Since being detected it has not spread far beyond this area, presumably due to its requirement for mild summer temperatures and moisture at ground level. Although tick infestations can be significant in the major endemic zones, this has not been seen in Western Australia, and routine control measures are not warranted.
The chief effect of the bush tick on host animals is due to the sucking of blood and in endemic areas, heavy infestations can cause severe anaemia, decreased growth rates and rarely, deaths in younger cattle. In older animals the combination of a continued low-grade anaemia and 'tick worry' can reduce production performance. However, significant effects have not been seen in WA, presumably because climatic conditions do not permit large populations to develop on individual animals. Other stock species rarely acquire the large burdens of bush ticks seen in cattle, but sheep can develop a particularly severe reaction at the site of tick attachment, which can cause extreme irritation. Ticks can be found on dogs, wildlife and occasionally on humans.
Cattle which have been exposed to ticks develop a degree of resistance to further tick infestation, and hence calves in their first year usually acquire heavier burdens than older stock. Cattle introduced from tick-free areas are likely to acquire more ticks than locally raised stock. The number of ticks seen on individual cattle usually varies greatly and they may be seen on only a small proportion of a herd. The bush tick is a different species from the cattle tick (Rhipicephalus microplus), which in WA occurs only in the Kimberley region, or various tick species seen on wildlife.
The bush tick is known to transmit the protozoal disease Theileriosis in cattle on the eastern seaboard and in WA this has been detected on a number of properties, mostly in the Denmark-Walpole area.
Biology of the tick
The bush tick orginated from the northern Pacific and is primarily a parasite of cattle, but readily infests many other warm blooded animals. Adult ticks are usually dark brown in colour and grow to about the size of a pea when fully engorged with blood from a host. The favoured sites of attachment are around the tail, on the udder, inside the legs, on the brisket, in the ears, and occasionally on the face and neck. On sheep, non-woolled areas are preferred.
The bush tick is referred to as a three host tick, because each of three life cycle stages must attach to a host for a few days before continuing development. Adult ticks suck blood from a host before dropping to the ground and laying several thousand eggs. Over some weeks the eggs develop to larvae, which climb onto the vegetation to await a passing host. After several days on a host animal, the larvae fall off and moult to become nymphs. Nymphal ticks again seek a host and suck blood for several days, before dropping off to moult to the adult stage. Both larvae and nymphs are very small and are not easily seen with the naked eye. The life cycle occurs over about 12 months.
Adult ticks are seen mainly during early summer, larvae from late summer to early winter and nymphs, mainly in spring. Larvae and nymphs which have not found a host can reportedly remain alive in the vegetation for over 12 months if conditions are favourable (though this is unlikely under WA conditions). On any one property, adult bush ticks are usually seen for only a few weeks in each year, at most, before disappearing.
The free-living (ground dwelling) stages of the bush tick have very specific climatic requirements. Moisture must be continually available and extremely hot temperatures are unfavourable. This limits the tick's distribution to areas where some rainfall occurs year round and summer temperatures are not excessive. In Australia, bush ticks are especially abundant in a narrow strip along the northern coast of New South Wales, and found as far north as Gympie in Queensland and south to Gippsland in Victoria. Even within favourable climatic zones, bush ticks require a moist, sheltered ground environment. Short, open pasture is usually too dry to permit survival of the vulnerable free living stages.
In the sub-tropical areas on the eastern Australian coast, where bush tick populations are heaviest, control measures are based on tickicide chemicals and pasture management, though this is not routinely necessary. The most effective products have a persistent effect, as individual ticks attach for only a few days, and hence protection against continual infestation is needed. These chemicals are available as dips, sprays, or pour-on products. This level of control has not been necessary in WA. If tick infestations were of concern, the risk of re-infestation would be largely eliminated by preventing susceptible stock (especially calves) from grazing paddocks containing scrub or heavy dense pasture.
The bush tick in WA
Following the discovery of the bush tick at Walpole in 1983, farms in the district were inspected annually for several years to determine the extent of establishment. The tick is now established on cattle farms in the Walpole, Hazelvale and Denmark districts, with recent reports closer to Albany, but it appears unlikely to spread significantly further, or to become of major significance in its own right. However, as the bush tick is the vector for the Theileria organism, disease related to this could potentially occur on farms within the tick’s distribution range.
No other tick species in WA outside the Kimberley region are considered to be of importance to agriculture and ticks found on humans may cause irritation but pose no disease risk.