What Are the Odds of Getting Lyme Disease?

There is no single percentage
Risk varies from one tick bite to another
Lyme is not the only infection to consider

What are the odds of getting Lyme disease after a tick bite? There is no single percentage that accurately describes every bite.

An often-cited estimate places the average risk after a tick bite at roughly 2% to 3%. But that number can be misleading when applied to an individual patient. The actual risk depends on several factors, including the species of tick, geographic location, whether the tick carries Borrelia burgdorferi, how long it has been attached, and how engorged it has become.

In some circumstances, the risk can be substantially higher than the overall average.

Where Does the 2% to 3% Lyme Disease Risk Come From?

A large prospective European study by Hofhuis and colleagues examined 3,525 reported tick bites. Among participants with known outcomes, the overall risk of developing Lyme borreliosis was estimated at 2.6%.

But “overall” is the important word.

The 2.6% estimate averaged together bites with different attachment times, levels of engorgement, tick stages, and Borrelia infection status. It was not intended to mean that every tick bite carries a 2.6% risk.

Within the same study, risk increased as the tick became more engorged and remained attached longer. The highest modeled risk was 14.4% after the bite of a substantially engorged tick that tested positive for Borrelia.

Can the Risk of Lyme Disease Reach 20%?

Yes, a U.S. prospective study identified a small group in which the observed risk reached 20%.

Sood and colleagues studied deer tick bites in a Lyme-endemic area. Among 109 evaluable bites, four people developed evidence of Borrelia burgdorferi infection.

Attachment duration made a striking difference. Infection occurred after 3 of 15 bites in which the tick had been attached for at least 72 hours—an observed rate of 20%. In comparison, infection occurred after 1 of 94 bites when estimated attachment was less than 72 hours.

The numbers in the prolonged-attachment group were small, so 20% should not be presented as the universal risk from every engorged tick. However, the study demonstrates why a low average risk across all tick bites can underestimate risk in selected higher-risk exposures.

Why Does Tick Engorgement Matter?

A tick becomes progressively engorged as it takes a blood meal. Engorgement can therefore provide useful information about how long a tick may have been feeding.

In the Hofhuis study, estimated Lyme disease risk increased from 1.4% with low engorgement to 5.5% with substantial engorgement when other predictors were averaged. When substantial engorgement was combined with detection of Borrelia in the tick, the estimated risk reached 14.4%.

Engorgement is not a perfect clock, but it is clinically relevant. A flat tick that has only recently attached does not represent the same exposure as a markedly engorged tick that may have been feeding for days.

For more about transmission timing, see How Long Does It Take to Get Lyme Disease After a Tick Bite?

How Much Does Attachment Time Matter?

In general, the risk of Lyme disease increases as an infected blacklegged tick continues to feed. However, attachment time is often estimated rather than known with certainty.

People may not notice a nymphal tick because it is extremely small, and the tick may be discovered only after it has already fed for a substantial period.

Attachment time should therefore be considered together with tick species, engorgement, geographic exposure, symptoms, and other clinical information rather than used as an absolute cutoff.

Does Every Blacklegged Tick Carry Lyme Disease?

No. The percentage of blacklegged ticks infected with Borrelia burgdorferi varies considerably by geographic area, tick life stage, and local ecology.

This creates another reason why there cannot be one universal probability for developing Lyme disease after a tick bite. A bite in an area where infected ticks are common does not represent the same exposure as a bite in an area where Borrelia infection in ticks is uncommon.

Even when a tick carries Borrelia, transmission is not automatic. Conversely, an apparently low-risk exposure cannot guarantee that transmission did not occur.

Lyme Disease Is Not the Only Risk From a Blacklegged Tick

The probability of developing Lyme disease should not be confused with the total probability of acquiring a tick-borne infection.

Ixodes scapularis, the blacklegged or deer tick, can transmit several pathogens. These include Borrelia burgdorferi, which causes Lyme disease; Babesia microti, which causes babesiosis; and Anaplasma phagocytophilum, which causes anaplasmosis. Other pathogens can also be associated with blacklegged ticks.

That means a percentage estimating the risk of Lyme disease alone does not describe the entire infectious risk associated with the bite.

Learn more about this distinction in Can One Tick Carry Multiple Pathogens?

What Are the Odds of Getting Babesia From a Tick Bite?

The risk is harder to express as a single bite-by-bite percentage.

Babesia microti is transmitted by blacklegged ticks and is endemic in parts of the Northeast and upper Midwest. However, the probability that one particular tick bite will result in babesiosis depends on local prevalence of Babesia in ticks, tick stage, feeding characteristics, and other factors.

Studies of patients with diagnosed tick-borne disease can tell us how often Lyme disease and babesiosis occur together, but those percentages should not automatically be converted into the probability of acquiring Babesia from one particular tick bite.

This distinction is important. A patient may hear that the Lyme disease risk from a bite is relatively low and assume that the same number represents the risk of every infection carried by that tick. It does not.

For a broader discussion of the available coinfection data, see How Common Are Tick-Borne Coinfections?

Can One Tick Transmit More Than One Infection?

Yes. A blacklegged tick can carry more than one pathogen, although finding multiple pathogens in a tick does not prove that all of them will be transmitted or cause illness.

Clinical guidelines recognize Lyme disease coinfection with babesiosis and anaplasmosis, particularly in geographic areas where these infections overlap.

This is one reason symptoms after a tick bite should not always be interpreted solely through the question of Lyme disease risk.

What Should You Do After an Engorged Tick Bite?

An engorged blacklegged tick deserves more attention than simply applying an average 2% to 3% risk estimate.

The clinical assessment should consider the tick species, degree of engorgement, likely attachment time, geographic location, symptoms, and the possibility of other tick-borne infections.

The decision about whether to monitor or treat after a tick bite remains an area where recommendations and clinical approaches differ. I discuss my approach in What to Do After a Tick Bite: Wait or Treat?

Frequently Asked Questions

How likely am I to get Lyme disease from a tick bite?

There is no single percentage that applies to every tick bite. One large European study estimated an overall risk of 2.6%, but risk increased with greater engorgement, longer attachment, and detection of Borrelia in the tick. A U.S. study found infection in 20% of a small subgroup whose ticks had been attached for at least 72 hours.

What are the odds of getting Lyme disease from an engorged tick?

The risk depends on how long the tick fed, whether it carried Borrelia, and where the exposure occurred. In one European study, the estimated risk reached 14.4% when a substantially engorged tick tested positive for Borrelia. A U.S. study observed a 20% infection rate among 15 bites with estimated attachment of at least 72 hours. Neither percentage should be treated as a universal risk for every engorged tick.

Can I get bitten by a tick and not get Lyme disease?

Yes. Most recognized tick bites do not result in Lyme disease. Not every tick is an Ixodes tick, not every blacklegged tick carries Borrelia, and an infected tick does not transmit the organism with every bite.

Does removing a tick quickly eliminate the risk of Lyme disease?

Prompt removal lowers the risk, but it should not be interpreted as a guarantee that transmission could not have occurred. Attachment time is also frequently uncertain. Symptoms developing after a tick bite still deserve appropriate evaluation.

Can the same tick transmit Babesia and Lyme disease?

Yes. Blacklegged ticks can carry both Borrelia burgdorferi and Babesia microti. Carrying both organisms does not guarantee that both will be transmitted, but Lyme disease and babesiosis can occur together.

Is the 2% to 3% Lyme disease risk also the risk of Babesia?

No. The often-cited 2% to 3% estimate refers to Lyme borreliosis after a tick bite in a particular study population. It should not be used as the probability of developing babesiosis or another tick-borne infection after an individual bite.

Clinical Takeaway

The odds of getting Lyme disease cannot be reduced to one number. The frequently cited 2% to 3% estimate is an average across tick bites with very different characteristics.

Studies show that risk rises with longer attachment and greater engorgement, and selected higher-risk exposures have produced substantially higher estimates. At the same time, Lyme disease represents only one potential consequence of a blacklegged tick bite. Babesiosis, anaplasmosis, and other infections may also need to be considered depending on geography and the clinical picture.

The practical question after a tick bite is not simply “What is the average risk?” but “What were the characteristics of this particular exposure, and what infections could this tick have transmitted?”

Related Articles

These articles address related questions about tick-bite prevention, warning signs, and the limitations of prophylaxis.

Can One Dose of Doxycycline Prevent Lyme Disease?
Bad Signs After a Tick Bite: 7 Warnings You Shouldn’t Ignore
If a Tick Carries Two Infections, Will It Transmit Both?

References

1. Sood SK, Salzman MB, Johnson BJB, et al. Duration of tick attachment as a predictor of the risk of Lyme disease in an area in which Lyme disease is endemic. J Infect Dis. 1997;175(4):996-999.
2. Hofhuis A, van de Kassteele J, Sprong H, et al. Predicting the risk of Lyme borreliosis after a tick bite, using a structural equation model. PLoS One. 2017;12(7):e0181807.
3. Yeh MT, Bak JM, Hu R, Nicholson MC, Kelly C, Mather TN. Determining the duration of Ixodes scapularis attachment to tick-bite victims. J Med Entomol. 1995;32(6):853-858.
4. Centers for Disease Control and Prevention. How Babesiosis Spreads. CDC.
5. Centers for Disease Control and Prevention. Transmission and Epidemiology: Anaplasmosis. CDC.
6. Lantos PM, Rumbaugh J, Bockenstedt LK, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America, American Academy of Neurology, and American College of Rheumatology: 2020 Guidelines for the Prevention, Diagnosis and Treatment of Lyme Disease. Clin Infect Dis. 2021;72(1):e1-e48.
7. Krause PJ, Auwaerter PG, Bannuru RR, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America: 2020 Guideline on Diagnosis and Management of Babesiosis. Clin Infect Dis. 2021;72(2):e49-e64.

Dr. Daniel Cameron, MD, MPH
Lyme disease clinician with over 30 years of experience and past president of ILADS.

Symptoms • Testing • Coinfections • Recovery • Pediatric • Prevention