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Tick-Borne Diseases, Introduction
Updated: Dec 9, 2008
Introduction to Tick-Borne Diseases
Ticks are excellent vectors for disease transmission.1 More than 800 species of these obligate blood-sucking creatures inhabit the planet. They are second only to mosquitoes as vectors of human disease, both infectious and toxic.
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, 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 and feed. Once on a host, the tick attaches 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, such as 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.
In North America, the following diseases are caused by tick bites: Lyme disease, human granulocytic and monocytic ehrlichiosis, babesiosis, relapsing fever, Rocky Mountain spotted fever, Colorado tick fever, tularemia, Q fever, and tick paralysis. In Europe, the list is similar, but other diseases should be considered as well; these include boutonneuse fever (caused by a less virulent spotted fever rickettsial organism Rickettsia connori) and tick-borne encephalitis.
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 due to developed hypersensitivity from the prior bites of uninfected ticks.2
For excellent patient education resources, visit eMedicine's Bites and Stings Center. Also, see eMedicine's patient education article Ticks.
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| Clinical Image Atlas Click to view clinical images on the features, causes, epidemiology, diagnosis, and treatment of Lyme disease. |
| Clinical Image Atlas Click to view clinical images on the features, causes, epidemiology, diagnosis, and treatment of Lyme disease. |
Biology and Life Cycle of Ticks
A brief understanding of the biology of the tick is important in understanding its role in the various tick-borne diseases and the prevention of these diseases. 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) have medical importance. The principle difference between the 2 groups is the presence of the hard plate, or scutum, that hard ticks possess.
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. These ticks generally feed for many days, a fact that has some bearing on the treatment of tick bites. More details are discussed in the articles dealing with specific tick-borne illnesses. The following representative cycle is that of I scapularis in the northeastern United States.
The larvae hatch from eggs in summer and begin seeking hosts in August; these ticks 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 for 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. 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. Then, the nymph either dies (if it fails to find a blood meal) or lives in the soil to molt into an adult in the fall season.
This photo is of an adult female, Amblyomma americanum, and a nymphal form of the same species (shown next to a common match for scale). This tick is the vector for monocytic ehrlichiosis and a Lymelike disease that occurs in the midcentral and southern United States. The agent for this latter disease has not yet been isolated. Photo by Darlyne Murawski; reproduced with permission.
The 8-legged adult tick is somewhat larger and seeks a larger host for its required blood meal. The white-tailed deer, Odocoileus virginianus, is the preferred host for adult ticks, which mate on deer over the winter months. Because the deer plays a key role in the mating of ticks, the increase in the deer population in many parts of the country is an important factor in the epidemic of some tick-borne diseases, such as 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 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 h), and disease can be transmitted in less than 1 minute.
This is an example of a soft-bodied tick of the genus Ornithodoros. These ticks transmit various Borrelia species that cause relapsing fever. Photo courtesy of Julie Rawlings, MPH, Texas Department of Health.
Prevention of Tick-Borne Diseases
Prevention of all tick-borne diseases can be divided into several strategies.3,4 These are environmental, personal, and prophylactic (after a tick bite has occurred).
- Environmental strategies (control of the population of deer and other vectors and tick control measures) are beyond the scope of this section.
- Personal strategies include avoiding grassy areas with shrubs that attract ticks, wearing white or light-colored clothing so that attached ticks can be seen easily 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 face and hands), applying permethrin to clothing, and performing daily tick checks and removing ticks as soon as they are detected.
- Diethyltoluamide concentrations of about 30% are recommended; neurotoxicity (eg, seizures) is reported in children, and some authorities recommend avoiding repeated use in children.
- Tick removal is best accomplished by grabbing the tick as close to the skin as possible with a very fine forceps and pulling it gradually, but firmly, out from the skin. Gloves should be worn when removing ticks, and the bite site should be thoroughly disinfected with alcohol or another skin antiseptic solution. Care should be taken to avoid squeezing the tick during removal, since squeezing may inject infectious material into the skin. 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 is to inject a wheal of lidocaine with epinephrine intradermally beneath the tick. By blanching the area and, thus, removing the blood, ticks have been reported to crawl out on their own accord. This method is intuitive but 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 sections.
- Last, some individual tick-borne diseases, specifically Lyme disease and relapsing fever, can be prevented by antibiotics. See the individual articles on these diseases for specifics.
Multimedia
 | Media file 1: This photo shows the relative sizes of the adult forms of Ixodes scapularis (right) and Dermacentor variabilis (left). These ticks are shown next to a common match for scale. I scapularis also is referred to as Ixodes dammini. Photo by Darlyne Murawski; reproduced with permission. |
 | Media file 2: This photo is of an adult female, Amblyomma americanum, and a nymphal form of the same species (shown next to a common match for scale). This tick is the vector for monocytic ehrlichiosis and a Lymelike disease that occurs in the midcentral and southern United States. The agent for this latter disease has not yet been isolated. Photo by Darlyne Murawski; reproduced with permission. |
 | Media file 3: This photo shows 2 nymphal Ixodes scapularis ticks. The one on the right is unfed. Note the clear linear brown markings that are the midgut diverticula. The nymph on the left has been feeding on a mouse for 48 hours. Note that its body is larger compared with the scutum and that the midgut diverticula are less distinct. Photo by Darlyne Murawski; reproduced with permission. |
 | Media file 4: This is an example of a soft-bodied tick of the genus Ornithodoros. These ticks transmit various Borrelia species that cause relapsing fever. Photo courtesy of Julie Rawlings, MPH, Texas Department of Health. |
Keywords
tick-borne diseases, tick bite, prevention of tick-borne disease, Amblyomma, Dermacentor, Ixodes, Ornithodoros, hard tick, soft tick, vector-borne disease, Lyme disease, human granulocytic and monocytic ehrlichiosis, babesiosis, relapsing fever, Rocky Mountain spotted fever, Colorado tick fever, tularemia, Q fever, tick paralysis, Ixodes scapularis, I scapularis, Rickettsia conorii, R conorii, boutonneuse fever, tick-borne encephalitis, life cycle of tick
More on Tick-Borne Diseases, Introduction |
| References |
References
Anderson JF, Magnarelli LA. Biology of ticks. Infect Dis Clin North Am. Jun 2008;22(2):195-215, v. [Medline].
Burke G, Wikel SK, Spielman A, et al. Hypersensitivity to ticks and Lyme disease risk. Emerg Infect Dis. Jan 2005;11(1):36-41. [Medline].
Clark RP, Hu LT. Prevention of lyme disease and other tick-borne infections. Infect Dis Clin North Am. Sep 2008;22(3):381-96, vii. [Medline].
Wilson ME. Prevention of tick-borne diseases. Med Clin North Am. Mar 2002;86(2):219-38. [Medline].
Doan-Wiggins L. Tick-borne diseases. Emerg Med Clin North Am. May 1991;9(2):303-25. [Medline].
Edlow JA. Lyme disease and related tick-borne illnesses. Ann Emerg Med. Jun 1999;33(6):680-93. [Medline].
Edlow JA. Preface: tick-borne diseases, part II. Infect Dis Clin North Am. Sep 2008;22(3):xiii-xv. [Medline].
[Best Evidence] Hasin T, Davidovitch N, Cohen R, et al. Postexposure treatment with doxycycline for the prevention of tick-borne relapsing fever. N Engl J Med. Jul 13 2006;355(2):148-55. [Medline].
Needham GR. Evaluation of five popular methods for tick removal. Pediatrics. Jun 1985;75(6):997-1002. [Medline].
Shapiro ED. Tick-borne diseases. Adv Pediatr Infect Dis. 1997;13:187-218. [Medline].
Spach DH, Liles WC, Campbell GL, et al. Tick-borne diseases in the United States. N Engl J Med. Sep 23 1993;329(13):936-47. [Medline].
Further Reading
Keywords
tick-borne diseases, tick bite, prevention of tick-borne disease, Amblyomma, Dermacentor, Ixodes, Ornithodoros, hard tick, soft tick, vector-borne disease, Lyme disease, human granulocytic and monocytic ehrlichiosis, babesiosis, relapsing fever, Rocky Mountain spotted fever, Colorado tick fever, tularemia, Q fever, tick paralysis, Ixodes scapularis, I scapularis, Rickettsia conorii, R conorii, boutonneuse fever, tick-borne encephalitis, life cycle of tick
