Ticks

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Medical Author:

Charles Patrick Davis, MD, PhD

Charles Patrick Davis, MD, PhD

Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

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Medical Editor:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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• Ticks Overview
• Tick Bite Symptoms and Signs
• When to Seek Medical Care
• Tick Bite Diagnosis
• Tick Bite Treatment
• Self-Care at Home
• Medical Treatment
• Tick Prevention
• Tick Bite Prognosis
• For More Information
• Synonyms and Keywords
• Authors and Editors
• Take the Lyme Disease Quiz
• Pictures of Lyme Disease - Slideshow
• Pictures of Bad Bugs - Slideshow
• Read more on Ticks from Healthwise
• Viewer Comments: Ticks - Preventive

Ticks Overview

Ticks are the leading carriers (vectors) of diseases to humans in the United States, second only to mosquitoes worldwide. It is not the tick bite but the toxins, secretions, or organisms in the tick's saliva transmitted through the bite that causes disease.

Ticks are arthropods, like spiders. There are more than 800 species of ticks throughout the world. Many organisms that bite humans for a blood meal are not ticks and should not be confused with ticks. Some common examples are bedbugs and fleas (both are insects, not arthropods). If it is possible to bring into the doctor's office what has caused a "bite," the physician may be able to determine what potential vector caused the "bite."

Two families of ticks, Ixodidae (hard ticks) and Argasidae (soft ticks), are important to humans because of the diseases or illnesses they can transmit or cause. Hard ticks have a tough back plate or scutum that defines their appearance. The hard ticks tend to attach and feed for hours to days. Disease transmission usually occurs near the end of a meal, as the tick becomes full of blood. Figure 1 shows several hard ticks and the various stages in their life cycle. The stages are part of the life cycle of ticks; the smallest stages, larva and nymph, are sometimes generally referred to as "seed ticks" because they resemble small plant seeds.

Figure 1: The life cycle of ticks. Source: CDC

Soft ticks have more rounded bodies and do not have the hard scutum found in hard ticks. These ticks usually feed for less than one hour. Disease transmission from these ticks can occur in less than a minute. The bite of some of these ticks produces intensely painful reactions. Ticks can transmit disease to many hosts; some cause economic harm such as Texas fever (bovine babeiosis) in cattle that can kill up to 90% of yearling cows. Figure 2 shows the body of a soft tick; there is no hard scutum, only the soft body. The adult soft ticks are about the same size as adult hard ticks (see Figure 1).

Figure 2: Picture of a soft tick. Source: CDC

The following is a list of tick-borne diseases, the usual tick vector(s), and the pathogen(s) the tick transmits:

• Lyme disease (borreliosis) -- Ixodes species including deer ticks (hard ticks) -- vectors for Borrelia species of bacteria (a spirochete or spiral-shaped bacterium)



• Babesiosis -- Ixodes species (hard ticks) -- vectors for Babesia, a protozoan



• Ehrlichiosis -- Amblyomma americanum or lone star ticks (hard ticks) -- vectors for Ehrlichia chaffeensis and Ehrlichia ewingii bacterial species



• Rocky Mountain spotted fever -- Dermacentor variabilis (American dog tick) and Rocky Mountain wood tick (Dermacentor andersoni) (hard tick) are the primary vectors and occasionally the brown dog tick (Rhipicephalus sanguineus); Amblyomma cajennense (hard tick) is the vector in countries south of the United States -- vectors for Rickettsia bacteria



• Southern tick-associated rash illness (STARI) -- Amblyomma americanum or lone star tick (hard tick) -- infectious agent not yet identified according to U.S. Centers for Disease Control and Prevention (CDC)



• Tick-borne relapsing fever -- Ornithodoros moubata or African tick (soft tick) -- vectors for Borrelia species of bacteria



• Tularemia -- Dermacentor variabilis (American dog tick) (hard tick) and Amblyomma americanum or lone star tick (hard tick) -- vectors for Francisella tularensis bacteria



• Anaplasmosis (human granulocytic anaplasmosis or HGA) -- Ixodes species (hard tick) -- vectors for Anaplasma phagocytophilum bacteria



• Colorado tick fever -- Dermacentor andersoni (hard tick) -- vectors for Coltivirus, a RNA virus



• Powassan encephalitis -- Ixodes species and Dermacentor andersoni (both hard ticks) -- vectors for Powassan encephalitis virus, an RNA arbovirus



• Q fever -- Rhipicephalus sanguineus, Dermacentor andersoni, and Amblyomma americanum (all three are hard ticks) -- vectors for Coxiella burnetii, a bacterium

Outbreaks of tick-related illnesses follow seasonal patterns (about April to September in the U.S.) as ticks evolve from larvae to adults. Different ticks go through complex life cycles (for example, see Figure 3) that involve mating and larval formation and usually have several hosts; humans are usually not an essential part of the normal tick life cycle, but wherever a mammalian host is pictured in a tick life cycle, usually a human can replace the normal host animal. For example, in Figure 3, people could replace the deer or cow. However, in most cases, the life cycle is not completed with human hosts.

Figure 3: This is the life cycle of ticks; humans are alternate hosts. Source: CDC

Ticks hide in low brush; this location allows them to physically contact a host. One study suggested that leaning against a tree or sitting on an old log was the quickest way to acquire ticks (about 30 seconds) in tick-infested areas. Ticks require a "blood meal" to grow and survive, and they are not very particular upon whom or what they feed. If ticks don't find a host, they may die.

• Once a tick finds a host (such as a human, a pet dog or cat, a deer, or a rabbit) and finds a suitable site for attachment, the tick begins to burrow with its mouthparts into exposed skin. Tick mouthparts are barbed, which helps to secure them to the host.



• Often the tick secretes "cementum" to more firmly attach its mouthparts and head to the host. Ticks may secrete or regurgitate small amounts of saliva that contain neurotoxins. These nerve poisons cleverly prevent the host from feeling the pain and irritation of the bite. Consequently, individuals may never notice the tick bite or its feeding. The saliva may contain a blood thinner to make it easier for the tick to get its blood meal. Some people are allergic to these secretions and may have a quick and severe allergic reaction to a tick bite; a few may develop other symptoms listed below.