Tick-borne diseases, which afflict humans and other animals, are caused by infectious agents transmitted by tick bites. Tick-borne illnesses are caused by infection with a variety of pathogens, including rickettsia and other types of bacteria, viruses, and protozoa. Because individual ticks can harbor more than one disease-causing agent, patients can be infected with more than one pathogen at the same time, compounding the difficulty in diagnosis and treatment. As of 2016, 16 tick-borne diseases of humans are known (four discovered since 2013).
As the incidence of tick-borne illnesses increases and the geographic areas in which they are found expand, health workers increasingly must be able to distinguish the diverse, and often overlapping, clinical presentations of these diseases.
Diagnosis and treatment
In general, specific laboratory tests are not available to rapidly diagnose tick-borne diseases. Due to their seriousness, antibiotic treatment is often justified based on clinical presentation alone.
Ticks tend to be more active during warmer months, though this varies by geographic region and climate. Areas with woods, bushes, high grass, or leaf litter are likely to have more ticks. Those bitten commonly experience symptoms such as body aches, fever, fatigue, joint pain, or rashes. People can limit their exposure to tick bites by wearing light-colored clothing (including pants and long sleeves), using insect repellent with 20%–30% DEET, tucking their pants legs into their socks, checking for ticks frequently, and washing and drying their clothing (in a hot dryer).
For a person or companion animal to acquire a tick-borne disease requires that that individual gets bitten by a tick and that that tick feeds for a sufficient period of time. The feeding time required to transmit pathogens differs for different ticks and different pathogens. Transmission of the bacterium that causes Lyme disease is well understood to require a substantial feeding period.
For an individual to acquire infection, the feeding tick must also be infected. Not all ticks are infected. In most places in the US, 30-50% of deer ticks will be infected with Borrelia burgdorferi (the agent of Lyme disease). Other pathogens are much more rare. Ticks can be tested for infection using a highly specific and sensitive qPCR procedure. Several commercial labs provide this service to individuals for a fee. The Laboratory of Medical Zoology (LMZ), a nonprofit lab at the University of Massachusetts, provides a comprehensive TickReport  for a variety of human pathogens and makes the data available to the public. Those wishing to know the incidence of tick-borne diseases in their town or state can search the LMZ surveillance database.
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Major tick-borne diseases include:
- Lyme disease or borreliosis
- Organism: Borrelia burgdorferi sensu lato (bacterium)
- Vector: deer tick (Ixodes scapularis (=I. dammini), I. pacificus, I. ricinus (Europe), I. persulcatus (Asia))
- Endemic to: North America and Eurasia
- Symptoms: Fever, arthritis, neuroborreliosis, erythema migrans, cranial nerve palsy, carditis, fatigue, and influenza-like illness
- Treatment: Antibiotics - amoxicillin in pregnant adults and children), (doxycycline in other adults
- Relapsing fever (tick-borne relapsing fever, different from Lyme disease due to different Borrelia species and ticks)
- Organisms: Borrelia species such as B. hermsii, B. parkeri, B. duttoni, B. miyamotoi
- Vector: Ornithodoros species
- Regions : Primarily in Africa, Spain, Saudi Arabia, Asia in and certain areas of Canada and the western United States
- Symptoms: Relapsing fever typically presents as recurring high fevers, flu-like symptoms, headaches, and muscular pain, with less common symptoms including rigors, joint pain, altered mentation, cough, sore throat, painful urination, and rash
- Treatment: Antibiotics are the treatment for relapsing fever, with doxycycline, tetracycline, or erythromycin being the treatment of choice.
- Typhus Several diseases caused by Rickettsia bacteria (below)
- Rocky Mountain spotted fever
- Organism: Rickettsia rickettsii
- Vector: Wood tick (Dermacentor variabilis), D. andersoni
- Region (US): East, Southwest
- Vector: Amblyomma cajennense
- Region (Brazil): São Paulo, Rio de Janeiro, Minas Gerais.
- Symptoms:Fever, headache, altered mental status, myalgia, and rash
- Treatment: Antibiotic therapy, typically consisting of doxycycline or tetracycline
- Helvetica spotted fever
- Organism: Rickettsia helvetica
- Region(R. helvetica): Confirmed common in ticks in Sweden, Switzerland, France, and Laos
- Vector/region(s)#1: Ixodes ricinus is the main European vector.
- Symptoms: Most often small red spots, other symptoms are fever, muscle pain, headache and respiratory problems
- Treatment: Broad-spectrum antibiotic therapy is needed, phenoxymethylpenicillin likely is sufficient.
- Human granulocytic anaplasmosis (formerly human granulocytic ehrlichiosis or HGE)
- Bartonella: Bartonella transmission rates to humans via tick bite are not well established  but Bartonella is common in ticks. For example: 4.76% of 2100 ticks tested in a study in Germany 
- Organism: Francisella tularensis, A. americanum
- Vector: D. variabilis, D. andersoni
- Region (US): Southeast, South-central, West, widespread
- Tick-borne meningoencephalitis
- Powassan virus/deer tick virus
- Colorado tick fever
- Crimean-Congo hemorrhagic fever
- Severe febrile illness
- Severe febrile illness, headaches, coma in 1/3 patients
- Organism: tentatively Alongshan virus, jingmenvirus group in the flavivirus family
- Vector: tick (unspecified)
- Region: Inner Mongolia but potentially more widespread
- Organism: Babesia microti, B. equi
- Vector: Ixodes scapularis, I. pacificus
- Region (US): Northeast, West Coast
- Organism: Cytauxzoon felis
- Vector: Dermacentor variabilis (American dog tick)
- Region (US): South, Southeast
- Tick paralysis
- Cause: Toxin
- Vector (US): D. andersoni, D. variabilis West
- Region (US): East
- Vector (Australia): Ixodes holocyclus
- Region (Australia): East
- Alpha-gal allergy - Alpha-gal syndrome caused by immune reaction to the Alpha-gal sugar molecule introduced by ticks. The immune reaction can leave people with an allergy to red meat.
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- Relapsing fever at eMedicine.
- Relapsing fever~treatment at eMedicine.
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- "Ticks". medent.usyd.edu.au. Department of Entomology, University of Sydney and Westmead Hospital. November 7, 2003.
- Tick-Borne Diseases: Recommendations for Workers and Employers—National Institute for Occupational Safety and Health
- Tickborne Diseases—National Center for Infectious Diseases (CDC)
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- Ixodes Scapularis—3D animation of Deer or Blacklegged Tick from US Army site
- Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance Wikibooks