- Author: Tracy Sanson, MD, FACEP; Chief Editor: Jeter (Jay) Pritchard Taylor, III, MD more...
Ticks are excellent vectors for disease transmission; consequently, tick-borne diseases are common.[1, 2] They are second only to mosquitoes as vectors of human disease, both infectious and toxic. More than 800 species of these obligate blood-sucking creatures inhabit the planet. The image below depicts the Ixodes scapularis and the Dermacentor variabilis.
See Lyme Disease and 4 Emerging Tick-Borne Illnesses, a Critical Images slideshow, to help identify and treat several tick-borne conditions.
Also, see the Bug Bites You Need to Know This Summer slideshow for helpful images and information on various bug bites.
From the perspective of disease transmission to humans, the essential characteristic of ticks is their need to ingest a blood meal to transform to their next stage of development. Not picky in their eating habits, they take their requisite blood meal from all classes of vertebrates (eg, mammals, reptiles, and birds), with the exception of fish.
Ticks feed by perching in low vegetation and waiting (questing) for a susceptible host on which they can attach themselves and feed. Once on a host, the tick inserts its hypostome, a central piercing element with hooks, into the host’s skin. Some ticks secrete a cementing material to fasten themselves to the host.
In addition, Ixodes ticks secrete anticoagulant, immunosuppressive, and anti-inflammatory substances into the area of the tick bite. These substances presumably help the tick to obtain a blood meal without the host’s noticing. These same substances also help any freeloading pathogens to establish a foothold in the host.
Ticks can carry and transmit a remarkable array of pathogens, including bacteria, spirochetes, rickettsiae, protozoa, viruses, nematodes, and toxins. A single tick bite can transmit multiple pathogens, a phenomenon that has led to atypical presentations of some classic tick-borne diseases. In the United States, ticks are the most common vectors of vector-borne diseases.
North American tick-borne diseases
In North America, the following diseases are caused by tick bites:
Red meat allergy
Red meat allergy
Over the past decade, the lone star tick, a medium-sized, reddish-brown tick common to the southeastern United States, has been implicated as the likely cause of severe red meat allergies, which have been increasing in incidence. This allergy has been found in North America, Europe, Asia, Central America, and Africa. The lone star tick is believed to produce a sugar from its gut called galactose-alpha-1,3-galactose (“Alpha-Gal”), which is injected into the host during a tick bite. The sugar is also found in red meat (eg, beef, pork, venison, rabbit) and some dairy products. The primary management of mammalian meat allergy is avoidance of mammalian meat. The allergy can cause symptoms ranging from urticaria to angioedema and anaphylaxis. Delayed anaphylactic shock has also been reported to occur 4-6 hours after consumption of red meat. These symptoms are treated in the usual manner.
Powassan virus disease
Powassan (POW) virus may also be transmitted to humans by infected ticks, with approximately 60 US cases reported in the last decade, mostly in the Northwest United States and Great Lakes region.
Signs and symptoms of POW infection may include headache, fever, weakness, vomiting, seizures, confusion, and memory loss. Long-term neurologic problems have been reported.
POW virus infection has no specific treatment, but infected persons often require hospitalization for respiratory support, intravenous fluids, or medications to reduce brain swelling.
Bourbon virus disease
In June 2014, a newly discovered virus (Bourbon virus, named after the Kansas county in which it was found to have infected a farmer, ultimately leading to his death) is presumed to have been transmitted via tick bite, although this has not been proven. The patient’s symptoms, including fever, low red and white blood cell counts, elevated liver enzyme levels, and loss of appetite, suggested a tick-borne illness such as ehrlichiosis or the Heartland virus, but test results were negative. A laboratory with the Centers for Disease Control and Prevention (CDC) in Colorado finally determined that the virus was one that had never been encountered in the Western hemisphere.
The Bourbon virus belongs to the orthomyxovirus family and possesses a genome similar to that of viruses in Eastern Europe, Africa, and Asia.
Tests are planned to determine if prior undiagnosed but similar cases may have been caused by Bourbon virus.
European tick-borne diseases
In Europe, the list is similar to that in North America, but the following diseases should be considered as well :
Boutonneuse fever (caused by a less virulent spotted fever rickettsial organism, Rickettsia conorii)
Tick-borne encephalitis [9, 10]
Most tick bites do not result in transmission of infection; in the case of Lyme disease, for example, only about 2-3% of all persons bitten by Ixodes scapularis ticks in endemic areas develop Lyme disease.
Secondary infections and allergic reactions to proteins in tick saliva are also possible. In fact, one study suggests that repeated tick bites may actually protect against Lyme disease, possibly because of hypersensitivity developed in response to previous bites of uninfected ticks.
Biology and Life Cycle of Ticks
A basic familiarity with the biology of ticks is important for understanding their roles in the various tick-borne diseases and the ways in which these diseases might be prevented. Ticks are arthropods of the class Arachnida, which includes spiders, scorpions, and mites. Of the 3 families of ticks, only hard ticks (family Ixodidae) and soft ticks (family Argasidae) are of medical importance. The principal difference between the 2 groups is the hard plate, or scutum, that hard ticks possess and soft ticks do not.
The life cycles of hard and soft ticks differ. Most hard ticks undergo a 2-year life cycle in which they begin as 6-legged larvae. Amblyomma, Dermacentor, and Ixodes are the 3 genera of hard ticks that transmit diseases to humans in the United States (see the images below). These ticks generally feed for many days, a fact that has some bearing on the treatment of tick bites. Specific tick-borne illnesses are discussed in greater detail elsewhere, in the articles devoted to individual tick-borne diseases.
The following representative cycle is that of I scapularis (also known as Ixodes dammini) in the northeastern United States. The larvae hatch from eggs in summer and begin seeking hosts in August; at this stage, they have only 6 legs and are the size of the period at the end of this sentence. If the larvae do not find a host from which to obtain a blood meal, they die. The preferred host is the white-footed mouse, Peromyscus leucopus. Larvae that successfully feed then fall off the host and live in the soil and decaying vegetation over the winter.
The next spring, most often in May and June, the larvae molt into 8-legged nymphs (see the image below). These nymphs are quite small and seek their blood meal from a small vertebrate. Humans may be infected as accidental hosts at this point in the cycle. The nymphs then either die (if they fail to find a blood meal) or live in the soil and eventually molt into adults in the fall.
The 8-legged adult ticks are somewhat larger and therefore seek a larger host. The white-tailed deer, Odocoileus virginianus, is the preferred host for adult ticks, which mate on deer over the winter. Because the deer plays a key role in tick mating, the increase in the deer population in many parts of the country is an important factor in the epidemiology of some tick-borne diseases (eg, Lyme disease). The adult female lays several thousand eggs and then dies. Eggs that survive the winter hatch into larvae the next season, and the 2-year cycle begins anew.
Soft ticks have no hard shell (scutum). In the United States, only ticks of the genus Ornithodoros (see the image below) transmit human disease—namely, relapsing fever. The biology of soft ticks differs from that of hard ticks in that meals last for only short periods (< 1 hour) and disease can be transmitted in less than 1 minute.
Prevention of Tick-Borne Diseases
Prevention strategies for tick-borne diseases can be divided into 3 general categories: environmental, personal, and prophylactic (after a tick bite has occurred).[14, 15, 16] Environmental strategies (eg, control of the population of deer and other vectors and tick control measures) are beyond the scope of this article.
Personal strategies for preventing tick-borne disease include the following:
Avoiding grassy areas with shrubs that attract ticks
Wearing white or light-colored clothing so that attached ticks can be easily noticed and removed
Tucking pant legs into socks
Walking in the center of paths to avoid vegetation on which ticks lie in wait of a host
Applying lotion containing diethyltoluamide to the skin (avoiding the face and hands) – Diethyltoluamide concentrations of about 30% are recommended; neurotoxicity (eg, seizures) is reported in children, and some authorities recommend avoiding repeated use in children
Applying permethrin to clothing
Performing daily tick checks and removing ticks as soon as they are detected
Tick removal is best accomplished by grabbing the tick as close to the skin as possible with a very fine forceps and pulling it with gentle upward traction. The bite site should be thoroughly disinfected with alcohol or another skin antiseptic solution. The use of household tweezers is discouraged, as they compress the tick and its feeding chamber within the host's skin and release allergen into the host's vascular bed. Use of gasoline, petroleum, and other organic solvents to suffocate ticks, as well as burning the tick with a match, should be avoided.
Often, the complete mouthparts do not come out with the rest of the tick. Leaving these in does not increase the risk of disease transmission, but they may cause a local infection or foreign body reaction.
Another method of tick removal that has been suggested is to inject a wheal of lidocaine with epinephrine intradermally beneath the tick. By blanching the area and thereby removing the blood, the ticks may be induced to crawl out of its own accord. This method seems intuitively feasible, but it has not been tested in any large clinical trials.
Prophylactic measures include the use of vaccines, which are available for some tick-borne diseases and are discussed in the individual articles bearing on those diseases. In addition, certain tick-borne diseases—specifically, Lyme disease and relapsing fever —can be prevented by administering antibiotics (see Lyme disease and Relapsing Fever).
Edlow JA. Lyme disease and related tick-borne illnesses. Ann Emerg Med. 1999 Jun. 33(6):680-93. [Medline].
Edlow JA. Preface: tick-borne diseases, part II. Infect Dis Clin North Am. 2008 Sep. 22(3):xiii-xv. [Medline].
Herrmann JA, Dahm NM, Ruiz MO, Brown WM. Temporal and Spatial Distribution of Tick-Borne Disease Cases among Humans and Canines in Illinois (2000-2009). Environ Health Insights. 2014. 8:15-27. [Medline]. [Full Text].
Feder HM Jr. Lyme disease. N Engl J Med. 2014 Aug 14. 371(7):683-4. [Medline].
CDC, National Center for Emerging and Zoonotic Infectious Diseases, Division of Vector-Borne Diseases. Powassan virus. Centers for Disease Control and Prevention. Available at http://www.cdc.gov/powassan/. 2015 Feb 9; Accessed: April 17, 2015.
Lowes R. Discovery of Bourbon virus raises many questions. Medscape News from WebMD. Available at http://www.medscape.com/viewarticle/837229. 2014 Dec 24; Accessed: January 9, 2015.
Achazi K, Patel P, Paliwal R, Radonic A, Niedrig M, Donoso-Mantke O. RNA interference inhibits replication of tick-borne encephalitis virus in vitro. Antiviral Res. 2012 Jan. 93(1):94-100. [Medline].
Sips GJ, Wilschut J, Smit JM. Neuroinvasive flavivirus infections. Rev Med Virol. 2012 Mar. 22(2):69-87. [Medline].
Grygorczuk S, Parczewski M, Moniuszko A, et al. Increased concentration of interferon lambda-3, interferon beta and interleukin-10 in the cerebrospinal fluid of patients with tick-borne encephalitis. Cytokine. 2014 Oct 29. 71(2):125-131. [Medline].
Burke G, Wikel SK, Spielman A, et al. Hypersensitivity to ticks and Lyme disease risk. Emerg Infect Dis. 2005 Jan. 11(1):36-41. [Medline].
Anderson JF, Magnarelli LA. Biology of ticks. Infect Dis Clin North Am. 2008 Jun. 22(2):195-215, v. [Medline].
Dumitrache MO, Gherman CM, Cozma V, et al. Hard ticks (Ixodidae) in Romania: surveillance, host associations, and possible risks for tick-borne diseases. Parasitol Res. 2012 May. 110(5):2067-70. [Medline].
Clark RP, Hu LT. Prevention of lyme disease and other tick-borne infections. Infect Dis Clin North Am. 2008 Sep. 22(3):381-96, vii. [Medline].
Wilson ME. Prevention of tick-borne diseases. Med Clin North Am. 2002 Mar. 86(2):219-38. [Medline].
Pages F, Dautel H, Duvallet G, Kahl O, de Gentile L, Boulanger N. Tick repellents for human use: prevention of tick bites and tick-borne diseases. Vector Borne Zoonotic Dis. 2014 Feb. 14(2):85-93. [Medline].
Hasin T, Davidovitch N, Cohen R, et al. Postexposure treatment with doxycycline for the prevention of tick-borne relapsing fever. N Engl J Med. 2006 Jul 13. 355(2):148-55. [Medline].