Benign theileriosis is a tick-borne disease caused by intracellular blood parasites belonging to the Theileria orientalis group (BATOG). This disease represents no threat to human health. To date the disease has been found in Australia in Victoria, Queensland and New South Wales.
Although the prevalence of BATOG remains relatively small compared to some cattle health problems, it is becoming more widespread.
It is estimated that the disease can cost the Australian beef industry up to A$100,000 in prevention measures and production losses. The average cost to affected beef herds is A$11,600 or about A$67/head.
Cause
Bush ticks are mainly a cattle parasite, but are able to attach to other mammals including wildlife, birds, livestock (including horses, sheep, goats and poultry) and domestic animals such as dogs and cats. In sheep, bush ticks prefer to attach mainly on body parts not covered by wool.
The most common sites of attachment on cattle are around the tail, on the udder, inside the legs, on the brisket, in the ears, and occasionally on the face and neck.
Although it may cause tick irritation and local reactions in all species, H. longicornis only transmits benign theileriosis to cattle.
Symptoms
The disease is known as bovine anaemia. Signs are those associated with severe anaemia and include: lethargy, lack of appetite, exercise intolerance (weak cattle that lag behind the mob if moved).
If forced to run they may stagger and gasp for breath and some may collapse and die. Their gums will be pale and/or yellow.
Pregnant cows may abort and still births are common. In dairy cows a drop in milk production will occur. Death rates are highest in heavily pregnant cows.
Testing of animal blood samples has been demonstrated as an effective detection tool for the main strains of Theileria orientalis parasites found in Australian herds.
Disease is generally seen when calves are 8-12 weeks old. By about six months of age, immunity develops and it is rare to see disease in calves older than six months and adults who have been resident in the area.
Treatment
Treatment options for benign theileriosis are limited to supportive care and symptomatic treatment.
Blood transfusion has been performed occasionally on valuable animals. Animals improve following transfusion but it is expensive and not practical if multiple animals are involved.
Most importantly, stress and movement of affected cattle should be minimised or their reduced ability to transport oxygen throughout the body may lead to collapse. They should be rested, nursed and given high quality feed. Handling of affected cattle should be avoided where possible; if movement or yarding is necessary, move animals slowly.
Researchers have found that the rate of spread differs on individual properties, possibly due to vectors and other factors that need to be investigated to develop better prevention methods. Drugs registered in Australia for other diseases have low efficacy against Theileria parasites.
A number of chemicals are known to work against the parasite, including buparvaquone (BPQ), which was effective in one small-scale Australian trial. Investigations and negotiations are underway to make available an effective drug as soon as possible, including compliance with regulatory requirements and location of a willing supplier.
Of the few drugs that could potentially be used against Theileria orientalis infections, the most promising is buparvaquone (BPQ), which has been used effectively against exotic Theileria parasites in Asia since the 1980s.
Prevention
In districts where Theileria is commonly found (endemic areas) and most adult cattle are immune, calves should be closely inspected when they are 6-12 weeks old.
Introduced cattle should be examined closely when they have been in the district for three to eight weeks.
In districts where Theileria is normally not present, but cattle from Theileria-infected areas have been introduced, check home cattle regularly between two and six months after the introductions. If signs of disease are noted, seek veterinary advice as treatment when animals are mildly affected has been most successful.
Following simple biosecurity procedures is the best action producers can take to help prevent the spread of the disease. Here are some specific preventative steps for producers:
- When buying in new stock, ascertain their health status. Avoid importing animals from known affected properties or localities.
- Where the health status of bought-in stock is unknown, treatment with a registered tick treatment may be advisable prior to introduction. When using insecticides, always consult with your veterinarian and remember to observe the prescribed withholding periods before marketing products of treated animals.
- Rotational grazing practices may also help control ticks; the use of non-bovine species may act as ‘vacuum cleaners’ to remove ticks from pasture before the introduction of cattle.
- Cattle showing clinical benign theileriois must not be stressed. They should be rested, nursed and given high quality feed.
Tick control
Reducing tick numbers using a registered acaricide should reduce the likelihood of cattle becoming infected. Although suppression of tick numbers will not do anything for animals already infected, it may reduce new instances of transmission.
It is highly unlikely that bush ticks from a property can be eradicated as they spend more than 9 months living on the ground and can attach to other animal hosts such as wildlife. A nil tick population through chemical treatment should therefore not be the aim of any tick control program.
Where a producer is experiencing severe problems with benign theileriosis, a more intensive acaricide regime may be unavoidable. Cattle will require treatment every two to three weeks for a few months during summer and autumn. It must also be noted that frequent usage of acaricides may engender chemical resistance in the tick population and will adversely affect livestock product marketing due to the lengthy withholding periods applicable. All aspects of tick control must be discussed in detail with livestock owners prior to implementing a control program.
It is important to note that on some properties, benign theileriosis has occurred where no bush ticks have been seen on cattle, and the actual mode of spread on all properties has not been determined. It is suspected that other vectors or methods of spread could be involved.
Rotational grazing practices may also help to control ticks; the use of non-bovine species may assist with removal of ticks from pasture prior to the introduction of cattle.