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.
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.
Medical Authors: Barbara K. Hecht,
PhD, Frederick Hecht, MD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
Ticks carry the bacteria that cause Lyme disease. People
contract Lyme disease by being bitten by these ticks.
In order to investigate whether specific human behavior increases the risk of
tick exposure, researchers from the University of California at Berkeley took to
the woods. They found that sitting on a log carried with it the greatest risk of
picking up a tick. If you sit on a log (at least in Northern California) for
only five minutes, you have a 30% chance of getting a tick on you! Gathering wood
was also cited as a risky activity as well as leaning up against a tree.
Comment: Aside from issuing a press release (below) about this study,
it was also published in the current issue of the Journal of Medical Entomology.
In case you aren't familiar with this journal, it is chock-full of everything
you might want to know (or not know) about the dangers that mosquitoes, cat
fleas, ticks, flies, scorpions, mange mites and other insects pose to you. We
don't recommend it for bedtime reading.
Ticks are small bloodsucking arthropods and are composed of two families, Ixodidae (hard ticks) and Argasidae (soft ticks), that each contain different genera and species of ticks.
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
diseases cause economic harm such as Texas fever (bovine babesiosis) 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 Figures 1
and 2).
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 organism responsible for the disease that the tick transmits:
Lyme disease (borreliosis) -- Ixodes species (also known as black-legged ticks) 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); some researchers suggest Borrelia lonestari may be the infectious agent.
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
African cattle disease -- Rhipicephalus evertsi also known as red ticks -- vector for parasites or bacterial infections in cattle
Outbreaks of tick-related illnesses follow seasonal patterns (about April to September in the U.S.) as ticks evolve from larvae to adults. Mild winters with an early spring often result in a high number of ticks and an increased frequency of the diseases they transmit. 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 live and 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.
Lyme disease, sometimes referred to as Lyme infection, is a bacterial illness, transmitted to humans by the bite of deer ticks (Ixodes ticks) carrying a bacterium known as Borrelia burgdorferi. The disease has been reported in the Northeast, Mid-Atlantic, North Central, and Pacific coastal regions of the United States (see map) and in Europe, where it was first described almost 100 years ago. It is most prevalent in the northeastern states of the United States, with about half of all cases clustered in New York and Connecticut.
Doctors at New Haven's Yale Medical Center first described and named Lyme disease in the United States in the late 1970s. An unexpected number of residents in Lyme, Connecticut, were found to have a "new" and unusual illness.
Fortunately, less than 5% of tick bites in high-contact areas result in Lyme infections.
Doctors have learned a great deal about the illness since th...
