Borrelia is a genus of corkscrew-shaped bacteria responsible for Lyme disease and relapsing fever, two of the most significant tick-borne infections worldwide. These bacteria belong to a larger family called spirochetes, named for their distinctive spiral shape. An estimated 129,000 cases of Lyme borreliosis alone are reported each year in Europe, and hundreds of thousands more occur in North America and Asia, making Borrelia one of the most clinically important bacterial groups transmitted by insects and ticks.
Shape, Size, and Movement
Borrelia bacteria are extremely thin and long relative to their width, roughly 0.3 micrometers in diameter and 5 to 20 micrometers in length. To picture that: if a human hair is about 70 micrometers wide, you could line up more than 200 Borrelia cells across its cross-section. Their hallmark feature is a flat, wave-like body that corkscrews through tissue with surprising efficiency.
What makes them so mobile is a set of internal motors called endoflagella, ribbon-like structures anchored at each end of the cell. Borrelia burgdorferi, the primary Lyme disease species, carries 7 to 11 of these flagella at each pole. Unlike the external flagella most bacteria use to swim, these internal motors do double duty: they both propel the cell and physically create its spiral shape. When the motors stop, the cell loses its characteristic form and can no longer move through tissue. This motility is central to how the bacteria cause disease. Laboratory studies have shown that paralyzed Borrelia cells injected into mouse skin simply stay put at the injection site, unable to spread.
Two Major Disease Groups
The genus Borrelia splits into two genetically and biologically distinct groups: one causes Lyme disease and the other causes relapsing fever. The differences between them are significant enough that some researchers have proposed formally splitting the genus into two separate genera. For now, both remain under the Borrelia name, but the distinction matters because the diseases they cause, the ticks that carry them, and the way infections unfold are all quite different.
Lyme Disease Borrelia
The Lyme disease group is known as the Borrelia burgdorferi sensu lato complex, a collection of at least 18 identified species. Three of these commonly infect humans: B. burgdorferi (the dominant species in North America and also found in Europe), B. afzelii, and B. garinii (both primarily found in Europe and Asia). Eleven species in the complex are strictly associated with Eurasia, while five were initially thought to exist only in North America. The clinical picture varies by species, ranging from a skin rash to joint inflammation and neurological problems.
Lyme borreliosis has earned the nickname “the great imitator” because its symptoms can mimic many other conditions. The earliest sign is typically an expanding circular rash called erythema migrans, which appears at the site of the tick bite. Left untreated, the infection can progress to arthritis, heart rhythm disturbances, or neurological complications over weeks to months.
Relapsing Fever Borrelia
Relapsing fever Borrelia species cause recurring episodes of high fever separated by periods where the person feels relatively well. Some of these species are carried by soft-bodied ticks in the Ornithodoros genus, which feed quickly, completing a blood meal in as little as five minutes to an hour depending on their life stage. These ticks can live for up to ten years, maintaining the bacteria across many feeding cycles. Other relapsing fever species are transmitted by human body lice, particularly in crowded living conditions.
A newer addition to this group is Borrelia miyamotoi, which blurs the line between the two categories. It’s a relapsing fever species, but it’s transmitted by the same hard-bodied blacklegged ticks (Ixodes scapularis and Ixodes pacificus) that carry Lyme disease. The CDC classifies the resulting illness as hard tick relapsing fever. Symptoms typically appear about two weeks after a bite, though the window ranges from 3 days to 6 weeks. Most people experience a single fever episode along with chills, fatigue, headache, nausea, and muscle or joint aches. About 10% of cases involve the classic relapsing pattern of recurring fevers.
How Borrelia Spreads Through the Body
After a tick deposits Borrelia into the skin, the bacteria multiply locally and begin triggering an inflammatory response. What happens next depends on whether and how quickly the infection is treated. The organisms are highly invasive. They use their corkscrew motility to push through tissue, and they produce surface proteins that help them stick to blood vessel walls, slip through vessel linings, and anchor themselves in tissues on the other side.
The most efficient route for early spread is through the bloodstream and lymphatic system. Once in circulation, the bacteria can reach the joints, heart, and nervous system. They also produce proteins that help them evade the immune system, which is why untreated infections can persist for months or even years. This combination of tissue adhesion, immune evasion, and aggressive motility is what allows a single tick bite to eventually produce symptoms in organs far from the original bite site.
How Tick Feeding Enables Transmission
Not all tick bites transmit Borrelia, and timing matters. For Lyme disease, the bacteria initially live in the tick’s midgut. When the tick begins feeding, the organisms migrate to its salivary glands and then into the host. This process takes time. Optimal transmission of B. burgdorferi occurs after about 48 hours of tick attachment, and the hard ticks that carry it feed slowly over five to seven days. This delay is why prompt tick removal significantly reduces infection risk.
Relapsing fever ticks operate differently. Ornithodoros ticks feed rapidly, sometimes in under an hour, which means transmission can happen before a person even realizes they’ve been bitten. For B. miyamotoi carried by hard ticks, the same general principle applies as with Lyme: the longer the tick stays attached, the greater the chance of transmission.
Diagnosis
Diagnosing Borrelia infections, particularly Lyme disease, relies on a two-step blood test that detects antibodies your immune system produces in response to the bacteria. The first step is a screening test. If that comes back negative, no further testing is needed. If it’s positive or borderline, a second, more specific test is run on the same blood sample. The result is only considered positive when both steps are positive.
Laboratories traditionally used two different types of tests for each step, but increasingly they’re using a modified approach where both steps use the same type of antibody test, which can be faster and easier to standardize. One important limitation: antibodies take time to develop, so testing done in the first few days after a tick bite may come back negative even if infection has occurred. In early Lyme disease, the characteristic rash is often the most reliable indicator, and treatment may begin before blood tests turn positive.
Treatment
Most Lyme disease infections respond well to a 10 to 14 day course of oral antibiotics. The antibiotics most commonly prescribed are doxycycline, amoxicillin, or cefuroxime axetil. Earlier treatment generally leads to faster and more complete recovery, which is one reason recognizing the early rash matters.
Preventive antibiotics after a tick bite are not routinely recommended. However, in areas where Lyme disease is common, a single dose of doxycycline given shortly after a bite can lower the risk of developing infection. This approach is typically reserved for situations where the tick was attached long enough to plausibly transmit the bacteria.
Relapsing fever caused by other Borrelia species is also treated with antibiotics, though the specific approach varies depending on the species and severity. Most people recover fully with appropriate treatment, but delayed diagnosis, particularly with relapsing fever species that may not be on a clinician’s radar, can lead to more prolonged illness.