Theileria spp. are found in blood of both normal healthy cattle and those cattle clinically affected by Theileriosis.
The increase in incidence of Theileriosis in beef cattle over the past few years has realised a need to better understand the underlying prevalence of Theileria spp. in beef cattle and allow the clinician to interpret laboratory results in context.
Prior to work published by Stewart et al. in 1992, it was believed Theileria spp. was more prevalent east of the Great Dividing Range. Region 6 from Stewart's study which includes the Darling Downs, demonstrated 100% (29/29) of herds sampled were serologically positive for Theileria buffeli. Further west of Roma where no Theileria was previously believed to exist, 75% (30 /40) of herds tested were sero-positive. Stewart et al. reported individual sample prevalence's of 76.8% and 36.7% respectively for these two regions.1
A prevalence study undertaken on the northern tablelands by New England LHPA in 2009 demonstrated a herd prevalence of 72% and individual animal prevalence of 22%.2
A study commissioned by Meat and Livestock Australia in 2010 undertook PCR testing in cattle herds across Queensland, NSW and Victoria. 3 In that study 20 NSW herds tested reported a herd prevalence of 45% and an individual animal prevalence of 23.7%. In Victoria, the 10 herds tested reported a herd prevalence of 80% and an individual animal prevalence of 34%. This study also demonstrated that using serology or polymerase chain reaction (PCR) in parallel with blood films would identify additional infected herds who may be sero-positive but not have detectable organisms visible on blood smears.1,4,5
Detection of Theileria spp. in a smear or PCR confirms the presence of Theileria spp. but does not confirm the diagnosis of Theileriosis as positive results for Theileria spp. are of limited value when viewed in isolation.
The level of Theileria spp. parasitaemia in individuals may give an indication of clinical significance particlulary when the levels are significant. However low levels of parasitaemia do not preclude Theileriosis.
Similarly, the presence of a regenerative anaemia is helpful however given the temporal variability of individual PCV levels and stages of regeneration, the lack of regenerative anaemia does not preclude the diagnosis of Theileriosis. Where ever possible, cohorts should also be blood sampled. This increases the likelihood of meaningful results and may demonstrate Theileria spp presence with regenerative anaemia and or significant parasitaemia levels in other animals.
An epidemiological approach should be taken to increase the likelihood of diagnosis and remove some of the error associated with individual and temporal variability. A thorough history of the mob should be sought including movements, management activities undertaken (e.g. tick control) and previous clinical history.
There is a need to work to a case definition for Theileriosis to ensure that Theileria spp. does not become the replacement of snakebite for the diagnostically destitute.
In diagnosing Theileriosis an attempt should always be made to exclude other diseases which cause similar clinical signs of anaemia, icterus, lethargy, abortion, illthrift and death.