The agent responsible for Lyme disease is a type of bacteria. This microscopic organism is the underlying cause of the illness and its range of symptoms. The disease is named after Lyme, Connecticut, where an unusual outbreak of arthritis in children in 1975 led researchers to investigate its origins. This investigation revealed the bacterial source of the condition.
Identifying the Borrelia Bacterium
The bacterium that causes Lyme disease belongs to a group known as Borrelia. The primary species in North America is Borrelia burgdorferi, first identified in 1982 by scientist Willy Burgdorfer. This bacterium is a spirochete, meaning it has a spiral or corkscrew shape. This form allows it to move in a twisting motion, drilling through tissues and enabling its spread throughout the body.
While Borrelia burgdorferi is the most common causative agent in the United States, it is not the only one. A species named Borrelia mayonii has also been found to cause Lyme disease, primarily in the upper Midwest. In Europe and Asia, other species are more prevalent, including Borrelia afzelii and Borrelia garinii. These different species can be associated with variations in symptoms; for instance, B. afzelii is often linked to skin manifestations, while B. garinii is more commonly associated with neurological symptoms.
The Tick Transmission Cycle
The Borrelia bacterium is transmitted to humans through the bite of an infected black-legged tick. In the eastern and midwestern United States, the primary vector is Ixodes scapularis, also known as the deer tick. On the Pacific coast, the western black-legged tick, Ixodes pacificus, is responsible for spreading the bacteria.
The tick’s life cycle consists of three stages: larva, nymph, and adult. Ticks are not born with the Borrelia bacterium; they acquire it by feeding on an infected host, typically a small mammal like a mouse or a vole. These animals act as reservoirs for the bacteria, and the tick becomes infected during its larval or nymphal stage.
Transmission to humans most often occurs from the bite of a nymphal tick. Nymphs are very small, about the size of a poppy seed, which allows them to feed unnoticed for the extended period required for bacterial transmission. For transmission to occur, an infected tick must remain attached and feeding for 36 to 48 hours. This duration provides the necessary window for the bacteria to move from the tick’s midgut to its salivary glands and then into the human bloodstream.
How the Agent Causes Disease
Once the Borrelia bacterium enters the human body, it multiplies and spreads from the site of the tick bite. The bacteria travel through the bloodstream to different parts of the body, establishing infections in tissues like the joints, nervous system, and heart. This dissemination is what leads to the wide array of symptoms associated with Lyme disease.
A key aspect of the bacterium’s ability to cause persistent infection is its capacity to evade the human immune system. Borrelia can change the proteins on its outer surface, which makes it difficult for the body’s immune cells to recognize and eliminate it. This process of altering surface proteins allows the bacteria to hide from the immune response and establish a long-term presence in the body.
The presence of the bacteria in different tissues is what causes the specific symptoms of Lyme disease. When the bacteria invade the joints, they can cause the inflammation that leads to Lyme arthritis. If they penetrate the nervous system, they can cause conditions like facial palsy or meningitis. The bacteria’s ability to spread widely explains why Lyme disease can present with such a diverse set of symptoms.
Methods for Detecting the Agent
Confirming the presence of the Lyme disease agent can be approached through two main types of methods: direct and indirect detection. Direct methods look for the bacterium itself, while indirect methods search for the body’s response to the infection. Direct detection is not the standard method used for routine diagnosis.
Direct detection often involves a technique called polymerase chain reaction (PCR). A PCR test can identify the genetic material (DNA) of the Borrelia bacterium in a sample of body fluid, such as blood, synovial fluid from a joint, or cerebrospinal fluid. While highly specific, PCR tests for Lyme disease are most useful in specific clinical situations, such as diagnosing Lyme arthritis, where the bacteria may be concentrated in the joint fluid.
The more common approach for diagnosing Lyme disease is through indirect methods, specifically serological testing. This involves a two-tiered process that detects antibodies produced by the immune system in response to the infection. The first step is typically a sensitive enzyme-linked immunosorbent assay (ELISA), and if the result is positive or equivocal, it is followed by a more specific test called a Western blot. This two-step process confirms the presence of an immune response targeted specifically at Borrelia burgdorferi, providing evidence of infection even when the bacteria themselves are difficult to find.