Feral cats carrying the disease toxoplasmosis are the target of a predator programme that could save Hawke's Bay farmers in excess of $4.5 million dollars a year.

A monitoring programme testing ewes on six farms, as part of the Cape to City predator programme, has found that up to 30 per cent of sheep carry the disease, which causes a high abortion rate in pregnant ewes.

Three "experimental" farms within the 26,000-hectare Cape to City footprint tested feral cats and mice for toxoplasmosis while three control farms outside of the footprint tested mice only.

Sixty sheep on each farm have also been sample tested to form a baseline across the farms that have been matched in size, stocking density and habitat.

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Hawke's Bay Regional Council Biosecurity adviser Rod Dickson said the baseline was high but "that was expected" and by reducing feral cats, it is hoped abortion rates will decrease.

"Feral cats are one of the main carriers of toxoplasmosis and if we can reduce the numbers of feral cats, we have a good chance of reducing the high abortion rate in ewes.

"This could provide a significant economic benefit for farmers," he said.

Mr Dickson said toxoplasma is highly prevalent in New Zealand sheep flocks with a recent survey testing 198 ewe flocks revealed 85 per cent of sheep had been exposed to the disease.

Sheep become infected from eating contaminated food such as pasture, concentrate feeds and hay.

Once ingested, the disease spreads to the sheep's muscles and brain " and also to the placenta. Shielded from the ewe's defence system the parasite multiplies rapidly, killing cells as infection spreads.

Feral cats and mouse tissue samples have been tested by Landcare Research with results due in early 2016.

The farms will be tested again in late September 2016 which will give the first insight into the success of feral cat control programme.

"The testing we've done gives us a good baseline to measure and monitor over the five year duration of Cape to City. The outcome will provide a very good measure of potential economic success.

Cape to City is a wide scale predator control and ecological restoration programme targeting ferrets, stoats, rats, hedgehogs, feral cats and possums over 26,000ha of land between Hastings and Cape Kidnappers.