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*Require Romanowsky stains
*Require Romanowsky stains
*Host and cell type specificity
*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
*