Having sheep measles on your farm is like having an STD — embarrassing but not life threatening, Dan Lynch says.

He's the national project manager for Ovis Management, which attempts to keep the parasites that cause it at bay. The cysts in sheep meat called sheep measles are a stage in the life cycle of tapeworms that affect dogs.

Whanganui — and especially Fordell and Mangamahu — have had a higher than average level of sheep measles for the past three years.

Lynch is not sure why, because farmers appear to be doing the right thing — dosing their dogs monthly with pills that cost $1 or $2 each.

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What's frustrating is not being able to pinpoint the source of the problem, an affected Mangamahu farmer said.

"Is someone bringing an unwormed dog on to our property? Are people you trust to be worming doing it conscientiously?"

The prevalence in Whanganui could be because our large farms breed sheep in the hills and fatten them on the coast. Or it could be because there are lots of sheep, and lots of people who take dogs out on rural roads or into paddocks.

"The key message we are trying to get out there is that if people are taking dogs into rural or sheep areas, dose them at least 48 hours before," Lynch said.

Nationally, 0.57 per cent of lambs for slaughter have the cysts of sheep measles, or the lesions they cause. That percentage has dropped from 0.64 per cent in 2016.

The tapeworm that has sheep measles as its intermediate stage, Taenia ovis, has come into focus since hydatids was wiped out and mandatory dog dosing for parasites ended in 2002.

Dogs with tapeworms excrete millions of tiny eggs with their faeces. The eggs blow around, are carried around by flies and last up to six months on pasture. Sheep eat them, and cysts and lesions form in their flesh. When a dog eats that flesh the adult tapeworm can develop inside it and start laying eggs.

The parasite is spread through dogs eating untreated sheep meat — the cysts are killed by freezing or cooking — or by eating dead sheep left exposed in a paddock.

Meat affected by sheep measles is not a human health risk, but it's unsightly and can be downgraded at meatworks, losing income for farmers. The best prevention is to worm all dogs on farms monthly, even dogs that are pets.

Town dogs walking on rural roads or farms should be wormed at least 48 hours before their outing and some farmers require vet certification before letting new dogs on their land.