Among the many tasks that mean careful planning for farmers is that of 'scratching lambs'.

Non-farming readers will be intrigued to learn that this is not a description of a disease, but refers to its prevention. Orf (an old Norse word) is an unpleasant skin disease of sheep and, importantly, humans. It takes the form of raised, crusty lumps mostly on lambs’ mouths, ewes’ udders and on the fingers of those handling infected sheep. It is, I am told, acutely painful in people. The disease is caused by a virus that passes between sheep when the crusty areas of one sheep rub against broken skin of another sheep. There is no effective treatment and the lamb usually self-cures in a few weeks.

However, a severely infected lamb may be unable to eat properly and infection by normal skin bacteria can make the situation worse. In ewes, infection of the scabs on the teats can lead to mastitis and permanent udder damage. Lots of treatments have been tried over the years, with one of the most popular being 'Ovaloids' – small brown capsules, resembling tiny rugby balls, that were given to each lamb. As with many treatments for viral infections, they don’t work.

The best means of control is vaccination and this is where the, 'scratching lambs' comes in. To be effective, the vaccine has to be given in the form of a scratch made in the skin, into which the live vaccine trickles.

With a few days, the scar becomes raised and lumpy as the body responds to the vaccine and 'takes'. The scar is, in effect, the same as the lumps from normal infection and is capable of spreading the infection. As a result, it is vital that the vaccine is only ever used on a farm where orf is present. It is equally important that the vaccine is given somewhere that lambs can’t get at the scar – which usually means under the front leg.

Even with vaccination however, some farms struggle to control orf and there is a lot we still don’t know about how it remains on farms.