Lyme Disease - Pathophysiology

Pathophysiology

Borrelia burgdorferi can spread throughout the body during the course of the disease, and has been found in the skin, heart, joint, peripheral nervous system, and central nervous system. Many of the signs and symptoms of Lyme disease are a consequence of the immune response to the spirochete in those tissues.

B. burgdorferi is injected into the skin by the bite of an infected Ixodes tick. Tick saliva, which accompanies the spirochete into the skin during the feeding process, contains substances that disrupt the immune response at the site of the bite. This provides a protective environment where the spirochete can establish infection. The spirochetes multiply and migrate outward within the dermis. The host inflammatory response to the bacteria in the skin causes the characteristic circular EM lesion. Neutrophils, however, which are necessary to eliminate the spirochetes from the skin, fail to appear in the developing EM lesion. This allows the bacteria to survive and eventually spread throughout the body.

Days to weeks following the tick bite, the spirochetes spread via the bloodstream to joints, heart, nervous system, and distant skin sites, where their presence gives rise to the variety of symptoms of disseminated disease. The spread of B. burgdorferi is aided by the attachment of the host protease plasmin to the surface of the spirochete.

If untreated, the bacteria may persist in the body for months or even years, despite the production of B. burgdorferi antibodies by the immune system. The spirochetes may avoid the immune response by decreasing expression of surface proteins that are targeted by antibodies, antigenic variation of the VlsE surface protein, inactivating key immune components such as complement, and hiding in the extracellular matrix, which may interfere with the function of immune factors.

In the brain, B. burgdorferi may induce astrocytes to undergo astrogliosis (proliferation followed by apoptosis), which may contribute to neurodysfunction. The spirochetes may also induce host cells to secrete products toxic to nerve cells, including quinolinic acid and the cytokines IL-6 and TNF-alpha, which can produce fatigue and malaise. Both microglia and astrocytes secrete IL-6 and TNF-alpha in the presence of the spirochete. This cytokine response may contribute to cognitive impairment.

In Lyme encephalopathy, diffuse white matter pathology can disrupt grey matter connections, and could account for deficits in attention, memory, visuospatial ability, complex cognition, and emotional status. White matter disease may have a greater potential for recovery than gray matter disease, perhaps because neuronal loss is less common. Resolution of MRI white matter hyperintensities after antibiotic treatment has been observed.

A developing hypothesis is that the chronic secretion of stress hormones as a result of Borrelia infection may reduce the effect of neurotransmitters, or other receptors in the brain by cell-mediated proinflammatory pathways, thereby leading to the dysregulation of neurohormones, specifically glucocorticoids and catecholamines, the major stress hormones.

This process is mediated via the hypothalamic-pituitary-adrenal axis. Additionally tryptophan, a precursor to serotonin, appears to be reduced within the central nervous system (CNS) in a number of infectious diseases that affect the brain, including Lyme. Researchers are investigating if this neurohormone secretion is the cause of neuropsychiatric disorders developing in some patients with borreliosis.

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