ANAPLASMATACEAE, Rickettsiales

 https://www.sciencedirect.com/topics/immunology-and-microbiology/anaplasmataceae

Anaplasmataceae

The family Anaplasmataceae includes intracellular organisms that reside within vacuoles of hematopoietic cells (erythrocytes, monocytes, neutrophils, and platelets) and includes (but is not limited to) the genera Ehrlichia, Anaplasma, and Neorickettsia.

From: Small Animal Pediatrics, 2011

Chapters and Articles

You might find these chapters and articles relevant to this topic.

Etiologic Agents of Infectious Diseases

Joanna J. Regan, William L. Nicholson, in Principles and Practice of Pediatric Infectious Diseases (Fourth Edition), 2012

Description of the Pathogens

The family Anaplasmataceae is in the order Rickettsiales and contains six genera of obligate intracellular bacteria. Three of the genera contain members that can infect humans. These zoonotic agents are transmitted by invertebrate vectors and cause potentially life-threatening diseases in humans and animals. As of 2010, four species in the genus Ehrlichia and one species in the genus Anaplasma have been documented causes of human disease (Table 170-1). Four species responsible for disease in the United States, Ehrlichia chaffeensis, E. ewingii, E. muris-like, and Anaplasma phagocytophilum, are all transmitted by ticks and have been described since 1987. In addition to these species, an Ehrlichia canis-like agent has been identified from blood of an asymptomatic patient in South America.¹ It is possible that additional Ehrlichia and Anaplasma species capable of causing human disease will be described in the U.S. and worldwide.

Ehrlichia and Anaplasma are small (0.5 to 1.5 µm) gram-negative cocci.² The genomes of these organisms are relatively small with a size of 1.18 Mb for E. chaffeensis and 1.47 Mb for A. phagocytophilum.³ Ehrlichia and Anaplasma species that infect humans are specialized for infecting leukocytes. After attachment to membrane receptors and subsequent induction of phagocytosis,⁴ the organisms multiply in early endosomes that fail to develop into mature phagocytic or lysosomal vacuoles. Although the complete repertoire of cell receptors used is not known, A. phagocytophilum binds to the P-selectin binding domain of the P-selectin glycoprotein ligand PSGL-1 on human granulocytes before cell internalization.⁵ Within the cell cytoplasm of infected leukocytes, organisms replicate within membrane-bound vesicles, developing into microcolonies called morulae. The morphologic features of morulae vary among the species in size, number of individual organisms, and the presence of a fibrillar matrix.⁶ Individual morulae can contain more than 50 bacteria, which are released into the extracellular space after cell lysis or by exocytosis after fusion of the vacuole membrane with the plasma membrane.² Ehrlichiae in morulae stain with eosin-azure-type stains and occasionally are observed in circulating leukocytes of acutely ill patients.

How Ehrlichia and Anaplasma cause disease in humans is incompletely understood, although they may kill infected leukocytes directly.² Perivascular inflammatory infiltrates become apparent in many organs, and morulae can be visualized within the leukocyte populations present in these infiltrates.⁷ However, factors leading to the inflammatory response are not known. Mechanisms of many of the hematologic and biochemical perturbations (e.g., cytopenia and abnormal serum hepatic enzyme levels) are not clear. Infections do not cause the vasculitis or endothelial damage characteristic of other rickettsial diseases (e.g., Rocky Mountain spotted fever (RMSF) caused by Rickettsia rickettsii).