2020–2021 BCSC Basic and Clinical Science Course™
8 External Disease and Cornea
Chapter 10: Infectious Diseases of the External Eye: Microbial and Parasitic Infections
Chlamydiae are spherical or ovoid obligate intracellular parasites of mucosal epithelium with a dimorphic life cycle. The infectious form is the elementary body (EB), which develops within an infected host eukaryotic cell into the intracellular replicating form, the reticulate body (RB). Only the EB survives outside the host, and only the EB is infectious. Reticulate bodies divide by binary fission to produce 1 or more EBs within a cytoplasmic vacuole, seen on light microscopy as a cellular inclusion.
Borrelia species are obligate parasites, best visualized with Giemsa stain. B burgdorferi, the etiologic agent of Lyme disease, is transmitted to humans by the deer tick. The clinical manifestations of Lyme disease are divided into 3 stages, with the most characteristic feature of stage 1 (local disease) being a macular rash, known as erythema chronicum migrans. The pathogenic factors of B burgdorferi include the expression of proteinases that facilitate tissue invasion, the induction of proinflammatory cytokines upon binding to phagocytes, and the activation of the complement cascade. Although the organism can be cultured from biopsy specimens taken from erythema migrans skin lesions, the diagnosis of Lyme disease is determined by the results of serologic testing and typical clinical findings. See BCSC Section 1, Update on General Medicine, and Section 9, Uveitis and Ocular Inflammation, for further discussion.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.