Research shows sheep may help spread bovine anaemia parasite

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New Zealand researchers have proven that sheep can carry and potentially spread the parasite behind the bovine anaemia epidemic.

Sheep were asymptomatic carriers of the tick-borne parasite Theileria orientalis, which meant they were unaffected by the disease but could spread it to the ticks that feed on them.

Theileria orientalis is a tick‒borne intracellular parasite of red blood cells that causes severe and mild infections in various ruminants worldwide.

To date there have been 11 types identified within this species, of which four types are presently found in New Zealand cattle, researchers said.

Since 2012, New Zealand suffered a substantial epidemic of infectious bovine anaemia in both dairy and beef cattle associated with the Ikeda type.

Researchers believed the speed at which the disease spread through the North Island suggested that other species could have been involved in transmission.

Therefore, the aim of a series of related experiments was to test the null hypothesis that sheep can't maintain T. orientalis Ikeda type infection or infect ticks that feed on them.

Several studies were conducted over two years to address this hypothesis.

The studies showed that sheep can have detectable levels of T. orientalis Ikeda type infection, in both the acute and chronic phase, and that Haemaphysalis longicornis larvae can become infected when feeding on sheep.

Researchers recorded no anaemia, weight loss or clinical disease in the sheep in the acute phase of infection.

The levels of infection recorded in the sheep were much lower than those found in cattle, consistent with the sheep being asymptomatic carriers of T. orientalis Ikeda-type infection, researchers said.

The researchers suggested it could explain how the disease spread so quickly through the North Island after the outbreak in 2012.

The findings are published in Veterinary Parasitology on sciencedirect.com.