You are in: eMedicine Specialties > Emergency Medicine > INFECTIOUS DISEASES Tick-Borne Diseases, IntroductionArticle Last Updated: Oct 16, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 6
  Author: Jonathan A Edlow, MD, Associate Professor of Medicine, Department of Emergency Medicine, Harvard Medical School; Associate Chief, Department of Emergency Medicine, Beth Israel Deaconess Medical Center Jonathan A Edlow is a member of the following medical societies: American College of Emergency Physicians Editors: Dan Danzl, MD, Chair, Department of Emergency Medicine, Professor, University of Louisville Hospital; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Jon Mark Hirshon, MD, MPH, Associate Professor, Department of Emergency Medicine, University of Maryland School of Medicine; John D Halamka, MD, MS, Associate Professor of Medicine, Harvard Medical School, Beth Israel Deaconess Medical Center; Chief Information Officer, CareGroup Healthcare System and Harvard Medical School; Attending Physician, Division of Emergency Medicine, Beth Israel Deaconess Medical Center; Charles V Pollack, Jr, MD, MA, FACEP, Professor, Department of Emergency Medicine, University of Pennsylvania College of Medicine; Chairman, Department of Emergency Medicine, Pennsylvania Hospital Author and Editor Disclosure Synonyms and related keywords: Amblyomma, Dermacentor, Ixodes, Ornithodoros, hard tick, soft tick, tick bite, 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, prevention of tick-borne diseases, life cycle of tick INTRODUCTION TO TICK-BORNE DISEASESSection 2 of 6
  Ticks are excellent vectors for disease transmission. 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. For excellent patient education resources, visit eMedicine's Bites and Stings Center. Also, see eMedicine's patient education article Ticks. BIOLOGY AND LIFE CYCLE OF TICKSSection 3 of 6
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. 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. PREVENTION OF TICK-BORNE DISEASESSection 4 of 6
Prevention of all tick-borne diseases can be divided into several strategies. These are environmental, personal, and prophylactic (after a tick bite has occurred).
MULTIMEDIASection 5 of 6
REFERENCESSection 6 of 6
Tick-Borne Diseases, Introduction excerpt Article Last Updated: Oct 16, 2006 | ||||||||||||||||||||||||||||
