P.PSH.1312 - Immunisation against virulent genotypes of Theileria orientalis

Summary

The intracellular protozoal parasite Theileria orientalis has rapidly spread across South-eastern Australia, substantially impacting local cattle industries since 2006. NSW-DPI have estimated an average cost of $59K for dairy producers and $11.6K for beef producers, which equates to AUD $131/head for dairy cattle and AUD $67/head for beef cattle for farms impacted by the parasite (Bailey 2012); all costing around $20m pa nationally (Lane et al., 2015). Interestingly, several studies from Australia and New Zealand have indicated that the carrier state arising in recovered dairy cattle did not compromise subsequent productivity (Perera et al., 2014; Lawrence et al., 2019).

Objectives

The research in project P.PSH.1312 were to:

• confirm that inoculation of naïve cattle with bovine blood infested with Theilerial genotypes (buffeli, ikeda, or chitose) will protect cattle from development of clinical theileriosis, preventing anaemia and death.
• determine whether this blood immunisation will prevent the weight loss incurred when cattle arrive in an endemic region of virulent theileriosis and are immediately subjected to tick challenge.
• Integrate successful outcomes into control measures on-farm through communications to Industry.

Key findings

Subcutaneous inoculation of blood containing T.orientalis ikeda or buffeli developed the “carrier state” in preinfected cattle after 4-6 weeks. In 2 field trials, when cattle were moved into a region of endemic theileriosis, the preinfection significantly reduced the first peak of ikeda parasitosis.

Depending on the intensity of tick challenge, animals preinfected with ikeda blood fared better than those given buffeli blood. Significant reductions in weight loss in preinfected cattle also depended on the level of tick challenge and was only significant when high intensity challenge occurred where controls lost between 5-20kg around the first peak of parasitaemia at 6-9 weeks after arrival. Weight loss could be recovered by compensatory gain if nutrition was available.

This result confirms speculation that carrier cattle possess some “premunity” that protects against the severity of repeated, seasonal tick challenges. It is noted that preinfection will not work for newborn calves in endemic areas as the virulent genotypes reach the first peak of parasitaemia by 6 weeks before any preinfection could induce protective coverage.

Benefits to industry

The 3-host tick is a proven biological vector and final host for transmission of Theileria orientalis ikeda. It would appear capable of effective transmission for T. orientalis chitose, but is confirmed as a poor vector for T. orientalis buffeli. In endemic Theilerial zones in NSW and southern states, this host-parasite relationship means that the virulent genotypes appear first after tick infestation, effectively negating any ability of T. orientalis buffeli or blood inoculation to generate any effective immunity unless this is deliberately undertaken in non-endemic zones prior to introduction. The tick also has the advantage of carrying theileria through 2 moults. If infected as larvae, the nymph does not require a boost to infection on the second host, whether this is an uninfected cow, a dog or a wildlife host.

Similarly, it must be appreciated that the nymphal stage is the usual stage to over-winter, and this stage can remain infective for at least 6 months.

Therefore, killing ticks in coastal areas by paddock spelling is difficult due to favourable climatic conditions and the presence of alternative hosts such a kangaroos, to transport and re-contaminate paddocks.

Future research

Should development of isoxazoline preparations (red in Table 2) for use in cattle be forthcoming, these would be a substantial support for cattle producers and without doubt, marketing from the relevant companies would ensure widespread uptake, especially in Queensland for control of babesiosis and anaplasmosis (Tick Fever).

It is also considered that the seasonal distribution of H. longicornis be more definitively mapped to determine danger zones for cattle movements. This is especially the case for the fringe regions such as the New England areas in NSW and in the South Australian borderlands and in Tasmania.