Category:Rickettsiales

Overview

• Cause systemic diseases in animals
• Usually use arthropod vectors
• Host and cell type specificity
• Q fever and Rocky Mountain spotted fever are zoonoses

Characteristics

• Non-motile, pleomorphic Gram-negative organisms
• Obligate intracellular pathogens
• Require live cells for culture such as tissue culture cells or embryonated eggs
• Require Romanowsky stains

• Include two families, Rickettsiaceae and Anaplasmataceae
• Rickettsiaceae have cell walls that contain peptidoglycan; they target endothelial cells and leukocytes
• Anaplasmataceae lack cell walls; they target erythrocytes

Epidemiology

• Rickettsiae replicate in gut epithelial cells of arthropod vectors and spread to other organs such as salivary glands and ovaries
• Transmission occurs during feeding on the animal host
• Transovarial or trans-stadial transmission occurs in the arthropod vectors
• Most ricketsiae have limited survival in the environment, apart from Coxiella burnetii, which undergoes aerosol transmission

Pathogenesis and pathogenicity

• Many rickettsiae target endothelial cells of small blood vessels; they produce phospholipase which damages phagosome membranes, escaping into the cytoplasm
• Ehrlichia target leukocytes or platelets, and inhibit phagosome/lysosome fusion
• Anaplasmataceae localise within vacuoles or on the surface of red blood cells; they may alter red cell antigens causing immune-mediated damage. Anaemia may result from haemolysis or removal of red blood cells

Identification

• Giemsa-stained blood or tissue smears identify blue/purple organisms
• Fluorescent antibody technique for specific identification
• Isolation in embryonated eggs or tissue culture lines
• Nucleic acid probes and PCR
• Inoculation of susceptible animals