Success in breeding sheep that are resistant to scrapie has so far been mixed, Professor Steve Bishop of the Roslin Institute in Scotland told an audience of animal scientists in Wageningen this week. Bishop talked about four projects in which he and colleagues are trying to breed salmon, pigs and sheep that are resistant to infectious diseases.
In the Roslin scrapie project, the researchers have examined the prion protein PrP that all sheep possess. The gene occurs in different variants, some of which are associated with an increased susceptibility to scrapie and others which do exactly the opposite. There are now breeding programmes directed at developing sheep strains with the gene that makes them least likely to develop scrapie. In the laboratory tests it also appears that the animals with this gene are less susceptible to BSE. However, the situation is not quite as straightforward as it appears. “Animals with the ‘right’ gene can still develop scrapie, we discovered recently,” says Bishop. “They are not resistant, but they take longer to develop the disease after being infected, which is also good.”
At Roslin researchers are also working on a type of salmon with resistance to the IPN (infectious pancreatic necrosis) virus found in many fish farms on the North European coasts. The virus affects salmon whose immune system is negatively affected during transport when they are released from the nursery ponds. At present there is a twenty percent mortality rate due to IPN among farmed salmon.
“We are carrying out experiments with a large number of families of salmon, for which we have made gene arrays,” explained Bishop. “We release them in ponds where we know that IPN is present. Divers retrieve the dead fish from the water so that we can determine which genes are responsible for weaker animals.”
Justifying the research Bishop sketched the economic background to animal disease. The costs of animal diseases are high, he said. “In the developed countries one in five farm animals dies from disease, and in the third world the situation is worse: over a third succumb to an infection.” Bishop has no illusions however that breeding is ‘the’ solution to animal diseases such as BSE and IPN. “Hygiene is also important, as are vaccines and medicines, and that’s unlikely to change. And there will always be diseases that no genetic combination can stand up to. But the costs to farmers are so high that we definitely need to use this instrument as well, if we can get it to work.”
The price that agriculture must pay for these genetic techniques is herds with less genetic variation. You can breed animals that are resistant to a certain disease. “But does this not increase the chance of another disease simply wiping out a whole herd?” was the question raised by a member of the audience. Bishop thinks not. “In genetically varied populations you will indeed get a few percent that are resistant to another disease, and this will not be the case for a homogeneous population. In nature that few percent may make the difference between survival and extinction, but in the world of animal husbandry those differences are no longer relevant.”