Red blood cells infected with Anaplasma marginale

On this page:

• Cause
• Occurrence and spread
• Animal susceptibility
• Clinical signs
• Post-mortem examination
• Diagnosis
• Treatment and control of outbreaks
• Prevention

In Australia, bovine anaplasmosis is a form of tick fever. The disease was probably introduced to this country as early as 1829 by cattle from Indonesia infested with the cattle tick Boophillus microplus.

Losses during an outbreak can be as high as 20-30%, although mortality rates of 5-10% in newly infected herds are more common. In persistently infected herds, losses of 1-2% each year are common if control procedures are not instituted.

Cause

Anaplasmosis in cattle is caused by an organism called Anaplasma marginale. It invades and multiplies in red blood cells. As the disease progresses, infected and even uninfected red blood cells are destroyed mainly in the liver and spleen, resulting in anaemia and even death in severe cases.

Occurrence and spread

Bovine anaplasmosis is spread primarily by the cattle tick Boophilus microplus. It is only found in the northern and eastern parts of Australia where the cattle tick is present.

The cattle tick can become infected at any stage of its life cycle after feeding on an animal carrying Anaplasma organisms in its blood stream. The organism multiplies in the tick and passes to later stages of the tick life cycle. However, the infection is not passed on to the eggs in Boophillus microplus. Consequently, the next generation of ticks will not be infected unless they also feed on a carrier animal. Therefore an infected stage of the tick must transfer to a susceptible animal for transmission to occur. Because the adult male tick is more mobile and lives longer than other stages, it is the most likely to transmit the disease.

Biting flies can transmit the disease mechanically but are less efficient vectors than ticks. Mechanical transmission via veterinary instruments (needles, dehorners etc.) is also possible and the organism can cross the placenta to the foetus.

Animal susceptibility

Calves from immune mothers receive temporary protection (maternal antibody) from the colostrum (first milk) which prevents clinical disease in the short term, although calves can still be infected in that time. This protection lasts about three months and, in most cases, is followed by an age resistance, which lasts until the animals are about 9-12 months old.

Calves exposed to bovine anaplasmosis when the maternal or age resistance is high rarely show clinical symptoms but develop a solid, long lasting immunity. It is therefore possible to have both Anaplasma marginale and cattle ticks present on a property without animal losses or clinical disease. This situation is known as endemic stability.

The age resistance gradually wanes and, if cattle have not been exposed to Anaplasma as calves, they will become increasingly susceptible to the disease. If susceptible adult cattle are mixed with infected cattle in the presence of the cattle tick, serious losses due to bovine anaplasmosis can occur.

Cattle that recover from bovine anaplasmosis remain carriers of the organism (often for life) and are immune to further disease.

There is no clear evidence that Zebu (Bos indicus) cattle are any more resistant to disease than European breeds (Bos taurus)

Clinical signs

The severity of symptoms is age related, but acutely affected animals will show:

• rapid loss of condition
• transient fever (40-41 ⁰C at peak infection)
• weakness and respiratory distress, particularly after exercise
• depression and loss of appetite
• mucous membranes pale (anaemia) then yellow (jaundice)
• urine is often brown due to bile pigments, but not red as in babesiosis
• severely affected animals may die.

Post-mortem examination

Cattle that die of bovine anaplasmosis are generally very anaemic and jaundiced. Blood is thin and watery and the spleen is enlarged. The liver may be mottled and yellow-orange, and the gall bladder is often enlarged and contains thick brown or green bile.

Diagnosis

History, clinical signs and post-mortem lesions are often suggestive of anaplasmosis, but it is impossible to make an accurate diagnosis based on these signs alone. The best method of identifying the agent in infected animals is by microscopical examination of stained thin blood films.

Treatment and control of outbreaks

Oxytetracycline antibiotics as well as Imidocarb (Imizol^® or Imidox^®) can be used to treat bovine anaplasmosis. Prompt treatment in the early stages of acute disease usually ensures survival, but severely infected cattle may die in spite of treatment.

Note: Oxytetracycline and imidocarb have withholding periods and restrictions apply in Australia to the use of Imidocarb in lactating dairy cattle.

Prevention

There are several options for the prevention of bovine anaplasmosis in Australia, ranging from keeping animals tick-free to vaccinating with tick fever vaccine.