Gene expression profiling as a tool for understanding the host response to cattle ticks

The cattle tick, 'Rhipicephalus Boophilus microplus' (formerly 'Boophilus microplus'), is the most significant parasite of cattle in Australia and threatens cattle production in tropical and subtropical countries worldwide. Some cattle breeds of mainly 'Bos indicus' origin develop a strong resistance to infestation with 'R. B. microplus', while other breeds of mainly 'Bos taurus' origin will succumb to anaemia and 'tick worry' in tick-infested pastures. Composite cattle ('B. indicus' x 'B. taurus') can have very wide ranges of tick resistance (Utech et al., 1978). Resistance to cattle tick infestation is primarily manifest against the larval stage and results in the immature tick failing to make a successful attachment and obtain a meal (Seifert, 1971). It is widely accepted that resistance to tick infestation in cattle is immunologically mediated and involves both cell-mediated and humoral immune responses. Previous research has demonstrated a role for the hypersensitivity response in the rejection of larvae from resistant 'B. taurus' cattle. Gene expression profiling using microarrays and quantitative PCR (qPCR) are popular tools to complement more classical measurements of immune reactivity and function. The immune response to 'R. B. microplus' infestation in 'B. indicus' and 'B. indicus' x 'B. taurus' cattle has not been studied in great detail. Using a combination of gene expression tools, together with measurements of immune system function such as histology and ELISA, we aimed to describe those responses of highly tick-resistant cattle that differ from those of tick-susceptible cattle.