Ticks have four pairs of legs. The head portion of the tick is referred to as the capitulum and is comprised of a median hypostome seated below the mouth and a pair of chelicerae (both arising from a capitular base), and an outer pair of palps. The purpose of the chelicerae is to open an incision so that the hypostome can penetrate into the softer tissue and obtain its blood meal. The hypostome has recurved teeth to insure attachment is secure to the host. The palps do not enter the bite but spread to act as a secondary anchor. The capitulum of each species of ticks have unique characteristics that can be used to identify the genus and species.
Ticks are members of the Acari which include the mites and are therefore described as large mites. Within the ticks there are two main families, the Argasidae (soft-bodied ticks) and the Ixodidae (hard-bodied ticks). Males of the Ixodidae have a shield that covers the entire dorsum of the male while covering only the anterior part of the female as seen below in the two Dermacentor variabilis images.
Disease / Pathology
Ticks are important vectors of various rickettsial bacteria and Piroplasms (Babesia spp.). Diseases such as Babesiosis, anaplasmosis, Lyme disease, and Rocky Mountain Spotted Fever to name a few.
Some ticks are also important in their potential to cause life threatening progressive paralysis. Ticks engorged on the host in close proximity to the spinal column can induce this progressive paralysis. It is thought that a substance in the saliva of the tick induces this reaction. Fortunately the progression of the paralysis can be stopped and reversed by the simple removal of the tick. Importantly, the mouth parts are important to be removed from the site to ensure that the paralysis is stopped from progressing and reversal occurs.
Another example of a tick inducing an unexpected reaction is Amblyomma americanum (Lone Star Tick), which has been found to have in it’s saliva a as of yet identified substance that can have a quite the unusual affect on the host. The saliva can induce in the host a hypersensitivity to red meat. Specifically, the host becomes sensitive to a carbohydrate called alpha-gal that is found in the saliva of the tick and is similarly found in meat. The allergy or hypersensitivity is not rapid in onset compared to most allergic reactions to foods such as peanuts or shellfish. Symptoms may take up to three to six hours before they appear and can be mild to severe. Symptoms include hives, headache, wheezing, swelling of lips-tongue-face, and life threatening anaphylaxis.
Location on the Host
Ticks feed on the blood of mammals and attach to them to feed. Some ticks such as Otobius spp. will preferably locate in the ear. Avoidance of brush, wearing protective clothing, and the administration of tick repellent will help avoid contact with ticks.
Ticks are cosmopolitan in distribution.
Ticks go through a rather complex life cycle that includes four stages – egg stage – six legged larvae stage – eight legged nymph stage – and finally the eight legged adult stage.
As vectors of various diseases, the stage, geographic location, and the amount of time feeding will determine the best chance of acquiring any one of the diseases transmitted. As an example, a tick identified as Ixodes scapularis on a patient with a recent travel history to Nantucket Island in the United States, and morphologically identified as a nymph that appears to have been feeding for an extended amount of time, would most likely place the patient at risk for Babesiosis or Lyme disease.
Morphology & Diagnosis
The ticks described below are members of the “hard” ticks, as opposed to “soft” ticks which are less common in temperate regions and more common in subtropical and tropical environments. Identifications of the three are performed based on the shape; scutum markings or lack of; anal orifice; and mouth parts.
The presence or lack of mouth parts in addition to the engorgement or non-engorgement of the tick is important information provided to the clinician. The lack of mouth parts are important in determining if the patient is at risk for developing possible complications such as secondary infections or tick paralysis. The size of the engorged tick can give the microbiologist and clinician a measurement of how long the tick may have been feeding on the patient. The larger the engorged tick the longer the tick has been feeding and therefore the greater the chance for the transmission of any of a number of possible tick transmitted diseases (Rocky Mountain Spotted Fever; Lyme disease; Babesiosis; Anaplasmosis; etc.).
Images & Descriptions
The following are some images of the most commonly encountered and clinically significant ticks found in North America. A brief discussion of each precedes each image.
Dermacentor variables (dog tick)
Left dorsal view of Dermacentor variablis female – note how the scutum extends less then half of the entire body and is ornate while that of the male on the right is highly ornate and extends the entire body
Ixodes scapularis (deer tick)
Dorsal view of Ixodes scapularis – female and male left and center respectively. Right photo illustrates the characteristic ventral located anal groove curving away from the front or head.
Amblyomma americanum (lone star tick)
The Loe Star tick has a characteristic white spot on its scutum. The tick has thick mouth parts that unlike other ticks will produce a larger area of penetration. What is unusual about the Lone Star tick is a syndrome associated with its bite called Alpha-gal Syndrome. This syndrome consists of the development of a sensitivity to Alpha-gal (galactose-alpha-1,3-galactose). Alpha-gal is a sugar molecule found in most mammals but not found in fish, birds or humans. It can be found in meat and meat by products, gelatin, cow’s milk and milk products. The result is red-meat allergy or Alpha-gal allergy. The allergy can be severe enough to be life threatening requiring emergency care.
Haemaphysalis longicornis (Asian longhorn tick)
Asian longhorn ticks are not as common as the previously mentioned ticks as they have been recently introduced into the United States. However they have been spreading rapidly due to the ability of the female to lay eggs and reproduce without mating. Connecticut, Arkansas, Missouri, New Jersey, Pennsylvania, South Carolina, Rhode Island, Virginia, Ohio and New York are some of the areas where the tick has been found. Where they are found it is not uncommon to find thousands at a time in the grass or shrubs. Or even on animals such as pets, livestock and wildlife. Though humans can also be a source of a meal, these ticks are prone to be less attracted to human skin.
The tick has been found to transmit Rickettsia rickettsii and future epidemiological studies will most likely result in discovering other bacterial and possibly parasitic diseases.