CANINE ERLICHIOSIS

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CANINE ERLICHIOSIS

Nature of the disease
Canine ehrlichiosis (tropical canine pancytopaenia — TCP) is a tick-borne rickettsial disease of dogs caused by Ehrlichia canis.
Classification
SPC List D disease
Susceptible species
The dog and some species of wild canidae are susceptible. Humans can become infected, but only through a tick bite. The disease cannot be spread directly from dogs to humans.
Distribution
TCP occurs in tropical and sub-tropical regions. It occurs over most of the range of its tick vector, the common brown dog tick (Rhipicephalus sanguineus), in Africa, the Middle East, east Asia, South East Asia, South America, the Caribbean and the USA (southern states and Hawaii).

The disease has been suspected in northern Australia, but has not been confirmed.

Clinical signs 
There is a wide range of clinical syndromes both acute and chronic. Carrier states can also exist. Commonly, four stages of disease are recognised
Febrile stage 
  • Fever
  • Lethargy, anorexia and loss of condition
  • Hind limb oedema in about 50% of cases
  • May be diarrhoea, epistaxis or other haemorrhages

Distemper-like stage 

  • Serous then mucopurulent rhinitis
  • Tonsillitis
  • Sub-mandibular lymphadenopathy
  • Coughing
  • Corneal capacity in many cases
  • Occasionally bronchopneumonia and death

Sub-clinical stage 

  • Apparent recovery, but progressive deterioration in haematological indices
  • Neurological signs — ataxia and hyperaesthesiaTerminal stage 

Death may occur following:

  • Extensive haemorrhaging with epistaxis
  • Pancytopaenia with associated weight loss, decreased stamina, bacterial infections, corneal opacity, and oedema
Post-mortem findings 
In severe form of the disease, the following may be seen:
  • Emaciated carcase
  • Pale mucous membranes
  • Oedema of the limbs, ascites and hydro-pericardium
  • Haemorrhages in gastro-intestinal tract, internal organs, sub-cutaneous tissues and eye
  • Enlarged lymph nodes and spleen
Differential diagnosis 
Other causes of anaemia, haemorrhage, fever, and skin conditions should be considered, such as:
  • Canine babesiosis
  • Trauma
  • Warfarin poisoning
  • Nasopharyngeal tumours
  • Canine distemper
  • Infectious canine hepatitis
  • Cutaneous leishmaniasis
  • Filarial dermatosis
Specimens required for diagnosis 
Blood samples — thin blood smears, whole blood for clinical pathology, for transmission tests (collected with EDTA or heparin anticoagulant), and serum.

At post-mortem, specimens of lung, spleen, liver and kidney should be collected in duplicate (one set in sterile containers, the other in neutral buffered for histopathology).

Any ticks found on the dog or in the vicinity should be collected for identification.

Transmission   
The disease is transmitted by the tick R. sanguineus. Trans-stadial transmission occurs and all tick stages can pass the infection on to dogs.
Risk of introduction   
Imported dogs and dogs visiting on yachts, fishing vessels etc. could introduce the disease to a previously free country. 

Apparently healthy carrier dogs may continue to carry the rickettsia for long periods. 

Suitable tick vectors need to be present for the disease to establish.

Control / vaccines  
Treatment of canine ehrlichiosis involves supportive care (intravenous fluids, blood transfusions may be required) and antibiotics. Tetracycline is the antibiotic of choice and needs to be given for a minimum of 2–3 weeks. Treatment with imidocarb is very effective but has a renal toxicity.

There is no vaccine available. Prevention depends on avoiding exposure to ticks that are likely to be carrying the disease.

References
  • Canine ehrlichiosis, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 562-564
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 440p