Category:Rickettsiales

This article is still under construction.

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

Cowdria ruminantium

Anaplasma marginale

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