Rickettsiales

BACTERIA

Overview

• Cause systemic diseases in animals
• Usually use arthropod vectors

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
• Host and cell type specificity
• 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
• Q fever and Rocky Mountain spotted fever are zoonoses

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

Rocky Mountain spotted fever

• Caused by Rickettsia rickettsii
• Affects humans and dogs
• Various tick vectors, which acquire the organism from small mammals
• Transovarial and trans-stadial transmission within the tick population
• Organisms replicate in endothelial cells of infected dogs, cause vasculitis, increased vascular permeability and haemorrhage

• Clinical signs
  • Incubation period 2-10 days; course less than 2 weeks
• Fever, depression, conjunctivitis, retinal haemorrhage, muscle and joint pain, coughing, dyspnoea, oedema of extremities
• Neurological signs in dogs include stupor, ataxia, neck rigidity, seizures and coma
• Death from cardiovascular, neurological or renal damage in severely-affected animals
• Gross pathology includes haemorrhage, splenomegaly and lymphadenopathy

• Diagnosis
  • Rising antibody titre during an indirect fluorescent antibody test or ELISA
  • Thrombocytopenia and leukopenia during early phase

• Treatment and control
  • Tetracycline therapy for two weeks
  • Tick removal from dogs

Canine monocytic ehrlichiosis

• Generalised disease of dogs in tropical and subtropical regions
• Caused by Ehrlichia canis
• The brown tick, Rhipicephalus sanguineus, is the main vector
• Dogs may carry the organism for 2 years after resolution of clinical signs

• Clinical signs
  • Incubation period of 3 weeks
  • Acute, subclinical or chronic phases
  • Acute phase: fever, thrombocytopenia, leukopenia and anaemia
  • Subclinical phase: low blood cell numbers but minimal clinical signs; can progress to a severe disease, tropical canine pancytopenia
  • Chronic phase: bone marrow depression, haemorrhages, neurological disease, peripheral oedema, emaciation and hypotensive shock

• Diagnosis
  • E. canis morulae present in mononuclear cells in Giemsa-stained blood smears
  • Rising antibody titre detected by indirect immunofluorescence
  • Culture in canine macrophages

• Treatment and control
  • Doxycycline for 10 days
  • Fluid therapy or blood transfusion
  • Prophylactic tetracyclines

Canine cyclic thrombocytopenia

• Caused by Ehrlichia platys
• Affects platelets
• Recurrent thrombocytopenia, but dogs usually asymptomatic
• Seroconversion detected by indirect immunofluorescence

Potomac horse fever

• Caused by Ehrlichia risticii
• Occurs during the summer
• Fluke vector
• Infects epithelial cells of colon and monocytes, macrophages and mast cells

• Clinical signs
  • Fever, anorexia, depression, diarrhoea, colic, leukopenia, laminitis
• 30% mortality
• Can cause abortion
• Hyperaemia of large intestine at post mortem

• Diagnosis
  • Seroconversion detected by indirect immunofluorescence or ELISA

• Treatment and control
  • Oxytetracycline for 1 week
  • Inactivated vaccines

Equine granulocytic ehrlichiosis

• Caused by E. equi
• Clinical signs: fever, depression, ataxia, limb oedema, icterus and petechial haemorrhages
• Low mortality
• Diagnosis: presence of morulae in neutrophils during early disease; seroconversion; leukopenia
• Treatment: tetracyclines

Bovine petechial fever

• Caused by E. ondiri
• Disease of cattle in Kenya and other countries of East Africa
• Tick vector
• Clinical signs: intermittent fever, depressed milk yield, petechiation of mucous membranes and conjunctiva
• Death from pulmonary oedema
• Diagnosis: presence of organisms in Giemsa-stained blood smears
• Treatment: tetracyclines

Tick-borne fever

• Caused by E. phagocytophila
• Affects ruminants in Europe
• The tick Ixodes ricinis is the main vector
• Animals remain carriers for up to 2 years, but are immune to reinfection

• Clinical signs
  • Incubation period of 2 weeks; recovery in 2 weeks
  • Fever, anorexia, reduced growth rates in young animals
  • Reduced milk production, abortion, still birth
  • Leukopenia and thrombocytopenia
  • Causes immunosuppression in young lambs causing susceptibility to tick pyaemia and louping ill

• Diagnosis
  • Intracytoplasmic morulae in neutrophils of Giemsa-stained blood smears during acute phase
  • Seroconversion detected by immunofluorescence

• Treatment and control
  • Oxtetracycline
  • Tick control
  • Prophylactic tetracyclines for lambs during first few weeks of life

Heartwater

• Severe disease of ruminants in sub-Saharan Africa caused by Cowdria ruminantium
• Vectors include Amblyomma ticks