Benign theileriosis has the potential to cause significant losses in beef herds. Despite theileriosis being associated mainly with introduced adult cattle, a number of properties in the southern part of the Northern Tablelands have suffered significant losses in home-bred calves over the past three years.
In October 2011, a 400 cow beef breeder enterprise at Yarrowitch on the eastern escarpment of the Northern Tablelands, had four calves, aged 12 weeks, die within a week. A fifth calf was reported to the New England LHPA as being 'down' and was following a similar pattern to the previously dead calves. Calf marking had not taken place but was imminent. The herd was split into nine separate mobs each set stocked on improved pasture. The dead calves were only in one mob.
The calf had died at the edge of a dam, not having moved from where it had been observed recumbent earlier that day. The calf appeared a good weight and was heavily muscled.
The carcass was very pale (Figure 1). The liver enlarged being approximately 1.5 times the normal size and was a diffuse bright ochre colour (Figure 2). Despite the colour of the liver there was no generalised jaundice of the carcase. The heart had splash haemorrhaging across the surface. The blood appeared not to clot and there was no heart clot present. The spleen was enlarged. There were no other abnormalities detected.
After the post mortem another calf, approximately 12 weeks of age, was found recumbent. This calf was depressed, with very pale mucous membranes and temperature of 41.8°C.
Blood was taken for smears to confirm theileriosis.
Smears from the recumbent calf revealed organisms consistent with Theileria sp present in <1% of RBCs were found. The PCV was 8.0% (normal range 20-50%).
Liver histology on samples from the dead calf indicated bilirubin accumulation. Marked hepatocyte injury was not noted, suggesting haemolysis as the likely cause.
This property was due to undertake calf marking. Despite the fact that theileria affected mobs are often advised to avoid any forms of stress, the current pasture conditions were conducive for clostridial disease. As a result, calf marking (and vaccination) was advised to go ahead.
A previous theileria prevalence study undertaken by the New England LHPA identified a possible link between the reduced presence of theileria and properties that undertook tick control measures, so the use of an acaracide was advised to be undertaken at the time of marking.
The percentage of theileria organisms found in red blood cells was 'rare' however the author has found that this is often the case in acutely ill animals and is exacerbated by a low PCV.
Benign theileriosis is often associated with introduced adult cattle however it can also be seen in calves approximately 8-14 weeks of age. Severe anaemia and pyrexia is often observed. The lack of jaundice in calves differs from that seen in adults affected by theileriosis. The post mortem findings in this case are consistent with benign theileriosis. It is the author's experience that affected calves are more likely to die than clinically affected adult cows.
The age of calves that are affected is consistent with the pre-patent period for theileria if the calf is seen as a 'recent introduction' to the herd.
This property had no recent introductions aside from a bull from a property near Inverell. This mob was grazing adjacent a neighbour who trades in cattle. It is possible that these traded cattle, which often consist of cattle from the coast, may have been the source of the theileria and combined with the high tick activity seen at this time of year.
* Postscript: At the time of writing this article, the author used the term "benign theileriosis". This term has been in use to distinguish this disease from the far more serious diseases caused by other species of Theileria not present in Australia, such as east coast fever (caused by Theileria parva) that occurs in Africa, and Tropical Theileriosis (caused by T. annulata). As the disease typified by this case report can result in significant mortalities, the disease occurring in Australia is now referred to as "Bovine Anaemia caused by Theileria orientalis group."