B101 - ANAPLASMOSIS

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B101 - ANAPLASMOSIS

Nature of the disease
Anaplasmosis is a form of ‘tick fever’ in cattle, caused usually by the rickettsia Anaplasma marginale and sometimes by Anaplasma centrale. It is characterised by initial high fever and progressive anaemia.
Classification
OIE List B disease
Susceptible species
Cattle, Bos indicus type cattle are less susceptible because of their greater resistance to tick infection.
Distribution
A. marginale is widely distributed throughout the tropics and sub-tropics. It occurs in Africa, the Middle East, Asia, Australia, the USA, Central and South America, southern Europe. In Australia the cattle tick, Boophilus microplus, is the main vector for anaplasmosis and the infection is widespread throughout the tick’s endemic area.
Clinical signs 
There is considerable variability in the severity of anaplasmosis with severity generally increasing with the age of the animal. 

Affected cattle have:

  • Steadily increasing temperatures
  • Anaemia, weakness and respiratory distress particularly after exercise
  • Depression and anorexia become more obvious as the disease progresses
  • Jaundice and frequently a marked loss of condition
  • Urine is often brown due to the presence of bile pigments
  • Severely affected animals may die
Post-mortem findings 
Gross pathology findings are due to anaemia and resulting anoxia. These include:
  • Pale and often jaundiced tissues
  • Watery blood and an enlarged spleen with a soft reddish-brown pulp
  • Liver is also enlarged , yellow-brown and often mottled
Differential diagnosis 
Other causes of haemolytic anaemia may need to be considered. These include: 
  • Babesiosis
  • Leptospirosis
  • Bacillary haemaglobinuria
  • Rapeseed poisoning
  • Eperythrozoonosis
  • Chronic copper poisoning
Specimens required for diagnosis 
Anaplasmosis can be confirmed by microscopic examination of stained blood smears. Serological testing for antibodies to A. marginale may also be useful, particularly in convalescent animals, or for assessing the status of an individual or herd.
Transmission   
Anaplasma is a parasite of red blood cells and vectors are necessary for its transmission between animals. 

Ticks are the natural vectors and a range of tick species has been shown to be capable of transmitting infection (Boophilus, Dermacentor, Rhipicephalus, Ixodes, Hyalomma, Argas and Ornithodoros). In the absence of ticks, biting flies can maintain the disease though these are less efficient vectors.

 The cattle tick, Boophilus microplus, is the major vector for anaplasmosis in Australia, Rhipicephalus sanguineus has also been shown to be capable of transmitting infection. Biting flies are considered important in the USA. 

Mechanical transmission of infected blood via veterinary instruments is also possible.

Risk of introduction   
The major risk of introducing anaplasmosis is through the importation of infected livestock. Whether the disease would establish depends on the presence of suitable vectors.
Control / vaccines  
Effective control of tick fevers has been achieved by a combination of measures directed at both the disease and the tick vector. Tick control by acaracide dipping is widely used in endemic areas.

Acaracides used for this purpose include various synthetic pyrethroids, amitraz, and some organophosphates. Dipping may be done as frequently as every 4-6 weeks in heavily infested areas. 

The occurrence of resistance of ticks, chemical residues in cattle and environmental concerns over the continued use of insecticides has led to use of integrated strategies for tick control. 

Anaplasmosis vaccines are readily available and are highly effective. The most commonly used vaccine is a live Anaplasma centrale vaccine used either singly or in combination with Babesia bovis vaccine. Vaccination is particularly important for susceptible cattle entering endemic areas. 

An anti-tick vaccine is also commercially available in Australia. This vaccine is of limited use, but can be used as part of an integrated program for the control of ticks.

References
  • Anaplasmosis, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 21-23
  • Anaplasmosis, In Veterinary Medicine, Saunders, Eight ed, 1997, London p.1146-1150
  • Office International des Epizooties, 2002