Experts believe that ensuring the high-health status of bought-in animals is a main way of helping stem the spread of BVD and Johne's disease.

Observing good on-farm hygiene and bio-security and a co-ordinated testing, vaccination and culling programme will also help contain outbreaks and ensure herds and flocks return to good health.

Producers of stock afflicted by either or both of the incurable diseases have suffered crippling financial losses.

Over 80 attended a meeting on Monday evening in Thurso as part of a nationwide animal health roadshow run by the Moredun Research Institute in Edinburgh.

Senior veterinary virologist Peter Nettleton said there are very reliable tests to detect BVD or Bovine Virus Diarrhoea.

Vaccines are also very effective and have led to the elimination of the disease in a number of parts of the world, including Shetland and Norway.

However, he warned that BVD remains common in Scotland and producers have to be switched-on to the risk of infection. A critical time is up to midway through the gestation period for calves.

He said: "If the foetus is infected up to 120 days, the virus stays in the system and the calf will carry it throughout its life and pass it on to other animals."

Dr Nettleton said producers should be very wary not to buy in an infected animal. Checks should be done to ensure the replacement comes from a herd accredited as being free of BVD. Should any doubts come to light, the animal should be quarantined and tested before being allowed to mix with its new herd.

Dr Nettleton said care should also be taken to prevent cattle mixing with potentially infected animals during winter or summer lets or at agricultural shows.

He added: "If your herd is BVD-free you're going to have to work extremely hard to keep it out. You need to take active measures to keep it out through good hygiene and bio-security."

Dr Nettleton recommends producers joining an animal health scheme. He is keen to see the establishment of a nationally accredited assurance scheme which would proscribe the sale of the calves carrying the virus.

Moredun colleague and Johne's disease expert Karen Stevenson said the condition is very difficult to diagnose until its latter stages.

Telltale signs of chronic diarrhoea, lameness and severe weight loss tend to exhibit two to four years after the animal is infected. It is an intestinal disorder, which affects cattle, sheep and other ruminants. Transmitted by the ingestion of faeces, young animals are particularly vulnerable to contracting it.

Infected stock will exhibit poor milk yield and low fertility, and result in a significant fall in the market value of herds and flocks.

Her advice to keep Johne's disease at bay and to contain its spread is similar to Dr Nettleton's recommendations for checking BVD.

"Doing nothing is not an option," said Dr Stevenson, a microbiologist. "If you don't act to control it, it is going to be bad news for you."

Dr Stevenson advocates the preparation of individual farm action plans to protect the health and welfare of livestock.

Apart from good on-farm management practices, she recommends close monitoring of affected herds and flocks and the culling of infected members of breedstock.

Vaccinations for Johne's disease are problematic as they do not prevent infection and interfere with the test for TB. Dr Stevenson said a DNA-based test has recently been introduced for the disease while research is ongoing to come up with a test that can detect it at a much earlier stage.

Frank Stephen, of Thurso vets D.S. MacGregor & Partners, gave a rundown of the impact of the two diseases in the Far North. He believes the incidence of BVD in the area would reflect a recent national survey which found that 16 per cent of beef herds are infected.

"We've no reason to believe that Caithness is any different from the rest of Scotland," said Mr Stephen. "We've no reason to be smug."

He said he is aware of between 30 and 40 Far North herds in which BVD has either been eradicated or an eradication programme started.

"The biggest, single problem we've found is getting people to vaccinate their cattle after their herds are free of the disease," he explained.

"I know of four herds which were free of the disease where it has since come back."

Referring to the potential dangers of bought-in stock, Mr Stephen said this applies to purchases Far North producers make from Orkney.

He said: "We get a lot of calves coming from Orkney. I know there is an active BVD eradication programme in Orkney but it's only 90 to 95 per cent clear.

"I'm not slagging off Orkney but just warning people to be careful and not think that everything coming off the island is BVD-free."

He said producers can also let their guard slip when a bull does not perform in season and a replacement is bought at short notice without regard to its health status. Alternatively, infection can be spread when a bull is leant to another holding. Mr Stephen said the financial consequences of BVD can be catastrophic.

A recent study had shown that the loss over a 10-year period to a 100-cow herd could be £40,000, with the main hit being in the first two years.

Mr Stephen said that BVD-affected herds typically suffer problems with scour, pneumonia, fertility and lameness.

He said if producers cannot keep a closed herd, they need to buy replacement stock from accredited sources and carry out a regular vaccination programme.

He added: "If you don't know the health status of stock you buy in, you need to quarantine them got a month and test them."

Mr Stephen said Johne's disease has been a sporadic problem in Caithness.

"It's probably been under-reported," he stated. "We tend to pick up on one or two cases on individual farms.

"I know of two or three herds which have had a real problem and have dealt with it by testing and culling infected animals."

He said good hygiene, particularly in and around calving time, is vital in preventing the spread of the disease.

Mr Stephen also recommended the cull of infected animals and their offspring.

A control programme, he said, will pay dividends though it could take three years to clear the disease.

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