac abnormalities, and muscle aches. Late dissemination may be chronic and disabling, but rarely, if ever, fatal. No attempt to describe treatment beyond antibiotics is presented here; the reader is advised to consult a professional and visit the comprehensive site of the Centers for Disease Control and Prevention (CDC) at www.cdc.gov/ncidod/dvbid/lymeinfo.html for a range of information from epidemiology to disease diagnosis. Another reliable site with substantial information is the American Academy of Physicians at www.acponline.org/lyme. If you have questions about symptoms of Lyme disease, get information and discuss it with your doctor.
A deer tick and Lyme disease prevention program, based on Cape Cod, Martha’s Vineyard and Nantucket, supported by the Massachusetts Department of Public Health (www.magnet.state.ma.us/dph/) has been funded since 1998. Goals of this program include the creation and provision of educational materials that describe the tick, its life history and Lyme disease prevention, and a field surveillance component that surveys over fifty sites during the high-risk period from May through August (Falco and Fish, 1992).
Readers may be interested in procuring a colorful, descriptive poster, an informative brochure with updated inserts, identification cards, and signs for golf courses, conservation areas or other public access ways. Contact the Barnstable County Department of Health and the Environment (www.capecod.net/bcdhe/) or BC public health nurse Rita Mitchell (www.rita@cape.com) for more information, and free samples. Education and reliable information about the deer tick and its helical passenger are paramount to reducing the incidence of exposure and infection.
The objective of the field surveillance project is to collect and document the abundance of deer tick nymphs within the study sites in Barnstable and Dukes counties and Nantucket. As noted, the nymph stage is responsible for perhaps 75% of infections. Study sites have been established in conservation areas in each town. Site selection criteria includes size, a deciduous canopy and public access. Sample values, as the number of nymph ticks per hour, are averaged in each town to provide a snapshot of risk. Ticks are collected and counted, placed in a humidified vial, (a tick humidor!) and transported to a cooperating laboratory. The level of spirochete infection in these collected ticks is then estimated by a visual technique known as direct immunofluorescence; simply, the ticks are dissected and treated with agents that render the spirochete visible under magnification.
The product of these two factors, the local tick abundance times the local infection, generates an entomological risk index, (ERI) of LD presence in a town each year (Mather et al. 1996). This index serves to determine the yearly infection and risk of sites within towns, and to generate a database for comparison following intervention procedures, such as acaricide treatment or the widening of trails.
The surveillance process is simple and repeatable. Field personnel visit a site every two weeks, beginning in May, and sample the nymph tick population by dragging a white flannel flag (0.5 m2) through the wooded understory for twenty minutes. Checking the cloth and counting tick nymphs every thirty seconds results in a consistent and comparable estimate. A summary of this project, to date, is presented; details are published (Simser 1999, 2000).
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Comparison of tick population, local infection and resultant ERI from 1998-2000 on Cape Cod and the Islands.
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All fifteen towns on Cape Cod have the LD spirochete; on Martha’s Vineyard, the bacterium has been found in four towns, and on Nantucket has been confirmed at all three sites. Levels of infection within a town may vary considerably, relative to sampled ticks. One site may show no infection, while another town site several miles distant may have an infection rate above 50%.
It appears that the local infection within a site could be a function of host infection therein. That is, a preponderance of competent reservoir species, e.g. white-footed mice, could elevate the infection of ticks, while a diversity of less competent host species could reduce the local infection. Other confounding factors, such as unrelated tick or host mortality, density independent variables (weather), housing development and habitat destruction, etc., no doubt account for this disparity.
Comparison of the percentage of infection during the sampling regime reveals a generally uniform rate; for example, the current (2000) infection rate ranged from 6-18% in collected tick nymphs in a sixteen week period, with a seasonal average of 11%.
The tick population decreased substantially in 1999, relative to 1998, but has since increased. The spring of 1999 was parched, with the lowest recorded precipitation in 110 years, and it is possible that the drought was fatal to a segment of the overwintering tick population, including nymphs and adults. Curiously, the rate of infection also dropped sharply in collected ticks between 1998-1999, from 15% to about 5%, but has since increased to 11%. The surveillance project will continue in 2001 and results will be documented.
Ticks have existed for more than 300 million years and have traded a transient life for the parasitic way, this mode being a really intimate association with a stranger. Only the parasite benefits from this acquaintance, and the host must defend its resources, or be forced to share. Ticks have responded to the gamut of host defenses with a thorough complement of mechanical and biochemical countermeasures. With unlimited patience, fantastic numbers and an unwavering focus on food, ticks are here to stay, despite our contrary attentions. We must comprehend that a life clinging so tightly to its nature should be recognized -- albeit from a distance -- rather than maligned and eradicated. The deer tick, its complex life cycle so intrinsically interwoven with its many vertebrate hosts, symbolizes the natural landscape. Although we are quick to ask, "What good are ticks?" we are merely ignorant of a world that breathes sans humans. Try as we may with our chemical arsenals, our antibiotics and ointments, we ultimately cannot be selective in choosing to complement our existence with only benign or beneficial neighbors. Sooner or later we will encounter a hungry tick, its elongated forelegs reaching out, as if begging, "Got Blood?"
1. Falco R. and D. Fish. 1992. A Comparison of Methods for Sampling the Deer Tick, Ixodes dammini, in a Lyme Disease Endemic Area. Exp. And Appl. Acarol. 14: 165-173.
2. Ginsberg, H. S. (ed.) 1993. Ecology and Environmental Management of Lyme Disease. Rutgers University Press. New Brunswick, NJ.
3. Mills, C. 1998. Blood Feud. The Sciences. March/April 34-38.
4. Mather, T.N., M. C. Nicholson, E. F. Donnely, and B. T. Matyas. 1996. Entomological Index for Human Risk of Lyme Disease. American J. Epidemiology 144: 1066-1069.
5. Ostfeld, R. S. 1997. The Ecology of Lyme Disease Risk. American Scientist. 85: 338-346.
6. Piesman, J., T. N. Mather, R. J. Sinsky and A. Spielman. 1987. Duration of Tick Attachment and Borrelia burgdorferi Transmission. J. Clinical Micro. 25: 557-558.
7. Simser, D. H. 1999. Deer Tick Surveillance Project on Cape Cod, Martha’s Vineyard and Nantucket. Environment Cape Cod 2: 1-9.
8. Simser, D. H. 2000. Deer Tick (Ixodes scapularis) Surveillance Project on Cape Cod, Martha’s Vineyard and Nantucket: 2000. Environment Cape Cod 3: 9-19.
All photographs by D. Simser, ©2001. Photographs cannot be reproduced without permission of photographer