Theileria Associated Bovine Anaemia (TABA)

The recent NZ Vet Journal had a whole issue on the TABA epidemic. Here are a few highlights gleaned from the MPI and Massey authors:

Theileria orientalis was first reported in NZ in 1984, but the anaemia epidemic spanned from August 2012 to March 2014. In the first 18 months there were 496 case herds (reported to MPI); 79% were dairy and 21% beef.

Of 882 individual cases (submitted) 93% were positive for Theileria orientalis Ikeda. This is a protozoan parasite. There are 10 other types of T. orientalis, 3 of which have been found in NZ (chitose , buffeli and type 5). Chitose occasionally causes anaemia. Australia and Japan have also been significantly affected by Theileria orientalis, although it is found worldwide.

Theileria is carried and transmitted by the cattle tick Haemaphysalis longicornis. The larvae, nymphs and adults all feed on ruminants, but different individuals. The infection is not passed onto the tick eggs. A tick can pick up the parasite from the blood of an infected cow, and then pass it on to the next cow it feeds from.  The tick-borne parasite is a protozoan, and the piroplasm form causes destruction of red blood cells, leading to anaemia. Animals can recover from infection, but the parasites may persist possibly for life; periods of stress such as calving can lead to a relapse. Disease is more frequently seen when naïve animals are introduced into an endemic area, or when infected animals are introduced to properties with ticks. The ticks are found throughout the North Island, plus Nelson and Golden Bay, and TABA has only occurred in tick areas.

Some interesting figures: Incidence risk 1%, cumulative mortality 0.23%, case fatality rate 17%.

It is believed the epidemic’s origin was the importation of infected cattle. There appear to be two scenarios of Theileria spreading. Firstly via cattle movements: farms that had Theileria were more likely to have brought cattle onto the farm, and from a greater distance. Trucking in infected cattle to a naive area would probably initiate each of these local outbreaks. Obviously the peak movement of 500,000 cattle happens on June 1st, leading to the mixing of infected and naïve cattle and ticks, and spread of the disease. Then a second localized spread via tick vectors: cases were seen in clusters within 20-30 days and up to 15-20km from a case farm. Infected tick nymphs can be carried to neighbouring properties by wildlife such as hares, horses, deer, dogs, goats and sheep. 100 ticks have been noted on the ears of a hare!

The time period from infection by tick bite to the parasite being found in the bloodstream is 10 days, and the number of bugs increase to 30-40 days, which is when the animal has the most severe anaemia and clinical signs. The peak of TABA in October 2013 therefore probably arose from infection soon after calving via the re-emergence of over-wintered nymphs. By the time you see a cow sick from it, she has been infectious to ticks for over a month already!

Beef calves were at greater risk of TABA than dairy calves, and calves were the most affected age group in Northland. This is probably because beef calves are born later (coinciding with the peak questing period of overwintered nymphs), dairy calves are housed for the first few weeks, and tick density is higher in Northland with earlier resumption of tick activity. Data from Africa suggests that as an area becomes endemically stable, the incidence of obvious disease falls and is often restricted to young animals; this may explain why there was lower risk of disease in Northland. In contrast, the Waikato bore the brunt of TABA with adult dairy cattle.

The prediction is that some areas will reach endemic stability (with clinical disease not being seen much as ticks and theileria are a constant factor), but that epidemics (outbreaks of disease) may continue in areas of marginal tick habitat. Practical treatments still focus around pasture management, tick control, animal husbandry (minimizing stress), supportive therapy and, if needed, blood transfusion. Buparvaquone is effective but the with-holding is too long to be practical. Some research work for a potential vaccine was being done in Japan.

Theileria

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