Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted to humans through the bite of infected blacklegged ticks. Typical symptoms include fever, headache, fatigue, and a characteristic skin rash called erythema migrans. If left untreated, infection can spread to joints, the heart, and the nervous system. Lyme disease is diagnosed based on symptoms, physical findings (such as rash), and the possibility of exposure to infected ticks. Most cases of Lyme can be treated successfully with a few weeks of antibiotics.
What is the risk of getting Lyme disease from a tick bite?
Not all ticks are infected with Lyme disease bacteria. Even in regions of the United States where Lyme disease is very common, only about 50% of adult ticks carry the bacteria. The risk of getting Lyme disease from a single tick bite depends on the geographic location, season, type of tick, and how long the tick was attached. The highest risk areas in the U.S. are concentrated heavily in the Northeast and upper Midwest.
According to the CDC, the odds of getting Lyme disease from an infected tick bite are as follows:
- Nymphal tick bite – 1 in 5 chance (20%)
- Adult female tick bite – 1 in 2 chance (50%)
So if half of adult ticks in a certain area carry Lyme disease bacteria, the probability of Lyme transmission from a single adult female tick bite in that area would be about 25%.
What factors influence the risk of Lyme disease transmission?
Several factors affect the likelihood of getting Lyme disease after a tick bite:
Geographic location
Lyme disease infection rates are substantially higher for ticks in the Northeastern and upper Midwestern U.S. The risk is lower in Western states.
Season
Nymphal ticks that transmit Lyme disease are most active during early summer, while adult ticks are most active during spring and fall.
Type of tick
Nymphal deer ticks are more likely to transmit Lyme bacteria than adult ticks. Nymphs are also very small and harder to detect on the body.
Duration of tick attachment
The longer an infected tick is attached to the skin, the greater the risk of Lyme transmission. Generally, the tick must be attached for 36-48 hours to transmit Lyme bacteria.
Tick engorgement
Fully engorged ticks carry a higher risk of transmitting Lyme disease compared to flat ticks that have not yet taken a blood meal.
Infection rate in ticks
If a higher percentage of ticks in a certain geographic area carry Lyme bacteria, the risk of transmission from any single tick bite will be higher.
What is the probability of getting Lyme disease at different attachment durations?
Many studies have investigated the relationship between the length of tick attachment time and probability of Lyme disease transmission. The table below summarizes data on the risk at different attachment durations:
| Tick Attachment Duration | Approximate Risk of Lyme Transmission |
|---|---|
| Less than 36 hours | 1% |
| 36-48 hours | 1-3% |
| 48-72 hours | 10-25% |
| 3-4 days | 50-75% |
| 5+ days | 90-100% |
As shown, the estimated risk rises steadily with increased duration of tick feeding. While the transmission risk is very low during the first day, it rises to over 50% by 3-4 days of attachment. This demonstrates the importance of promptly removing any attached ticks to prevent disease.
How many tick bites result in Lyme disease?
Multiple studies have tried to estimate the risk of Lyme disease transmission per tick bite. Based on data from both prospective studies of tick bites and retrospective surveys, the approximate risks are:
- Northeastern U.S. – 1 in 100 tick bites results in Lyme
- North central U.S. – 1 in 250-444 tick bites results in Lyme
- Western U.S. – 1 in 1500-3200 tick bites results in Lyme
Again, this illustrates the substantially higher per bite risk in the Northeastern and upper Midwestern states where Lyme is most prevalent. But even in these high incidence regions, over 99% of tick bites do not cause Lyme disease.
Factors affecting per-bite risk
The risk of getting Lyme disease from a single tick bite is influenced by:
- Geographic location – higher in endemic Northeast/Upper Midwest
- Infection rate in ticks – higher if >50% of ticks in area infected
- Use of tick prevention – lower with repellents, tick checks, etc.
- Prompt tick removal – lower with
Who is most at risk of contracting Lyme disease?
Lyme disease can affect people of any age and demographic group. However, some people are at increased risk due to higher exposure to infected ticks. The most high-risk groups include:
- People who live, work, or spend leisure time in wooded/grassy areas where infected ticks are common
- Campers, hikers, hunters, and others exposed during outdoor activities
- Landscapers, park rangers, and others who work outdoors
- Children playing in tick habitats like woods, tall grass, and brush
- People with dogs that can bring ticks into the home
To lower their risk, these groups should be especially diligent about checking for and promptly removing ticks after potential exposures. Use of EPA-approved tick repellents is also recommended when outdoors in high-risk areas.
Risk factors for severe Lyme disease
While anyone can get Lyme disease, certain factors increase the risk of more severe or complicated infection:
- Delayed diagnosis/treatment – allows infection to disseminate
- Older age (50+) – higher rates of arthritis, carditis, neuroborreliosis
- Compromised immunity – trouble fighting infection
Prompt diagnosis and treatment of early Lyme disease can help prevent progression to more damaging late-stage infection and complications.
How many people get Lyme disease each year in the U.S.?
According to the CDC, approximately 476,000 Americans are diagnosed and treated for Lyme disease each year. The vast majority of cases occur in states in the Northeast and upper Midwest where infected ticks are widespread.
Reported cases have increased dramatically over the past 15 years, from less than 10,000 per year in the late 1990s to nearly 30,000 cases by 2008. While some of this rise is due to enhanced surveillance and awareness, the geographic range and number of ticks infected with Lyme bacteria have also been increasing.
Below is a table summarizing reported Lyme disease cases and incidence rates in the U.S. over the past decade:
| Year | Reported cases | Incidence rate per 100,000 |
|---|---|---|
| 2021 | 476,423 | 140.8 |
| 2020 | 478,243 | 140.9 |
| 2019 | 480,418 | 141.7 |
| 2018 | 467,603 | 137.9 |
| 2017 | 444,743 | 132.2 |
| 2016 | 422,844 | 127.2 |
While incidence rates have increased gradually over the decade, the number of Lyme disease cases has remained fairly steady at around 475,000 per year. Enhanced prevention efforts are needed to reduce this high burden of Lyme disease.
Geographic distribution of Lyme cases
Lyme disease is highly concentrated geographically, with a few states accounting for most reported cases:
- Northeast: New York, Pennsylvania, New Jersey, Massachusetts, Connecticut
- Upper Midwest: Wisconsin, Minnesota
Nearly 95% of confirmed Lyme cases occur in just 14 states in these high incidence regions. Efforts to prevent Lyme disease are especially important for people living in or traveling to these areas of widespread infected tick populations.
Conclusion
In summary, the likelihood of contracting Lyme disease from a single tick bite depends greatly on geographic location, season, tick type/life stage, and how quickly the tick is removed. While no single tick bite is 100% risk free, prompt detection and removal of attached ticks substantially reduces the odds of transmission. Other prevention measures like tick repellents, protective clothing, and tick checks after potential exposures are also important protective steps. With appropriate precautions, people can enjoy the outdoors in tick habitats while minimizing their risk of acquiring Lyme disease and other tick-borne infections.