Glossary

Lyme disease glossary

Plain-English definitions of 148 Lyme disease terms, from Borrelia to tick drags. Where experts disagree, we label the term Debated and show both sides.

By the Beat Lyme editorial teamChecked against CDC, NIH and the 2020 IDSA/AAN/ACR guidelineUpdated October 11, 2026Not medical advice

Part of How to beat LymeStep 7 of 7: Learn and connectSee all 7 steps →

Use this glossary to decode a test report, a doctor visit or a news story. Search for a word, filter by topic, or jump to a letter. Every term has its own link, so you can share exactly the definition you need. This is general information, not medical or legal advice.

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0–9

2-UndecanonePrevention

An EPA-registered repellent ingredient originally derived from wild tomato plants. Check the label before using it on children.

Related: EPA-registered repellent

2020 IDSA/AAN/ACR guidelineOrganizations & debate

Joint U.S. guidelines for preventing, diagnosing and treating Lyme disease, written by a 36-member panel and published in Clinical Infectious Diseases in November 2020. CDC’s treatment advice follows them.

4-Poster deer stationPrevention

A feeding station that rubs tick-killing pesticide onto deer as they eat. Study results have been mixed, and the devices can be costly and labor-intensive and may need permits.

Related: White-tailed deer, Acaricide

4Dx testPets

Short for SNAP 4Dx Plus, an in-clinic dog screening test. It checks for heartworm and for antibodies to Lyme (using C6), Anaplasma and Ehrlichia.

Related: C6 test, Lyme disease in dogs

504 planLiving with Lyme

A school plan for supports a student with a disability needs, based on Section 504 of the Rehabilitation Act of 1973. Section 504 bars disability discrimination in programs that get federal funds, including public schools.

General information, not legal advice. Check details with the agency named.

Lyme disease and school →

A

AAN (American Academy of Neurology)Organizations & debate

A professional society of neurologists and a co-author of the 2020 Lyme guideline.

AcaricidePrevention

A pesticide that kills ticks. It can reduce ticks in treated areas, but CDC says not to rely on spraying alone to prevent infection.

ACIPOrganizations & debate

The Advisory Committee on Immunization Practices, CDC’s vaccine advisers. If the FDA approves a Lyme vaccine, CDC will work with ACIP to decide who should get it.

ACR (American College of Rheumatology)Organizations & debate

A professional society of rheumatologists (arthritis doctors) and a co-author of the 2020 Lyme guideline.

Acrodermatitis chronica atrophicans (ACA)Disease

A late skin form of Lyme disease seen mostly in Europe and caused by Borrelia afzelii. It starts as bluish-red or violet skin, often on a hand, elbow, ankle or knee, that slowly becomes thin and fragile. It is treated with antibiotics and responds best when caught early.

ADA (Americans with Disabilities Act)Living with Lyme

A federal civil rights law. It protects people with a physical or mental condition that substantially limits a major life activity, and its job rules apply to employers with 15 or more employees.

General information, not legal advice. Check details with the agency named.

Working with Lyme disease →

Alpha-gal syndrome (AGS)Look-alikes & co-infections

A serious, potentially life-threatening allergy to alpha-gal, a sugar found in most mammals but not in people. In the U.S. it is most often linked to lone star tick bites, and reactions follow eating red meat or using other mammal products.

Alpha-gal syndrome guide →

American dog tick (Dermacentor variabilis)Ticks

A common tick east of the Rocky Mountains. It cannot transmit Lyme bacteria, but it can spread Rocky Mountain spotted fever and tularemia.

Ticks that carry Lyme disease →

AmoxicillinTreatment

A penicillin-like antibiotic. It is one of the three standard oral choices for Lyme disease, along with doxycycline and cefuroxime axetil.

Related: Antibiotic, Doxycycline, Cefuroxime axetil

Lyme disease treatment guide →

AnaplasmosisLook-alikes & co-infections

An infection with Anaplasma phagocytophilum bacteria, spread mainly by blacklegged and western blacklegged ticks. Fever, chills, severe headache and muscle aches usually start 1 to 2 weeks after the bite. Doxycycline is the drug of choice.

AntibioticTreatment

A medicine that kills bacteria or stops them from growing. Antibiotics are the standard treatment for Lyme disease; CDC says most cases are treated with 10 days to 4 weeks of them. They do not work against viruses such as Powassan.

Lyme disease treatment guide →

AntibodyTesting

A protein the immune system makes to fight a specific germ. Most Lyme tests look for these antibodies rather than for the bacteria themselves.

Related: IgM antibodies, IgG antibodies

Asian longhorned tickTicks

An invasive tick first reported in the U.S. in 2017 and now found in many eastern and midwestern states. One experimental study found it is not likely to help spread Lyme bacteria.

Related: Tick

At-home Lyme test kitsTesting

Kits sold for testing at home. MedlinePlus warns that some use methods that are not proven to work, so results may be wrong.

Related: FDA-cleared test

Attachment timeTicks

How long a tick has been feeding on you. In general, an infected blacklegged tick must be attached for more than 24 hours to spread Lyme bacteria, so removing ticks quickly lowers the risk.

How long a tick must be attached →

AzithromycinTreatment

An antibiotic CDC lists as a backup for early Lyme disease when someone cannot take the standard drugs. It is less effective, so patients should be followed closely to be sure symptoms clear.

Related: Antibiotic, Erythema migrans (EM)

B

BabesiosisLook-alikes & co-infections

An infection with tiny Babesia parasites that invade red blood cells, spread mostly by blacklegged ticks and less often by blood transfusion. Many people have no symptoms, but it can be serious for people without a spleen, with weak immune systems or over 50.

BartonellaLook-alikes & co-infectionsDebated

A group of bacteria that some providers test for in people with suspected Lyme disease. CDC says there is no evidence that Bartonella is spread by ticks.

Blacklegged tick (deer tick, Ixodes scapularis)Ticks

The main tick that spreads Lyme disease across much of the eastern U.S. It can also spread anaplasmosis, babesiosis, Borrelia miyamotoi, one type of ehrlichiosis and Powassan virus.

Ticks that carry Lyme disease →

Borrelia afzeliiDisease

A Lyme-causing species found mostly in Europe. It is the cause of acrodermatitis chronica atrophicans, a late skin form of Lyme disease.

Borrelia burgdorferiDisease

The main bacterium that causes Lyme disease in the United States. It is a spirochete, a long, corkscrew-shaped germ, and it lives in ticks and small wild animals.

What causes Lyme disease →

Borrelia mayoniiDisease

A rarer Lyme-causing bacterium that CDC, Mayo Clinic and Midwest health officials reported in 2016. So far it has been found only in the upper Midwest. Early patients had nausea, vomiting and widespread rashes rather than a single rash.

Borrelia miyamotoi (hard tick relapsing fever)Look-alikes & co-infections

A bacterium spread by the same blacklegged ticks that carry Lyme disease; it causes hard tick relapsing fever. It usually brings a single bout of fever with chills, headache, aches and tiredness about 2 weeks after a bite. Many people recover on their own, and symptoms usually improve within days of starting antibiotics.

Brain fogLiving with Lyme

An everyday phrase for trouble thinking, focusing or remembering. CDC lists difficulty thinking among the symptoms some people have after Lyme treatment.

Lyme disease fatigue →

Brown dog tickTicks

A tick found around the world. In the southwestern U.S. and along the U.S.–Mexico border it can spread Rocky Mountain spotted fever. CDC does not list it as a Lyme carrier.

Bull’s-eye rashDisease

A popular name for an erythema migrans rash that clears in the middle and looks like a target. Many Lyme rashes are not bull’s-eyes, so a solid red, spreading patch can still be Lyme.

Lyme disease rash guide →

C

C6 testPets

An antibody test for the C6 peptide of Lyme bacteria, used as a first-step blood test in people and in in-clinic tests for dogs. A positive result shows exposure, not necessarily disease.

Lyme symptoms in dogs →

Canine Lyme vaccinePets

Lyme vaccines for dogs, available as bacterin and recombinant types, with protection lasting about a year. The Merck Veterinary Manual advises testing exposed dogs before vaccinating, and the vaccine does not treat an existing infection.

Related: Lyme disease in dogs, C6 test

Lyme vaccine for dogs →

CD57 testTesting

A count of one kind of white blood cell, sometimes promoted for Lyme. The 2020 guideline advises against using it to diagnose Lyme disease.

CDC (Centers for Disease Control and Prevention)Organizations & debate

The U.S. public health agency that tracks Lyme disease cases, studies ticks and publishes guidance for patients and clinicians.

Related: NNDSS, Surveillance case definition

CeftriaxoneTreatment

A cephalosporin antibiotic given into a vein (IV) or as a shot. For Lyme it is used in certain serious cases, such as severe meningitis or carditis, or arthritis that did not get better with oral antibiotics.

Lyme disease treatment guide →

Cefuroxime axetilTreatment

An oral antibiotic from the cephalosporin family. It is one of the three standard oral choices for Lyme disease.

Related: Antibiotic, Amoxicillin, Doxycycline

Lyme disease treatment guide →

Chronic Lyme diseaseDiseaseDebated

A term used in two very different ways. Mainstream view (CDC, IDSA): the term is discouraged because it implies an ongoing infection that research has not shown; “PTLDS” is preferred for symptoms after treatment. Patient-advocate view (ILADS and many patients): it describes long-lasting illness that may come from infection that persists, which they say can justify more treatment.

Chronic Lyme disease and PTLDS →

Clinical diagnosisTesting

A diagnosis based on symptoms, an exam and likely tick exposure rather than lab results. A typical EM rash in someone who may have been exposed to ticks is diagnosed this way.

Co-infectionLook-alikes & co-infections

Getting more than one tickborne germ at the same time, since a single blacklegged tick can carry several. Anaplasmosis and babesiosis are common co-infections; CDC reports Anaplasma co-infection in fewer than 10% of Lyme patients.

Confirmed, probable and suspect casesData

Labels for how strongly the evidence supports a reported case. In high-incidence states, cases are probable or suspect based on lab results; low-incidence states can also count confirmed cases.

CSF:serum antibody indexTesting

A test that compares Lyme antibodies in spinal fluid with those in blood. The 2020 guideline says spinal fluid antibody testing should include this comparison.

CSTEOrganizations & debate

The Council of State and Territorial Epidemiologists. Its 2021 position statement set the Lyme surveillance case definition used since 2022.

D

DEETPrevention

A synthetic, long-used skin repellent found in hundreds of EPA-registered products. It repels ticks as well as mosquitoes.

Related: EPA-registered repellent, Picaridin

Lyme disease prevention →

DoxycyclineTreatment

A tetracycline-type antibiotic and one of the main oral treatments for Lyme disease. It is also the recommended treatment for anaplasmosis, ehrlichiosis and Rocky Mountain spotted fever. It can make skin burn more easily in the sun, and MedlinePlus advises taking it with a full glass of water.

Eating while on antibiotics →

E

Early disseminated Lyme diseaseDisease

The stage when the bacteria have started to spread, usually weeks to months after the bite. Signs can include extra EM rashes on other parts of the body, facial palsy, nerve pain, meningitis and heart rhythm problems.

Stages of Lyme disease →

Early localized Lyme diseaseDisease

The first stage of Lyme disease, when the infection has not yet spread through the body. Signs can include the EM rash, fever, chills, headache, tiredness, swollen lymph nodes and muscle or joint aches.

Stages of Lyme disease →

EhrlichiosisLook-alikes & co-infections

A group of infections caused by Ehrlichia bacteria, spread mainly by lone star ticks and, for one type, blacklegged ticks. Fever, chills, headache and muscle pain begin 1 to 2 weeks after the bite, and doxycycline is the treatment of choice.

EIA / ELISATesting

An enzyme immunoassay, also called ELISA: a lab test that detects antibodies in a blood sample. In Lyme testing it is the first step, and in modified testing it is used for both steps.

EndemicTicks

Describes a disease that is regularly found in a certain area. A “Lyme-endemic area” is a place where Lyme disease is known to occur, and doctors weigh this when judging risk after a bite.

Lyme disease by state →

EPA-registered repellentPrevention

A repellent the Environmental Protection Agency has evaluated for safety and effectiveness. Look for the EPA registration number on the label; CDC recommends these products for preventing tick bites.

Related: DEET, Picaridin, IR3535, Minimum-risk repellent

Lyme disease prevention →

Equivocal resultTesting

A borderline result that is neither clearly positive nor clearly negative. In two-tier testing, an equivocal first test is followed by the second test.

Erythema migrans (EM)Disease

The expanding red rash that is the most common sign of early Lyme disease. It appears in about 70 to 80 percent of infected people, usually 3 to 30 days after the bite (about 7 days on average), and can grow to 12 inches (30 cm) or more. It often feels warm but is rarely itchy or painful.

Lyme disease rash guide →

F

Facial palsyDisease

Weakness or drooping on one or both sides of the face. In Lyme disease it is a sign that the infection has spread, and CDC lists oral doxycycline as its treatment.

Lyme disease and the face →

False negativeTesting

A test that reads negative even though the infection is there. Lyme antibody tests can be falsely negative in the first 4 to 6 weeks, which is why a typical EM rash is treated without waiting for a test.

False positiveTesting

A test that reads positive even though the infection is not there. Lyme tests can cross-react with other conditions, such as syphilis, relapsing fever, rheumatoid arthritis and Epstein-Barr virus infection.

FDA (Food and Drug Administration)Organizations & debate

The U.S. agency that reviews medical tests, drugs and vaccines. Any future Lyme vaccine must be FDA-approved before CDC can recommend it.

FDA-cleared testTesting

A test that the U.S. Food and Drug Administration has reviewed and allowed on the market. CDC recommends FDA-cleared Lyme tests and will only recommend new ones after FDA clearance.

FMLA (Family and Medical Leave Act)Living with Lyme

A federal law giving eligible employees up to 12 weeks of unpaid, job-protected leave in a 12-month period for a serious health condition, their own or a close family member’s. Leave can be taken in separate blocks (intermittent leave) when medically needed.

General information, not legal advice. Check details with the agency named.

Working with Lyme disease →

G

Gulf Coast tickTicks

A tick found mainly in the southeastern U.S. It can spread Rickettsia parkeri rickettsiosis, a type of spotted fever, and is not listed by CDC as a Lyme carrier.

Related: Tickborne disease

H

Heart block (AV block)Disease

A problem where the electrical signal from the heart’s upper chambers to its lower chambers is slowed or stopped. In Lyme carditis it can change quickly, and some patients need a temporary pacemaker while they are treated.

Related: Lyme carditis

Lyme carditis guide →

High-incidence stateData

A state averaging at least 10 confirmed Lyme cases per 100,000 people for three years in a row. Since 2022 these states count cases mainly from lab results; 15 states and DC met the bar when the rule was written.

Lyme disease by state →

I

IDSA (Infectious Diseases Society of America)Organizations & debate

A professional society of infectious disease specialists. It led the 2020 Lyme guideline with the AAN and ACR.

IEP (individualized education program)Living with Lyme

A written plan for a child with a disability who qualifies for special education and related services under the federal IDEA law.

General information, not legal advice. Check details with the agency named.

Related: 504 plan

Lyme disease and school →

IgG antibodiesTesting

A type of antibody that builds up later in an infection. Lyme antibodies can stay in the blood for months to years, even after the bacteria are gone, so a positive test cannot show whether someone is cured.

Related: IgM antibodies, Antibody, Seropositive

IgM antibodiesTesting

A type of antibody the body makes early in an infection. CDC says a positive IgM result should be disregarded once a person has been sick for more than 30 days.

ILADS (International Lyme and Associated Diseases Society)Organizations & debateDebated

A medical society whose 2014 treatment guidelines differ from IDSA’s. They favor longer first courses, support retreatment for ongoing symptoms based on clinical judgment and shared decisions, and rate their own supporting evidence as very low quality.

IncidenceData

The number of new cases in a group over a period of time, usually given per 100,000 people per year. It allows fair comparisons between places of different sizes.

Related: High-incidence state

Infection-associated chronic conditions and illnesses (IACCI)Disease

A CDC umbrella term for long-lasting symptoms that can follow many kinds of infection, not just Lyme. CDC groups lingering symptoms after Lyme treatment with these conditions and says the science is still developing.

IR3535Prevention

A synthetic skin repellent related to a natural amino acid (beta-alanine). It is EPA-registered for use against ticks and mosquitoes.

Related: EPA-registered repellent

IV antibioticsTreatment

Antibiotics given directly into a vein. For Lyme disease they are used for certain severe cases, often in the hospital; most people do well with antibiotics by mouth.

Lyme disease treatment guide →

J

Jarisch-Herxheimer reaction (“herx”)Disease

A short flare of symptoms, sometimes with fever, that can start within the first 24 hours of antibiotic treatment for early Lyme. The 2020 guideline says it is usually mild, goes away on its own and does not return later in treatment, so symptoms that start later should not be called a herx.

Related: Doxycycline, Antibiotic

L

Laboratory-developed test (LDT)Testing

A test designed, made and used within a single lab. A 2024 FDA rule to bring these tests under FDA oversight was struck down by a federal court in 2025, and FDA then withdrew it.

Larva (tick)Ticks

The first, tiny life stage after a tick hatches from its egg. Each stage needs one blood meal; a larva that feeds on an infected animal, such as a mouse, can pick up Lyme bacteria and pass them on at its next meal.

Related: Nymph, White-footed mouse, Reservoir host

Late Lyme disease (late disseminated)Disease

The stage that can appear months or even years after an infection that was not treated. Joint pain and swelling, often in the knee, are most common, and nerve problems can also occur.

Stages of Lyme disease →

Leaf litterTicks

The layer of fallen leaves on the ground. It keeps ticks moist and sheltered, which is why CDC suggests staying out of leaf litter and clearing it from yards.

Related: Questing, Tick-safe yard

Lone star tick (Amblyomma americanum)Ticks

An aggressive biting tick found in the South, Midwest and Northeast; adult females have a white dot on the back. It cannot transmit Lyme bacteria, but it is linked to ehrlichiosis, tularemia, STARI, Heartland and Bourbon viruses and alpha-gal syndrome.

Ticks that carry Lyme disease →

Long-term antibiotic therapyTreatmentDebated

Antibiotics taken for months or longer. Mainstream view: placebo-controlled trials found it no better than placebo for lingering symptoms, and it is linked to serious harm such as sepsis and colitis. Advocate view: ILADS supports retreatment when ongoing infection is judged a possible cause and quality of life is affected.

Chronic Lyme disease and PTLDS →

Low-incidence stateData

A state that has not met the high-incidence bar. These states still need clinical information, such as a rash or other Lyme signs, to count a case as probable or confirmed.

Lyme disease by state →

Lumbar puncture (spinal tap)Testing

A procedure that collects cerebrospinal fluid from the lower back with a needle. It may be used when Lyme could be affecting the nervous system, though CDC says it is not needed to diagnose Lyme meningitis.

Lyme arthritisDisease

Joint inflammation caused by Lyme bacteria, usually one to a few months after infection. It typically causes clear swelling in one or a few joints, most often the knee, and is first treated with oral antibiotics.

Lyme arthritis guide →

Lyme borreliosisDisease

Another name for Lyme disease, used often in Europe, in research and in veterinary medicine. “Borreliosis” simply means an illness caused by Borrelia bacteria.

Related: Lyme disease

Lyme carditisDisease

A heart problem that happens when Lyme bacteria get into heart tissue and disrupt its electrical signals. It occurs in about 1 in 100 reported cases and can cause lightheadedness, fainting, shortness of breath, palpitations or chest pain. Most people recover within 1 to 6 weeks of treatment.

Lyme carditis guide →

Lyme diseaseDisease

An infection caused by Borrelia bacteria that people get from the bite of an infected blacklegged tick. In the U.S. it is most common in the Northeast, mid-Atlantic and upper Midwest. Most people treated early with antibiotics recover quickly and completely.

What is Lyme disease? →

Lyme disease in catsPets

Cats can be exposed to Lyme bacteria, but illness from it is rare.

Related: Lyme disease in dogs

Lyme disease in cats →

Lyme disease in dogsPets

Most dogs that test positive never get sick. When signs appear, they can include fever, low energy, poor appetite, swollen lymph nodes and lameness that shifts from leg to leg.

Lyme disease in dogs →

Lyme disease in horsesPets

Horses can test positive for Lyme antibodies, but most seropositive horses show no signs of illness.

Related: Lyme disease in dogs, Seropositive

Lyme disease in horses →

Lyme meningitisDisease

Inflammation of the lining around the brain and spinal cord caused by Lyme bacteria. It can bring headache, fever, stiff neck and sensitivity to light, and it is treated with oral or IV antibiotics depending on how severe it is.

Neurological Lyme disease →

Lyme nephritis (Lyme nephropathy)Pets

A rare but serious kidney disease in dogs that is thought to involve immune complexes in the kidney filters and causes protein loss. A direct cause-and-effect link with Lyme has not been fully established.

Related: Lyme disease in dogs

Lyme symptoms in dogs →

Lyme-literate doctor (LLMD)Organizations & debateDebated

An informal patient-community term for clinicians who focus on Lyme disease, especially long-lasting symptoms. It is not an official credential or board certification, and approaches vary widely.

LYMErixPrevention

The only human Lyme vaccine ever licensed by the FDA, approved for people aged 15 to 70. The maker stopped selling it in 2002, citing low demand, and people vaccinated back then are no longer protected.

History of Lyme disease →

Lymphocyte transformation test (LTT)Testing

A test that measures how white blood cells react to Lyme proteins. The 2020 guideline advises against it because it lacks independent validation.

M

Mast yearTicks

A year when oak trees drop a very large crop of acorns. More acorns can mean more mice, which can raise Lyme risk about two years later.

Related: White-footed mouse

Minimum-risk repellentPrevention

A product made with ingredients such as citronella, cedar, geranium, peppermint or soybean oil that EPA exempts from registration because they pose minimal risk. EPA has not evaluated these products for effectiveness.

Related: EPA-registered repellent

Modified two-tier testing (MTTT)Testing

A newer two-step method that uses a second EIA instead of a Western blot. FDA cleared tests for this approach in July 2019, and more labs now use it.

Lyme disease testing guide →

N

Neuroborreliosis (neurologic Lyme disease)Disease

Lyme disease that affects the nervous system. It most often happens in the early disseminated stage and can cause facial palsy, meningitis or shooting nerve pain. Most people respond well to treatment and fully recover.

Neurological Lyme disease →

NIAIDOrganizations & debate

The National Institute of Allergy and Infectious Diseases, part of NIH. It leads federal research to better diagnose, treat and prevent Lyme disease.

NNDSSData

The National Notifiable Diseases Surveillance System. State and local health departments send it case data with personal details removed, and CDC publishes the national Lyme counts from it.

NymphTicks

The young tick stage between larva and adult, about the size of a poppy seed. Nymphs pose a high risk because they are small and plentiful and are most active in spring and early summer, so many people never notice the bite.

Related: Larva (tick), Attachment time

Ticks that carry Lyme disease →

O

Oil of lemon eucalyptus (OLE)Prevention

A repellent made from the lemon-scented gum tree and refined to concentrate its active ingredient, PMD (para-menthane-3,8-diol), which is also made synthetically. CDC says not to use OLE or PMD products on children under 3, and pure lemon eucalyptus essential oil is not the same thing.

Related: EPA-registered repellent

OspA (outer surface protein A)Prevention

A protein on the surface of Lyme bacteria. It was the target of the LYMErix vaccine and is the target of the newer candidate LB6V.

P

PacingLiving with Lyme

Planning activity to stay within your energy limits so a good day does not lead to a “crash.” CDC describes this approach, with frequent breaks and tasks split into smaller steps, for conditions with long-lasting fatigue.

Lyme disease fatigue →

PCR (polymerase chain reaction)Testing

A test that looks for the bacteria’s genetic material (DNA) instead of antibodies. For Lyme it is mainly used on joint fluid when more information is needed; guidelines advise against PCR of blood or spinal fluid for suspected nerve involvement because it misses many cases.

Related: Lyme arthritis

PermethrinPrevention

An insecticide used on clothing, boots and camping gear rather than on skin. CDC suggests 0.5% permethrin, which stays protective through several washings; pre-treated clothing is also sold.

Related: Tick check, Acaricide, Tick tubes

Lyme disease prevention →

Persistent (antibiotic-refractory) Lyme arthritisDisease

Joint swelling and pain that continue after two courses of antibiotics. CDC notes that more antibiotics have not been shown to help at that point; the cause is thought to be immune-driven, and a rheumatologist may help.

Related: Lyme arthritis, Ceftriaxone

Lyme arthritis guide →

PicaridinPrevention

A synthetic skin repellent that is an EPA-registered alternative to DEET for ticks and mosquitoes.

Related: EPA-registered repellent, DEET

Lyme disease prevention →

PICC lineTreatment

A thin tube placed in an arm vein that reaches a large vein near the heart, used for long courses of IV medicine. CDC has reported serious bloodstream infections, septic shock and a death in people given prolonged IV antibiotics through these lines for “chronic Lyme.”

Post-exposure prophylaxis (PEP)Treatment

A preventive antibiotic given after a tick bite, before any symptoms. The 2020 guideline recommends a single dose of doxycycline only for high-risk bites: an identified Ixodes tick, a highly endemic area, attachment of 36 hours or more, and treatment within 72 hours of removal.

Related: Doxycycline, Attachment time, Endemic

Tick bite: what to do →

Post-treatment Lyme disease syndrome (PTLDS)Disease

CDC’s term for fatigue, body aches or trouble thinking that last after standard Lyme treatment. The cause is unknown. Studies found long-term antibiotics work no better than placebo and can cause serious harm, and recovery can take many months.

Chronic Lyme disease and PTLDS →

Powassan virus diseaseLook-alikes & co-infections

A rare but increasing virus spread by blacklegged, groundhog and squirrel ticks. It can cause fever, headache and weakness, and sometimes serious brain swelling. There is no specific medicine or vaccine, so preventing bites matters.

Q

QuestingTicks

How ticks wait for a host. They hold onto grass or shrubs with their back legs and stretch out their front legs, then climb on when a person or animal brushes past.

R

Radiculopathy (radiculoneuritis)Disease

Pain, numbness, tingling or weakness caused by inflamed nerve roots near the spine. In Lyme disease it can cause sharp, shooting pain in the arms, legs or trunk with no injury to explain it.

Neurological Lyme disease →

Reasonable accommodationLiving with Lyme

A change at work, or in how things are usually done, that helps a qualified person with a disability do the job, such as a modified schedule, breaks or leave. Employers must provide one unless it would cause undue hardship, meaning significant difficulty or expense.

General information, not legal advice. Check details with the agency named.

Working with Lyme disease →

Relapsing feverLook-alikes & co-infections

A group of illnesses caused by Borrelia bacteria distantly related to the Lyme germ. They cause bouts of fever, headache and muscle pain that last several days, separated by stretches of feeling well.

Reported vs. estimated casesData

Reported cases are those sent to CDC by health departments (89,468 in 2023). Estimated cases come from insurance data: about 476,000 people a year are diagnosed and treated, which CDC says may be an overestimate.

Related: NNDSS, Underreporting

How common is Lyme disease? →

Reservoir hostTicks

An animal that carries a germ and passes it to ticks that feed on it, which keeps the germ going in nature. For Lyme disease, small mammals, especially white-footed mice, are key reservoirs.

Rocky Mountain spotted fever (RMSF)Look-alikes & co-infections

A tickborne bacterial disease that causes fever, headache and a rash that often appears a few days after the fever. It can be deadly if not treated early, and doctors start doxycycline right away without waiting for test results.

Rocky Mountain wood tickTicks

A tick of the Rocky Mountain states, usually found at higher elevations. It can spread Rocky Mountain spotted fever, Colorado tick fever and tularemia; CDC does not list it as a Lyme carrier.

Rodent bait boxPrevention

A device, placed by licensed applicators, that treats wild mice with a tick-killing product as they feed. Several studies found large drops in ticks near the boxes.

Related: White-footed mouse, Acaricide, Tick tubes

S

Septic arthritisLook-alikes & co-infections

A bacterial infection inside a joint that can look like Lyme arthritis, especially in children. Septic arthritis may need surgery, while Lyme arthritis generally does not.

Related: Lyme arthritis

SeroconversionTesting

The point when the body has made enough antibodies to show up on a test. It can take several weeks, and early antibiotic treatment may keep it from happening at all.

Related: False negative, Antibody

SeropositiveTesting

Having a positive antibody test. In people and pets, it shows past exposure but does not prove active infection on its own.

Related: IgG antibodies, C6 test

Specialty (“Lyme-literate”) labsTestingDebated

Private labs that market Lyme testing, sometimes outside insurance. CDC notes some offer tests not cleared or approved by the FDA. Advocates point out that standard tests can miss early infection; CDC and the 2020 guideline warn that tests without independent validation can give misleading results.

Lyme disease testing guide →

SpirocheteDisease

A type of bacterium with a long, spiral or corkscrew shape. The bacteria that cause Lyme disease are spirochetes.

SSDI (Social Security Disability Insurance)Living with Lyme

Monthly Social Security payments for people who have worked long enough and can no longer do substantial paid work because of a condition expected to last at least 12 months. SSI is a separate program based on financial need.

General information, not legal advice. Check details with the agency named.

Lyme disease and disability benefits →

Standard two-tier testing (STTT)Testing

The original two-step method: an EIA first, followed by a Western blot if needed.

Lyme disease testing guide →

STARI (Southern tick-associated rash illness)Look-alikes & co-infections

A rash illness that can follow a lone star tick bite, mostly in the southern U.S. Its red, expanding rash looks a lot like the Lyme rash, but STARI is not caused by Lyme bacteria, its cause is unknown, and there is no blood test for it.

Surveillance case definitionData

A uniform set of rules for counting cases in public health data. It is not meant for diagnosing patients. The 2022 Lyme update changed how cases are counted, which is a main reason reported numbers jumped that year.

How common is Lyme disease? →

T

TickTicks

A small parasite that feeds on the blood of animals and people. Ticks cannot fly or jump. In the U.S. they can spread bacteria, viruses and parasites that cause disease.

Ticks that carry Lyme disease →

Tick checkPrevention

A full-body search for ticks after time outdoors, including under the arms, behind the ears and behind the knees, plus pets and gear. CDC also suggests showering within two hours and tumble-drying clothes on high heat for 10 minutes.

Related: Tick removal, Permethrin

Lyme disease prevention →

Tick drag (tick dragging)Ticks

A way researchers count ticks by pulling a white cloth over grass or leaf litter. Questing ticks grab the cloth, and the count shows how many ticks live in an area.

Related: Questing

Tick paralysisTicks

Weakness or paralysis thought to come from a toxin in the saliva of certain ticks. It typically goes away within 24 hours after the tick is removed.

Related: Tick, Tick removal

Tick removalTicks

CDC advises grasping the tick close to the skin with fine-tipped tweezers and pulling away with steady, even pressure, without twisting. Skip folk remedies like nail polish, petroleum jelly or heat, then clean the bite and your hands.

Tick bite: what to do →

Tick testingTesting

Sending a removed tick to a lab to check it for germs. CDC and the 2020 guideline advise against using the result for treatment decisions: an infected tick does not mean you are infected, and a negative result can falsely reassure. Identifying the tick’s species is more useful.

Tick bite: what to do →

Tick tubesPrevention

Cardboard tubes stuffed with permethrin-treated cotton that mice carry off for nesting, killing ticks that feed on them. Study results have been mixed, partly because they depend on mice collecting the cotton.

Tick-bite bumpDisease

A small red bump that often shows up right after any tick bite, much like a mosquito bite. It usually fades in 1 to 2 days and is not a sign of Lyme disease by itself.

Related: Erythema migrans (EM), Tick removal

Tick bite: what to do →

Tick-safe yardPrevention

Yard changes that make it harder for ticks to reach people: clear leaf litter and brush, mow often, and put a 3-foot band of wood chips or gravel between lawns and woods.

Related: Leaf litter, Acaricide

Lyme disease prevention →

Tickborne diseaseLook-alikes & co-infections

Any illness spread by tick bites. CDC lists 16 tickborne diseases and conditions in the U.S., and many cause similar symptoms such as fever, chills and aches.

Related: Co-infection, Tick

Ticks that carry Lyme disease →

TularemiaLook-alikes & co-infections

A potentially serious illness caused by Francisella tularensis bacteria. People can get it from tick or deer fly bites, handling infected rabbits or rodents, or contaminated water or dust. Most cases can be treated with antibiotics.

Two-tier testing (two-step testing)Testing

The CDC-recommended way to test blood for Lyme antibodies. A first test is run; if it is positive or unclear, a second test is run on the same sample, and both must be positive for an overall positive result.

Lyme disease testing guide →

U

UnderreportingData

When surveillance misses real cases. CDC notes that every disease tracking system misses some cases and misclassifies others, so reported numbers are not an exact count.

Related: Reported vs. estimated cases

Unproven Lyme treatmentsTreatment

Therapies offered for lingering Lyme symptoms without good evidence that they work, such as IV hydrogen peroxide, hyperbaric oxygen, colloidal silver and stem cell transplants. A CDC report lists these among unproven treatments and describes serious infections tied to long IV treatment courses.

Lyme disease myths →

Urine antigen testTesting

A test that looks for Lyme proteins in urine. The 2020 guideline recommends against it because it has not been independently validated.

V

VLA15 (LB6V) vaccine candidatePrevention

An experimental human Lyme vaccine from Valneva and Pfizer, now called LB6V. It targets the OspA protein of several Lyme strains and has completed Phase 3 trials, but no Lyme vaccine is available yet.

Lyme disease vaccine →

W

Western blacklegged tick (Ixodes pacificus)Ticks

The tick that spreads Lyme disease along the Pacific coast, especially in northern California. It can also spread anaplasmosis and hard tick relapsing fever.

Ticks that carry Lyme disease →

Western blot (immunoblot)Testing

A more detailed antibody test used as the second step in standard two-tier testing. Results are reported separately for IgM and IgG antibodies.

White-footed mouseTicks

A small woodland mouse considered a leading reservoir of Lyme bacteria. Young ticks that feed on an infected mouse can pick up the bacteria and pass them on at their next bite.

White-tailed deerTicks

Deer are a major food source for adult blacklegged ticks, but they rarely become infected and are poor reservoirs for Lyme bacteria. Large deer herds can still help support tick populations.

Missing a term? Tell us which word you ran into, and we will check it against official sources and add it.

Ready for next steps? See How to beat Lyme, our step-by-step path from tick bite to living well.

Sources

Every definition was checked against these sources. We paraphrase rather than copy, and we update terms as guidance changes.