Lyme disease is the most common illness transmitted to humans by ticks across the Northern Hemisphere. This infection begins when a person is bitten by an infected tick, which transmits spiral-shaped bacteria, known as spirochetes, into the bloodstream. Understanding the true complexity of Lyme disease requires looking beyond a single agent, as the illness is caused by a diverse group of bacteria within the Borrelia genus. The severity of the disease and the specific symptoms a patient develops depend heavily on which bacterial type causes the infection.
The Borrelia Complex: Defining the Primary Pathogens
The question of “how many strains” is best answered by examining the different species of bacteria responsible for Lyme disease. All Lyme-causing bacteria belong to the Borrelia burgdorferi sensu lato (Bbsl) complex. This complex comprises over 20 distinct genetic species, or genospecies, that circulate in the environment, though only a smaller number cause illness in humans.
The most common human pathogens within this complex are currently recognized as four distinct species. Borrelia burgdorferi sensu stricto (Bbss) is the species most widely known and is found in both North America and Eurasia. The other two primary species, Borrelia afzelii and Borrelia garinii, are predominantly found across Europe and Asia. A fourth species, Borrelia mayonii, was recently identified and appears to be exclusive to the upper Midwestern United States.
The term “strain” refers to genetic variants that exist within a single species. These intraspecies variations are significant because they can alter the bacteria’s virulence, or its ability to cause disease. For instance, certain strains of B. burgdorferi sensu stricto are more invasive and likely to cause disseminated infection. This genetic diversity is one reason why the presentation of Lyme disease can vary even in the same geographic region.
Regional Differences in Lyme-Causing Bacteria
The distribution of the pathogenic Borrelia species is highly dependent on geography and the specific tick species that acts as the primary vector. In North America, the black-legged tick (Ixodes scapularis) and the western black-legged tick (Ixodes pacificus) are the main carriers. These ticks primarily transmit B. burgdorferi sensu stricto, which accounts for nearly all U.S. Lyme disease cases. B. mayonii is a rare exception, detected only in the upper Midwest.
The situation is far more complex across Eurasia, where the sheep tick (Ixodes ricinus) and the taiga tick (Ixodes persulcatus) are the principal vectors. The Eurasian tick populations are infected with a greater diversity of Borrelia species compared to North America. The two dominant species in this region are B. afzelii and B. garinii, though B. burgdorferi sensu stricto is also present.
This geographic difference has a substantial impact on public health and diagnostic methods globally. Diagnostic tests developed for use in North America are often designed to detect antibodies specifically against B. burgdorferi sensu stricto. These tests may not recognize the distinct proteins produced by the Eurasian species, such as B. afzelii or B. garinii. This discrepancy can lead to potentially missed or delayed diagnoses for individuals infected with non-North American species.
Varying Symptoms Driven by Bacterial Species
The genetic differences between Borrelia species are not merely academic distinctions; they directly influence the clinical outcome of the infection. Each species shows a preference, or tropism, for different tissues in the human body, which leads to distinct symptom profiles. This means that the species causing the infection can determine whether a patient is more likely to develop joint problems, skin issues, or neurological complications.
Infections caused by Borrelia burgdorferi sensu stricto, the species dominant in North America, have a strong affinity for joint tissue. This tropism frequently leads to Lyme arthritis, which typically involves swelling and pain in large joints, most commonly the knee.
In contrast, Borrelia garinii, one of the most common Eurasian species, has a pronounced tropism for the nervous system. Infections with B. garinii are the main cause of neuroborreliosis in Europe, often presenting with severe nerve pain and meningitis-like symptoms, sometimes referred to as Bannwarth syndrome.
The third major species, Borrelia afzelii, primarily targets the skin. While all Borrelia species can cause the initial expanding rash known as erythema migrans, B. afzelii is strongly associated with Acrodermatitis chronica atrophicans (ACA). This chronic condition causes a progressive, inflammatory skin atrophy that can develop months or years after the initial infection.