Category:Rickettsiales

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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

Rickettsia rickettsii

Ehrlichia canis

Ehrlichia platys



Ehrlichia risticii

Ehrlichia equi

Ehrlichia ondiri

Ehrlichia phagocytophila

Heartwater

  • Severe disease of ruminants in sub-Saharan Africa caused by Cowdria ruminantium
  • Vectors include Amblyomma ticks
  • Carrier status can occur for up to 8 months in adult cattle and wildebeest
  • Clinical disease in calves and lambs
  • Clinical signs
    • Incubation period 4 weeks
    • Acute fever
    • Neurological signs including chewing, twitching of eyelids, circling, high-stepping gait, convulsions and recumbency; can be fatal
    • Subacute cases suffer from hydropericardium, hydrothorax and pulmonary oedema and congestion; splenomegaly and haemorrhages may occur
  • Diagnosis
    • Clinical and post mortem findings in endemic areas suggestive
    • Orgnisms present in nuclei of endothelial cells in Giemsa-stained brain tissue
    • PCR, indirect immunofluorescence, ELISA and western blot
  • Treatment and control
    • Tetracyclines during early disease
    • Immunisation using infected blood


Bovine anaplasmosis

  • Disease of cattle in tropical and sub-tropical regions caused by Anaplasma marginale
  • Carrier status in endemic regions with clinical disease occuring in times of stress
  • High mortality rate in naive adult cattle
  • Morulae localise inside red blood cells close to the cell membrane
  • Transmitted via the Boophilus tick as well as flies and contaminated instruments
  • Clinical signs
    • Inculbation period 2-12 weeks
    • Anorexia, fever, depression, reduced milk yield, weight loss
    • Anaemia and icterus
    • Sudden death from hypoxia can occur
  • Diagnosis
    • Clinical signs and haematology suggestive
    • Organisms present in erythrocytes of Giemsa-stained blood smears
    • Immunofluorescence of blood smears, PCR, serology
  • Treatment and control
    • Long-acting oxytetracycline or imidocarb early in the disease
    • Supportive therapy
    • Vaccination of animals being introduced into an endemic region


Feline infectious anaemia

  • Caused by Mycoplasma haemofelis ( until recently, known as Haemobartonella felis)
  • Organism found on surface of erythrocytes
  • Common in roaming tom-cats of 1-3 years old
  • Possibly transmitted via bite wounds or arthropods
  • Recoverd cats become carriers
  • Clinical signs
    • Peracute disease is associated with severe anaemia and immunosuppression, and can be rapidly fatal
    • Fever, anaemia, depression, weakness and jaundice occur in the acute form
    • Anaemia, lethargy and weight loss occur in the chronic form
    • Recovery can occur with a regenerative bone marrow response
    • Severe feline infectious anaemia may occur in the presence of feline leukaemia virus
  • Diagnosis
    • Organism intermittently seen on the surface of red blood cells in Giemsa-stained blood smears
    • Immunofluorescence of blood smears
    • Reduced packed cell volume; regenerative anaemia
  • Treatment
    • Doxycycline for 3 weeks early in the course of the disease
    • Blood transfusion
    • Flea control

Canine haemobartonellosis

  • Dogs infected with Haemobartonella canis usually asymptomatic
  • Acute haemolytic anaemia may occur in dogs after splenectomy, immunosuppressive drug therapy or immunosuppressive infections


Q fever

  • Caused by Coxiella burnetti
  • Influenza-like disease of humans in contact with farm animals
  • Transmitted to humans by inhalation from parturient sheep, goats and cattle
  • Organisms replicate in female genital tract and mammary glands of ruminants
  • Shedding occurs in uterine discharges, foetal fluid and milk
  • Infections in animals usually subclinical
  • Sporadic abortions occur in sheep, goats, cattle, cats
  • Infertility may result in ruminants, as well as placentitis or endometritis
  • Hepatitis, myocarditis and interstitial pneumonia may occur in affected foetuses
  • Diagnosis: MZN-stained smears of placental tissue and uterine discharges; immunofluorescence; PCR; culture; serology
  • Control: disposal of infected placenta and separation of pregnant ruminants; inactivated vaccines