Wood Tick


A wood tick that lost part of one leg when I pulled it off.

Common Name: Wood Tick, American Dog Tick – The predominant species of tick in wooded areas in Eastern North America.

Scientific Name: Dermacentor variabilis – The genus name characterizes the behavior rather than the taxonomy of the tick; it is a skin (Latin derma) goader (Greek kentor meaning to stick, goad or prick). The species name is Latin for variable, probably a reference to its adaptability to a variety of habitats (and subsequent ubiquity).

Potpourri: Ticks are consummate parasites, so much so that the order to which they belong within the Arachnid Class of arthropods is named Parasitiformes. They live on blood and blood alone; their every action directed at its source, which includes mammals like humans. Because the life cycle of the tick involves three separate hosts for its sanguinary diet, it unwittingly transfers the fluids from one host to the other with not infrequently deleterious consequence. Ticks are second only to mosquitoes in the transmission of diseases to humans, an antibiosis that has profound effects on the health of those who traverse sylvan grasslands, most notably hikers.

The simplistic morphology of the tick belies a very complex physiology. Ticks are arthropods (jointed foot) like the insects with similar motile complexity; the legs have six segments: coxa, trochanter, femur, patella, tibia, and tarsus (the similarity to mammal appendages is evident in the names). The tarsi on the front legs are particularly important for ticks as they contain the Haller’s organ, a host- locator sensor; it is here that the scents and chemical markers are detected. Unlike the insects which have three body parts (head, thorax, abdomen), ticks have two parts: a “false head” called a capitulum which contains the complex mouth parts and the feeler-like palps; and a body which is comprised of a fusion of the thorax and the abdomen covered by a hard protective shell called the scutum. One of the reasons that the capitulum is called a false head is because it is; the eyes are located on the scutum on either side of an ornate marking. Wood ticks are also known as ornate-eyed ticks.

The life cycle of the tick, which extends for two to three years, is a marvel of evolutionary dynamics, requiring a sequence of coincidental encounters that are so unlikely as to seem implausible. An adult female tick lays thousands of eggs in one location, each hatching into a six-legged larva whose first and only order of business is to find a small mammal (rarely lizards). If successful, a peripety by no means assured, the larva feeds on the blood of the animal and drops off to molt over a two-week period into an eight-legged nymph, which must repeat the process on a (presumably but not necessarily) different mammal for a second blood meal to molt into an adult. As only adults can breed, both the male and the female must not only independently find a source of blood but must also find each other, with serendipity on the same host. Then and only then can the female lay her eggs after which she will expire, following on the heels of the male who succumbs after intercourse; quite the Greek tragedy. Their own mobility being limited by their miniscule legs (ticks do not jump or fly), a host blood source must happen by in close enough proximity for a direct encounter. Wood tick evolution favored light-sensitive eyes as a means of accentuating the crucial host encounter. In seeking light, they slowly and inexorably ascend the nearest grass stalk to its apex to reach out with the two foremost legs extended using the Haller’s organs to search for and ultimately attached to a host: this behavior is known as “questing”. That is how ticks get on your legs, and, unless detected and removed, to migrate to a more choice (perhaps prime) location, the goal of their quest.


Wood Tick Dermacenter variabilis Gerhard Shelter 14 June 2014
A wood tick lurking on a hiking shelter

The ability of the nearly immobile tick to connect with three different highly mobile hosts as a matter of survival has been a matter or philosophical interest for many years. To the noted German biologist Jakob von Uexküll, the tick exemplified what he called umwelt, German for the environment to which an animal interacts; he is credited with initiating the field of biosemiotics, the signs and symbols used by animals. The tick, according to umwelt, has three biosemiotics: the butyric acid redolence of the host mammal, the warm-blooded temperature of about 37 degrees Centigrade, and the hirsute skin covering to confirm placement. More recent research has revealed a panoply of sensory stimulants employed by ticks including carbon dioxide, ammonia, lactic acid, body heat, moisture, vibration, and, last but not least, visual.

Compared to the eviscerating rapacity of other carnivores, the blood-trophic tick’s feeding process is an elegant combination of engineering and subterfuge. Having settled in an appropriate spot as determined by the tactile palps, a wood tick tilts its scutum about 45 degrees upward to employ the paired chelicerae, specialized cutting organs that penetrate the epidermal layers; this can take as long as two hours. Once break-through occurs and a lesion is formed under the skin, the multifunctional and complex salivary glands are activated, a chemical plant nonpareil. To secure placement, the glands secrete a cement-like substance to anchor the tick to the extraction point, very like the stabilization of an oil rig at the well’s entry point. To ensure an unimpeded flow of the vital blood nutrients, ticks must overcome the platelets which would otherwise coagulate into a sealing clot. The saliva contains blood platelet aggregation inhibitors and anticoagulants to prevent the hemostasis of the wound. If the host is alerted to the tick’s presence in advance of firm emplacement (only humans have the tactile dexterity necessary to extract an embedded tick) dislodgement would possibly ensue. To prevent this, salivary anesthetics are injected to dupe the host into inaction. It is rare that you discern the presence of a tick by tactile sensation; you usually find them visually or by groping in the nether areas. During blood extraction, immunosuppressive and anti-inflammatory agents in the saliva turn back the mammalian defensive mechanisms and vasodilators increase the blood flow. The blood meal itself is consummated primarily in the last 24 hours of the operation, the intricate attachments and chemical adjustments take some time. The wound is kept free of impediments to flow during extraction by the continuous insertion of tick saliva, which is elevated beyond the chemical infusions to return water extracted from the blood back to the host. This allows the tick to collect only concentrated blood to store in its miniscule abdomen (which does swell considerably in some species). The net result is that there is a substantive exchange of body fluids between the tick and the host, an ideal habitat for another of life’s exploitive species, the bacteria. It is the tick saliva that is responsible for its virulence as a disease vector.

Wood ticks are carriers of the causal agents for at least three diseases that have been so far isolated and characterized: Rocky Mountain spotted fever, Tularemia and Tick Paralysis. Rocky Mountain spotted fever was the first tick borne disease to be isolated, its toponym due to its provenance in the Snake River Valley of Idaho late in the late 19th Century. The causal agent is the bacteria Rickettsia rickettsii, named for its discoverer. Its common name is a mnemonic; it results in a fever that is followed by a spotted rash. The name is now a misnomer, as it is most common in a latitudinal strip that runs from North Carolina through Tennessee to Arkansas, and it is a disease problem throughout the Western hemisphere, wherever ticks meet people. Before the advent of antibiotics (it is now normally treated with Doxycycline), thirty percent of all people infected died. Tularemia, also known as Rabbit Fever (the primary host) is the result of infection with the bacterium Francisella tularensis. Named for its most recent epicenter in Tulare County, California, Tularemia is thought to be an ancient disease, responsible for a number of epidemics that engulfed Mesopotamia at about the biblical time frame of the exodus from Egypt in 1300 BCE. While it was delisted by the CDC in 1994 due to a dearth of reported cases, but reinstated six years later in reaction to bioterrorism concerns. It is considered an especially viable biological agent because it is effective, capable of being sprayed, incapacitating with a low lethality, and easy to decontaminate. Tick Paralysis is a direct result of the salivary secretions of the wood tick that are immunosuppressive in nature. A progressive paralysis can ensue that extends to the lung muscles, causing death by respiratory failure.